Let me make something very I am speaking of the astronomer Edwin Hubble and his revolutionary discovery of the expansion of the universe (Hubble's Law) in 1925. Prior to that time (look him or his law up) no one knew the universe was expanding (not changing or moving, but expaning like the surface of a balloon being inflated). PERIOD! Lemaître postulated the "Big Bang" in 1927 to explain this (again, the notion of the universe "coming into being" was ORIGINAL, at least in astonomy and physical cosmology). The "Big Bang", as a scientific notion (let alone a theory, let alone part of the Standard Model) did not exist. PERIOD!
Einstein rejected both of these notions and it was only the 1931 Friedmann-Lemaître model of general relativity and Hubble's very quick (within weeks) endorsement of it that it displaced the earlier Einsteinian relativity (linear Robertson-Walker metric). By the thime de Sitter and other cosmologists were done relativity theory (and Einstein's assumption of a spatially homogeneous, isotropic, and steady-state universe was relegated to the dustbin of history). Of course the great man knew it when he said "My greatest mistake was the Cosmological Constant" (the number he fudged to make the model spatially homogeneous, isotropic, and steady-state).
Just read the references for Einstein, Hubble, Lemaître, and physical cosmology at wiki (while not 100% accurate, well within 90%)>