How was the big bang discovered?

Answer 1

In 1927, Georges LeMaitre was the first to propose, based on Edwin Hubble's findings, that the universe was expanding from an extraordinarily hot and dense initial state. Fred Hoyle coined, much later, the term Big Bang to describe this expansion. Later observations, including the key observation of a Cosmic Background Radiation, confirmed the basic model proposed by LeMaitre.

Basically, Edwin Hubble discovered that distant galaxies were red-shifted, and that their red-shift increased proportionally with their apparent distance from us. In other words, it looks as though the further something is away from us, the faster it is receding from us. Initially, some people thought that this might be the result of these objects moving away from us through space. But this would have meant that the Earth had to be at the center of the expansion, a notion so unlikely that it was immediately discarded. The only logical alternative then would be that space itself was expanding, and that we'd see the same thing (everything moving away from us with a speed relative to distance) wherever we were in the universe.