Among the top cosmological discoveries of the last century, Edwin Hubble’s analysis showing that the universe was expanding, or “exploding” as the Science News-Letter described it in 1931, is undoubtedly No. 1. Tom Siegfried, managing editor of Science News, has called the discovery “the greatest intellectual upheaval in the human conception of the cosmos since Copernicus.” From this point forward, the universe was no longer an unchanging backdrop for the unfolding of human dramas. The universe had its own drama, its own story to tell. All at once, there were a host of new questions to ask: Not only questions about the specifics of the expansion—how fast, how long, and why—but also questions about the beginning of the universe and its future.
Evidence of an expanding universe led some of the greatest minds in the history of physics to conclude that the universe began small, hot and dense and then shot outward, an event known today as the Big Bang. In a poetic moment, a reporter for the Science News-Letter wrote: “All the energies of which we are aware, from the bursting brilliance of giant stars that far outshine our sun down to the feeblest kicks of a dying protozoon, are but the varied expressions of that primal explosion, if this hypothesis holds good.” Solid evidence for a Big Bang came in the form of the cosmic microwave background radiation, essentially an echo of the universe’s birth, and future studies of that leftover radiation.
The Big Bang theory later led cosmologists to the idea of inflation, a brief period of superfast expansion that would have left most of the universe smooth but also planted the seeds for the structure we see today. Scientists thought they had a smoking gun to prove inflation correct with results from BICEP2, a South Pole telescope looking for signatures of primordial gravitational waves in the cosmic microwave background. But the finding was later attributed to galactic dust.
Beyond speaking to cosmic beginnings, the expansion rate of the universe, known as the Hubble constant, is a key value for calculating other properties of our universe: Its current age, its geometry (whether open, closed or flat) and what it’s made of. In 1998, two teams found that the expansion of the universe was actually accelerating, suggesting that some mysterious form of dark energy filled all of space. Next-generation telescopes should be able to pin down some of these properties by determining the Hubble constant with increasing precision. But as Siegfried describes in “Cosmic question mark,” the first article in this chapter, the road won’t necessarily be easy.
The Planck mission’s data put a kink in precision cosmology
By Tom Siegfried, April 5, 2014
For as long as humans have wondered about it, the universe has concealed its vital statistics—its age, its weight, its size, its composition. By the opening of the 21st century, though, experts began trumpeting a new era of precision cosmology. No longer do cosmologists argue about whether the universe is 10 billion or 20 billion years old—it was born 13.8 billion years ago. Pie charts now depict a precise recipe for the different relative amounts of matter and energy in the cosmos. And astronomers recently reached agreement over just how fast the universe is growing, settling a controversy born back in 1929 when Edwin Hubble discovered that expansion.
Except now the smooth path to a precisely described cosmos has hit a bit of a snag. A new measurement of the speed of the universe’s expansion from the European Space Agency’s Planck satellite doesn’t match the best data from previous methods. Just when all the pieces of the cosmic puzzle had appeared to fall into place, one piece suddenly doesn’t fit so perfectly anymore.
“Something doesn’t look quite right,” says astrophysicist David Spergel of Princeton University. “We can no longer so confidently go around making statements like all our datasets seem consistent.”
In other words, different ways of measuring the universe’s expansion rate—a number called the Hubble constant—no longer converge on one value. That calls into question the whole set of numbers describing the properties of the cosmos, known as the standard cosmological model. Accepting the new Hubble constant value means revising the recipe of ingredients that make up the universe, such as the dark matter hiding in space and the dark energy that accelerates the cosmic expansion.
Over the years, the Hubble constant’s value has been as elusive as it is important. Hubble himself badly overestimated the expansion speed, which depends on distance—the farther away two objects are, the faster space’s expansion pushes them apart. Hubble calculated that objects separated by a million parsecs (roughly 3 million light-years) would fly apart at 500 kilometers per second. At that rate, the universe would be, paradoxically, younger than the Earth.
Refined measurements gradually reduced the estimate to a more realistic realm. By the 1970s, experts argued over whether the Hubble constant is closer to 100 or to 50. By the late 1990s, Hubble Space Telescope observations of supernovas and other data placed the expansion rate value in the 70s, eventually settling in at around 73 km/s/megaparsec.
Confidence in that value was enhanced by measurements of the radiation glow left over from the Big Bang, primarily by a satellite probe known as WMAP. Its value for the Hubble constant was about 70, close enough to 73 that the margins of error for the two values overlapped.
But last year, the Planck satellite reported even more precise measurements of that glow—known as the cosmic microwave background radiation—implying a Hubble constant around 67. That was about 10 percent lower than the Hubble telescope value, a difference that most physicists found too big to ignore.
“We seem to be having some disagreement,” says Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., and leader of the team that measured the expansion rate using the Hubble telescope.
Freedman, Spergel and other experts expect that further refinements of the measurements will eventually resolve the conflict with no major repercussions. Nevertheless, the discrepancy was a constant topic of discussion in December at the Texas Symposium on Relativistic Astrophysics, held in Dallas. Krzysztof Górski of the Planck team acknowledged the disagreement during his talk at the symposium, but he noted that much of the Planck data has not yet been analyzed. “I think we should just stay calm and carry on,” he said.
It may just be that unknown problems in calibrating instruments afflict one or the other of the two methods. On the other hand, the chance remains that something might be wrong with science’s basic model of the universe.
“The latter possibility would be the most exciting,” says Spergel, “because it would point potentially to something about dark energy or some new physics that we could study.”
Already several speculative proposals have appeared offering novel ways to reduce or eliminate the disparity. Perhaps gravity itself has mass, leading the supernova method to overestimate the Hubble constant, one group proposes. Or maybe dark energy and dark matter, supposedly independent ingredients in the cosmic recipe, interact in a way that favors a Hubble constant higher than the Planck results suggest. And possibly an uneven distribution of matter in the universe means that the Hubble constant based on supernovas and other objects in the nearby (and therefore recent) universe won’t match the value derived from the cosmic microwave background, which dates to nearly the beginning of time.
Most experts are betting that more data will relieve the tension without the need for major cosmological surgery. And plenty more data are on the way. Planck’s report, for instance, was based on about 15 months of observations, corresponding to two sweeps of the sky. Eventually the Planck team will analyze 50 months’ worth of data, which should further reduce the margin of error when the analysis is released later this year.
Meanwhile, Freedman and colleagues with the Carnegie Hubble project have continued to refine the Hubble constant estimate. That endeavor combines data from the Hubble telescope, the Spitzer Space Telescope and ground-based telescopes to establish the cosmic distance scale.
Since the universe’s expansion rate depends on how far apart two objects are, measuring it depends on accurate knowledge of cosmic distances. Nearby, distances are calculated directly from parallax. Simple geometry can tell how far away a star is by viewing it from opposite sides of the Earth’s solar orbit. Farther out, distance measurements rely on “standard candles”—objects of known brightness whose distance can be inferred by how bright they appear.
Traditionally, astronomers have exploited the standard candle potential of a particular class of stars known as Cepheid variables. Cepheids periodically brighten and dim on a regular schedule, with the timing depending on their intrinsic average brightness. Parallax establishes the brightness-distance relation for nearby Cepheids; that relation can then be used to estimate distances for those much farther away.
For measuring the Hubble constant, though, even more distant standard candles are needed, and that’s a job for supernovas. Supernovas of one category, known as type 1a, are not exactly all equally bright, but their intrinsic brightness can be calculated based on how fast their light dims over time. Currently Freedman and colleagues are combining data on type 1a supernovas in galaxies that also contain Cepheids, allowing a calibration of the distance scale. Along with how fast objects are receding—inferred from the colors of light they emit—those distances determine the expansion rate. Combining supernova and Cepheid data “at present offer the best opportunity to measure the Hubble constant and minimize the systematic errors,” Freedman says.
She points out that this method offers a direct measure of the expansion rate, while the cosmic microwave background readings give merely indirect estimates. Planck measured temperature differences in space, relics of variations in the matter density in the baby universe. Combined with other data and assumptions, those measurements can be used to deduce other properties of the cosmos. Planck’s value for the Hubble constant is the best fit for a whole model of the universe, including a model for the nature of the dark energy.
“So maybe the disagreement is a clue that the dark energy is not quite as simple as you think,” says Harvard University cosmologist Robert Kirshner. “But it’s too soon to conclude that.”
If the Hubble constant is truly on the high side, as supernova data indicate, then the expansion of the cosmos is now a little faster than it has been, on average. That could imply that the dark energy’s strength changes over time. “It’s an interesting possibility,” says Kirshner. Dark energy that changes in strength has implications for the fate of the cosmos. Rather than expanding forever, the universe could someday get torn to shreds in a “Big Rip.”
If the Planck value for the expansion rate is correct, though, then the standard model of the universe’s ingredients needs to be adjusted to make all the numbers fit. Those ingredients include a little ordinary matter, a lot more dark matter and considerably more dark energy. Taken together, these components produce a consistent model of the universe in which the geometry of space is just about perfectly flat (meaning that ordinary Euclidean plane geometry describes it accurately).
Before Planck, the universe’s mass-energy recipe consisted of 4.5 percent ordinary matter, nearly 23 percent dark matter and almost 73 percent dark energy. Planck’s estimates shift the dark energy down to about 68 percent, with dark matter nearly 27 percent and ordinary matter at close to 5 percent.
Since the Planck results came out in March 2013, physicists have searched for ways that tinkering with the standard model could explain the Hubble constant inconsistency. One proposal calls for modifying Einstein’s general relativity, the theory of gravity that provides the foundation for all cosmological science. In this approach gravity itself would be massive: Gravitons, the supposedly massless particles that transmit gravitational force, would possess a small mass, adding a new field to space. That field could influence the acceleration of the universe’s expansion attributed to dark energy, Douglas Spolyar of the Institute of Astrophysics in Paris proposed in a paper published December 11 in Physical Review Letters with Martin Sahlén and Joe Silk of the University of Oxford.
If graviton mass diminishes the effect of dark energy, they point out, then dark energy would be stronger in regions with less gravity, such as nearby voids in space. More vigorous, expansion-producing dark energy in local voids could explain why data from nearby supernovas would yield a higher value of the Hubble constant than measures of the more distant universe probed by Planck.
In another departure from standard cosmology, André Costa of the University of São Paulo and colleagues propose a conspiracy between dark matter and dark energy, the two most mysterious ingredients in the cosmic recipe. In the standard view, the dark sides of the universe are independent components. One adds to gravitational attraction (playing an important role in forming galaxies); the other exerts gravitational repulsion, causing the cosmic expansion rate to accelerate. But if they renounce independence and interact in some way—like two medications that cure separately but kill in combination—Hubble constant measurements could be contaminated.
If, for instance, energy can flow from dark matter to dark energy, then the Planck data would underestimate the true Hubble constant, which would be closer to the value measured by supernova studies, Costa and colleagues pointed out in a paper posted last November at arXiv.org.
It’s possible, other researchers suggest, that resolution will come without such drastic challenges to current orthodoxy. It may just be that the two measurements differ because they probe different parts of the universe. And not only do the two methods measure the expansion at different eras of time, they also probe physics on different scales. Supernovas are big by human standards, but on the cosmic scale they’re like points in space. Measurements of the cosmic microwave background probe vastly bigger patches of sky, as a team of French cosmologists pointed out in a paper last year in Physical Review Letters.
Ultimately all the speculations will be filtered by more data—from Planck, supernova studies and other methods. In December, for instance, the European Space Agency’s Gaia probe was launched; it will produce more precise Cepheid parallaxes to feed into measurements of the Hubble constant using the supernova method. Already the BOSS project, part of the Sloan Digital Sky Survey, has contributed new fodder for the Hubble debate using clues called baryon acoustic oscillations.
Interaction of matter and light in the very young universe produced concentric pressure waves, or acoustic oscillations, much like the ripple in a pond produced when a rock falls in the water. The ridges of these sound-wave ripples would have deposited matter—small seeds that would grow into galaxies—at specific separation distances. Thus the signature of those ripples ought to be reflected in the distribution of galaxies in space today. BOSS observations show that the preferred distance between two galaxies is about 150 megaparsecs (roughly 450 million light-years). Combined with a redshift measurement, which reveals how fast a galaxy is moving away from Earth, BOSS data provide a new check on the cosmic distance scale. Adding in data from cosmic microwave background probes allows another computation of the Hubble constant.
In January, at the American Astronomical Society meeting near Washington, D.C., BOSS scientists presented their latest results, which yield a Hubble constant of 68 to 69. That result remains lower than the value from supernova measurements, said BOSS team member Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics.
“It’s not in sharp disagreement, but it is interesting, so there are a lot of groups trying to track this problem and trying to understand what the resolution might be,” he said. “Today we still have this mild tension. We’ll see where we’re at in six to 12 months.”
