* The broadest topic in the science of astronomy is the study of the entire Universe itself -- its origins, structure, and ultimate fate. This field is known as "cosmology". This chapter describes the history of modern cosmology.
* As astronomers learned more details about the Universe over the last few centuries, they became more interested in the "big picture" of the cosmos: What was the overall structure of the Universe? How big was it? Was it eternal? If not, how and when did it begin, and how and when would it end? Such questions and the search for their answers evolved into the field now known as "cosmology".
Such issues were nothing new, having been addressed in the creation theologies of most of the world's religions -- but religions were never particularly concerned with measurables. Even astronomers were stuck with guesswork until the 20th century.
The first major step towards a physical understanding of the cosmos came in 1919 when Albert Einstein published his "General Theory of Relativity". While General Relativity was specifically focused on a "spacetime" description of gravity, it also had implications for the structure of the Universe as a whole.
Einstein assumed in this work that the Universe was fundamentally static and unchanging. However, in 1922, the Russian theorist Alexander A. Friedmann (1888:1925) examined General Relativity in detail and realized that Einstein's Universe would be unstable, like a ball carefully balanced on a needle. The slightest disturbance would cause the Universe to either expand or contract. In fact, this problem was not really news. In 1692 a scholar named Reverend Richard Bentley (1662:1742) had written a letter to Isaac Newton asking why the gravitational force of the bodies of the Universe did not cause them all to collapse together in on themselves. Newton thought it over and said they wouldn't if they were in a neutral balance, but admitted that was an unstable configuration, and could only feebly suggest the Creator prevented it from happening.
* Nobody paid much attention to the matter in the 1920s, the general assumption at the time being that the Universe was eternal: it always had existed and it would always exist in the future. However, in the meantime, as discussed previously, observational astronomers were proving that some of the "nebulas" long listed in catalogs were actually galaxies, "island universes" like our own, giving the first hint of the true scale of the Universe. They also discovered the galaxies were flying away from each other: the Universe was expanding.
Vesto M. Slipher (1875:1969) of the Lowell Observatory in Arizona began to collect evidence that suggested that on the average, ignoring the random motions of the galaxies in one direction or another, these galaxies were moving away from each other. In 1929, Edwin Hubble, working with his colleague, observational astronomer Milton Humason (1891:1972), to collect data that suggested the Universe is expanding, galaxies are in fact moving away from each other, and the rate at which a galaxy is moving away from our own Milky Way galaxy must be proportional to its distance away.
Hubble based his theory on the redshifting of light emitted by distant galaxies. Hubble observed that the farther away a galaxy seemed to be, the greater its redshift. He expressed this relation in what is now know as "Hubble's Law", given by:
recession_velocity = Hubble_constant * distance
Hubble originally estimated the "Hubble constant" to be about 500 kilometers per second (KPS) per megaparsec (million parsecs / MPC). In other words, a galaxy a million parsecs away had a recession velocity of 500 KPS, one two million parsecs away had a recession velocity of 1,000 KPS, one three million parsecs away had a recession velocity of 1,500 KPS, and so on. Hubble's value turned out to be an overestimate by an order of magnitude due to the incorrect calibration of the Cepheid variable star yardstick, but that wasn't realized until the 1940s.
Fritz Zwicky, strongly inclined to the contrary view, claimed the Universe was not expanding, that idea that some process in the Universe sapped energy from light, with the drain proportional to distance. That would have created the observed redshift, but Zwicky's "tired light" theory suffered from the fatal flaw in that he never managed to come up with any valid physics for why it would happen -- though he did go on record to claim that Humason and Hubble had cooked their data. This time, Zwicky was wrong.
Incidentally, the redshifting of light from the distant Universe is not really due to the Doppler shift as such. The reddening of light is due to the expansion of space, which stretches out the wavelength of a photon traveling towards us. This might seem like an arbitrary distinction, but it's not: this "cosmological redshift" is calculated differently than a Doppler shift, and gives slightly different results.BACK_TO_TOP
* The expanding Universe implied that the cosmos began at a specific instant in time in a hot and dense state, and has been expanding outward ever since. This matched an idea proposed in 1927 by a Belgian Catholic priest and cosmologist named Georges Lemaitre (1894:1966), who suggested that the Universe might have begun in a superdense "primeval atom". Lemaitre had been ignored at the time, but Hubble's theory seemed to be strong evidence that the Universe had once begun in such a primeval atom, and was now expanding outward from its creation. Lemaitre was not timid in pointing out his priority for the idea, and he was given his due credit for the concept. Indeed, he became something of a celebrity, the public being impressed with a priest who was also a top-rank physicist.
