FILLING IN MORE DETAILS

Let us review the status of cosmology in the mid-twentieth century. By the early 1930s, the great discovery that we live in a vast, expanding universe composed of galaxies of stars hurtling away from one another at great speeds was well confirmed and astronomers busied themselves with filling in more details. The most powerful telescope in the world remained the 100-inch reflector on Mount Wilson, which had gone into operation in 1908. It retained that position for forty years, until 1948 when it was finally supplanted by the 200-inch reflector at the Palomar Observatory. Of course, these were not the only telescopes and many others had special designs that made them very useful for particular types of observations.

With these instruments, astronomers began surveying the skies and cataloging galaxies, a process that would continue for years with many surprising and dramatic results. One of the most productive catalogers was an idiosyncratic astrophysicist, Fritz Zwicky, already noted for proposing the unsuccessful tired-light mechanism for the redshift of galaxies and his observation of the missing mass of galaxies. He also conjectured that high-energy cosmic rays come from outside the solar system and result from the explosions of supermassive stars. He dubbed these supernovae. Zwicky used the Palomar 18-inch Schmidt telescope, invented in 1930 by German optician Bernard Schmidt, to search for supernovae. The Schmidt could accurately survey large patches of sky. After it went into operation in 1936, Zwicky discovered a dozen supernovae.1

In 1948, the larger 48-inch Schmidt telescope was used for the National Geographic Palomar Sky Survey. It spectacularly verified Zwicky's proposal, mentioned in chapter 8, that galaxies gather together in clusters. In 1958, UCLA astronomy professor George Abell produced a catalog of 2,712 clusters in the northern sky and by 1989 had catalogued 4,073 rich clusters over the entire sky. By the 1970s astronomers began to see the clusters themselves organizing into “spongy” structures of their own, with holes, filaments, and walls.

A HOT, DENSE PAST

In the meantime, the focus of cosmological physics shifted from general relativity to nuclear physics. It began to be realized that if the universe is expanding, it must have grown from a very small, hot, and dense region in the past when nuclear reactions played a major role. Georges Lemaître may have been the first to recognize this. However, his proposal that a primordial supernucleus decayed radioactively into the universe we have today was pure speculation and had no empirical or theoretical foundation.

As a result, few at the time took the notion seriously. What Lemaître did provide was a cosmological solution to general relativity for an expanding universe. This is now called the Friedmann-Lemaître solution since it was already implicit in Friedmann's equations. The model itself became known as the Lemaître-Eddington model, since Eddington developed it further.

In any case, Lemaître pictured a finite universe with a definite beginning. While this may have been suggested to him by his religious belief in a creator, as mentioned earlier he never based his arguments on theology and, indeed, objected to such an interpretation.

On the other hand, we saw in chapter 7 that Eddington found the idea of a beginning to the universe “repugnant.” His universe was expanding but eternal and most physicists of the time were inclined to agree. As for observational astronomers, they stuck to their telescopes.

Lemaître continued to develop his model and was cognizant of the need to make it testable. He realized that if the universe had once been hot, dense, and radioactive, it must have left a remnant of radiation that might be observable today. However, he did not think this radiation was electromagnetic, that is, photons, but instead would be composed of charged particles. Once again, most physicists were not convinced, although Einstein expressed mild interest. But the evidence simply was not there.²

Furthermore, a major problem for the finite-universe hypotheses lay in the timescale implied by the data. Recall, Hubble's law implies that the age of the universe is equal to the inverse of the Hubble constant. That value was coming out under two billion years, less than estimates of the age of Earth from geology and nuclear physics. In fact, it may come as a surprise to hear that Hubble himself questioned the expanding universe, which would ultimately bring him great fame. He wrote, “No effects of expansion—no recession factor—can be detected. The available data still favor the model of a static universe rather than a rapidly expanding universe.”³

However, the assumption that the age of the universe T = 1/H is based on zero cosmological constant. Lemaître's model included a cosmological constant and allowed for a much-older universe. Unfortunately, Einstein had disowned the cosmological constant and would have nothing further to do with it.⁴