In any case, as Freedman points out, the current Hubble constant debate is about a drastically narrower range than in the days when competing camps championed values as high as 100 or lower than 50.
“Fortunately the range of values for the Hubble constant that are now being measured, and the uncertainties in those measurements, have come down quite considerably,” she said at the Texas symposium. “And there’s also a huge amount of progress planned for the future that will set this controversy to rest. There remains the exciting opportunity that there is new physics. Or not.”
October 10, 1931
The universe is actually exploding and the galaxies are scattering apart at a terrific rate, Sir Arthur Eddington, professor of astronomy at Cambridge University, contended before the British Association for the Advancement of Science hundredth anniversary meeting. In support of his contention he presented computations based solely on pure mathematical and physical theory, without the use of astronomical data. The rate of nebular recession thus obtained is in close accord with Dr. Edwin P. Hubble’s Mt. Wilson Observatory figures for the red shift of nebular spectra.
Prof. Hubble said, “Detailed theories of stellar evolution are overshadowed by the fact that the time scale is again in the melting pot. With a rapid-expansion universe, a very long time scale of billions of years, fashionable recently, becomes exceedingly incongruous. We have to accept an age of ten to the tenth power or ten billion years for galaxies and presumably also for stars.”
Since the age of the earth alone, derived through the radioactive method, is over a billion years this embarrasses astronomers, geologists and biologists. Prof. Eddington derived the actual rate of expansion of the universe from the wave equation for the electron, which is the fundamental equation of the modern quantum theory. This equation, adapted to the curvature of space, contains the term: “the square root of the number of electrons in the universe divided by the radius of the universe in a state of equilibrium,” which term is the mass of the electron, usually written: M•C2/E2.
Combined with the formulae of the relativity theory, this gives the principal data of the size of the universe. Its original radius was 1,070,000,000 light-years, before it started expanding. Its rate of expansion is 528 kilometers per second per megaparsec, compared with 465 derived from the Hubble astronomical data. It is over a hundred miles per second for each million light-years’ distance. “The close accordance of the theory with observation forces acceptance of an alarmingly rapid dispersal of nebulae, with important consequences in limiting the time available for evolution,” Prof. Eddington concluded.
April 2, 1932
Prof. Albert Einstein, father of relativity, says that space may be and probably is the sort of uncurved, three-dimensional space that Euclid imagined and countless generations of schoolboys have learned. Although Prof. Einstein in a sense scraps the less familiar and more complicated brands of space-time that he has been using, this does not affect the validity of relativity, which has been at the foundation of so much scientific thinking for the past two decades.
Prof. Willem de Sitter, Dutch astronomer, who had built his own shape of universe on Einsteinian foundations, joins with Prof. Einstein in espousing space which is on the average Euclidean. These two eminent astrophysicists conceived the new kind of universe when working together recently at Mt. Wilson Observatory and their joint announcement was made to the world through the medium of the Proceedings of the National Academy of Sciences just issued. Prof. Einstein is now en route to his home in Germany while Prof. de Sitter is travelling in South America.
This joint announcement, that is sure to cause a furor in the world of science, means that the universe around us may be not only unbounded but infinite, instead of finite and unbounded as Einstein and his followers have previously believed.
In the Euclidean universe now re-enthroned, light travels in straight lines and goes on and on forever and ever. A ray of light would not traverse the circuit of the universe and come back to where it started as it would in the superseded Einstein and other varieties of space. Curvature of space is on the average banished from the universe.
“We must conclude that at the present time it is possible to represent the facts without assuming a curvature of three-dimensional space,” Profs. Einstein and de Sitter say in their report.
Two important developments made Einstein and de Sitter change their universes. One of these was the piling up of evidence at Mt. Wilson Observatory at Pasadena, Calif., by Dr. Edwin P. Hubble and others that the shift toward the red of spectrum lines in light from far distant nebulae is evidence that the universe is expanding at a terrific rate, as high as 15,000 miles per second and that the farther away the nebula the faster the recession.
The other factor was the demonstration by Dr. Otto Heckmann, privatdozent in astronomy at the University of Goettingen, Germany, that an expanding universe can have matter throughout it and still be Euclidean. When Einstein built his first universe he did not dream of an expanding space. He thought it static and constant in size and found himself forced to make space curved to fit this idea. This gave his famous finite but unbounded universe which, upon Dr. Heckmann’s suggestion, he and de Sitter now revise.
Into the equations of Einstein relativity which have stood the test of time, Profs. Einstein and de Sitter, following Heckmann’s lead, have inserted both Euclidean space and the recessional velocity of the nebulae indicated by the expanding universe idea and the Mt. Wilson measurements of red shift in light from the nebulae. The scientists were then able to compute the density of matter in the universe and found that it compares favorably with the ideas that are current as to how matter is spread throughout space on the average.
It is almost impossible to imagine how thinly spread on the average is the matter in the universe. One pound of matter spread throughout a sphere sixteen times the diameter of the earth would give this extremely small density of matter. And as the universe is expanding at a super-terrific rate at extreme distances outward, always getting larger as it were, the density of the matter in the universe must be getting less and less.
Profs. Einstein and de Sitter observe, however, that as more astronomical data are gathered it will undoubtedly be possible to determine with more precision the density of matter in the universe. If it should turn out that there is more matter per unit volume of space, then it will be necessary to return to the original Einstein space even with an expanding universe. If the matter is more sparsely distributed, it will be necessary to learn to live in a space of average negative curvature, such as Lobatschewski, the Russian scientist, dreamed of a century ago. In this strange space an infinite number of lines parallel to a given straight line can be drawn through any point.
The revision of the geometry of the universe by Profs. Einstein and de Sitter does not appreciably alter the geometry of the galaxy of stars in which we live. Consequently it leaves unaltered the theoretical predications originally made by Einstein which so triumphantly vindicated his theory. These are: The wriggling of the orbit of the planet Mercury, the red shift of the spectral lines in the sun and companion of Sirius, and the bending of light rays about the sun which is merely the Euclidean interpretation of a Riemann straight line. A straight line in Riemann curved space is curved when interpreted in Euclidean space. The geometry of an Einstein universe is based on the assumption that light travels in straight lines.
Whole universe began as explosion of a single giant atom two billion years ago, is the suggestion of a Belgian scientist. Uranium bearing rocks support theory.
September 8, 1945
Atomic explosions that wipe out whole cities in an instant seem awesome on a human, planetary scale. Seen from a cosmic grandstand, however, they are scarcely even sparks from a single powder-grain in the grand pyrotechnics of the universe.
Indeed, according to one bold theory, the whole visible universe, with great pinwheel galaxies containing millions of flaming stars, and with possible swarms of planets like the earth that have never been seen and only lately have been rather vaguely guessed at, got its start as a single super-atom of unimaginable energy content, that exploded a couple of billions of years ago—and is still exploding. All the energies of which we are aware, from the bursting brilliance of giant stars that far outshine our sun down to the feeblest kicks of a dying protozoon, are but the varied expressions of that vast primal explosion, if this hypothesis holds good.
The idea started with the notion of an expanding universe. Light from remote stars and galaxies, caught in astronomers’ instruments, is redder than it theoretically should be. One explanation of this so-called red shift is that all parts of the visible starry universe are rushing away from each other at terrific speeds—much faster than the pieces of an exploding bomb. About 15 years ago a young Belgian priest-scientist, the Abbé Georges Lemaitre, boldly suggested a backward extension of this expanding or exploding universe. Mathematical calculations carried him back to a beginning point where neither time nor space existed, and all the matter that eventually came to constitute all the stars and planets was present only potentially, as terrifically high-level energy in one single cosmic atom.
Chemical elements as we know them are discussed in terms of their atomic weights and atomic numbers, which are expressions of the number of electrons spinning around the sunlike nucleus or atomic heart. Since the number of electrons in the smallest pinpoint of ordinary matter—a single dust-grain, for example—must be reckoned in trillions, the atomic number of this primordial atom is simply unimaginable. We have to call it infinity and let it go at that.
What the first atomic explosion was like is also something that defies human imagination. What set it off is doubtless forever beyond our guessing. Theoretical considerations have led the Abbé Lemaitre to a tentative conclusion that it must have occurred something like two billion years ago. Analysis of uranium-containing rocks from the earth’s oldest known geological formations are of about that age, by other, independent methods of analysis and calculation. This would seem to require more time than the Lemaitre hypothesis allows; but it has been suggested that perhaps in the beginning the evolution of cosmic materials went on at a much more rapid rate, and that by the time the processes we know as geology could begin events could be ticked off by a slower clock.
If these dizzying ideas are valid, our most terrifying “city-buster” bombs are made of mere pinches of debris from the universe’s first enormous outburst, scraped up out of overlooked corners like a winter’s last snowballs.
Astronomers polled in a Science Service Grand Jury disagree on theories explaining the origin of the universe, as well as on the observations needed to answer the problem.
July 11, 1959
The world’s top astronomers do not agree on the origin of the universe.
Of 33 participating in a Science Service Grand Jury on this subject, there was a virtually equal division on whether or not the universe started with a “big bang” several billion years ago. To this question, 11 (33.3%) voted “Yes,” and 12 (36.4%) voted “No,” while 10 (30.3%) were counted as “Not Voting.”
Concerning the more recent theory that matter is being continually created and destroyed, opinion was more sharply divided among the 33. More than half of those responding, 18, or 54.5%, said they did not agree. Eight, or 24.2%, replied they did believe matter is being continually created, and seven, or 21.2%, did not vote.
Of the 33 experts, 23, or 69.7%, showed high hopes that one or the other of these opposing theories would be proved right within the next 41 years, while three, or 9.1%, thought they would never be solved. Seven, or 21.2%, did not vote.
Concerning a specific year in the future voted on by 23, fourteen, or 42.5%, were sufficiently optimistic to predict that either the big bang or the steady-state question would be solved by 1975, the other nine, or 27.2%, holding out for the year 2000 A.D. One wrote in a forecast for a solution within five years, as well as voting for 1975.
Concerning the kind of observation most likely to give the answer to the problem, many of the astronomers and cosmologists responding to the poll chose more than one method. Twenty, or 60.5%, said observations of radio waves from far-distant objects would yield the answer, while three, or 9.1%, predicted that radio astronomy would not provide a solution.
A telescope on a satellite would do the job in the opinion of 11, or 33.3%, although seven, or 21.2%, held that it would not. An earthbound telescope, either the 200-inch giant atop Mt. Palomar or others of more than a 100-inch aperture, would give an answer to the origin of the universe, 10 of the 33, or 30.3%, believe. The lone astronomer who thought a telescope mounted from a balloon held the key was voted down ten to one by his colleagues.
Five astronomers, or 15.2%, did not vote on the question of what kind of observation would be most likely to provide a solution to the problem.
Of the 61 scientists selected for the Grand Jury, 36 came from the United States, two from Canada and 23 from foreign countries. Of those answering, 26 are U.S. scientists, two Canadian and five from foreign countries.
Besides answering questions, the 33 astronomers polled were given an opportunity to make any comment they desired, with assurances of anonymity for their remarks. Not all astronomers agreed with the idea of a poll.
One said, “I do not believe that polls such as this one serve any useful scientific purpose and in fact are apt to be misleading. I prefer, therefore, not to participate.”
Another astronomer said that much of the “fun of astronomical research” would be removed if a sure answer to the question of the origin were ever found.
One German astronomer remarked: “Of course, these answers are quite tentative and new observations—as everywhere in science—may completely overthrow someday our present ideas about the origin of the world ‘as a whole.’ More important than any specific answer is the fact that these problems have become accessible to scientific methods and scientific judgment.”
A Netherlands astronomer said he thought the chief merit of the theory of continuous creation is “to force the cosmologists to realize the brittleness of all their inferences from observation.”
Big bang theorists take relic radiation as proof
By Dietrick E. Thomsen, June 15, 1968
For the last 50 years cosmologists have known that they have to deal with an expanding universe. Many theories of the universe’s evolution and history have been proposed in the light of this fact, and controversy over them has been continuous.
Until recently observation—the basis of most sciences—has had little to contribute to the argument because the effects that would distinguish among the theories were unobservable. However, radio astronomers in the last few years appear to have hit on one of the few observables in the business. They have identified in space radiation of the sort that would be expected from a black body at temperatures three degrees above absolute zero.
This level of blackbody radiation is predicted by the so-called big bang theory of cosmology, which postulates a universe supposed to have expanded from a small, extremely dense beginning and to be getting thinner and thinner as it grows. Finding the radiation, supposed to be a relic of physical conditions at the time of the primordial fireball, has given proponents of the theory almost a certainty that they are right, but the proponents of a radically different set of theories—the steady-state models—are not ready to give up. They continue to try to shake the confidence of their opponents.
