How was the Big Bang theory conceived?

Answer 1

When Einstein applied his General Theory of Relativity to the entire Universe, he noted that his own equations made a Universe of infinite age impossible -- a Universe obeying his laws would either be contracting or expanding, but it could never be infinitely stable. Because he could not accept any Universe with a beginning, Einstein added, out of thin air, a cosmological constant that made his equations "work," in the sense that the Universe could thus be stable. When others noted this was an ad hoc "solution," Einstein replied with near-ridicule.

In 1927 Jesuit priest Georges LeMaitre worked out the mathematics of an expanding Universe, and suggested that this would explain an apparent red-shift of distant nebulae (which turned out to be other galaxies, but LeMaitre did not know this at the time). LeMaitre later added to his work by noting that, if his description of our Universe was correct, then there had to be (in his words) "a day with no yesterday."

When Edwin Hubble showed that all distant galaxies had a red-shift perfectly in keeping with LeMaitre's predictions, Einstein admitted he had made (again, in his words), the "greatest blunder" of his career.