In 1933, Arthur Stanley Eddington published a popular science book titled THE EXPANDING UNIVERSE to broadcast the notion to the general public -- and to also dryly suggest that astronomers build new telescopes quickly lest all the galaxies disappear into the beyond before they did so. After World War II, astrophysicists began to flesh out Lemaitre's idea. Their research was pushed by Hubble's evidence for the expanding Universe, and pulled by new theoretical models for the synthesis of elements. Astronomers knew then as they knew now that the Universe is composed mostly of hydrogen, with a small proportion of helium and a smattering of heavier elements. They began to construct models how these elements had been created and why they had the proportions that they did.
* In 1948, George Gamow, then at George Washington University, postulated that at the creation the Universe was a hot plasma of neutrons, which he called "ylem" after a Greek word for "primordial matter". The ylem in part decayed into protons and electrons as the Universe expanded and cooled, eventually forming hydrogen and helium into roughly the distributions observed in the present Universe. Gamow was a brilliant ideas man but not good at plod, and delegated the dirty calculational work to his graduate student Ralph Alpher (1921:2007).
Alpher fleshed out Gamow's idea and found that the results provided a good match to the observed abundances of the hydrogen and helium, resulting in a landmark scientific paper published in 1948. Gamow liked corny gags and couldn't resist simply tacking on the name of his old friend Hans Bethe to the paper so it would become the "Alpher Bethe Gamow (alpha beta gamma)" paper. Bethe liked the gag and approved, but neither he nor Gamow realized that it denied recognition to Alpher by framing him between two giants.
Gamow had hoped that the investigation would be able to show that all the elements would be formed at the creation, but the calculations showed that wasn't the way things could happen. It would take the 1957 paper by the Burbidges, Fowler, and Hoyle on stellar nucleosynthesis to show how it had really happened.
In any case, Alpher then collaborated with Robert Herman (1914:1997) of the Johns Hopkins University's Applied Physics Laboratory to publish another paper later in 1948 that suggested there would be a lingering "afterglow" of the creation of the Universe all over the sky. It was not a completely new idea, since Georges Lemaitre had suggested in his musings on the primeval atom that it might have left a remnant of radiation behind. Alpher and Herman's analysis showed that the radiation would have had a spectrum matching that of a black body with a temperature of 5 degrees Kelvin. Gamow didn't like the low temperature, there being no gear available at the time that could measure it, making it unprovable. Possibly pushed by a bit of wishful thinking, he tweaked the calculations to come up with a value of 50 degrees Kelvin.
* In 1946, Hoyle and two British colleagues, Hermann Bondi (1919:2005), and Thomas Gold (1920:2004), had proposed an alternative cosmology. They did not like the complications introduced by a Universe with a beginning and an end, and suggested that the Universe is expanding forever, with new matter spontaneously created in the vacuum to keep the Universe supplied with new stars and galaxies. This theory became known as the "Steady State" theory of cosmology. Hoyle named the competing theory the "Big Bang". He meant it as mockery, but Gamow and his disciples cheerfully took the name as their own, and in fact it proved to be a simple and vivid label for their theory.
There was actually a good basis for believing in Steady State cosmology at the time. Hubble's overestimate of the Hubble constant indicated that the Universe was about two billion years old, while isotopic dating of the Earth's minerals showed our planet was much older than that. How could the Earth predate the origin of the Universe? Something had to be wrong. Another one of the flaws of Gamow's theory was that it did not explain the abundances of elements heavier than hydrogen and helium, and in fact it was one of Hoyle's motives in helping to write the 1957 stellar nucleosynthesis paper to undermine the Big Bang by showing that it wasn't needed to establish the heavy elements of the Universe.
The objection that the Steady State theory implied creation of matter out of nothing was easily answered: so did the Big Bang theory, the only difference was that the Big Bang created all the matter at once, while in the Steady State matter was created all the time.BACK_TO_TOP
* The debate between the Big Bang and Steady State theories went on until the mid-1960s. The two sides argued, sometimes loudly, but they never became enemies. The facts were slowly building up against the Steady State theory, but the evidence that decisively tipped the balance towards the Big Bang was the discovery of the cosmic background radiation.