The expansion of the universe began as a theoretical prediction by Dr. Willem de Sitter, who found in 1917 that one of the possible solutions of Einstein’s general relativity called for it. Observational evidence that there was indeed an expansion going on came in the next decade from studies of the velocities of distant galaxies by Dr. Edwin Hubble and others, but whether it fitted de Sitter’s model was not determinable; the field was open for theorists.
This expansion, if it has always been going on, raises serious problems of beginning and end in cosmological theories. If one follows time backward, the universe gets smaller and smaller until in the beginning everything is concentrated in a geometrical point. Going toward the future the universe gets ever more attenuated.
These ideas, especially the problem of infinite density at one end and infinite expansion at the other, were difficult for many scientists to accept, and some tried to get around them by postulating various combinations of static and expanding or pulsating universes.
A more radical way of getting around the density difficulties was to throw away the notion, implicit or explicit in other models, that the amount of matter and energy in the universe was fixed and constant once and for all. Doing this led to the steady-state or continuous-creation theories associated with the names of Profs. Hermann Bondi, Thomas Gold and Fred Hoyle.
Though Profs. Gold and Bondi differ from Prof. Hoyle, both theories are based on the notion that as the universe expands, matter is continually created out of nothing. This notion is shocking to many people, but, Prof. Bondi asks in his book, Cosmology (Cambridge University Press, 1961): “…why is it more of a hypothesis to say that creation is taking place now than that it took place in the past?” The continuous creation maintains the density of the universe at a constant value. As the universe gets bigger, more matter appears; in the past when the universe was smaller, there was less matter.
At about the time—1948—that Profs. Bondi and Gold came up with their steady-state theory, Drs. George Gamow, R.A. Alpher and R.C. Herman took a close look at the physical conditions that would have existed if the universe began in a point or a very small volume. They evolved a theory that has since become known as the big bang or cosmic fireball concept.
They found that if the matter and energy now in the universe were concentrated into a very small volume it would have been very hot; 10 billion degrees C. is the figure usually given. At these temperatures organized matter would not exist; electromagnetic radiation would dominate the situation. They found further that this radiation would be thermal or blackbody radiation; that is, it would have the characteristics of the radiation that comes from a hypothetically perfect radiator when only thermal motions are contributing to its generation.
The spectrum of a black body shows a characteristic pattern of variation of brightness with wavelength. The shape of the pattern is the same no matter what the temperature of the emitter, but the wavelength band it covers shifts with temperature: When the body is cool, it emits in the radio range, shifting its emissions into the infrared, visible, ultraviolet and X-ray regions as it gets warmer.
As the universe expanded, it would have cooled. Most of the original radiation would have been converted to matter, but some of the radiation would have stayed as such, cooled by now to a temperature less than 10 degrees above absolute zero, at which it would appear roughly as centimeter-band radio waves.
The idea got a good deal of publicity when it was presented, but then it fell into a kind of limbo. Prof. Robert H. Dicke of Princeton University suggested a similar blackbody condition in a theoretical study of a pulsating universe, but he forgot that he had done so until some students and colleagues reminded him of it years later.
It was in 1964 that the reminder took place and Prof. Dicke suggested that the cooled-off blackbody radiation might be looked for. A group of Princeton physicists prepared to look, aided by a microwave radiometer that Prof. Dicke had invented in 1945—this he had not forgotten—which used a technique of switching back and forth between the signal and a supercooled reference to record faint signals that were in danger of being masked by receiver noise caused by heating in the circuits.
While the Princeton cosmologists were at this work, they received word that Drs. Arno A. Penzias and Robert W. Wilson of the Bell Telephone Laboratories in Holmdel, N.J., had found a signal at 7.5 centimeters wavelength that fit a blackbody spectrum of about three degrees above absolute zero. (Absolute zero is minus 273.16 degrees C.) Shortly thereafter, Drs. Peter G. Roll and D.T. Wilkinson of the Princeton group found a signal at 3.2 cm that fit the same spectrum. As Prof. Wilkinson said recently while reviewing progress for the members of the American Physical Society, all this could have been done 15 years earlier than it was if the right people had talked to each other, read the right papers and made the proper mental connections.
But once the investigation was started, it was pursued with great vigor. Many observers in different parts of the world went to work to find other points of the spectrum. Up to now many points have been found ranging from about one-meter wavelength down to about eight millimeters; all that have so far been found fit the spectrum of a blackbody that is now figured to about 2.7 degrees K. (The Kelvin scale starts at absolute zero.)
The growing certainty of the blackbody radiation had led, for a while, to a general belief that steady-state theories had been pretty well discredited. But their proponents, especially Prof. Hoyle and his colleagues, have counterattacked in a series of recent papers, principally in Nature, dating back from March 30.
The steady-state theories suffer from insusceptibility to observation. It would take millions of years to be sure that the density of the universe is indeed constant—the motions are too slow to reveal in a shorter time period, whether it changes or not. The appearance of the new matter that the theory calls for is also impossible to observe. On the average, according to Prof. Bondi, the mass of one hydrogen atom is created in one liter of volume every 500 billion years. Nobody has bothered to calculate the chances of seeing one pop up.
The steady-state proponents have therefore concentrated on attempts to demolish the observational case for the cosmic blackbody. Since they cannot deny the existence of the radiation, they have tried to find other factors responsible for it. They have suggested interstellar dust grains, cosmic rays and heating of gas clouds by radiation from infrared stars. Success in attributing all or any part of the radiation to such causes would shake the fireball theory.
At the moment there is no firm evidence that any of these are at work, and the proponents of the fireball are not in the least daunted. They are moving on to the job of providing a detailed history of the universe under their theories.
If you believe the fireball theory, says Prof. P.J.E. Peebles of Princeton, who has been very active in work on this theory, you have a new handle on the evolution of the universe. A number of people are at work determining such things as at what stage various kinds of matter formed, when and how galaxies formed, and whether clusters of galaxies associate in superclusters.
And Prof. Wilkinson points out: One good practical use of the cosmic fireball is that it gives jobs to a lot of cosmologists.
November 8, 1975
Which is the way the world ends? Is it a bang or a whimper? T.S. Eliot opted for the whimper, but he did so largely on moral and theological grounds. Robert Frost couldn’t decide between fire and ice.
It appears that on purely scientific grounds cosmologists can’t completely decide either. Will the universe continue to expand forever, getting thinner and thinner and adiabatically colder until the game ends with a frozen whimper? Or will the expansion eventually stop, and a collapse ensue to a hot little ball in which saint and sinner alike will be barbecued? A “neighborhood meeting” was held in Cambridge, Mass., last week by the Smithsonian Astrophysical Observatory to discuss the question. No overwhelming consensus emerged, and none was really expected. In the opinion of George Field, the organizer of the meeting, the question may never be completely resolved.
The reason is that there are so many uncertainties and loose ends in the data, and so many assumptions to be made in drawing conclusions from them, that two equally competent observers can come up with virtually opposite conclusions from essentially the same data.
James E. Gunn of the California Institute of Technology and P.J.E. Peebles of Princeton University came to significantly different values of a crucial parameter in the argument, the ratio of the universe’s actual matter density to the critical density required for closure. If the universe is dense enough, the mutual gravitational attraction of its parts will bring the expansion to a stop and reverse the motion. If the ratio of actual density to critical density (called omega) is one or greater, there is closure; if the ratio is much less than one, the universe is open.
Both Gunn and Peebles use essentially the motions of galaxies and clusters of galaxies to deduce gravitational effects and therefore density. Gunn comes up with an omega equal to 0.1; Peebles makes it 0.7. Given the uncertainties, this is a factor of about six difference. Not only is that large: Gunn’s determination militates in favor of an open universe; Peebles’s comes close to a closed one.
Field asked the two men how come they differed so widely. Gunn replied that the mass-to-luminosity ratio of galaxies was at stake. It is assumed that a galaxy’s mass is related to its luminosity, and Gunn says his figure is 200 while Peebles used 400 or 500. Gunn also says he considers the luminosity density of the galaxies to be less than what Peebles thinks it is. Combining the two discrepancies gives the factor of six.
“That doesn’t sound like my calculation,” Peebles responds. “I didn’t mention M/L or luminosity density.” He believes the traditional figures applied to those concepts are unreliable, and he did his analysis, he insists, in a way that avoided recourse to them. Thus, the two men are even at cross purposes in discussing their differences.
There are a number of other tests both global and local that bear on omega or the deceleration parameter q0, which is related to it. They include such things as counting distant objects in a given volume of space and comparing the number to that of nearer ones; using the apparent sizes of certain objects as yardsticks to measure the curvature of space; comparing present to primordial abundances of certain elements to get a handle on the density. All these are complicated measurements requiring difficult data reduction; more detail on the ways and means will be considered in subsequent articles along with the promise of future observing techniques.
Meanwhile it should be remembered that the whole discussion rests on the axiom that Edwin Hubble was right. In studying galaxies Hubble noticed that the light of each was always redshifted. He assumed this was a Doppler shift arising from a difference in velocity between our galaxy and the distant one. Since all the differences were positive, every galaxy seemed to be flying away from every other galaxy.
So Hubble postulated the expanding universe and derived a relationship between distance and redshift that goes as a simple proportion, a linear relation. At the meeting was one devil’s advocate, a mathematician from the Massachusetts Institute of Technology named I.E. Segal, who proposes that this emperor has no clothes.
The problem on which he chooses to bite is the so-called Hubble diagram. The apparent brightness of galaxies also varies with distance, so it should be possible to graph apparent brightness against redshift and get a nice clean line representing Hubble’s linear relationship. In fact, the diagram comes out a broad smear. Astrophysicists explain this by saying that the galaxies are wrong, not Hubble: All galaxies don’t have the same intrinsic luminosity so the luminosity-distance relation is not exact. Segal says let’s forget this and simply apply statistics to the data as they stand. He finds the graph best fits a second-order or squared relationship rather than Hubble’s linear one, and he asserts that the expanding universe hypothesis is all wet.
The assertion was greeted by the assembled astrophysicists with a chill as cold as intergalactic space. After the formal close of the session, a heated argument ensued between Segal and several prominent astrophysicists over a number of points, including whether the galaxies whose redshifts are known are a fair sample for statistical analysis.
It’s a good question. Of the uncounted galaxies in the sky, only 3,000 have had their redshifts measured, and most of these have been special-interest items. A systematic field of redshifts, those in a given volume around our own galaxy, which would make a regular sample, extends only to 200. One of the great future needs is a much more exhaustive redshift catalogue. Getting it with ground-based observations is difficult, because each measurement is time consuming and must compete for telescope use with more glamorous observation programs. A not entirely facetious suggestion by Herbert Gursky of the Smithsonian’s Center for Astrophysics is to put up an orbiting telescope that could do them wholesale at a rate of 25,000 a year. In contrast Mt. Palomar does about 20 a year. If all of Palomar’s time were used, denying the world’s largest telescope to other investigations, it might increase that number by a factor of about 10.
Particle physicists are bringing symmetry to cosmology—a symmetry that seems to be turning out to be a null balance
By Dietrick E. Thomsen, February 12, 1983
Cosmology started out as a branch of astronomy. Lately it seems to be becoming a branch of particle physics, or at least a meeting and commingling place of astronomy and particle physics. If a dinner table conversation at the recent Eleventh Texas Symposium on Relativistic Astrophysics (held in Austin) is any indicator, some of the traditional astronomer cosmologists resent the invasion of the particle physicists. But as Alan H. Guth of Massachusetts Institute of Technology pointed out in a talk at the same meeting, particle physicists have nowhere else to go.
The theorists of particle physics have made much progress toward a theory that would unify most of that science, but in so doing they have left the realm of practical experiment behind. To test the several candidates proposed to be the Grand Unified Theory (GUT), physicists have to study phenomena that occur at fantastically high energies. There is no hope of producing such energies in a laboratory. As Guth remarks, it would take “a linear accelerator one light-year long—unlikely to be funded during the Reagan administration.” The only place to find such energies is in the early stages of the history of the universe, and so numbers of particle physicists are landing in cosmology.
They hit the ground running. The application of what Guth calls the simplest of the GUT theories, the one based on the mathematical symmetry group SU(5) and proposed by Howard Georgi and Sheldon Glashow, produces radical changes in cosmology. The standard astronomically derived big-bang theory has the universe expanding smoothly, causally and adiabatically from the moment of origin to the present time. (Adiabatic cooling is a drop in temperature due to expansion alone without loss of heat from the system.) GUT cosmology rejects this, proposing that the universe in its very early stages went through one or more phase transitions (like a freeze or onset of boiling), and that these transitions interrupted causality and adiabatics.
