In cosmology, the Big Bang is the scientific theory that describes the early development and shape of the universe.
The Big Bang Theory is the most accepted theory for the origin and evolution of our universe. The big bang theory states that at some time in the distant past there was nothing. It suggests that around 10 to 14 billion years ago, the part of the universe we can see today was only a few millimetres across. According to this theory, at the beginning of time, all of the matter and energy in the universe was concentrated in a very dense state, from which it "exploded" and this is known as the Big Bang.
The Big Bang marks the instant at which the universe began. From a dense, hot ball of gas, radiation and subatomic particles. This exploded and began expanding rapidly outward. As it expanded it cooled and electrons, protons and neutrons formed. As the universe grew in size, the temperature dropped, which eventually formed huge numbers of Hydrogen, Helium and Lithium nuclei. After many millions of years the expanding universe, at first a very hot gas, thinned and cooled enough to condense into individual galaxies and then stars. Stars and galaxies began to form about one billion years following the Big Bang. It has since expanded from this hot dense state into the vast and much cooler cosmos we currently inhabit.
Evidence for the Big Bang Theory
American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was generally shifted toward the red end of the spectrum, this is called the Red Shift. This happens when stars are moving rapidly away from Earth. This evidence means that it is obvious that the universe is expanding.
The second evidence is that this theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations.