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The universe is expanding
Until 1924 it was an open dispute whether the Milky Way was the whole universe. Hubble settled it by finding Cepheid variables in Andromeda and applying Leavitt's relation to them. Five years later he showed that redshift increases with distance — what is now called Hubble's law. A universe that is expanding must have been smaller before, and that is the starting point of all Big Bang cosmology. Georges Lemaître had published the same relation two years earlier, in French, in a little-read journal.
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