When was the big bang theory discovered and by whom?

Answer 1

In 1924, Alexander Friedman was the first to note that the equations of General Relativity would allow a universe with space that was either shrinking or expanding, but not a static universe. He made no effort to say what our Universe was doing.

In 1927, Jesuit priest Georges Lemaitre was the first to explicitly state that our Universe was expanding and that there was, in his words, "a day with no yesterday." He also suggested that present astronomical observations supported his idea. Einstein initially ridiculed Lemaitre's ideas, but Einstein later admitted he was wrong.

In 1929, Edwin Hubbell showed that the space was, indeed, expanding; but this fact did not also prove that matter was once far more dense than it is today. Our Universe could simply have been creating matter for all eternity, keeping mass density about the same as space expands. The latter idea was championed by Fred Hoyle, who referred to Lemaitre's view as the "Big Bang" (in 1949) and his own as "Steady State."

Big Bang Cosmology predicted an isotropic microwave radiation resembling a black body of temperature 3K; Steady State Cosmology didn't. When this microwave radiation was found in 1964 by Robert Wilson and Arno Penzias, BBC achieved near universal acceptance. Today, Steady State Cosmology borders on pseudo-science.

So take your pick: Friedmann, Lemaitre, Hubbell, Hoyle, or Wilson & Penzias.