The Big Bang Theory
The Big Bang Theory is a broadly accepted theory for the origin
and evolution of our universe. It postulates that the observable universe
started from an instantaneously expanding point, roughly ten to twenty billion
years ago.
Foundations of the Big Bang Theory
The hot Big Bang Theory is a broadly accepted theory for the origin
and evolution of our universe. It rests on two seeming sound pillars:
- The General Theory of Relativity: Over eighty years ago, Einstein
proposed this theory that describes how the distribution of mass
in the universe determines the geometry of the space. Originally,
the theory was able to account for peculiarities in the orbit of
Mercury and the bending of light by the Sun. In recent years,
the theory has passed a series of rigorous tests.
- On the largest scales, the distribution of matter in the universe
is nearly uniform. This assumption appears to confirmed both by galaxy
surveys and by the low level of
fluctuations in the cosmic microwave background radiation
In the hot Big Bang Theory, the observable universe began with an
instantaneously expanding point, roughly ten to twenty
billion years ago. Since then, the universe has continued to expand,
gradually increasing the distance between our
Galaxy and external galaxies. The expansion of the universe "stretches"
light rays converting blue light into red light and red light into infrared
light. Thus, distant galaxies, which are rapidly moving away from us, appear
redder. This expansion also cools the microwave background radiation. Thus,
the cosmic microwave background radiation, which today has a temperature
of 2.728 Kelvin, was hotter in the early universe. Gravity slows the
expansion of the universe. If the universe is dense enough, the expansion of
the universe will eventually reverse and the universe will collapse. If the
density is not high enough, then the expansion will continue forever. Thus,
the density of the universe will determine its ultimate
fate.
Tests of the Big Bang Theory
The hot Big Bang Theory is consistent with a number of important
observations:
- the observed expansion of the universe,
- the observed abundances of helium, deuterium and lithium, three elements
thought to be synthesized primarily in the first three
minutes of the universe,
- the thermal spectrum of the cosmic microwave background
radiation
- the cosmic microwave background radiation appears hotter in distant
clouds of gas. Since light travels at a finite speed, we see these
distant clouds at an early time in the history of the universe, when it was
more dense and thus hotter.
Beyond the Big Bang Theory
In its current form, the big bang theory is not complete. It does
not explain:
Many cosmologists suspect that the Inflation
Theory, an extension of the Big Bang Theory, may answer these questions.
Further Reading
- Peebles, P.J.E., Schramm, D.N., Turner, E.L. \& R.G. Kron 1991,
"The Case for the Relativistic Hot Big Bang Cosmology",
Nature, 352, 769 -- 776.
-
Peebles, P.J.E., Schramm, D.N., Turner, E.L. \& R.G. Kron 1994,
"The Evolution of the Universe'',
Scientific American, 271, 29 -- 33.
- Will, Clifford, "Was Einstein Right?"
Back to the Introduction to Cosmology Page
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Last updated: March 29, 1996
