Georges Lemaître first proposed what became the Big-Bang theory in 1927 after he realized how neatly this fitted the nonstatic models of general relativity and formulated the theory of t expanding universe.
Edwin Powell Hubble’s discovery of the galactic red shift in 1929 was the significant development. It indicated that all galaxies were receding from each other as part of an expanding universe. The color change happens when objects are moving away, making lightwaves stretch out and change color.
The more distant the galaxies are the faster they are rushing away.
Also in 1920s, George Gamow worked with a group of scientists and suggested that elements heavier than hydrogen, specifically helium and lithium, could be produced in thermonuclear reactions during the Big-Bang.
In 1948, Gamow and his student Ralph Alpher wrote a paper on ‘The Origin of Chemical Elements’ one of many contributions to the Big-Bang theory that Gamow made and a key step to the modern understanding of nucleosynthesis.
More evidence of the Big-Bang came in the 1960s, when astronomers detected faint microwave radiation coming from every point in the sky.
Theory of big-bang