Another “dramatic difference from earlier cosmology,” in Guth’s words, is that GUT cosmology seems to be a theory of creation truly ex nihilo, and the universe seems to remain perpetually nothing as long as it exists. That is, all of the quantities that are the subjects of conservation laws and so important to a physical analysis of the system (such as electric charge, angular momentum, “color” charge, etc.) seem to be so arranged that negative and positive amounts of them are equal and so always add up to zero, a situation “you can’t distinguish from nothing,” Guth says.
(Guth also says that he has been trying to persuade his colleagues to start abbreviating “theory” with Th instead of just T. If they do, it would be tempting to call this the world according to GUTh.)
GUT cosmology has three main consequences. It predicts first that there was one or more phase transitions at a time when the temperature of the universe was 1014 billion electron-volts (1014 GeV). In Guth’s use of units 1 GeV is about the equivalent of 1013 kelvins, so in kelvins that temperature comes out to 1027 compared to the universal mean temperature of about 3 kelvins at present.
The second prediction is that magnetic monopoles exist and that their mass is about 1016 GeV. In Guth’s units this equals about a hundred-millionth of a gram. The third consequence is that the law of conservation of baryons no longer holds. Baryons are a class of particles whose lightest member is the proton. They include the neutron and several dozen heavier, radioactively unstable varieties. The baryon conservation law, the proposition that the net number of baryons and antibaryons never changes (which means that baryons change into other baryons when they do change), was a pillar that held up the roof of the older particle physics and the older cosmology.
Application of the new particle physics to cosmology deals in particular with three serious problems, the horizon problem, the flatness problem and the magnetic monopole problem. Under the assumptions of the standard big-bang theory—that is, the old cosmology—the universe in its earliest moments expands too fast to maintain causal relations. The speed of light is too slow for messages to catch up, and different parts of the universe get out of communication with each other. However, at the present time we observe a high degree of isotropy in the universe: Things are very much the same in all directions. That tells us that all parts of the universe were in communication with each other throughout the expansion or at least through as much of it as serves to determine present conditions. Or, in other words, an unbroken chain of causes stretches back from now to the origin. The incompatibility between the two statements is the horizon problem.
Our present observation tells us also that space is very nearly flat. Any curvature that there is must be minute. In the standard model, the universe tends to evolve toward greater curvature, so if it is very flat now, it must have been that much flatter in the earliest times. This is an extremely special initial condition, and its happening just at random is highly improbable.
The creation of magnetic monopoles is governed by the topology of space-time at the moment they come into existence. One monopole is made in each volume of space of a certain critical size. The size of the volume is governed by a critical length, called in technical terms the coherence length of the Higgs field. (The Higgs field is an important mathematical link in the unification of the different force fields that we observe, electromagnetic, nuclear, etc., into a single overall description. Its coherence length is the maximum distance over which phenomena that arise from it can propagate themselves without getting out of phase.) Under the ordinary big-bang assumptions the Higgs coherence length is very short. The universe contains an astronomical multitude of such critical volumes, and a corresponding number of monopoles is made.
Monopoles are extremely massive for elementary particles. Such a large number of them would make the density of the universe extremely high. The high density speeds up the adiabatic cooling rate and so the evolution of the universe. The universe could go from the big bang to its present state in 30,000 years. Such a number is a blatant contradiction of evidence from several branches of science.
Guth and the other proponents of the new inflationary model contend that the insertion of a phase transition at a very early split second of history can cure these problems. (The possibility of such a happening is closely connected to the repeal of the law of baryon conservation by the GUT theory.) A phase transition is a sudden change of structure and order of a material system. Water undergoes a phase transition when it freezes or boils. In physics since Einstein, space-time has quasimaterial properties, and it can go through an analogous change. The universal phase transition occurs during a super-cooling period, an era when the universe cools much faster than the ordinary rate. The physicists speak of bubbles of the new phase appearing in the midst of the old, like bubbles of steam in boiling water. The bubbles grow and coalesce until all of space-time has changed to the new phase just as all the water in the pot eventually becomes steam.
The phase transition cures the horizon problem by legitimizing the interruption of causal relations. During it the different parts of the universe are not in communication with one another. After the phase transition is complete, the universe cools adiabatically, and the conditions we have now derive from those that obtained immediately after the phase transition. Thus, the initial conditions of the universe do not determine our current state. In fact, during the transition the universe actually expands at an exponential rate, faster than the simple adiabatic rate, but that no longer matters. Causal connections do not have to be preserved during that period. It is this extrafast expansion that contributes the word “inflationary” to the name of the new model.
When the general relativistic equations that describe space curvature are calculated in connection with the phase transition, it turns out that the phase transition requires a very flat space, and so expansion to the present state had to start from a very flat configuration. This solves the flatness problem. The phase transition solves the monopole problem by changing the topology so that only one monopole per universe or fewer appears.
The phase transition provides also for creation ex nihilo. In the language of mathematical field theory the phase transition can be seen as a change from a false vacuum to the true vacuum. “Vacuum” in this terminology means the zero energy level of the system, a state devoid of matter or energy. Any real phenomena in the system exist at energy levels above the vacuum, and all dynamical processes take place between these higher energy levels.
Energy scales are relative, and it turns out (though it seems very strange) that the zero, vacuum, levels at different stages in the history of the universe can be different. If so, only the lowest possible one of these is the “true vacuum.” The others are “false vacuums,” and in any era when dynamics is based on a false vacuum, a catastrophe may occur in which the bottom falls out, so to speak, and dynamics shifts to base itself on the true vacuum. The phase transition is such an occurrence. It is not so much that nothing somehow becomes less than nothing as that the range of something is increased. Phenomena that were not possible before can now appear.
Among the new phenomena that appear at this point, Guth says, are those that make our universe recognizably what it is: energy, entropy and matter. It can be said, therefore, that the false vacuum is the source of energy, entropy and matter as they arise somehow from its disappearance. Since the false vacuum is by definition nothing, it follows that these things come from nothing.
The original inflationary universe model, however, does not solve another important cosmological problem, that of homogeneity. Cosmologists believe that on the large scale the matter and energy in the universe are spread around evenly. (There are some recent astronomical observations that raise questions about this belief, but until there is a solid disproof of it, homogeneity will remain a basic input into cosmological theories.) To get homogeneity in the inflationary universe model, the bubbles that form in the phase transition must grow and coalesce in the proper way so that everything mixes together evenly. It turns out they do not.
This difficulty seemed insuperable until, after some cogitation, it was realized that only one bubble is enough, the one we are in. This bubble represents as much of the universe as we can observe, and within it we can have homogeneity. Why should we concern ourselves with what lies outside it? With this the “new inflationary universe” model was born.
“The details are still not quite right,” says Guth. There are in fact still some serious difficulties in fitting the theory to observations. “We are still looking for one theory to solve all problems,” he says, but he is hopeful that this is the right way to go. And if so, there we will be, ensconced in our bubble in a universe that comes from nothing or almost nothing. And Guth concludes, “The universe itself may be the ultimate free lunch.”
By training computers to simulate the histories of many plausible universes, cosmologists hope to better understand the particularly puzzling one we inhabit
By Penny D. Sackett, October 29, 1983
In 1965, two Bell Laboratory scientists named Arno Penzias and Robert Wilson discovered cosmic static in their radio receiver. Modern cosmology, the study of the origin, evolution and structure of the universe, was born. The realization that this cosmic microwave hiss was a signal from the universe’s extremely hot youth pounded the final nail into the coffin of some long-standing cosmological theories. Left as the strong front-runner was the “big bang” model—wherein the cosmos expanded from its birth in a gigantic explosion of space and energy.
Many of the later-discovered inadequacies of the original big bang model have been successfully addressed by new and shinier models, offspring of the recent marriage of cosmology and particle physics. Yet, some deceivingly simple questions remain unanswered: Why is the matter in the universe distributed in its present pattern? Is the universe composed of primarily luminous matter, or are some dark cosmic constituents eluding our vision?
Researchers first in the USSR and now also in the United States are hoping to solve these old astrophysical riddles by using information from particle physics as input for huge computer codes which simulate the evolution of several possible universes. These cosmological biographies are contrasted with observational knowledge of our own universe in an attempt to find a match.
All of this is, of course, quite a grand ambition. How could one possibly hope to trace the history of an entire universe? The answer lies in simplifying the model in question until it is stripped of all but a bare minimum of pertinent characteristics. Even so, to trace once through a particular cosmological history can require three continuous hours of computer time and computer codes so large that they strain presently available computer storage.
A simulation is initiated when the researcher tells the computer how matter interacts and describes initial particle speeds and positions believed to be representative of those at early times. Evolving the matter through the past to the present, the computer then does the rest, giving the researcher a “snapshot” of the speeds and positions of matter at any time in the life of this particular model universe.
Since computer space and time are limited, nearly all simulations done to date have followed the history of only a single kind of particle. The outcome of these simulations depends primarily on the “density perturbation spectrum” of the chosen particle, that is, on precisely how the matter is believed to be distributed when the universe is young—information donated by particle physics.
A good first guess might be to assume that at early times, hydrogen, the most abundant elemental constituent of the universe, was distributed randomly throughout the cosmos. Although the hydrogen we see now certainly is not randomly distributed (it’s gathered into lumps called galaxies and oceans and people), perhaps over time mutual gravitational attractions could clump initially random matter into an uneven distribution.
Computer simulations done in the late 1970s to test this hypothesis were encouraging. Beginning with an initially random pattern of particles distributed throughout the early universe, and assuming that the matter interacted only gravitationally, computer codes evolved the particles through time. Snapshots representing how such a model universe might look at present times showed very clumpy distributions of matter reminiscent of the blotchy character of our universe.
But our universe is lumpy in a particular way. The relatively small lumps of ignited hydrogen that we call stars are separated by vast wastelands nearly devoid of matter. Galaxies, which are aggregates of billions of stars, are further clumped by the hundreds and thousands into loose neighborhoods called clusters. Finally, the galaxy clusters themselves are grouped into enormous spaghetti-like strings known as superclusters—a single one may stretch one one-hundredth of the way across the entire universe with comparatively empty voids in between.
To measure the specifics of clustered matter distributions, astrophysicists use the “auto-correlation function.” Simply put, starting with any particular galaxy, the auto-correlation function measures the odds of finding another galaxy at a given distance away. This probability will vary, in general, as the distance is changed.
To test the success of the simulations that relied on initially random organizations of matter, the auto-correlations of the computer-generated snapshots were compared to those of real astronomical observations. Now the trouble began. The two auto-correlations did not match, especially at very large distances. Furthermore, the filamentary structure of superclusters and the huge voids seen in between had not been reproduced in the computer simulations.
“Although the people that did the simulations argued quite strongly that what the observers were seeing was just an ordinary statistical effect enhanced by their imaginations…I think a lot of people were never convinced by those arguments. And so people began looking for a different kind of initial condition which would produce a universe in which you could, in fact, find filaments and holes.” So relates Simon White, astrophysicist at the University of California at Berkeley, who is working with colleagues Marc Davis and Carlos S. Frenk on many ongoing computer simulations of just that type.
At first, one might suppose that by simply varying the initial lumpiness of the matter in just the right way, one could achieve a model universe that would then evolve to give the proper auto-correlation function observed today. But not only is this just plain cheating, the early distribution of hydrogen-like matter is fairly firmly determined by theory. It is not free to be varied at the whim of a cosmologist.
According to current cosmological theories, as positively charged protons began to latch onto negative electrons to form hydrogen in the cooling half-million-year-old universe, photons of light were freed from their bondage to this matter. Free from scattering, this “decoupled” light could then travel basically unheeded throughout the cosmos, cooling with it: a relic of the big bang and a sign of what the universe had once been like. This ancient light, cooled to microwave frequencies, was the source of the annoying noise in Penzias and Wilson’s horn-shaped antenna. Subsequent measurements of this background radiation have shown that after corrections are made for the earth’s motion through the cosmos, the earth receives the same amount of this microwave static, at the same temperature, from all directions in space. This uniformity has implications for the early cosmic distribution of hydrogen.
Since the photons were linked intimately to subatomic matter just before the “decoupling” time, the smoothness seen now in the cosmic microwave radiation indicates that the pairing protons and electrons were also distributed evenly at the decoupling time. The computer simulations have shown that, even with the help of gravity, the predicted smooth initial distributions of light and hydrogen cannot account for the observed hierarchical clumping of matter in the present universe.
Perhaps the researchers had over-simplified their computer models, overlooking some subtle but important effects. Perhaps quantum fluctuations at the time of the universe’s birth were more important than previously thought. Or perhaps another kind of matter is an important actor in the life of the universe.
