Big Bang Cosmology

The Big Bang Model is a broadly acceptedtheory for the origin and evolution ofour universe. It postulates that 12 to 14 billion years ago, the portion of theuniverse we can see today was only a few millimeters across. It has sinceexpanded from this hot dense state into the vast and much cooler cosmos wecurrently inhabit. We can see remnants of this hot dense matter as the now verycold cosmic microwave background radiation which still pervades the universe andis visible to microwave detectors as a uniform glow across the entire sky.

Foundations of the Big Bang Model

The Big Bang Model rests on two theoretical pillars:

General Relativity

Albert Einstein at the Chalkboard- source unknown

The first key idea dates to 1916 when Einstein developed his General Theory of Relativity which heproposed as a new theory of gravity. His theory generalizes Isaac Newton'soriginal theory of gravity, c. 1680, in that it is supposed to be valid forbodies in motion as well as bodies at rest. Newton's gravity is only valid forbodies at rest or moving very slowly compared to the speed of light (usually nottoo restrictive an assumption!). A key concept of General Relativity is thatgravity is no longer described by a gravitational "field" but ratherit is supposed to be a distortion of space and time itself. Physicist JohnWheeler put it well when he said "Matter tells space how to curve, andspace tells matter how to move." Originally, the theory was able to accountfor peculiarities in the orbit of Mercury and the bending of light by the Sun,both unexplained in Isaac Newton's theory of gravity. In recent years, thetheory has passed a series of rigorous tests.

The Cosmological Principle

APM Galaxy Survey

After the introduction of General Relativity a number ofscientists, including Einstein, tried to apply the new gravitational dynamics tothe universe as a whole. At the time this required an assumption about how thematter in the universe was distributed. The simplest assumption to make is thatif you viewed the contents of the universe with sufficiently poor vision, itwould appear roughly the same everywhere and in every direction. That is, thematter in the universe is homogeneous and isotropic when averaged over verylarge scales. This is called the Cosmological Principle. This assumption isbeing tested continuously as we actually observe the distribution of galaxies onever larger scales. The accompanying picture shows how uniform the distributionof measured galaxies is over a 70° swath of the sky. In addition thecosmic microwave background radiation, theremnant heat from the Big Bang, has a temperature which is highly uniform overthe entire sky. This fact strongly supports the notion that the gas whichemitted this radiation long ago was very uniformly distributed.

These two ideas form the entire theoretical basis for Big Bang cosmology andlead to very specific predictions for observable properties of the universe. Anoverview of the Big Bang Model is presented in a set ofcompanion pages.

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?"