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.

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