Early Expanding Universe FindingsIn 1922, Russian cosmologist & mathematician Alexander Friedman found that solutions to Einstein's general relativity field equations resulted in an expanding universe.
As a believer in a static, eternal universe, Einstein added a cosmological constant to his equations, "correcting" for this "error" and thus eliminating the expansion. He would later call this the biggest blunder of his life.
Actually, there was already observational evidence in support of an expanding universe. In 1912, American astronomer Vesto Slipher observed a spiral galaxy (considered a "spiral nebula" at the time, since astronomers didn't yet know that there were galaxies beyond the Milky Way) and recorded its redshift. He observed that all such nebula were traveling away from the Earth, though these results were quite controversial at the time and the full implications of them were not considered at the time.
In 1924, astronomer Edwin Hubble was able to measure the distance to these "nebula" and discovered that they were so far away that they were not actually part of the Milky Way ... he had discovered that the Milky Way was only one of many galaxies, and that these "nebulae" were actually galaxies in their own right.
Birth of the Big BangIn 1927, Roman Catholic priest and physicist Georges Lemaitre independently calculated the Friedman solution and again suggested that the universe must be expanding. This theory was supported by Hubble when, in 1929, he found that there was a correlation between the distance of the galaxies and the amount of redshift in that galaxy's light. The distant galaxies were moving away faster, which was exactly what was predicted by Lemaitre's solutions.
In 1931, Lemaitre went further with his predictions, extrapolating backwards in time find that the matter of the universe would reach an infinite density and temperature at a finite time in the past. This means the universe must have begun in an incredibly small, dense point of matter - a "primeval atom."
Philosophical Sidenote: The fact that Lemaitre was a Roman Catholic priest concerned some, as he was putting forth a theory which presented a definite moment of "creation" to the universe. In the 20's & 30's, most physicists - like Einstein - were inclined to believe that the universe had indeed always existed. In essence, the Big Bang theory was seen as "too religious" by many people.
Proving the Big BangWhile several theories were presented for a time, it was really only Fred Hoyle's steady state theory which provided any real competition for Lemaitre's theory. It was, ironically, Hoyle who coined the phrase "Big Bang" during a 1950's radio broadcast, intending it as a derisive term for Lemaitre's theory.
Steady State Theory: Basically, the steady state theory predicted that new matter was created such that the density and temperature of the universe remained constant over time, even while the universe was expanding. Hoyle also predicted that denser elements were formed from hydrogen & helium through the process of stellar nucleosynthesis (which, unlike steady state, has proved to be accurate).
George Gamow - one of Friedman's pupils - was the major advocate of the Big Bang theory. Together with colleagues Ralph Alpher & Robert Herman, he predicted the cosmic microwave background (CMB) radiation, which is a radiation that should exist throughout the universe as a remnant of the Big Bang. As atoms began to form during the recombination era, they allowed microwave radiation (a form of light) to travel through the universe ... and Gamow predicted that this microwave radiation would still be observable today.
The debate continued until 1965, when Arno Penzias & Robert Woodrow Wilson stumbled upon the CMB while working for Bell Telephone Laboratories. Their Dicke radiometer, used for radio astronomy & satellite communicate, picked up a 3.5 K temperature (a close match to Alpher & Herman's prediction of 5 K).
Throughout the late 1960's & early 1970's, some proponents of steady state physics attempted to explain this finding while still denying the Big Bang theory, but by the end of the decade it was clear that the CMB radiation had no other plausible explanation. Penzias & Wilson received the 1978 Nobel Prize in Physics for this discovery.
Cosmic Inflation TheoryCertain concerns, however, remained regarding the Big Bang theory. One of these was the problem of homogeneity. Why does the universe look identical, in terms of energy, regardless of which direction one looks? The Big Bang theory does not give the early universe time to reach thermal equilibrium, so there should be differences in energy throughout the universe.
In 1980, American physicist Alan Guth formally proposed inflation theory to resolve this and other problems. Inflation basically says that in the early moments following the Big Bang, there was an extremely rapid expansion of the nascent universe, driven by "negative-pressure vacuum energy" (which may be in some way related to current theories of dark energy). Alternatively inflation theories, similar in concept but with slightly different details, have been put forward by others in the years since.
The Wilkinson Microwave Anisotropy Probe (WMAP) program by NASA, which began in 2001, has provided evidence which strongly supports an inflation period in the early universe. This evidence is especially strong in the three-year data released in 2006, though there are still some minor inconsistencies with theory. The 2006 Nobel Prize in Physics was awarded to John C. Mather & George Smoot, two key workers on the WMAP project.
Existing ControversiesWhile the Big Bang theory is accepted by the vast majority of physicists, there are still some minor questions concerning it. Most importantly, however, are the questions which the theory cannot even attempt to answer:
- What existed before the Big Bang?
- What caused the Big Bang?
- Is our universe the only one?