The cosmological theory called the big bang theory implies that the universe began from a singular state of infinite density. This theory was explained in the complete solution of Albert Einstein's equations, obtained by Aleksandr Friedmann in 1922. In 1927, Georges Lemaitre used these equations to devise a cosmological theory incorporating the concept that the universe is expanding from an explosive moment of creation.
The theory was further developed in the 1940s by George Gamow and R. A. Alpher. They proposed a high-temperature state in the beginning and elaborated the concept to include a theory of element synthesis and background radiation. (The term big bang itself has been attributed to Fred Hoyle.) With the evidence currently available, including the discovery of the background radiation, the big bang theory appears to best fit for the evolution of the universe. In the later 20th century the theory was further elaborated by inflationary theory.
The so-called inflationary model of the events involved in the rapid expansion of the universe in its first moments of creation, as expected in the big bang theory, was developed by American physicist Alan H. Guth in the early 1980s. It is the first successful and comprehensive attempt to correlate particle physics with cosmology.
As we now know, the universe is expanding. The American Astronomer, Edwin Hubble proved this theory right by discovering that the spectra of all external galaxies are red-shifted. This implies that the galaxies are getting farther away from us meaning that the universe is expanding. But what’s expanding? According to Einstein the fabric of time-space is what’s expanding. This new theory changed the way we thought about the universe and gravity.