Cosmologists weren’t the only ones asking this last question. Clustered galaxies clocked at high speeds had been puzzling astrophysicists for years. The estimated mass of these galaxies was too small to hold them into a gravitationally bound cluster at such high speeds. But the mass estimates were based on the amount of luminous matter in the galaxies—perhaps some dark, unseen mass is providing the extra-gravitational tug.
Various candidates for this dark matter have been proposed, including: cosmic dust, cold planet-sized rocks, black holes and a variety of particle species from the heavily populated zoos of particle physics. If the dark matter exists in large amounts, cosmologists realized that they needed to know more about it so that the computer codes could be modified.
The amount of luminous matter that can be seen by astronomers suggests that the universe is diffuse enough to continue expanding forever. If the cosmic density is greater than “closure” density, however, the universe is so dense that its own gravitational attraction would eventually triumph over expansion and the cosmos would collapse back onto itself sometime in the distant future. Dark matter in the amount needed to explain galactic clusters would increase the density of the universe enough to push it to at least two-tenths of that needed for closure.
Some cosmologists feel that this effect rules out some of the proposed choices for the unseen matter on the basis of the existing theory of cosmic nucleosynthesis. At the time of nucleosynthesis in the universe’s infancy, high temperatures and densities worked as a fusion catalyst for the heavy so-called baryonic matter found in nuclei. Small amounts of helium and deuterium were formed from hydrogen.
Adrian Melott is a cosmologist at the University of Chicago who was prompted to do computer simulations of the universe’s history while studying the dark matter dilemma. He explains, “The nucleosynthesis arguments say that if the density of the universe were greater than about 0.2 [of closure density] in just baryonic matter (protons, neutrons, atomic nuclei), there would be much more helium produced than is observed and far too little deuterium. This says that the baryon density of the universe cannot be greater than about 0.2 of closure, so there’s a contradiction here with dark matter calculations. In order to resolve that contradiction, some of the matter that’s producing the gravity needs to be non-baryonic.”
And so it seemed that the dark matter nominees must not only be found in copious amounts in galactic clusters, but if the arguments concerning nucleosynthesis were correct, they must also be composed of leptons—a lightweight class of weakly interacting particles.
This latter criterion eliminated cosmic dust, rocks and black holes from the competition and left particle physicists searching their theoretical zoos for leptons that would fill the cosmological bill.
The most obvious candidate was the well-known and abundant (about 100 to the thimble-full) neutrino. Because they decoupled from the rest of cosmic matter quite early on—about 100 seconds after the cosmic birth—neutrinos may have had lumpier distributions than hydrogen at the photon decoupling epoch, which is the usual starting point for computer simulations. Although this evidence was encouraging, the neutrinos were missing one key ingredient—mass!
Then, in 1980, a controversial Soviet experiment claimed to show that neutrinos have a minuscule, but definitely non-zero, rest mass. The proposed neutrino mass would provide about the right amount of the “missing mass” in galactic clusters. Furthermore, if the Soviets were right, the neutrinos would also add enough mass to the cosmic density that the universe’s ultimate fate could be a monstrous collapse sometimes known as the “big crunch.”
Particle physicists and cosmologists alike immediately began studying the implications of non-zero neutrino rest mass. Computer simulations were developed by the Berkeley group and independently by Melott and Joan Centrella (then at the Universities of Pittsburgh and Illinois-Urbana respectively) to test how a universe dominated by the light but ubiquitous neutrinos would evolve through time. Theories from particle physics provided the necessary input concerning the likely initial neutrino distributions.
Progressive snapshots of these neutrino-dominated universes were striking. Over time, gravitational interactions did cause clumping in these models, just as in the hydrogen models—but the end results looked quite different.
In the early neutrino pictures, the matter seemed to form large, flat pancake structures. As the computer evolved the model, matter could be seen collecting along the stringlike intersections of these pancakes and then further clumping at the intersections of these strings. Large voids separated the stringlike structures from one another. The filaments seen in the computer simulations had about the same length as those reported by the astronomical observers.
Although only neutrinos were studied in these simulations, researchers believed that the ordinary “hydrodynamic” baryonic matter (found in stars and human beings) would probably tag along with the neutrinos, reacting to their strong gravitational tug. When the weakly interacting neutrino clouds collided and passed through one another with little effect, the hydrodynamic matter, it was thought, would get trapped in the collision zones, shock and warm to high temperatures. Perhaps superclusters, clusters and galaxies could fragment from this ordinary matter squashed in the neutrino-formed pancakes.
But the proof of the pudding still remained in the auto-correlation test. A snapshot was found in the neutrino album that had the proper auto-correlation function. Unfortunately, says White, the snapshot showed a universe too young to correspond to the present time. In it, matter would be just starting to clump; galaxies just beginning to form.
He explains, “We had to go back to such an early time in the simulation that we were saying that galaxies formed yesterday—which is not consistent with observation.”
Possible loopholes exist, since the computer simulations are known to have shortcomings. The “shocking” interactions of hydrodynamic matter may play an important role in galaxy formation. Even though the assumption had been made that the neutrino density dominated the universe, perhaps information about the heavier but more sparse baryonic matter should have been incorporated. This model is much more complicated than the pure neutrino models, however, and therefore requires more computer time and storage.
Astrophysicists Jim Wilson of Lawrence Livermore National Laboratory (LLNL) in California; Centrella, currently at the University of Texas at Austin; and Melott are working together to produce these more detailed simulations.
Meanwhile, both they, Soviet researchers in Moscow and Estonia, and Davis, Frenk and White have moved on, undaunted, to consider still more controversial particles as the mysterious dark matter. The latest models rely, not on existing particles that may or may not have mass, but on theoretical massive particles that may or may not exist: axions, gravitinos and photinos.
The results are preliminary, but encouraging. The relatively slow speeds of these particles at early times in cosmic history allow them to clump more easily, even in small groups. The result is that in the axion, gravitino and photino models, small cosmic structures seem to evolve along with the larger ones, and at more appropriate times. The snapshots corresponding to recent cosmological history bear more resemblance to astronomical observations than did the neutrino snapshots. But White warns that more work needs to be done on these models before firm conclusions can be drawn. The appearance of small clumped regions of matter in these models requires that further sample histories be done that are capable of distinguishing fine structures.
Present work by these two U.S. teams and by astrophysicists in the USSR will concentrate on incorporating baryonic matter into their codes, testing new types of dark matter candidates and improving the spatial resolution of their computer models.
If they succeed in demonstrating that the dominant actor in galaxy formation and the overwhelming constituent of the cosmos could be a heretofore obscure lepton, it would be more ego shattering than Copernicus’ heliocentric solar system. Not only is our position in the heavens completely undistinguished, but our very baryonic make-up might then be only so much inconsequential dust riding about on great gusts of leptonic gravity.
By Michael Stroh, May 2, 1992
After searching for nearly three decades, scientists have uncovered evidence that may solve one of cosmology’s oldest riddles: How did primordial matter evolve into the stars, galaxies and galactic clusters we see today?
Instruments on NASA’s Cosmic Background Explorer (COBE) satellite have picked up temperature fluctuations in the cosmic microwave background, the ubiquitous energy left over from the creation of the universe. The fluctuations represent tiny gravitational ripples—variations in the density of matter. “This is like looking at the invisible man and seeing the footprints,” says COBE scientist George F. Smoot of the University of California, Berkeley.
Cosmologists believe these ripples unbalanced the primordial universe enough to cause matter to begin lumping together and, after 15 billion years, evolve into the cosmic structures found today. Smoot’s team announced its findings last week at an American Physical Society meeting in Washington, D.C.
“It’s the missing link,” says cosmologist Joseph Silk of the University of California, Berkeley. “The lack of fluctuations has been a major obstacle in having many people accept not just [theories of] galaxy formation but the basic premises of the Big Bang.”
In its simplest form, the Big Bang theory predicts that the cosmic microwave background will have a perfectly uniform temperature. Soon after COBE’s launch in 1989, the satellite’s preliminary measurements indicated that the microwave background was a perfectly smooth 2.73 kelvins, which fit with the basic Big Bang theory. Still, this finding puzzled cosmologists because a smooth primordial universe, they believed, couldn’t have evolved so quickly into the galaxies and galactic clusters visible today.
Now, after analyzing hundreds of millions of measurements taken during COBE’s first year in orbit, Smoot’s team has found hot and cold spots in the cosmic microwave background. These spots differ barely thirty-millionths of a kelvin from the 2.73-kelvin background. The new data support some Big Bang add-on theories, such as the inflationary model, in which galaxy formation springs from small gravitational disturbances.
But theorists still have their work cut out for them. The COBE measurements suggest that the gravitational ripples probably acted in concert with other, yet unknown mechanisms. “We’ve seen how strong the gravitational forces in the early universe are, and they’re not strong enough to cause ordinary matter to collect in clusters of galaxies,” says COBE scientist Edward L. Wright of the University of California, Los Angeles.
Some cosmologists have speculated that cold dark matter, an invisible substance that hypothetically makes up a significant chunk of the universe, provided the extra gravitational push. “The nature of dark matter is still mysterious, but it seems to be required in order to make the structures that we see today,” says Wright.
While the COBE findings support the cold dark matter theory, they don’t rule out other possibilities. Cosmologists have suggested more exotic models—involving cosmic strings, for example—that both fit with the new findings and seem to explain the lumpy cosmos.
COBE is still gathering data, so more concrete answers may arrive soon. “What I think we’re going to see is really a breakthrough and a revolution in our understanding of the early universe,” says Smoot, “because we’re going to have hard facts.”
Competing theories about how the universe got its lumps
By Ron Cowen, May 22, 1993
“The time has come,” the Walrus said,
“To talk of many things:
Of shoes—and ships—and sealing-wax—
Of cabbages—and kings—
And why the sea is boiling hot—
And whether pigs have wings.”
—Lewis Carroll, Through the Looking Glass
Among the many riddles about the nature of the universe, none seems more puzzling than this: If the cosmos began as a hot, uniform soup of radiation and particles, how did it evolve into clumps of stars and clusters of galaxies? For more than a decade, cosmologists have argued over which of a myriad of scenarios best explains how the modern cosmos got its lumps.
One year ago, after astronomers announced that a U.S. satellite had discovered evidence for the seeds of lumpiness early in the history of the universe, some researchers expected the arguing to diminish. But if anything, the debate has intensified. While the findings from the satellite, known as the Cosmic Background Explorer (COBE), pose problems for some theories, tinkering with details of the models seems to have kept many of the proposals afloat. “I don’t think COBE has ruled out any theory that wasn’t ruled out before,” says cosmologist P.J.E. Peebles of Princeton University.
But the time has come, says Paul J. Steinhardt of the University of Pennsylvania in Philadelphia, “to pursue each model to its limit, so that we can find the smoking gun that will eliminate some of the theories.”
Many scientists hope to do this by piecing together the COBE findings with experiments that look for the seeds of cosmic structure in patches of sky much smaller than the satellite can study. COBE looks for lumpiness on angular scales greater than 7°, while other experiments search for lumpiness on scales of 1° or less. “We’re likely to get very important microwave background observations on the degree, arcminute, and arcsecond scales,” says David N. Spergel of Princeton University. “I think once we have the full set of observations, we’ll be able to know a great deal [more] about the initial conditions for forming galaxies.”
Several experiments now monitor the faint microwave background—the relic radiation from the Big Bang—one small patch of sky at a time. These surveys have not yet provided results as definitive as COBE’s. But like COBE, some of these studies have detected tiny hot spots or cold spots in the cosmos, which may prove to be small temperature fluctuations in the seemingly uniform microwave background. Understanding the nature of these temperature fluctuations may help solve the riddle of cosmic evolution.
Many astronomers believe that the microwave background represents a snapshot of the universe as it appeared a mere 300,000 years after the Big Bang. Before that time, the cosmos consisted of a foggy mixture of radiation, ions, and electrons. Electrons would have scattered the radiation this way and that, obscuring how lumpy the universe was at that early time.
But as the cosmos cooled, much of the celestial fog lifted: Ions and electrons combined into atoms, which absorb and scatter radiation less. During that crucial era, when the cosmos was about one-thousandth its current size, the universe became transparent. Light and matter went their separate ways. As they did, matter left an indelible mark on the radiation, which some 15 billion years later has now reached Earth.
Variations in the temperature of the microwave background radiation mark how smoothly matter was distributed at the time the universe became transparent, Steinhardt notes. That relationship stems from Einstein’s general theory of relativity, which holds that mass exerts an influence on particles of light, even though these particles are massless.
