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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
the scientific method that
scientific theories cannot be proven to be correct. As of now, the Big
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
- 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
- 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 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
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April 29, 2013
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