Up until this point, the late 1930s, the only physics involved in theoretical cosmology was general relativity. Lemaître's primeval atom was mainly speculation, with some tentative attempts to develop a quantitative model. But then, a great breakthrough occurred in 1938–1939 when the German physicists Hans Bethe (working in America) and Carl Friedrich von Weizäcker independently proposed nuclear-fusion mechanisms responsible for the energy of stars. Bethe's process was particularly simple. Four protons unite to form helium by a series of two-body collisions involving only fundamental particles: protons, neutrons, electrons, photons, and (as we know today) neutrinos. Weizäcker's mechanism was more complex, involving carbon, oxygen, and nitrogen isotopes.5

Weizäcker also suggested that his theory might account for the formation of the elements.⁶ However, his model failed to give an accurate account of cosmic abundances of the elements.⁷ Still, nuclear physicists were sufficiently intrigued to begin taking part in cosmological research.

YLEM

A major step toward establishing the validity of the big-bang model was made by a Russian-Ukrainian physicist who emigrated to the United States, George Gamow.⁸ In 1924, Gamow was in the classroom in Leningrad when Alexander Friedmann gave a series of lectures, “Mathematical Foundations of the Theory of Relativity.” Gamow wanted to study under Friedmann, who sadly died young just a year later.

After earning a doctorate in quantum theory from Göttingen with a thesis on the atomic nucleus, Gamow worked in Copenhagen with Niels Bohr, in Cambridge with Ernest Rutherford, and in 1931, at age twenty-eight, became a member of the Academy of Sciences of the USSR. His many accomplishments in nuclear physics include demonstrating quantitatively that nuclear alpha decay (emission of helium nuclei, also called alpha particles) can be understood as resulting from quantum tunneling. This process also plays an important role in fusion reactions in stars. We will see later how this uniquely quantum mechanical process has been recognized in cosmology as the mechanism by which our universe may have come into existence.

In 1934, Gamow moved to the United States and worked with Edward Teller at George Washington University in Washington, DC. During World War II, Teller went off to work on the Manhattan Project. However, Gamow was not cleared to work on the nuclear bomb because he had been made a commissioned officer in the Red Army in order that he could teach at the Soviet military academy. He remained in Washington and consulted for the US Navy. After the war, Gamow was cleared for nuclear research at Los Alamos.9

Besides continuing to work on nuclear physics, with a developing interest in astrophysics, Gamow became famous as the author of bestselling, popular books including One, Two, Three…Infinity, The Birth and Death of the Sun, Mr. Tompkins in Wonderland (a series of six books), and many others. I devoured them as a teenager and they undoubtedly contributed to my becoming a physicist. This was just one more indication of Gamow's immense genius; he didn't even speak English until graduate school.

In 1948, Gamow, Ralph Alpher, and Hans Bethe published a short letter in the Physical Review, “The Origin of the Chemical Elements,” reinvigorating the notion that the nuclei of the atoms that make up the periodic table of the chemical elements were produced in the early universe.¹⁰ Bethe's name was added so the paper could be referred to as “Alpher, Bethe, and Gamow.” While not an original author, Bethe nevertheless made valuable contributions. The paper does not cite Lemaître or Weizäcker.

Alpher, Bethe, and Gamow proposed that, in the beginning there existed a compact core of primordial stuff, which they would later dub “ylem,” composed of neutrons at high density and temperature. Some neutrons decayed into protons by beta decay, emitting an electron and (we now know) an antielectron neutrino, leaving a bath of these particles.

The elements then formed as the protons and neutrons recombined by a process known as radiative capture in which photons are emitted, adding to the mix. In this manner, a proton and a neutron fuse to make a deuteron (a hydrogen nucleus with two neutrons). Add another neutron and you get a triton (a hydrogen nucleus with three neutrons). A triton and a proton, or two deuterons, can fuse to make helium, with a substantial release in energy.

As an aside, attempts at controlled nuclear fusion utilize these reactions, which require lower temperatures than those reactions taking place in the cores of stars. Even so, that temperature is still incredibly high, on the order of 100 million degrees, and despite over a half century of effort, this source of energy is still far from being realized.

Gamow and collaborators envisaged building up the complete periodic table by a series of nuclear reactions in the early universe. However, despite their intense efforts, the process was found not to continue. Adding a neutron or a proton to helium does not produce a stable nucleus with five nucleons. Fusing two helium nuclei together also does not produce a stable state of eight nucleons.

