The Big Bang Theory is the name given to the concept that the Universe started out as a single point of extremely high (possibly infinite) density and temperature, which underwent a rapid expansion. This theory was arrived at following the observation by Edwin Hubble that the Universe is expanding, with galaxies receding away from us in every direction in space. Extrapolating this expansion backwards in time naturally leads to the conclusion that the Universe started out as a point of high density, at some finite time in the distant past.
It should be noted, however, that both Georges Lemaître and Alexander Friedmann had independently proposed that the Universe is expanding, prior to the publication of Hubble’s results, from a solution to Einstein’s field equations. Lemaître had used the term ‘primeval atom’ to refer to the point where the fabric of spacetime came into existence.
The term “Big Bang theory”, was originally coined during a 1949 BBC radio interview with the astronomer Fred Hoyle who criticised “the hypothesis that all matter of the universe was created in one big bang at a particular time in the remote past”. Hoyle was a proponent of the rival steady-state theory, that the density of matter in the expanding universe remains unchanged due to a continuous creation of matter.
Evidence for the Big Bang Theory
In addition to the evidence of the expanding universe from the observation of the red-shift of distant galaxies, in accordance with Hubble’s law, the most compelling evidence supporting the big bang theory is the Cosmic Background Radiation (CBR). This was discovered in 1964 by Arno Penzias and Robert Wilson, and was predicted from the big-bang model by George Gamow Ralph Alpher and Robert Herman in the 1950s. The discovery of the CBR led to the demise of the steady-State Theory, that had been advocated by Hoyle.
This radiation, in the microwave band of the electromagnetic spectrum, is consistent with the spectrum from a black-body at a temperature of 2.725 Kelvin, and is approximately uniform in all directions in space. The CBR is interpreted as the ‘afterglow’ of the big bang, which has gradually cooled, or red-shifted, to this temperature over the billions of years since the creation of the Universe.