In 1922, Russian mathematician Alexander Friedmann answered Albert Einstein's equations of general relativity. Friedmann's answers led to the theory that the universe is dynamic, meaning that it changes in size over time. Friedmann introduced the term "expanding universe" into the scientific lexicon.
General relativity suggests that space is not solid. It can bend into many different shapes and is, in fact, not expanding. It is the space between points in the universe that is expanding. The distances between these points become greater over time.
The space between clusters of galaxies can expand as well. However, the gravity within clusters of galaxies prevents the galaxies from expanding.
In 1931, six years after Friedmann's death from typhoid fever, Belgian physicist Georges Lemaitre proposed the idea known as the big bang theory with the aid of Friedmann's works. This theory suggests that the universe began out of singularity and has been expanding since day one.
Initially, Einstein rejected Friedmann's idea of an expanding universe, writing that Friedmann's answer had "no physical meaning." However, Einstein eventually accepted Friedmann's theory and rejected the cosmological constant as part of his general relativity work. Einstein was quoted as telling Friedmann that the cosmological constant was the "biggest blunder" of his career.