The big bang theory was developed through experimental observation and theoretical deduction of the structure of the universe. In terms of experimental observation, in 19 12, Vesta Slipher first measured the Doppler shift of a "spiral nebula" (spiral nebula was the old name for spiral galaxy at that time), and later he and Carl Wertheim Wirtz confirmed that most similar nebulae were retreating from the earth. However, Silver did not think of the significance of this observation to cosmology, because at that time, there was a high degree of controversy about whether these "nebulae" were "island universes" outside our galaxy.
In terms of theoretical research, in 19 17, Einstein applied the theory of general relativity to the whole universe and published the article Cosmology Research Based on General Relativity, which marked the establishment of physical cosmology. However, the model of the universe based on general relativity is not static, which is not consistent with the mainstream view of believing in the static universe at that time. Einstein added a cosmological constant to the field equation to correct it. 1922, Soviet cosmologist and mathematician Alexander Friedman assumed the uniformity and isotropy of the universe on a large scale, and derived Friedman's equations describing the spatial uniformity and isotropy by using the gravitational field equation, in which the cosmological constant can be eliminated. By choosing an appropriate equation of state, the universe model obtained from friedmann equations is expanding.