4
ALMOST THE VERY BEGINNING: A VERY GOOD PLACE TO START
A rather funny and outspoken Russian theoretical physicist startled everyone over coffee recently when he was describing the colloquium he was planning for the following week. A physics colloquium is a general talk that is geared toward students, postdoctoral fellows, and professors—all of whom have a background in physics, though not necessarily with a focus in the speaker’s more narrowly defined field. This particular physicist’s description of his proposed colloquium was “I will talk about cosmology.” When it was pointed out this might be a bit broad—after all, cosmology is an entire discipline—he argued that there are only a few ideas and quantities worth measuring in cosmology and he could cover all of them—along with his own contributions—in an hour-long talk.
I’ll let you be the judge of whether this extreme view of cosmology is actually true—for the record I’m doubtful. Many issues remain to be explored and understood. But indeed, part of the beauty of the Universe’s early evolution is that in many respects it is surprisingly simple. By looking at the single sky that astronomers and physicists observe and study today, we can extrapolate facts about the composition and activities of the Universe billions of years ago. In this chapter, we’ll explore the stunning progress in our understanding of the Universe’s history that the past century’s beautiful theories and measurements have brought us.
THE BIG BANG THEORY
We don’t have the tools to reliably characterize the very beginning. But not knowing how the Universe started doesn’t mean that we don’t know quite a lot. Unlike its very beginning, which no known theory can describe, the Universe’s evolution only a tiny fraction of a second after its beginning hewed to established laws of physics. By applying the equations of relativity and using simplifying assumptions about the Universe’s contents, physicists can determine a great deal about the Universe’s behavior only a minuscule interval after it began—perhaps 10-36 seconds or so later—at which time the Big Bang theory, which describes the expansion of the Universe, applies. The Universe at this early time was filled with matter and radiation that was uniform and isotropic—the same at all places and in all directions—so only a few quantities suffice to describe its early physical properties. This characterization makes the early evolution of the Universe simple, predictable, and understandable.
The lynchpin of the Big Bang theory is the Universe’s expansion. In the 1920s and 1930s, the Russian meteorologist Alexander Friedmann, the Belgian priest and physicist Georges Lemaître, the American mathematician and physicist Howard Percy Robertson, and the British mathematician Arthur Geoffrey Walker—the last two working together—solved Einstein’s equations of general relativity and deduced that the Universe must grow (or contract) with the passage of time. They moreover calculated how the expansion rate of space would respond to the gravitational influence of matter and radiation, both of whose energy densities change too as the Universe evolves.
The expansion of the Universe is perhaps an odd concept, given that the Universe has very likely been infinite all along. But it is space itself that is expanding, meaning that the distances between objects like galaxies increases with time. I am frequently asked, “If the Universe is expanding, what is it expanding into?” The answer is that it is not expanding into anything. Space itself grows. If you imagine the universe as the surface of a balloon, the balloon itself stretches. (See Figure 4.) If you had marked two points on the balloon’s surface, those two points would grow farther apart, just as galaxies recede from each other in an expanding universe. Our analogy isn’t perfect since the surface of the balloon is only two-dimensional and it does in fact expand into three-dimensional space. The analogy works only if you imagine that the balloon’s surface is all there is—it is space itself. If this were true—even with nothing else already there to expand into—the marked points would nevertheless still grow apart.
[FIGURE 4] Galaxies move away from each other as the Universe expands, much as points on a balloon move away from each other as it is blown up.
THE BALLOONIVERSE
For the analogy to be even better, only the space between the marked points would expand—not the points themselves. Even in an expanding universe, stars or planets or anything else that is sufficiently tightly bound by other forces or by stronger gravitational effects won’t experience the expansion that drives galaxies apart from each other. Atoms, which consist of a nucleus and electrons that are kept in close proximity by the electromagnetic force, don’t get any bigger. Neither do relatively dense strongly bound structures such as galaxies—or our own bodies for that matter, which have a density in excess of a trillion times the mean density of the Universe. The force driving the expansion acts on all these dense bound systems too, but because other force contributions are so much more powerful, our bodies and the galaxies don’t grow with the Universe’s expansion—or if they do, it is by such a negligible amount that we would never notice or measure the effect. Matter that is bound more strongly than the force driving the expansion remains the same size. It is only the distance between such objects that gets bigger as the growing space drives them farther apart.