While gravity can’t slow the speed of light, it does shift this radiation to lower or higher energies. At the moment the universe became transparent, it was as if the cosmos were a rubber sheet riddled with tiny hills and valleys. Clumps of matter created the valleys; the bigger the clump, the deeper the valley. In effect, light near a mass clump would have had to climb out of a valley—expend energy—in order to reach Earth. When radiation lost energy, it shifted to a longer wavelength and appeared colder. In contrast, light would easily depart a region with a lower-than-average mass density. Unencumbered by the gravitational tug that light elsewhere in the universe must contend with, this radiation effectively got an extra kick, as if it were sliding down a hill. Light emitted from low-density parts of the cosmos thus gained energy and now appears shifted to shorter wavelengths, or hotter temperatures.
In this way, explains Steinhardt, large-scale cold spots in the microwave background reveal regions where matter has clumped together, while hot spots indicate regions with an unusually low density of matter.
The vast hot and cold spots measured by COBE, as well as other experiments that measure temperature fluctuations on an angular scale greater than 2°, may have special significance. They apparently correspond to clumps that came into being a tiny fraction of a second after the birth of the universe.
With its limited resolution, COBE only detects hot and cold patches so large that a flash of light emitted at one end couldn’t have reached the other end in the 300,000 years it took the universe to become transparent. Since nothing travels faster than light, that means the two ends were entirely oblivious to each other. Matter at one edge of the patch couldn’t influence matter at the other edge.
So if such a patch—indicating a large lump of matter—truly existed, it could not have formed gradually over an extended period. Instead, it would seem to represent a primordial ripple in the fabric of space that must have resided there as far back as a tiny fraction of a second after the birth of the universe. Moreover, these large-scale hot and cold spots in the microwave background would signify structures 10 times larger than any so far observed in the universe.
Smaller hot and cold spots, examined by ground-based and balloon-borne experiments that have a much higher spatial resolution than COBE, also provide a snapshot of the universe’s clumpiness when it became transparent. But, because such spots are smaller, light emitted at one end of a patch can reach the other side in 300,000 years.
This means that changes in the lumpiness of matter at one end could trigger changes in the lumpiness of matter at the other end. Thus, these smaller hot and cold spots would seem to trace two sources of lumpiness: the primordial, initial lumpiness of the universe as well as further clumping that took place sometime later during those first 300,000 years. Moreover, these finer-scale ripples in the uniform microwave background signify the seeds of smaller structures—those on the scale of clusters or superclusters of galaxies—that astronomers have actually observed.
Because of the different information each experiment provides, combining data about temperature fluctuations in the microwave background on both small and large angular scales provides a more rigorous test of theories of how the universe evolved, notes Neil Turok of Princeton University. “COBE was never intended to settle all our questions about cosmology,” he says. “It’s an opening of a window.”
Most cosmologists don’t dispute the primordial nature of the fluctuations seen by COBE and other large-angle experiments. But that’s where the agreement ends. In one scenario, large hot and cold spots began as tiny quantum fluctuations during the explosive birth of the universe. In a mere 10-34 second, according to this model, these microscopic fluctuations became enormously magnified as the universe underwent a cosmic burp—a period of rapid expansion known as inflation.
Theorists have proposed that the vast majority of matter in the inflation model consists of a hypothetical, invisible material known as cold dark matter. Under the influence of gravity, this weakly interacting, slow-moving material can clump earlier and faster than ordinary matter. As a result, concentrations of cold dark matter might more easily account for the lumpy nature of the modern universe.
Inflation plus cold dark matter creates a “flat” universe—one in which the total mass density is such that the cosmos is poised between collapse and expansion. Cosmologists describe the balance by invoking the parameter omega, defined as the ratio of the actual mass density in the universe to the critical density needed to create a flat universe. Many astronomers favor inflation because in this model omega equals one and remains constant with time. In other models, omega changes with time. For example, consider a universe in which the total density of mass is too small to stop the cosmos from expanding forever. In this “open universe” omega would become smaller and smaller as the universe grew bigger and bigger.
Such a model would indicate that we live at a special time, when the ever-changing density in the universe is just right to sustain structures such as galaxies and galaxy clusters. At an earlier time, the mass density of the universe would have deviated from that required for a flat universe by no more than about one part in a million. And at a later time, the mass density would have been so low that the cosmos could not have made clusters of galaxies.
“I find the open universe a fairly unpleasant idea,” says Steinhardt. “It requires us to be living in a very peculiar epoch.” He notes that inflation explains the near-uniform glow of the microwave background and the formation of large-scale structures without making any assumptions about the current era of the universe.
Spergel, however, says the observed motion and distribution of galaxies “are more consistent with an open universe than with a flat [inflationary] universe.”
Inflation plus cold dark matter has several testable features. The model fashions a cosmos in which temperature fluctuations are tiny and evenly distributed: Rather than having a few very hot and very cold spots, the sky should be littered with spots that show just a tiny deviation from the average temperature of the microwave background. Moreover, at angular scales down to about 1°, the temperature variations should be nearly identical on all angular scales. In other words, big patches of sky should show the same variations in temperature as smaller patches. And finally, on any given angular scale, the fluctuations have a high probability of looking the same no matter what region of the sky is studied.
But the temperature variations in the microwave background seen by COBE, only a few ten-millionths of a kelvin, are about twice as large as those predicted by the standard cold dark matter model—if the theory is to account for the observed distribution of galaxies and clusters of galaxies. Conversely, if cosmologists adjust the theory to match the fluctuations seen by COBE, then it predicts too much clumping on the scale of galaxies and galaxy clusters.
In addition, a balloon-borne experiment called MAX has found tentative evidence for dramatically different variations in temperature from two different patches of the sky. Those variations might simply represent spurious microwave emissions from our galaxy or the experiment itself. But if future balloon flights verify the MAX results, the standard cold dark matter scenario cannot survive, Turok says.
Notes Peebles: “You can wiggle a little bit with one angular scale measurement; you can’t wiggle nearly as much with two different measurements.”
To fit their theory with COBE and other findings, some researchers have proposed an ad-hoc assumption: Dark matter, they say, actually consists of a mixture of cold and hot dark material, the latter composed of low-mass particles moving at the speed of light. Because hot dark matter moves at high speed, only large aggregates of the stuff can easily form lumps; smaller groupings would fly apart. Thus, the additional presence of hot dark matter would limit the amount of clumpiness on smaller angular scales. Peebles calls the modification “bells and whistles” added solely to keep intact a problematic theory.
Researchers also tinker with cold dark matter theory in other ways. Some suggest that the universe has a cosmological constant, which is equivalent to assuming that empty space has a constant energy density associated with it. Albert Einstein first made such an assumption in 1917 in order for his equations of general relativity to yield a static, rather than expanding, universe. He later called the idea the worst mistake of his career. But modern-day cosmologists say that if 80 percent of the energy density of the universe comes from a cosmological constant and just 20 percent comes from cold dark matter, it may explain the COBE findings as well as telescope observations of large-scale clustering of galaxies.
One alternative theory, proposed by Turok, explains the primordial fluctuations seen by COBE by a mechanism far different from inflation. Unlike inflation, this scenario requires the universe to have begun with a perfectly smooth distribution of matter. He suggests that topological defects created tiny fluctuations in the uniform distribution of matter about 10-30 second after the birth of the universe. Such fluctuations could act as seeds for the eventual formation of galaxies.
The defects may have arisen as parts of the universe underwent a phase transition from a state of high energy to a state of lower energy, much as the molecules of liquid water lose energy to become ice. Turok proposes that the transition is not perfectly smooth; some high-energy regions become trapped during the transition, akin to cracks in the formation of an ice crystal.
One type of defect, called a cosmic texture, resembles a knot that unwinds at the speed of light. Matter—predominantly cold dark matter in Turok’s model—congregates around the texture as it unwinds. Thus, textures could act as seeds for the tiny clumps of matter in the young cosmos. Under the influence of gravity, some clumps could grow to form clusters of galaxies. Although quite different in its assumptions, the texture model yields “patterns very similar to that predicted by inflation on large scales,” says George R. Efstathiou of the University of Oxford in England.
Turok notes that a cosmic texture begins as a defect confined to a tiny region in space. Some regions have defects while others do not. In this way, the model explains why different parts of the universe would become slightly clumpier than other parts—a feature consistent with the preliminary MAX results. Textures would also enable clusters of galaxies to form relatively early in the history of the universe. This feature dovetails with observations of some distant quasars and galaxies, which appear fully developed even though astronomers see them as they looked when the cosmos was 10 percent of its current age.
In contrast, cold dark matter plus inflation requires most large-scale structures in the universe to have formed relatively recently.
Alas, Turok and other researchers note, the texture model also has its downside. It predicts temperature fluctuations that are about twice as big as those suggested by the COBE results so far reported, which are based on one year’s worth of data from the satellite. Turok hopes that the mismatch will not be as great when the COBE team releases data, possibly this summer, from the second year of the four-year mission.
“There are problems with the theory,” admits Turok, “but it’s not dead yet.”
Recently, several scientists have focused new attention on an older idea. According to some models of inflation, the early universe could have generated ripples in space-time known as gravity waves. Just as a stick striking a drumhead sets off vibrations, upheavals in the rubber-like fabric of space-time could create gravity waves. These waves would scatter radiation in the early universe. As a result, over large patches of sky, primordial gravity waves could mimic the influence of stationary mass clumps, leaving their imprint on the microwave background by shifting the radiation to longer or shorter wavelengths.
Thus, COBE may have viewed hot and cold spots in the microwave background due to two sources: clumps of matter, which form the precursors of galaxies; and gravity waves, which disperse and don’t give rise to structure. But as long as lumps of matter far outnumbered primordial gravity waves, interpretation of the COBE findings wouldn’t change much. However, some researchers, including Steinhardt and COBE team member George F. Smoot of the University of California, Berkeley, now calculate that primordial gravity waves could have created as many as half of the ripples in the microwave background recorded by COBE. That highly speculative notion could send some cosmologists back to the drawing board.
For starters, it would mean the universe began with fewer lumps of mass than the COBE data at first suggested—a result that would alter the fit between the inflation plus cold dark matter theory and the satellite’s findings.
No one knows whether primordial gravity waves actually exist. But Smoot says that smaller angular-scale experiments now under way could test for their presence. It turns out, he says, that the greater the proportion of gravity waves to mass clumps in the early universe, the greater the likelihood that there will be smaller temperature fluctuations on smaller angular scales. It’s too soon to tell if the ground-based and balloon experiments that search for smaller-scale ripples in the microwave background are seeing such an effect, Steinhardt says. But as more data accumulate, these surveys will become crucial for making or breaking the gravity-wave theory.
While the continuing search for small-angle hot and cold spots will give cosmological theories “a kick in the pants,” Peebles notes, the new maps of the clustering of galaxies in the universe are providing additional constraints. A repaired Hubble Space Telescope might finally indicate when galaxies first formed. The new Keck Telescope atop Mauna Kea in Hawaii and the European Southern Observatory’s Very Large Telescope in La Serena, Chile, may look back in time far enough to catch clusters of galaxies in the act of forming.
In a separate effort, British astronomers have embarked on a project to measure the distances of nearly a million galaxies. This survey should more accurately determine how galaxies cluster in the sky and over how large a scale. Outfitting the 4-meter Anglo-Australian Telescope in Siding Spring, Australia, with a spectrograph and special optics will allow the astronomers to examine a wide swath of sky at high resolution. The researchers expect to begin their study late next year.
Other researchers, including Edmund Bertschinger of the Massachusetts Institute of Technology in Cambridge, are working to improve maps of the velocities of galaxy clusters. Assuming that galaxy clusters move solely in response to the tug of matter, regardless of its composition, velocity maps have the advantage of indicating the distribution of all mass in the universe—whether it’s visible mass or dark matter. Notes Efstathiou: “We can see how the galaxies are distributed, but if 95 percent of the matter in the universe is dark, how can we be sure of how matter is distributed?”
If researchers find that cold dark matter clumps less strongly than the visible mass in galaxies, the theory would better match the COBE results, Efstathiou notes. But Bertschinger says it appears so far that visible mass and dark matter have about the same clumpiness. He adds, however, that these results are still preliminary. And Efstathiou cautions that the velocities of galaxy clusters, gleaned by a crazy quilt of different observational studies, are notoriously difficult to compute—let alone compile on the same map.
“I hope no one is making up their mind right now about which theory [of the evolution of the universe] is correct,” Turok says. “For 10 years cosmologists have tried to explain complex phenomena with simplistic theories. Now it’s the experimentalists’ turn. Things may change dramatically in the next few years.”
By Ron Cowen, June 17, 1995
Astronomers this week reported that they have found the unmistakable fingerprints of ionized helium in the early universe. The finding, a confirmation of earlier hints, supports a key prediction of the Big Bang theory, which holds that hydrogen, helium, and trace amounts of lithium were forged in the first few minutes after the birth of the universe.
