The universe

Alan Guth's 1980 fix for three specific puzzles about the early universe made a precise prediction that satellites later confirmed, and it also implies an eternally inflating multiverse that one of the theory's own inventors now argues against.

Hubble's 1929 pairing of galaxy distances with their redshifts became the observational basis for cosmic expansion, though his own constant was wrong by nearly a factor of seven and its modern successor is still disputed.

Vera Rubin and Kent Ford measured how fast stars orbit at the edges of spiral galaxies and found speeds that refused to fall off with distance, evidence that most of a galaxy's mass is invisible.

Mapping galaxy positions in three dimensions revealed a vast web of filaments and voids, a structure dark-matter simulations had already anticipated, though the largest structures found still test how uniform the universe is supposed to be.

In 1920 two astronomers each gave a forty-minute talk on whether the Milky Way was the whole universe or one galaxy among many, and neither had quite enough evidence to win outright, until a third astronomer settled it a few years later.