Einstein famously first derived the expansion of the universe from his equations of relativity. He did so, however, before any expansion was measured, so he didn’t accept or advocate his result. In an attempt to reconcile his theory’s predictions with a static universe, Einstein introduced a new source of energy that looked to him like it could thwart the predicted expansion. Edwin Hubble proved this kludge misguided in 1929 when he discovered that the Universe was indeed expanding, with galaxies moving farther away from each other over time (though incredibly, as an observer who didn’t trust any particular theory, he didn’t accept this interpretation of his results). Einstein readily dispensed with the fudge he had made, and is known (perhaps apocryphally) to have called it his “biggest blunder.”
The modification was not entirely erroneous, however, in that the type of energy that Einstein proposed does exist. More recent measurements have shown that the new type of energy he added, which we now call “dark energy”—though not of the magnitude or of a type that would curb the expansion of the Universe—is actually necessary to account for recent observations of precisely the opposite effect—the Universe’s accelerated expansion. But I think Einstein really thought his blunder—if indeed he actually called it that—was in not recognizing the correctness and significance of his initial expansion prediction, which could have been viewed as a key prediction of his theory.
In fairness, before Hubble presented his results, very little was known about the Universe. Harlow Shapley had measured the size of the Milky Way to be 300,000 light-years across, but he was convinced that the Milky Way was all that the Universe contained. In the 1920s, Hubble realized that this was not true when he discovered that many nebulae—which Shapley had thought were clouds of dust that merited this uninspiring name—were in fact other galaxies, millions of light-years away. Toward the end of the decade, Hubble made his even more famous discovery—the redshift of galaxies—the shift in frequency of light that told scientists the Universe was expanding. The redshift of galaxies—like the lowering in pitch of a moving ambulance’s siren that tells us it is moving away—demonstrated that other galaxies were receding from ours, indicating that we live in a Universe in which galaxies grow farther apart.
Today, we sometimes talk about a Hubble constant, which is the rate at which the Universe currently expands. It is a constant in the sense that today, its value everywhere in space is the same. But actually the Hubble parameter is not constant. It changes with time. Earlier in the Universe, when things were denser and gravitational effects were stronger, the Universe expanded far more rapidly than it does today.
Until rather recently, a fairly broad range of “measured” values for the Hubble parameter, which quantifies the expansion rate today, meant we could not precisely pin down the Universe’s age. The lifetime of the Universe depends on the inverse of the Hubble parameter, so if that measurement is uncertain by a factor of two, so too is its age.
I remember reading in the newspaper when I was a kid that some recent measurements had caused the age of the Universe to be revised by this amount. Not knowing this represented the expansion rate measurement, I remember my astonishment at the radical revision. How could something as important as the age of the Universe have been at liberty to change? It turns out we can understand a good deal of the Universe’s evolution at a qualitative level, even without knowing its precise age. But better knowledge of its age does foster a better understanding of the Universe’s contents and the underlying physical processes at work.
This uncertainty is in any case now under much better control. Wendy Freedman, who was then at the Carnegie Observatories, and her collaborators measured the expansion rate and ultimately quelled the debate. In fact, because the value of the Hubble parameter is so important for cosmology, a concerted effort was made to ensure the greatest accuracy possible. Using the Hubble Space Telescope (given the name, it seems only fair), astronomers measured a value of 72 km/sec/Mpc (meaning something at a distance of a megaparsec moves away at 72 km/sec) with an accuracy of 11 percent—a far cry from Hubble’s original and very inaccurate measurement of 500 km/sec/Mpc.
A megaparsec (Mpc) is a million parsecs, and a parsec, like many astronomical units, is a historical relic from the way distances were measured in early times. It is a shortened version of “parallax second” and has to do with the angle subtended by an object on the sky, which is why it has an angular unit in it. Although many astronomers still use the units, as they do with many other nonintuitive, historically motivated measures, most people prefer not to think in terms of parsecs. To convert to what is perhaps a slightly more familiar measure of distance, a parsec is about 3.3 light-years. It is a fortuitous coincidence that the arcane measure is roughly equivalent to the more readily interpreted quantity.