In 1946, Robert Dicke (1916:1997), then at the Massachusetts Institute of Technology, reported on observations of the sky with a ultrasensitive microwave radiation receiver or "microwave radiometer". His paper only said that the observed microwave energy was at a low level and did little to suggest further investigation. Making a connection between the usefulness of the microwave radiometer and the cosmic background radiation would take some time, and then it would happen somewhat by accident.
In 1960, Arthur B. Crawford of AT&T Bell Labs had built a 6 meter (20 foot) long "horn" antenna focused on a microwave radiometer at the Bell Labs facility at Crawford Hill, near Holmdel, New Jersey. The antenna had been built to pick up signals from the "Echo" experimental communications satellite. Echo was a balloon with a metalized envelope that was launched into orbit to bounce communications signals over long distances. The Echo satellite worked, but since it was just a passive relay the receiving stations had to be very sensitive to pick up the faint signals bounced off the balloon, making the receiving stations very expensive. Communications satellites quickly moved up to electronic platforms that could carry active "transponders" -- which received signals from transmitting ground stations, then amplified and relayed them to receiving ground stations. There was no need for supersensitive ground stations.
That left the horn antenna at Crawford Hill available for other tasks, and astronomers Arno A. Penzias (born 1933) and Robert W. Wilson (born 1936) of Bell Labs asked to use it for radio astronomy studies. Their first task was to reduce or compensate for noise, but while they were working to this end, they found a low-level noise signal that they couldn't get rid of, and which came from all over the sky. After eliminating all the possible sources of noise -- which included executing a pair of pigeons that had decided to nest in the antenna and were fouling it with their droppings -- the researchers found that it seemed to correspond to the radiation of a black body at a temperature of 3.5 degrees Kelvin.
They contacted Dicke, who was then at Princeton with his colleague P. James E. Peebles (born 1935). Dicke and Peebles were at the time actually working on cosmic background radiation studies to back up their own cosmological theories. Penzias and Wilson didn't know what their noise source was, but Dicke and Peebles realized that the two Bell Labs scientists had just found the afterglow of the Big Bang.
All four researchers published papers on the phenomena in the 1 July 1965 issue of ASTROPHYSICAL JOURNAL. The prediction of Gamow, Alpher, and Herman of the Big Bang radiation was not mentioned in the papers. Partly the snub was because that prediction had been made almost two decades previously and few were aware of it. In 1978, Penzias and Wilson would win the Nobel Prize for their discovery, and Penzias would make sure that the priority of Gamow, Alpher, and Herman was recognized in his acceptance speech.
Later the cosmic background radiation would become known as the "cosmic microwave background (CMB)", to distinguish it from the "cosmic infrared background (CIB)", which is due to the faint glow of distant galaxies at the edge of space and in the early Universe. The discovery of the CMB was a blow to the Steady State theory, which predicted there would be no CMB.
The Steady State theory also predicted that the Universe would look much the same at any time in the past, and Bondi very specifically pointed out that this permitted the Steady State theory to be proven or disproven by observation. By that time, observational astronomers peering farther into the Universe and deeper into the past found galaxies that appeared much different than they do now, featuring many hot blue young stars. Probing even farther revealed an era when many galaxies had bright energetic cores, turning them into quasars. The astronomers could see that the Universe had undergone evolution as they looked back in time towards the limit of the Big Bang.
The Steady State faction was forced to concede defeat, Hoyle saying that the CMB revelations and other new evidence had "knocked the stuffing out of me." He would later renege, trying to propose modified steady-state theories that were generally seen as highly contrived and found few converts -- which may have been one of the major reasons Hoyle didn't win the Nobel prize. Hoyle had an increasing tendency to take on contrarian causes as he got older, provoking considerable derision later in life with half-baked attacks on evolutionary science.
Ironically, Hoyle had almost allowed Gamow to spot the CMB in 1956. They were personally on good terms, their differences over cosmology being seen as a friendly rivalry, and one day Gamow was driving Hoyle around, telling Hoyle that his calculations gave a CMB of about 50 degrees Kelvin. Hoyle then replied that he had seen a 1941 paper by astronomer Andrew McKellar, in which McKellar pointed out that if the average temperature of space were more than 3 degrees Kelvin, that would excite carbon-hydrogen (CH) and carbon-nitrogen (CN) radicals dispersed in space. McKellar was able to link the spectral emissions of CN radicals to the CMB temperature and concluded it was about 2.3 degrees Kelvin. Hoyle wasn't looking for the CMB at all and Gamow was thinking it was a much higher value, and so neither recalled the matter again until they were scooped nine years later.