As we will see, Fred Hoyle and his collaborators were able to show later how the heavier nuclei form inside stars by what is called stellar nucleosynthesis, a process first suggested by Arthur Eddington and also studied by Bethe. When Gamow's primordial nucleosynthesis failed to account for the full periodic table of the elements, the big bang once again fell out of favor.

However, the Alpher-Bethe-Gamow model, also contributed to by Ralph Herman, had another implication that was realized in a remarkable paper by Alpher and Herman published in 1949. The reaction rates involved are sufficiently fast compared to the expansion rate of the universe, given by the Hubble parameter H, that a kind of quasi-thermal equilibrium with slowly decreasing temperature occurs in the plasma of interacting particles. Alpher and Herman estimated that the temperature “when the neutron capture process became important” would be about 600 million degrees.11 Including in the mix at the time were photons. Applying the theory of cosmological expansion, they estimated that today this temperature would have cooled to “on the order of 5 K,” that is five degrees Kelvin.12

Although they were not quite so explicit, Alpher and Herman had made the world-shaking prediction that the universe should be bathed in thermal radiation, that is, radiation following the spectrum of a blackbody at 5 K, which is in the microwave region. This only applies to the photons, which remained in thermal equilibrium as the universe expanded while matter did not. A 5 K blackbody spectrum peaks at a wavelength of about one meter. By comparison, the optical spectrum of the sun (5000 K) peaks at 550 billionths of a meter.

This prediction failed to excite any interest among physicists and astronomers, probably because it was tied to the big-bang nucleosynthesis mechanism that was unable to reach beyond the first few elements of the periodic table.

Furthermore, there still was the age paradox. So, yet again, the big bang was forgotten. Despite the enthusiasm of Pope Pius XII, after 1953, only one paper on the big bang was published over the period of a decade.¹³ In its place was a model of an eternal, unchanging universe that attracted far more attention than it deserved, perhaps because of the eminence of its proposers.

THE STEADY-STATE CHALLENGE

As mentioned, “big bang” was a derisive term coined by Fred Hoyle in an interview on the BBC in 1948. That year, Hoyle,¹⁴ along with Hermann Bondi and Thomas Gold,¹⁵ developed an alternative to the big bang called the steady-state universe. James Jeans had first suggested a steady-state cosmology in 1928.¹⁶ With the aid of Margaret and Geoffrey Burbidge, and Jayant Narlikar, Hoyle continued to promote the steady-state model well after the empirical evidence for the big bang had become overwhelming.¹⁷

These authors felt strongly that the universe had to obey what they called the perfect cosmological principle, which they interpreted to mean that the universe must look the same everywhere and everywhen, that is, at every place and at every time. Not only is there no special place in space that we can regard as center of the universe, there is no special moment in time when it all began.

Now, in an expanding universe with a fixed total mass, the mass density must decrease with time. Since, in the steady-state authors’ minds, the average mass density of the universe must remain constant or else the universe will “look different,” matter must be continuously and uniformly created. The rate was very small, however, only 10^–43 grams per cubic centimeter per second.

This would seem to violate energy conservation—as the authors realized. However, various versions of the model that were proposed over the years provided for energy conservation with the introduction of a field of negative pressure. It was noted that this special field was equivalent to the cosmological constant. A positive cosmological constant produces a constant negative pressure in an expanding gas, resulting in an increase in internal energy fully consistent with conservation of energy. The energy to create the new mass from the work is done on the system by its own negative pressure. However, this interpretation seems to have been rejected by the steady-staters.18

In any case, numerous observations would soon prove conclusively that the universe looked quite different at different times. In the 1950s, radio astronomer Martin Ryle and his group at Cambridge showed, after some controversy (Hoyle's office was down the hall), that the density of radio sources at large distances, and thus early times, was greater than it is now. When, as we will discuss later in this chapter, quasars and other active galaxies were discovered, it was found that they also were far more abundant in the deep past.

Ryle would share the 1974 Nobel Prize for Physics with his Cambridge colleague Antony Hewish, whose student Jocelyn Bell discovered the first pulsar (more about this in a moment). This was the first Nobel Prize ever awarded for astronomical accomplishments. It would not be the last.