The Hubble Telescope’s more accurate result for the Hubble parameter might have been uncertain by 10 to 15 percent, but it was not uncertain by a factor of two. More recent results relying on measurements of the cosmic microwave background radiation data do even better. The age of the Universe is now known to within a couple of hundred million years, and measurements have continued to improve. When I wrote my first book it was 13.7 billion years old but we now believe it to be a bit older—13.8 billion years from the so-called Big Bang. Note that it is not only the changing Hubble parameter, but the discovery of the dark energy that I mentioned in Chapter 1, that led to this more refined result, since the age of the Universe depends on both.
PREDICTIONS OF BIG BANG EVOLUTION
According to the Big Bang theory, the very early Universe originated 13.8 billion years ago as a hot, dense fireball consisting of many interacting particles with temperature higher than a trillion trillion degrees. All known (and presumably as-yet unknown) particles zipped around everywhere at close to the speed of light, constantly interacting, annihilating, and being created from energy in accordance with Einstein’s theory. All types of matter that interacted sufficiently strongly with each other had a common temperature.
Physicists call the hot, dense gas that filled the Universe in its early stages radiation. For cosmological purposes, radiation is defined as anything that moves at relativistic speeds, which means at or very close to the speed of light. To count as radiation, objects have to possess so much momentum that their energy far exceeds the energy stored in their mass. The early Universe was so outrageously hot and energetic that the gas of fundamental particles that comprised it readily satisfied this criterion.
Only fundamental particles were present in this Universe, and not, for example, atoms, which are made of nuclei bound together with electrons—or protons—that are made from the more fundamental particles called quarks. Nothing could remain trapped in a bound object in the face of so much heat and energy.
As space expanded, the radiation and particles that permeated the Universe became more dilute and cooled down. They behaved like hot air trapped inside a balloon, which becomes less dense and cooler as the balloon expands. Because each energy component’s gravitational influence affects the expansion differently, the study of the Universe’s expansion over time allows astronomers to disentangle the separate contributions of radiation, matter, and dark energy. Matter and radiation dilute with the expansion but radiation, which redshifts to lower energy—much like a siren decreases in frequency as it moves away—dilutes even more rapidly than matter. Dark energy, on the other hand, doesn’t dilute at all.
As the Universe cooled, notable events occurred when its temperature and energy density no longer sufficed to produce a particular particle. This happened at times when a particle’s kinetic energy no longer exceeded mc2, where m is the mass of that particular particle and c is the speed of light. One by one, massive particles became too heavy for the cooling Universe. By combining with antiparticles, such heavy particles annihilated, converting into energy that then heated up the remaining light particles. The heavy particles thereby decoupled and essentially disappeared.
But even though the Universe’s contents changed, nothing observable happened until a few minutes into the Universe’s Big Bang evolution. So we’ll jump ahead to when the contents of the Universe changed substantially—and did so in a verifiable way. The Hubble expansion mentioned above was one confirmation of the Big Bang theory. Two other significant measurements—both involving the Universe’s contents—solidified physicists’ confidence that it was correct. We’ll first consider the prediction of the relative fractions of the different types of nuclei that were formed in the very early Universe, which match pretty closely the densities that have been observed.
A few minutes after the “Big Bang,” protons and neutrons stopped flying around in isolation. The temperature dropped sufficiently that these particles became bound into nuclei in which they were held together by strong nuclear forces. Also by that time, matter interactions that initially kept the number of protons and neutrons the same were no longer effective. Because neutrons could still decay into protons through the weak nuclear force, their relative number changed.
Because neutron decay takes place sufficiently slowly, a substantial fraction of neutrons survived long enough to be absorbed into nuclei along with the protons that were present. Helium, deuterium, and lithium nuclei were then formed and the cosmic relic amount of these elements, as well as of hydrogen—whose density was depleted when helium was created—was established. The residual amounts of different elements were set by the relative number of protons and neutrons as well as by how quickly the required physical processes took place relative to the speed at which the Universe expanded. So the predictions of nucleosynthesis (as this process is known) test the theory of nuclear physics as well as the details of the Big Bang expansion. In a significant confirmation of both the Big Bang theory and nuclear physics, observations agree with predictions spectacularly well.
