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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 a 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 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 Big Bang
theory's major competitor, called 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 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 abundance of hydrogen, helium, deuterium, lithium agrees with that predicted by the Big Bang theory. The abundances are checked from the spectra of the the oldest stars and gas clouds which are made from unprocessed, primitive material. They have the predicted relative abundances.