The helium discovery came as researchers at last glimpsed the tenuous fog of gas that fills the space between galaxies in the young cosmos. This gas, the diffuse intergalactic medium, has eluded detection for more than 25 years.
Using the new observations, principal investigator Arthur F. Davidsen and his colleagues at Johns Hopkins University in Baltimore also estimated the total abundance of both helium and hydrogen in the early universe. Although these gases had very low densities, they account for 5 to 10 times as much mass as the known population of stars and galaxies, the researchers say. This excess is consistent with a Big Bang prediction that baryons—the protons and neutrons that make up ordinary particles—should constitute not just the visible matter in the universe, but also a few percent of the invisible, or dark, matter.
Davidsen and his colleagues Gerard A. Kriss and Wei Zheng presented their findings at a meeting of the American Astronomical Society in Pittsburgh. The new work provides “absolute evidence that at early times there was a lot of helium around,” says cosmologist Gary Steigman of Ohio State University in Columbus.
Davidsen and his collaborators base their results on the detection of characteristic gaps in the spectrum of ultraviolet light emitted by a distant quasar. As the beacon of light traverses the vast expanse of space between the quasar and Earth, it encounters intergalactic hydrogen and helium. Gas completely ionized by the quasar light can’t absorb any more radiation. The light therefore passes unimpeded, as if it were traveling through a transparent window. This appears to be the case for diffuse hydrogen, which is easily stripped of its one electron.
But it takes more energy to ionize a helium atom, which has two electrons. Although the quasar beacon fully ionizes most of the helium it encounters, some of the atoms manage to retain one of their electrons. When the radiation passes through singly ionized helium, the ions absorb light of a particular wavelength, leaving behind a fingerprint—a dark line, or gap, in the quasar’s spectrum. Because of the redshift of light caused by the expansion of the universe, gaps due to helium ions at different distances along the line of sight to the quasar will appear at different wavelengths to an observer on Earth. Thus, the helium ions collectively create a series of dark absorption lines in the quasar spectrum.
The Hopkins Ultraviolet Telescope (HUT), part of the Astro 2 Observatory that flew aboard the space shuttle last March, recorded a series of such gaps in the spectrum of the quasar HS1700+64, which lies about 10 billion light-years from Earth. Light from this quasar illuminates the universe as it appeared about 10 billion years ago, when the cosmos was roughly one-third its current age.
The singly ionized helium detected by HUT represents only a tiny fraction of the total amount of helium that resided in the early universe, since most of the gas is completely ionized. “We are only seeing the tail of the dog,” notes Davidsen. “[But] it’s enough of a tail to know that it’s a very big dog.”
HUT’s ability to analyze the quasar light at high resolution enabled the team to distinguish absorption from two intergalactic sources: denser clouds of neutral hydrogen and helium, and the more diffuse distribution of singly ionized helium. In contrast, says Davidsen, a previous detection by the Hubble Space Telescope could not differentiate between the two.
The HUT findings, he adds, support the notion that quasars, rather than galaxies, provide the ultraviolet glow bathing the youthful cosmos.
Experiment supports theory of Big Bang element production
By Andrew Grant, August 9, 2014
An underground experiment has imitated conditions just after the Big Bang to produce the universe’s most confounding element, lithium. The experiment’s result supports the prevailing theory while reinforcing what scientists call the lithium problem, a discrepancy between theoretical calculations for how much of the element should have been produced 13.8 billion years ago and the amounts observed in ancient stars.
Scientists are confident that all of the universe’s lithium, and most of its helium and deuterium (heavy hydrogen), formed just minutes after the Big Bang, when the expanding cosmos cooled enough for protons and neutrons to bind into lightweight atomic nuclei. The theory that describes this primordial element production, called Big Bang nucleosynthesis, successfully predicts the abundances of deuterium and helium that astronomers observe in ancient stars.
Yet the theory does not successfully forecast the universe’s current lithium supply. Old stars contain one-quarter to one-half as much lithium-7 (made of three protons and four neutrons) as theory predicts and contain 1,000 times more lithium-6 (three protons and three neutrons) than expected.
Before resorting to radical explanations for this discrepancy, scientists want to make sure their theory correctly accounts for how lithium formed in the early universe. So Alessandra Guglielmetti, a nuclear physicist at the University of Milan, and colleagues set out to re-create the universe’s production of lithium-6 in the lab.
Using the Laboratory for Underground Nuclear Astrophysics, or LUNA, located beneath the Gran Sasso, a mountain in central Italy, the researchers fired a beam of helium nuclei at a deuterium target. Unlike the sites of earlier, similar experiments, LUNA is shielded from aboveground particles by about 1.4 kilometers of rock. LUNA can also probe energies equivalent to about a billion degrees Celsius, the temperature at which elements probably formed during Big Bang nucleosynthesis.
The experiment created nearly as much lithium-6 as theory predicts, and far less than is observed in ancient stars, the researchers report in a paper published July 21 in Physical Review Letters. “It’s a really beautiful measurement,” says Brian Fields, an astrophysicist at the University of Illinois at Urbana-Champaign. When combined with similar findings from LUNA and other labs about the production of lithium-7, the result bolsters the Big Bang nucleosynthesis theory.
The finding also eliminates the possibility of a simple solution to the lithium problem. Scientists must now either find errors in measurements of lithium levels in space or come up with ideas for exotic early-universe processes that could account for the discrepancy, Fields says.
Fields and colleagues are exploring the possibility that dark matter, the unknown material that makes up most of the mass of the universe, interfered with lithium production.
“There are ways of introducing mischief in the early universe,” he says.
By Ron Cowen, May 11, 1996
For the past few years, two groups have held center stage in the controversy over one of the most fundamental quantities in cosmology—the age of the universe. Using the sharp eye of the repaired Hubble Space Telescope, these researchers have reported significantly different values of the Hubble constant, a measure of the expansion rate of the cosmos that’s linked directly to its age.
In press releases, journal articles, meetings, and public forums, the teams have duked it out. Now, new findings, some reported this week at a meeting at the Space Telescope Science Institute in Baltimore, have narrowed the age gap.
“We’re on the path to convergence, and everyone is excited about it,” says Abhijit Saha of the science institute and a member of both teams.
Each of the groups used Hubble to look for Cepheid variables, a kind of star whose brightness, and therefore distance, can be inferred from its pulsations. Researchers led by Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif., measured the distance to the Fornax galaxy cluster. They then used that distance as a yardstick to gauge the distance of faraway clusters whose velocity measures cosmic expansion. At the meeting, Freedman reported a Hubble constant of about 73 kilometers per second per megaparsec, which corresponds to a universe between 9 and 12 billion years old.
The other group, led by Allan R. Sandage, also of Carnegie, used Cepheids to calibrate distances to a specific type of exploded star, or supernova, which the team then found in more distant galaxies. This group reported a Hubble constant of 57 in the March 20 Astrophysical Journal Letters. According to Saha, this puts the age of the universe between 11 billion and 16 billion years.
The range of ages depends on the amount of matter in the universe. The younger ages assume the universe has a critical density—just enough matter to teeter between perpetual expansion and ultimate collapse. The older ages assume much lower cosmic densities.
Although the new measurements of the Hubble constant agree to within 25 percent, half the difference of just five years ago, not everyone is smiling. The findings may spell trouble for cosmologists who argue that the development of structure in the universe can best be explained if it has a critical density.
The problem arises from estimates of the ages of globular clusters, assemblages of the oldest known stars. Two teams of researchers now argue that globular clusters are, on average, 14.7 billion years old and no younger than 12 billion years. Don A. VandenBerg of the University of Victoria in British Columbia presented these calculations last week at a meeting of the American Physical Society in Indianapolis.
Astronomers believe the universe may be about a billion years older than the globular clusters. Thus, its age must lie at the high end of the ranges derived from the Hubble constant, indicating a low-density universe. “It’s just about impossible to reconcile these values” with the type of universe desired by theorists, says VandenBerg. New Hubble observations over the next 18 months may shed further light on the issue.
By Ron Cowen, April 29, 2000
Intrepid explorers braved the unknown to find that the world is round. Now, a detector suspended from a balloon circling the frigid Antarctic has measured the curvature of the universe and revealed that it’s perfectly flat.
The balloon-borne experiment detected tiny fluctuations in the temperature of the cosmic microwave background, the whisper of radiation left over from the Big Bang. This energetic radiation cooled to microwave energies as it traveled through space for some 13 billion years.
The new data, from the Italian-U.S. experiment BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), represent the most detailed images ever taken of the infant universe, says Andrew E. Lange of the California Institute of Technology in Pasadena.
Several fundamental parameters lie hidden in the maps of the microwave background that Lange and his colleagues have constructed, researchers say.
The new findings, several cosmologists note, strongly support a theory called inflation, in which a burst of expansion enlarged the cosmos from subatomic size to cosmic proportions—all within a minuscule fraction of a second. In the process, tiny fluctuations in density would have been amplified, giving rise to today’s superclusters of galaxies.
Lange and his colleagues describe their findings in the April 27 Nature.
“This is what we’ve been hoping for the last few years, that we would enter into an era of precision cosmology, where we’re able to study the properties of the early universe,” comments Wayne Hu of the Institute for Advanced Study in Princeton, N.J. In 1992, the Cosmic Background Explorer first revealed fluctuations in the microwave background but averaged temperature variations over huge patches of sky. Such large-scale measurements can’t trace in detail the conditions in the infant cosmos, Hu notes.
BOOMERANG measures the microwave background on much smaller scales. Its data provide the clearest evidence for primordial sound waves, which theorists have suggested are part and parcel of the microwave background. These sound waves “probe the conditions of the early universe as a kind of cosmic ultrasound,” Hu writes in a commentary in the same issue of Nature.
In the very hot, very young universe, he explains, matter and photons—particles of light—were tightly coupled. Photons bounced between electrons and couldn’t travel freely. Whenever gravity compressed the matter, the pressure exerted by the photons offered resistance, reversing the motion and setting up acoustic oscillations—alternations of higher and lower pressure with the same physical form as sound waves. The compression raised the temperature of the microwave background ever so slightly, while expansion lowered it—creating the hot and cold spots seen by BOOMERANG and similar experiments.
After about 300,000 years, the cosmos cooled enough for electrons and protons to combine to form hydrogen atoms. Because photons aren’t bounced back and forth by atoms, they could suddenly stream freely into space. This radiation, detected by BOOMERANG 13 billion years later, reveals the pattern of the sound waves, which bears the imprint of the shape and other characteristics of the early universe.
Cosmologists have long tried to measure cosmic curvature, the extent to which matter and energy curve space according to the principles of Einstein’s theory of general relativity, notes Michael S. Turner of the University of Chicago and the Fermi National Accelerator Laboratory in Batavia, Ill. A host of other experiments, he says, including a test flight of BOOMERANG, had already suggested that the universe isn’t curved. This means that the cosmos has just the right density of matter and energy to expand forever instead of collapsing in a Big Crunch. The new data, from BOOMERANG’s 11-day flight in late 1998, show “that the pattern of hot and cold spots on the microwave sky is undeniably that of a flat universe,” Turner adds. “Our equations really seem to mean something.”
Combined with other measurements, the new work confirms a gap in the cosmic ledger book. Because the mass that’s been measured isn’t enough to make the universe flat, there must be some additional “dark energy,” Turner says. This energy could cause the universe to rev up its rate of expansion, a bizarre notion that recent observations support.
BOOMERANG reveals that the temperature fluctuations in the microwave background are greatest when measured over patches of sky of a certain size. This size, theorists say, corresponds to the longest sound wave that existed when the universe was 300,000 years old. If the inflation theory is correct, the microwave background must exhibit a series of such peaks corresponding to shorter wavelengths, just as a musical instrument plays several overtones. With only 5 percent of the 1998 experiment’s data analyzed, BOOMERANG neither reveals nor excludes a second peak, Lange says.
It’s too soon to worry, says Turner. Other tests now under way and the expected launch next year of the Microwave Anisotropy Probe may yet reveal the missing peaks, he says.
Microwave glow powers cosmic insights
By Ron Cowen, March 15, 2008
New observations of the oldest light in the universe have enabled astronomers to determine the age of the cosmos with unprecedented precision, infer the existence of a vast sea of neutrinos, and better gauge the start and duration of the long-ago era when the first stars switched on.
The findings come from an analysis of five years of observations of the cosmic microwave background—the radiation left over from the Big Bang—using NASA’s Wilkinson Microwave Anisotropy Probe (WMAP).
The glow was generated at the birth of the universe, but WMAP sees the radiation as it appeared when the universe was about 380,000 years old. That’s when the cosmos became cool enough for electrons and ions to combine into neutral atoms, releasing the radiation these charged particles had trapped. A snapshot of the early universe, the radiation is riddled with regions slightly hotter or colder than average-markings of the primordial lumps that grew into galaxies and galaxy clusters. The microwave background also carries the fingerprints of what it has encountered during its multibillion-year journey to Earth.