Not only did these measurements verify existing theories, but they constrain new ones too. That is because the expansion rate when nuclei abundances were established is mostly accounted for by the energy carried by the types of matter we already know about. Whatever new stuff existed at the time better not have contributed too much energy back then or the expansion rate would have been too rapid. This constraint is important for me and my colleagues when we consider more speculative ideas for what can exist in the Universe. Only small amounts of novel forms of matter could have been in equilibrium and had the same temperature as known matter at the time of nucleosynthesis.
The success of these predictions also tells us that, even today, the amount of ordinary matter cannot be much greater than what has been observed. Too much normal matter and the predictions of nuclear physics wouldn’t match the observed heavy element abundances in the Universe. Along with the measurements described in the previous chapter, which tell us that luminous matter does not suffice to explain observations, the successful predictions of nucleosynthesis tell us that ordinary matter cannot account for all the observed matter in the Universe—largely dispelling the hope that it was invisible just because it wasn’t burning or reflective enough. If there were much more ordinary matter than is observed in luminous matter, the successful nuclear physics predictions would no longer apply unless there were some new ingredient. Unless ordinary matter can somehow hide during nucleosynthesis, we have to conclude that dark matter must exist.
But perhaps the most significant milestone in the Universe’s evolution, at least in terms of detailed testing of cosmological predictions, occurred somewhat later on—about 380,000 years after the Big Bang. The Universe was originally filled with both charged and uncharged particles. But at this later time, the Universe had cooled sufficiently that positively charged nuclei combined with negatively charged electrons to form neutral atoms. From that time forward, the Universe consisted of neutral matter, which is matter that carries no electric charge.
For photons, the particles that communicate the force of electromagnetism, this sequestering of charged particles into atoms was a substantial change. In the absence of charged matter to deflect them, photons could traverse the Universe unhindered. This meant that radiation and light from the early Universe could reach us directly, essentially independent of any more complicated evolution in the Universe that might occur later on. The background radiation we see today is the radiation that existed 380,000 years into the Universe’s evolution.
This radiation is the same radiation that was present immediately after the Universe began its Big Bang expansion, but it is now at a much lower temperature. The photons cooled, but they didn’t disappear. The temperature of the radiation today is 2.73 kelvin,* which is extremely cold. The radiation’s temperature is only a few degrees warmer than zero kelvin—also known as absolute zero, the coldest anything can be.
The detection of this radiation was in some sense the “smoking gun” for the Big Bang theory, perhaps the most convincing evidence that the equations were correct. The German-born astronomer Arno Penzias and the American Robert Wilson accidentally discovered this cosmic microwave background radiation in 1963, while using a telescope at Bell Labs in New Jersey. Penzias and Wilson weren’t actually looking for cosmological relics. They were interested in radio antennas as a way to do astronomy. Of course, Bell Labs, which was associated with a telephone company, was interested in radio waves too.
But when Penzias and Wilson tried to calibrate their telescope, they recorded a uniform background noise (like static) that came from all directions and didn’t change with the seasons. It never went away so they knew they couldn’t ignore it. Because it had no preferred direction, it couldn’t be coming from nearby New York City, the Sun, or the test of a nuclear weapon the previous year. After cleaning off the droppings from the pigeons nesting inside the telescope, they concluded it couldn’t come from the pigeons’ “white dielectric material,” as Penzias politely called it, either.
Robert Wilson told me the story of how lucky they were in the timing of their discovery. They didn’t know anything about the Big Bang, but the theoretical physicists Robert Dicke and Jim Peebles at nearby Princeton University did. The Princeton physicists were in the process of designing an experiment to measure the relic radiation that they recognized was a crucial implication of the Big Bang theory when they discovered they had been scooped—by the Bell Lab scientists who hadn’t yet realized what they had found. Luckily for Penzias and Wilson, the Massachusetts Institute of Technology astronomer Bernie Burke, whom Robert Wilson described to me as his personal early Internet, knew about both the Princeton research and also the mysterious finding of Penzias and Wilson. Burke put two and two together and clinched the connection by bringing the relevant players into contact. After consulting with the theoretical physicist, Robert Dicke, Penzias and Wilson recognized the import and value of what they had discovered. Along with the much earlier discovery of the Hubble expansion, this discovery of the background radiation, which subsequently earned the two Bell Lab physicists the Nobel Prize in 1978, clinched the Big Bang theory of a cooling, expanding Universe.
