Big Bang theory

The Big Bang theory places the age of the universe at approximately 13.8 billion years, based on measurements of the cosmic microwave background by the Planck spacecraft. This estimate is further corroborated by the observed expansion rate of the universe, known as the Hubble constant.

Belgian physicist and Catholic priest Georges Lemaître first proposed what he called the 'hypothesis of the primeval atom' in the late 1920s, deriving it from solutions to Einstein's general relativity equations. The name 'Big Bang' was later coined dismissively by astronomer Fred Hoyle during a 1949 BBC radio broadcast.

Three key observational pillars support the theory: the cosmic microwave background radiation discovered by Arno Penzias and Robert Wilson in 1965, the Hubble expansion showing redshift proportional to distance, and the observed relative abundance of light elements like hydrogen and helium matching Big Bang nucleosynthesis predictions.

The theory describes the universe beginning as an extremely hot, dense state, followed by rapid expansion and cooling. Within the first fractions of a second, the universe underwent a period called cosmic inflation, during which it expanded faster than the speed of light, and fundamental forces separated as temperatures dropped.