The fact that the universe has changed its appearance over billions of years constitutes a failure of the perfect cosmological principle as it was defined by Hoyle. However, instead of asserting that the universe must appear the same at all times and places, we can introduce a cosmological principle that serves the same purpose as that proposed by Hoyle, which was to extend the Copernican principle to include time as well as space. We simply require that the models scientists construct to describe the universe apply at all times and places. The most distant objects such as quasars, which are also being viewed in the deep past, exhibit the same spectral lines and other basic physics properties as we measure in the laboratory here and now on Earth, confirming this version of the cosmological principle.

STELLAR NUCLEOSYNTHESIS

Hoyle's views received a tremendous boost when he and his team of collaborators were able to develop a successful theory of element formation in stars called stellar nucleosynthesis, published in 1957.19 This undermined the big-bang model because of the failure of big-bang nucleosynthesis.

In 1952, physicist Edwin Salpeter had found a way to jump the instability gap of five and eight nucleons in building up the elements heavier than helium. In his so-called triple alpha process, two alpha particles, that is, helium-4 nuclei (He⁴) first unite to from beryllium-8 (Be⁸), a nucleus containing four protons and four neutrons.²⁰ The beryllium nucleus is unstable, however, which we have seen is the major bottleneck that prevents big-bang nucleosynthesis from going further. Salpeter showed that at sufficiently high temperature and density, a Be⁸ nucleus can capture another He⁴ nucleus to form stable carbon-12 (C¹²) before decaying. Here are the reactions:

He⁴ + He⁴ → Be⁸

He⁴ + Be⁸ → C¹²

Of course, carbon is the most important element in the formation of life as we know it. Other nuclei important to life, such as oxygen and calcium, can also be formed from He⁴ fusing with other nuclei:

He⁴ + C¹² → O¹⁶

He⁴ + O¹⁶ → Ca²⁰

These processes can, in principle, occur in the big bang. But the temperature has to drop below one billion degrees because above that temperature nuclei break apart by photodisintegration as fast as they synthesize. At that temperature, the density of the early universe has dropped to 10^–4 grams per cubic centimeter, far too low for the Salpeter process to occur.

In 1954, Hoyle showed that when a star burns up all its hydrogen and collapses under gravity, its core will reach a temperature on the order of one hundred million degrees and density of about ten thousand grams per cubic centimeter, allowing the triple alpha process to take place.21

THE STEADY STATE AND GOD

One of the strong objections Hoyle, Bondi, Gold, and other proponents of the steady-state universe had for the big-bang model was what Hoyle termed “esthetic.” Any explanation for an abrupt origin would have to rely on “causes unknown to science.”²² By this he meant metaphysical. Hoyle was outspokenly atheist and as late as 1982, he was assailing those scientists who supported the big bang, by then the majority, unfairly attributing to them religious motives:

I have always thought it curious that, while most scientists claim to eschew religion, it actually dominates their thoughts more than it does the clergy. The passionate frenzy with which the big-bang cosmology is clutched to the corporate scientific bosom evidently arises from a deep-rooted attachment to the first page of Genesis, religious fundamentalism at its strongest.23

In his 1994 autobiography he declared, “Big-bang cosmology is a form of religious fundamentalism.”²⁴

However, as we will now see, the big-bang model emerged triumphant while the steady-state model failed, both for the best of reasons. The big-bang model agreed with the data and the steady-state model did not. To the everlasting credit of Hoyle and his collaborators, stellar nucleosynthesis also triumphed. But it never had anything to do with the steady-state universe in the first place and its success should never have been held against the big-bang model.

Perhaps Hoyle's atheism may have prejudiced him against the big bang and motivated him to seek another explanation for nucleosynthesis than element production in the early universe. In any case, today stellar nucleosynthesis is an integral part of cosmology, with big-bang nucleosynthesis providing for just the right amount of the lighter elements to supplement what is not produced in stars. In science and Denver Bronco football, things do not always work out the way you expect.

ACTIVE GALAXIES

Perhaps the most important discovery of cosmological importance prior to the discovery of the cosmic microwave background in 1964, which will be discussed in the next chapter, was the observation of quasi-stellar objects, now popularly known as quasars. By 1960, radio astronomy had begun to flourish and a hundred or so curious radio objects were recorded that seemed to have a small angular size. One, 3C 48, was tied to an optical source by John Bolton. In 1963, Maarten Schmidt using the 200-inch Hale telescope at the Palomar Observatory optically identified the radio source 3C 273.25 Actually, 3C 273 is visible with relatively small, amateur telescopes—looking just like a star (hence “quasi-stellar”).