This was a lovely example of science in action. The research was done for a specific scientific purpose but had ancillary technological and scientific benefits. The astronomers weren’t looking for what they found, but because they were extremely technologically and scientifically skilled, they didn’t dismiss their finding. The research—while looking for relatively small discoveries—resulted in a discovery with tremendously deep implications, which they found because others were simultaneously thinking about the big picture. The discovery by the Bell Lab scientists was accidental, but it forever changed the science of cosmology.
Furthermore, within a few decades of its discovery, this radiation also helped advance major new insights into cosmology. In a spectacular achievement, detailed measurements of this radiation helped verify the predictions of cosmological inflation—in which an explosive stage of expansion occurs very early on.
[FIGURE 5] History of the Universe with inflation and Big Bang evolution, including the formation of nuclei, structure beginning to form, the cosmic microwave background radiation imprinted on the sky, and the modern Universe—in which galaxies and galaxy clusters have been established.
COSMOLOGICAL INFLATION
Many scientific breakthroughs have emerged from an underlying debate about whether change happens gradually, or suddenly, or even—as with our initial ignorance of the Universe’s expansion—if change happens at all. Although people frequently neglect the relevance of this important factor, accounting for the rate of change in today’s world can be very useful when considering the consequences of technology, for example, or when evaluating environmental transformations.
Debates about the pace of change underscored many of the central nineteenth-century conflicts over Darwinian evolution too. As we will see in Chapter 11, the debates contrasted gradualism as espoused by Charles Lyell, in the case of geology, and his acolyte Charles Darwin with the arguments in favor of sudden geological changes posed by the Frenchman Georges Cuvier. Cuvier also recognized another kind of radical change, suggesting, controversially, that not only do new species emerge, as Darwin had so notably demonstrated, but that they also disappear through extinction.
Debates about the pace of change were also central to our understanding of the development of the cosmos. With the Universe, the first surprise was that it evolves at all. When the Big Bang theory was first proposed in the early twentieth century, its implications were very different from those of the theologically favored static Universe, which was what most people had accepted at the time. But another, later surprise was the recognition that very early on, our Universe underwent a phase of explosive expansion—cosmological inflation. As with life on Earth, both gradual and catastrophic processes played a role in the Universe’s history. For the Universe, the “catastrophe” was inflation. And by catastrophe, I mean only that this phase occurred suddenly and rapidly. Inflation destroyed the contents of the Universe that had been there initially, but it also created the matter that filled our Universe when the explosive phase came to an end.
The history presented so far is the standard Big Bang theory of an expanding, cooling, aging Universe. It is remarkably successful, but it’s not the whole story. Cosmological inflation occurred before the standard Big Bang evolution took over. Even though I cannot tell you what happened at the very beginning of the Universe, I can say with reasonable certainty that at some time very early on in its evolution—perhaps as early as 10−36 seconds in—this sensational event called inflation took place. (See Figure 5.) During inflation, the Universe expanded far more rapidly than it did during standard Big Bang evolution—most likely exponentially—so that the Universe kept multiplying in size for the duration of this inflationary phase. Exponential expansion means, for example, that when the Universe was sixty times older than when inflation began, the Universe would have increased by more than a trillion trillion–fold, whereas without inflation, the Universe’s size would have increased only by a factor of eight.
Once inflation ended—also only a fraction of a second into the Universe’s evolution—it left behind a large, smooth, flat homogeneous Universe whose later evolution is predicted by the traditional Big Bang theory. The inflationary explosion was in a sense the “bang” that started the cosmological evolution toward the smoother, slower evolution that was just described. Inflation diluted away the initial matter and radiation as the rapid cooling sent the temperature very close to zero. Hot matter was reintroduced only when inflation ended and the energy driving inflation was converted to a tremendous number of elementary particles. The conventional, slower expansion took over when inflation ended. From this stage forward, the old Big Bang cosmology applies.
The physicist Alan Guth developed the theory of inflation because the Big Bang theory—successful as it is—left several issues unresolved. Why, if the Universe grew from an infinitesimally sized region, is there so much stuff contained within? And why has the Universe been so long-lived? Based on the theory of gravity, you might have expected a Universe containing so much stuff to have expanded away into nothingness or to have collapsed very quickly. Yet despite the enormous amount of matter and energy it contains, the three infinite spatial dimensions of the Universe are very nearly flat and the Universe’s evolution has been sufficiently slow for us to celebrate its 13.8 billion years of existence.
One further major omission in the original Big Bang cosmology was an explanation for why the Universe is so uniform. When the cosmic radiation we now observe was emitted, the Universe was only about one-thousandth its current size, meaning that the distance light could have traveled was far smaller. Yet when observers view the radiation emitted from different regions of the sky from this time, the radiation appears to be identical, meaning that the deviations in temperature and density are minuscule. This is puzzling because according to the original Big Bang scenario, the age of the Universe at the time when the cosmic radiation decoupled from charged matter was too low for light to have had enough time to travel even one percent of the way across the sky. That is to say, if you go back in time and ask whether the radiation that ends up in these separated patches of the sky could have ever sent or received any signals between them, the answer would be no. But if the separate regions never communicated with each other, why would they look the same? It would be as if you and a thousand strangers from different places with different stores and different magazines as inspiration entered a theater dressed identically. If you never had any contact with each other or with shared media outlets, it would be a remarkable coincidence for you to all end up dressed alike. The sky’s uniformity is even more remarkable since the uniformity applies at a precision of one in 10,000. And it looks like the Universe began with more than 100,000 regions that didn’t communicate with each other.
The idea that Guth proposed in 1980 seemed very attractive in light of these deficiencies. He suggested an early epoch during which the Universe expanded extraordinarily rapidly. Whereas in the standard Big Bang scenario, the Universe grew calmly and steadily, in the inflationary epoch, the Universe underwent a phase of explosive expansion. According to the theory of cosmological inflation, the very early Universe grew from a tiny region to an exponentially larger region in an extremely short period of time. The size of a region that a light ray could have crossed might have increased by a factor of a trillion trillion. Depending on when inflation began and how long it lasted, the original region a light ray could cross might have begun as 10−29 meters in size but expanded during inflation to be at least about a millimeter big—a little bigger than a piece of sand. With inflation, you do in some sense have the Universe in a grain of sand—or at least in the size of a grain of sand, as William Blake would have you believe, if you measure the size of the Universe as the observable region at that time.
The inflationary Universe’s extremely rapid expansion explains the Universe’s enormity, uniformity, and flatness. The Universe is enormous because it grew exponentially—in very little time it became very big. An exponentially expanding Universe covers far more territory than one expanding at the far slower rate of the original Big Bang scenario. The Universe is uniform because the enormous expansion during inflation smoothed out the wrinkles in the spacetime fabric, much as stretching out your jacket sleeve eliminates the creases in its fabric. With the inflationary Universe, a single very small region in which everything was close enough to communicate via radiation grew into the Universe we see today.
Inflation also explains flatness. From a dynamical perspective, the flatness of the Universe means that the density in the Universe as a whole is at the borderline where it can last a very long time. Any larger energy density would have made for positive curvature of space—the kind of curvature that a sphere has—which would have made the Universe quickly collapse. Any less density would have caused the Universe to expand so rapidly that structure would never have coalesced and formed. Technically, I am overstating slightly. With a very tiny amount of curvature, the Universe could have lasted as long as it has. But that curvature would have had to have been mysteriously small without inflation to justify its value.
In an inflationary scenario, the Universe is currently so large and flat because it grew so much early on. Imagine that you could blow up a balloon to be as large as you wanted. If you focus on some particular region of the balloon, you would see that it became flatter as the balloon grew larger. Similarly, people originally thought the Earth was flat because they saw only a small region of the surface of a much larger sphere. The same thing is true for the Universe. It flattened as it expanded. The difference is that it expanded by a factor exceeding a trillion trillion.
The extreme flatness of the Universe was the chief confirmation of inflation. This might not come as a surprise, since flatness was after all one of the problems inflation was supposed to address. But at the time inflation was conceived, it was known that the Universe was flatter than naive expectations would suggest, but with nowhere near the precision required to test inflation’s extreme prediction. The Universe has now been measured to be flat at the level of a percent. Had this not been true, inflation would have been ruled out.