By using WMAP to measure the size of the hot and cold spots as they appear on the sky today, along with knowledge of their size when the radiation was first released, researchers have pegged the age of the universe to 13.73 billion years, give or take 0.12 billion.
According to theory, immediately after the Big Bang, positrons and electrons collided and annihilated, producing both photons and vast numbers of nearly massless elementary particles called neutrinos. The neutrinos would have slightly smoothed out variations in the microwave background. For the first time, WMAP data reveal just such a smoothing. “Had we not seen this, it would have implied that that was something missing in our understanding of the first second after the Big Bang,” says WMAP scientist David Spergel of Princeton University.
He and his colleagues unveiled their findings last week in online articles.
The team also more accurately determined when the first stars were born. Soon after those stars turned on, they emitted enough ultraviolet light to reionize the universe, stripping atoms of their electrons. These electrons created a thin fog that scattered the microwave background radiation and polarized it. WMAP measurements indicate that the first stars began to shine when the universe was no older than 430 million years. Combined with ground-based surveys of ancient quasars, the findings also indicate that reionization was an extended process, lasting for half a billion years.
| Universal Stats |
| Age of the universe: 13.73 billion years |
| Universe contents: |
| Dark matter: 23.3 percent |
| Ordinary matter: 4.6 percent |
| Dark energy: 72.1 percent |
| Maximum age when stars began reionizing universe: 430 million years |
That provides a guide for future telescopes—such as Hubble’s proposed successor, the James Webb Space Telescope—as to “when in time they need to begin looking for the first stars,” says Spergel.
The results provide “another milestone in precision cosmology,” comments theorist Max Tegmark of the Massachusetts Institute of Technology. Though the findings mostly confirm previous results based on three years of WMAP data, the added precision is critical for testing models for the origin of the universe and the formation of galaxies, says Nick Gnedin of the Fermi National Accelerator Laboratory in Batavia, Ill.
Astrophysicists interrogate one of their most successful theories
By Alexandra Witze, July 28, 2012
Ask any astronomer what inflation is, and you’ll hear about the moment when the universe’s primordial fireball expanded like a balloon on steroids, smoothing and flattening its initial wrinkles before it grew into the cosmos seen today.
Now, some physicists are trying to let a little air out of that scenario.
Generally regarded as one of the most successful theories about the early universe, inflationary cosmology is not exactly under attack. But a few scientists are questioning whether it deserves its reputation as completely untouchable. Inflation may be the best-developed explanation for many features seen in the modern universe, these researchers say, but it still has problems.
“The picture doesn’t really hold together,” says Paul Steinhardt, a theoretical physicist at Princeton University. “Either inflation needs a major overhaul or we have to think about some other approach to cosmology.”
In a paper posted online at arXiv.org in April, physicist Robert Brandenberger of McGill University in Montreal argues that scientists should continue exploring alternatives to inflation rather than just taking for granted that it’s right.
One such alternative, developed over the last decade, holds that the universe may not have begun with a single Big Bang, but rather experiences cycle after cycle of contraction and expansion. Another approach posits a world with a collection of tiny vibrating strings whose movements generate cosmic features currently explained by inflation.
Within the next few years, telescopes may collect enough data to distinguish among the options. Only then, say the inflation agnostics, will the picture hold together or fall apart.
“We really don’t know what happened in the early universe,” says Jean-Luc Lehners, a cosmologist at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. “We know what the result was, but we don’t know how the universe got there.”
Those on both sides of the issue are quick to point out that inflation could turn out to be right. Inflation, says Andrei Linde of Stanford University, is “the only presently existing internally consistent theory of the early universe.” Linde developed some of the first versions of inflation, and thinks those who question it are either intellectually off-course or led astray by journalists looking for a story. “It is quite possible that eventually this theory will be generalized and extended,” he says. “But so far all attempts to replace it by something better failed.”
In 1981 Alan Guth, now of MIT, proposed inflation and showed that it could explain two mysteries about the universe: why it is so smooth and why it is so flat.
Distant reaches of the cosmos look very much alike, even though they are too far apart to have had any contact after the universe was born in the Big Bang, 13.7 billion years ago. But if the infant universe had ballooned outward ridiculously quickly before slowing to a more leisurely expansion rate, this inflation would have smoothed out primordial disorder differentiating one region of space from another. Just a minuscule fraction of a second of inflation would have spread matter out uniformly except for tiny clumps—fluctuations in the background density—to serve as seeds around which the first protogalaxies could grow.
Such an “inflationary” period would also put the universe into the finely balanced state seen today, in which space on large scales seems very close to flat.
In 1992, the Cosmic Background Explorer satellite, or COBE, confirmed a key part of Guth’s idea by measuring slight fluctuations in the leftover heat from the Big Bang. Later data from the Wilkinson Microwave Anisotropy Probe (WMAP for short) brought those cosmic fluctuations into even greater focus. Astronomers began talking about the arrival of an era of precision cosmology, in which detailed observations produced hard numbers that supported inflation.
But others aren’t so sure. “We can’t count the fact that our calculations agree with current observations as a success,” says Brandenberger.
He has several problems with inflation. For starters, its math doesn’t mesh nicely with emerging notions of particle physics; inflation doesn’t play well with ideas like string theory that attempt to unify quantum mechanics with general relativity.
Another problem, he says, is that inflation requires density fluctuations in the infant universe to have wavelengths smaller than the Planck length, below which regular notions about space break down. “This is not to say that the calculations are wrong, but the calculations are extrapolations into regions where we cannot trust them,” Brandenberger says.
Other recent work attempts to deal with new problems that inflation created. One phenomenon of concern is eternal inflation—inflation that never stops.
Guth’s original concept called for inflation to end after a fraction of a second. But Steinhardt and others soon discovered that inflation would continue forever in a few rare spots, spawning rogue areas that went on ballooning. “That now turns the story inside out,” Steinhardt says. “Instead of most of the universe being like us, most of the universe is inflating.”
With eternal inflation, an infinite number of “pocket” universes can pop into existence. And in a universe where anything that can happen will happen an infinite number of times, it becomes impossible to determine what events are more or less likely.
Guth himself has wrestled with that last point, known as the “measure problem.” In a paper posted at arXiv.org last year, he and Stanford’s Vitaly Vanchurin describe efforts to define probabilities of events in an eternally inflating universe.
Not understanding eternal inflation doesn’t mean inflation is wrong, though. “Many cosmologists, including me, believe that eternal inflation is the almost unavoidable consequence of our best understanding of the fundamental laws of physics,” says Guth—meaning even alternative theories would have to cope with eternal inflation somehow.
Of alternative ideas, the one with the most traction comes from scientists including Steinhardt and Neil Turok, now of Canada’s Perimeter Institute. It involves cycles of contraction and expansion.
In this “cyclic scenario,” the Big Bang isn’t the beginning of space and time, but simply a transition from an earlier period in which the universe was contracting. It gets around the eternal inflation problem by smoothing out matter clumps during contraction. Any rogue areas are thus shrinking and don’t become a problem.
Only after a period of contraction does the universe reverse itself and expand outward, so that astronomers today see distant galaxies rushing away at an accelerating rate. Yet this universe has its own problems, most notably that researchers can’t properly describe the change from contraction to expansion.
Other alternatives to inflation include the “matter bounce”—which also relies on a switch from a contracting to an expanding universe, but using different mathematics. Brandenberger’s favorite, “string gas cosmology,” calls for a gas of tiny vibrating strings in the early universe, rather than a gas of particles, and thus meshes with string theory, he says.
Still, most scientists say inflation remains a much stronger candidate than any of the other proposals. “All these alternative models are not justified either by observations or theoretically,” says Viatcheslav Mukhanov of Ludwig Maximilians University Munich.
Ongoing experiments should reveal whether inflation will triumph in the end. Several efforts are now looking for a sign of inflation called gravitational waves. These disturbances ripple through spacetime from violent cosmic events like colliding black holes—or the Big Bang. Other clues may come from the European Space Agency’s Planck satellite, launched in 2009 to build on the success of COBE and WMAP. Planck is hunting for another subtle imprint on the cosmic microwave background, with initial results expected next spring.
One final approach may be to bundle the alternatives to inflation together. Lehners, for instance, has been working to combine eternal inflation with the cyclic universe. Each pocket universe created by eternal inflation, he says, could replay the cyclic scenario over and over again, in a sort of best of both worlds.
In the end, physicists will undoubtedly keep exploring both inflation and its alternatives, and the final solution may lie somewhere in between. “It’s really harmful,” says Lehners, “to assume that we know what the answer is going to be.”
New data offer no evidence to support BICEP2 results
By Christopher Crockett, October 8, 2014
New data from the European Space Agency’s Planck satellite spell more trouble for the claimed discovery of ripples in the fabric of space created moments after the Big Bang. The Planck data strongly suggest that dust in the Milky Way galaxy might account for the entire signal interpreted as gravitational waves by researchers using BICEP2, the Antarctic telescope responsible for the initial discovery announced in March.
If the BICEP2 observations hold up, they would be the first direct peek at the long-hypothesized epoch of inflation, a period of explosive cosmic expansion that followed the birth of the universe.
In the months after the announcement, however, doubts surfaced. Many researchers wondered whether the BICEP2 team properly accounted for the amount of dust in the Milky Way, which might interfere with the observations.
Planck’s results, reported September 21 at arXiv.org, strongly suggest that BICEP2 didn’t see gravitational waves, only dust in our galaxy.
“There is more dust in the BICEP2 signal than they accounted for,” says Jan Tauber, an ESA astronomer and Planck team member. But, he notes, there’s enough uncertainty that gravitational waves might still lurk in the data.
Both BICEP2 and Planck observed radiation called the cosmic microwave background—the faint glow of the first light released into space as atoms formed out of the primordial fog 380,000 years after the Big Bang. BICEP2 looked for gravitational waves by hunting for twirling patterns imprinted on the alignment, or polarization, of this microwave light. The telescope stared at one patch of sky for nearly three years.
But interstellar dust—sootlike grains of carbon and silicon—can mimic the polarization pattern of gravitational waves. BICEP2 researchers took this into account by relying on six estimates of Milky Way dust. They also picked a part of the sky where dust should be sparse.
BICEP2 is more sensitive than Planck but it measured light at only one frequency, 150 gigahertz, where dust is hard to detect. Planck mapped the polarization of the entire sky at seven frequencies, many of which are more sensitive to dust. Those higher frequencies provided the first direct measurement of dust polarization over the entire sky.
“This is a significant change,” says Lloyd Knox, a cosmologist at the University of California, Davis, who also works with the Planck team. BICEP2 relied on calculations to guess the interference from dust, he says. “Now there’s a much better estimate of the contamination solidly grounded in data.”
Jamie Bock, a Caltech cosmologist and one of the leaders of the BICEP2 team, admits that the initial analysis probably overestimated the strength of gravitational waves. “The dust level is significant,” he says. But it’s too early to know whether dust makes up the entire signal. “The analysis is not a one-to-one comparison with the signal reported by BICEP2,” he says.
That’s because the instruments on BICEP2 and Planck are very different. Also, Planck’s interpretation of the BICEP2 data relies on extrapolating from observations of the sky at 353 gigahertz down to BICEP2’s frequency of 150 gigahertz. The extrapolation is guided by observations at intermediate frequencies of the entire sky. But there’s no guarantee that dust seen in the patch of sky monitored by BICEP2 behaves the same as dust from other parts of the sky.
In July, the teams announced that they would share data to resolve the controversy. The teams plan to publish that analysis in late November.
Meanwhile, many cosmologists, including members of both BICEP2 and Planck, emphasize that the current maps are not the final word. “The results are not definitive,” Knox says.
Scott Dodelson of the Fermi National Accelerator Laboratory in Batavia, Ill., agrees. While the Planck results imply dust is the culprit, he says, “there’s lots of room to go one way or the other.”
Planck discovered that no part of the sky is dust-free, but some patches are cleaner than the one chosen by BICEP2. “This will affect strategies for the future,” Knox says, for the many other experiments hunting for gravitational waves.
“These data are invaluable,” says Princeton cosmologist William Jones, who is in charge of SPIDER, a balloonborne experiment designed to hunt for gravitational waves with observations at two frequencies. The balloon will launch over Antarctica in December.
The Planck maps, he says, will help the SPIDER team plan observations and sample patches of sky with minimal dust. If the data from different parts of the sky agree with one another, Jones says, then they probably have a common origin in the cosmic microwave background. Then the team can be confident that SPIDER is seeing gravitational echoes from the birth of the universe.