* The confirmation of the Big Bang explained the fallacy behind Olber's Paradox: If there are infinite numbers of stars in the sky, why isn't the sky completely bright? The reality is that the Universe is not infinite, being bounded relative to any locality by the instant of the Big Bang, and so the radiation in it is not infinite either, and such radiation as exists in it has been diluted over an expanding region of space.
Interestingly, at a certain distance, the expansion of the Universe may exceed the speed of light. That doesn't contradict Einstein's special theory of relativity, the predecessor to his general theory. The special theory states that nothing can exceed the speed of light; this remains true, but space itself can expand faster than light. However, this means that light emitted by any galaxy or other object that has a recession velocity faster than lightspeed can never reach us, in exactly the same way that light emitted by an object that falls below the event horizon of a black hole can never reach us. Ultimately, the expansion of the Universe may mean that distant galaxies will disappear beyond a boundary, and will never be seen again.BACK_TO_TOP
* At that time, the general features of the Big Bang were well understood, though there was and still is disagreement over the details. The Big Bang apparently took place about 13.7 billion years ago. In the instants following the event, temperatures and densities put the Universe in a state where matter and energy were indivisible and the four forces of the Universe, including gravity, electromagnetism, and the strong and weak nuclear forces, were unified as a single force. This is now referred to as the "era of Grand Unification".
Within instants, the four forces had emerged, and the basic constituents of matter, the "quarks", roamed in a sea of energy. Within about a minute, the Universe had expanded and cooled to the extent that the quarks combined into protons and neutrons, the basic constituents of atoms. Some of the protons and neutrons combined to form isotopes of hydrogen and helium. However, the Universe was still much too hot to permit these atomic nuclei to capture electrons.
That event occurred 300,000 years after the Big Bang, when the Universe was 1,000 times smaller than it is now. At that time, the Universe as we know it began to form, with gas clouds forming into stars. By the time the Universe had grown to a fifth of its present size, the stars had formed into young galaxies. By the time it had grown to half its present size, the biggest stars had synthesized heavy elements and then died in supernova explosions, seeding the cosmos with the materials for planets and life-forms.
In outline -- there being discussion over the exact timings -- the history of the Universe following the Big Bang proceeded as follows:
The Universe is described as having the size of an atom or a grapefruit at various early stages of the cosmic expansion, which immediately leads to the question of what it was exploding into. Cosmologists reply that this is a bogus question. The Universe may have been infinite at the time of the Big Bang; it's just that the Universe that we see today was confined to a space the size of, say, a grapefruit. The Big Bang was an explosion of everything, everyplace.
At the time of the creation of the first generation of stars, the Universe consisted only of hydrogen, helium, plus traces of lithium, and so rocky planets could not have existed. The largest stars went through their life cycles quickly, synthesizing "metals" and then spewing them out in supernova explosions; it is believed that conditions in the early Universe encouraged the creation of very large stars. The oldest stars observed are poor in "metals", and are referred to as "Population II" stars, in contrast to younger, "metal"-rich stars, which are referred to as "Population I". The first generation of stars that had no "metals" is sometimes referred to as "Population III", though any that were long-lived enough to survive to our era are necessarily small, faint, and difficult to spot.
* Having accepted the Big Bang, cosmologists then had to consider whether the Universe was going to continue its expansion forever, or whether at some time in the future it was going to slow down, stop, and then gradually begin to fall in on itself, ultimately collapsing into a "Big Crunch" that would lead to another "Big Bang" and a new cycle of the Universe.
A Universe that expands forever is doomed over the long run. The galaxies will become ever more isolated, kept company only by partner and satellite galaxies. Stars will slowly fade out into black cinders. The galaxies will finally evaporate away as their burned-out stars scatter into space, with everything in an ever-growing emptiness whose temperature steadily drops toward absolute zero.
A Universe that stops and falls back in on itself would take a very different path. Redshifting will no longer dilute the energy emitted by distant stars; blueshifting will instead increase it. The energy of the stars in the Universe will slowly accumulate to the level where the skies become bright at all times, rising to a fiery incandescence as all that exists falls together into a explosive apocalypse, presumably leading to an equally violent rebirth.BACK_TO_TOP