When I was a graduate student in the 1980s, inflation was considered an interesting idea, but not one that most particle physicists took very seriously. From a particle physics point of view, the circumstances required for a long-lived exponential expansion seemed extremely unlikely. In fact they still do. Inflation was supposed to address the naturalness of the initial conditions for the expansion of the Universe. But if inflation itself is unnatural, the problem isn’t really solved. The question of how inflation occurred—its underlying physics model—remains a matter of speculation. The model-building issues that plagued us in the 1980s are still a concern. On the other hand, people like Andrei Linde, a Russian-born physicist now at Stanford, who was one of the first to work on inflation, thought it had to be correct even when the idea was first proposed simply because no one had found any other solutions to the puzzles of size, flatness, and homogeneity, which inflation was able to solve in one fell swoop.
In light of recent detailed measurements of the cosmic microwave background radiation, most physicists now agree. Despite the fact that we have yet to determine the theoretical underpinnings of inflation, and that inflation happened long ago, it leads to testable predictions, which have convinced most of us that inflation, or something very similar to inflation, has occurred. The most precise of these observations concerned details about the 2.73 degree background radiation that Penzias and Wilson had discovered. NASA’s Cosmic Background Explorer (COBE) measured this same radiation, but more comprehensively and over a large range of frequencies—establishing its extremely high degree of uniformity across the sky.
But the most spectacular COBE discovery—one that won over almost all inflationary skeptics—was that the early Universe was not exactly uniform. Overall, inflation made the Universe extremely homogeneous. But inflation also introduced very tiny inhomogeneities—deviations from perfect uniformity. Quantum mechanics tells us that the exact time at which inflation ends is uncertain, which means that it ended at slightly different times in different regions of the sky. These tiny quantum effects were imprinted in the radiation as small deviations from perfect uniformity. Though far smaller, they are like the perturbations that rise in the water when you drop a pebble into a pond.
In what is certainly among the most mindblowing discoveries of the last few decades, COBE discovered the quantum fluctuations that were generated when the Universe was roughly the size of a grain of sand, and which are ultimately the origin of you, me, galaxies, and all the structure in the Universe. These initial cosmological inhomogeneities were generated when inflation was ending. They started on minuscule-length scales but they were stretched by the expansion of the Universe to sizes where they could seed galaxies and all other measurable structure, as the following chapter will explain.
Once the discovery of these density perturbations—as these small deviations in temperature and matter density are known—was made, it was only a matter of time before they were investigated in detail. Beginning in 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) measured density perturbations with even more accuracy and on smaller angular scales. WMAP, along with telescopes at the South Pole, observed the ripples—perturbations—in the density of the radiation that encapsulates the complexity that had just begun to be created. The details of these measurements confirmed the Universe’s flatness, determined the total amount of dark matter, and verified the predictions of an early exponential expansion. Indeed, one of the most fabulous results from WMAP was its experimental confirmation of the inflationary paradigm.
The European Space Agency launched its own satellite—the Planck mission—in May 2009 to study the perturbations in even more exquisite detail. Indeed, the satellite’s results have improved the precision with which most cosmological quantities are known and helped solidify our knowledge of the early Universe. One of the Planck satellite’s most important accomplishments was that it pinned down one further quantity that hints at the dynamics that drove the inflationary expansion. Just as the Universe is mostly homogeneous, with small perturbations that violate this homogeneity, the amplitude of perturbations in the sky is mostly independent of their spatial extent, but exhibits a small dependence on scale. The dependence on scale reflects the changing energy density of the Universe at the time inflation ended. In an impressive confirmation of inflationary dynamics, WMAP and the more precise Planck satellite measured that scale-dependence, determining that an early stage of rapid expansion gradually came to an end, and measuring a value that constrains inflationary dynamics.
Although our understanding is far from complete, cosmologists have now established that inflation and the subsequent Big Bang expansion are part of our Universe’s history. We can establish these theories in detail because the early Universe, with its high degree of uniformity, is relatively easy to study. Equations can be solved and the data can be readily evaluated.
However, billions of years ago when structure formation took place, the Universe changed from a relatively simple system to a far more complex one, so cosmology faces greater challenges when addressing the Universe’s later evolution.. The distribution of the contents of the Universe became more difficult to predict and interpret as structures such as stars, galaxies, and galaxy clusters were formed.
Nonetheless, a great deal of information is buried in this ever-evolving structure of the Universe—which observations, models, and computer power should ultimately reveal. As we’ll see in the later part of the book, measuring and predicting this structure promises to teach us quite a lot—including the relevance of dark matter to our world. But for now, let’s explore how this structure came about in the first place.