Why is the big bang theory?

Answer 1

There is a vast quantity of astronomical observations upon which this theory is based. We can tell by Doppler shifts in stellar spectra that the universe as a whole is expanding, and has been expanding for quite some time. If we try to calculate what the universe was like in the past, it must have been smaller than it is in the present, since expansion makes it larger. The farther back into the past you go, the smaller the universe must have been. We then extrapolate back about thirteen and a half billion years, at which time the universe was compressed into a very tiny package (sometimes called the Cosmic Egg). The time when the expansion began, therefore, would have resembled a Big Bang. In addition to the extrapolation of the overall size of the universe, we have additional confirmation in the form of our observations of the cosmic background radiation that is left over from the Big Bang. It has been analysed in great detail and it is consistent with the theory.