The Big Bang theory is the prevailing cosmological model for the early development of the universe. According to the theory, the Big Bang occurred approximately 13.798 ± 0.037 billion years ago, which is thus considered the age of the universe. At this time, the universe was in an extremely hot and dense state and began expanding rapidly. After the initial expansion, the universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
Monseigneur Georges Henri Joseph Édouard Lemaître, ( 17 July 1894 – 20 June 1966) was a Belgian priest, he was astronomer and professor of physics at the Université catholique de Louvain. He was the first person to propose the theory of the expansion of the Universe, widely misattributed to Edwin Hubble. He was also the first to derive what is now known as Hubble’s law and made the first estimation of what is now called the Hubble constant, which he published in 1927, two years before Hubble’s article. Lemaître also proposed what became known as the Big Bang theory of the origin of the Universe, which he called his ‘hypothesis of the primeval atom.