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The Big Bang Theory is a natural result of Einstein's Theory of General Relativity as Lemaître showed back in the 1930s. What evidence is there for thinking the Big Bang theory is correct? The Big Bang theory may be nice but it has to pass the judgement of observation. Nature and experiments are the final judge of the correctness of scientific ideas. Though some details of the Big Bang still need to be perfected, the general scheme of an early hot universe with a definite beginning is accepted by most astronomers today. Even so, we have to be open to the possibility that future observations could show it to be wrong. The observations given below are sometimes said to be ``proof'' of the Big Bang theory. Actually, the observations are consistent with the Big Bang theory, but do not provide proof. Recall from the discussion in the chapter on the scientific method that scientific theories cannot be proven to be correct. As of now, the Big Bang theory is the only one that can explain all of these observations.
The galaxies (or galaxy clusters) are systematically moving away from us such that the farther away galaxies are moving faster away from us. As a result of General Relativity this means that space itself is expanding carrying the galaxies with it. Both the Big Bang Theory and its major competitor, the Steady State Theory, could explain it. Recall that the Steady State Theory used the perfect cosmological principle while the Big Bang uses the cosmological principle.
The cosmic microwave background radiation can be explained only by the Big Bang theory. The background radiation is the relic of an early hot universe. The Steady State theory could not explain the background radiation, and so fell into disfavor.
The amount of activity (active galaxies, quasars, collisions) was greater in the past than now. This shows that the universe does evolve (change) with time. The Steady State theory says that the universe should remain the same with time, so once again, it does not work.
The number of quasars drops off for very large redshifts (redshifts greater than about 50% of the speed of light). The Hubble-Lemaître Law says that these are for large look-back times. This observation is taken to mean that the universe was not old enough to produce quasars at those large redshifts. The universe did have a beginning.
The observed abundance of hydrogen, helium, deuterium, lithium agrees with that predicted by the Big Bang theory. The abundances are checked from the spectra of the oldest stars and gas clouds which are made from unprocessed, primitive material. Even better observations are those made of light from very distant quasars that have passed through gas in regions of the universe where are no stars that could have contaminated the gas. The intervening intergalactic primordial gas imprints its signature on the quasar light giving us the composition of the primordial gas. All of those places have the predicted relative abundances.
The American Astronomical Society and the Astronomical Society of the Pacific published a beautifully-illustrated guide for teachers, students, and the public called An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time. (PDF document: 800 kb in size!) This guide for Teachers, Students and the Public was written by a subcommittee of the American Astronomical Society's Astronomy Education Board. This is a local copy from the AAS Education Board.
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last updated: June 28, 2022