Measuring the optical spectrum, Schmidt found that the spectral lines of hydrogen were shifted by 47,400 kilometers per second or 15.8 percent of the speed of light. If the speed measured from the redshift of the source is used to determine its distance by Hubble's law, 3C 273 was two billion light-years away when it emitted the observed light. Note that it is much farther away now since the universe has been expanding during all that time. It obviously was not a single star.

Schmidt estimated that the object, if at the distance implied by Hubble's law, had to be one hundred times brighter than any of the galaxies that had so far been associated with radio sources, and that the light was coming from a nucleus less than three light-years across. Having somewhat more than an amateur instrument at Palomar, Schmidt also observed an optical jet about 150 light-years long that he associated with a radio feature. He inferred that the object was of galactic dimensions.

In the paper immediately following the one published in Nature by Schmidt, Jesse Greenstein and Thomas Matthews reported a redshift for 3C 48 corresponding to a recessional speed of 110,200 kilometers per second, 37 percent of the speed of light and implying a distance of almost five billion light-years.²⁶

While alternate explanations for quasars that placed them much nearer Earth were debated for a while, it was soon established that they are indeed distant objects with enormous, unprecedented energy emission.²⁷

Eventually quasars were identified as members of a class of astronomical objects called active galaxies. These are galaxies that are far more luminous than normal galaxies with strong emission lines usually associated with a central core as well as radio and x-ray emissions. They often have radio jets thousands of light-years long pointing out from the center. They also often exhibit a highly variable luminosity that can change by a factor of two in a few days, which is very unusual for an object of galactic dimensions. This implies that the sources of the enormous energies of active galaxies are confined to regions of just a few light-days in size, tiny by comparison with the size of a galaxy such as the Milky Way, which is one hundred thousand light-years in diameter. It now appears that these sources are supermassive black holes.

Active galaxies fall into three subclasses:

Seyfert galaxies

These are named after astronomer Carl Seyfert, who first noticed them in 1943. Seyferts are galaxies with very bright nuclei containing broad emission lines of hydrogen, helium, nitrogen, and oxygen. The broadening is interpreted as the Doppler shifts from gases moving at speeds from five hundred to four thousand kilometers per second.

Radio galaxies

These are very bright radio sources emitting huge double-lobed structures of radio emission usually shooting out in opposite directions from an optical core. When the jets point along a line toward Earth so we can't see them, they are called blazars. BL-Lac objects form a subclass of blazars. As we will see, blazars send to Earth very high-energy gamma-rays and, perhaps, neutrinos.

Quasars

It is now realized that quasars are active galaxies that are so distant that they appear as point sources.

All of the galaxies that form our local group are “normal.” The nearest active galaxy is Centaurus A, which is ten million light-years away. Most are quite distant. In fact, the preponderance of active galaxies existed around two billion years after the big bang. Their population was reduced to less than 10 percent by six billion years.28

If ever there was conclusive evidence against a steady-state universe, this is it. The universe that we see when we study active galaxies looks quite different than the universe we see when we study nearby galaxies. Apparently, when galaxies first formed they went through a period known as the bright phase when they were far more luminous than are the galaxies of today. These earlier galaxies died off while later galaxies formed that were much more benign. The explanation is simple. The early galaxies contained many more giant stars than do current galaxies. These emit more light but are shorter-lived.

PULSARS

Although not of great cosmological significance from the perspective of this book, another unexpected discovery in the 1960s bears mention. In 1967, astronomy graduate student Jocelyn Bell, mentored by Antony Hewish in Cambridge, discovered the first pulsar, an astronomical object emitting pulses of radio emission 1.33 seconds apart.²⁹ Soon many more were observed, some with pulse periods as low as milliseconds, and they were eventually associated with neutron stars.

A neutron star is the remnant of a supernova that has blown off most of its mass, leaving behind a highly dense sphere that is mostly composed of neutrons formed by gravitational collapse. Typically it is about 1–3 solar masses with a radius of about twelve kilometers. Its density is comparable to that of an atomic nucleus. If it has a strong magnetic field and is rotating rapidly, it will emit the short period electromagnetic pulses as are observed.