ALBERT EINSTEIN when: 1916 what he did: Introduced the "cosmological constant," a form of antigravity, to make general relativity agree with a cosmos that seemed not to be expanding Person of the Century: Albert Einstein

EDWIN HUBBLE when: 1927 what he did: Discovered that faraway galaxies all seem to be flying away from Earth, which suggested that the universe was in fact expanding. In doing so, Hubble gave Einstein scientific license to abandon the cosmological constant, whose creation the brilliant physicist dubbed the greatest blunder of his career. In retrospect, it was an inspired guess that could have won him another Nobel Prize TIME 100: Astronomer Edwin Hubble

FRITZ ZWICKY when: 1936 what he did: Noted that the orbits of distant galaxies were impossible without the gravitational tug of some sort of "dark matter." The idea was ridiculed; now it is mainstream Unofficial Zwicky Site: "Supernovae, an alpine climb and space travel."

ARNO PENZIAS AND ROBERT WILSON when: 1965 what they did: Accidentally detected the leftover glow from the Big Bang — which astronomers are now deciphering to learn the composition and ultimate fate of the cosmos Winning the Nobel Prize: 1978 Nobel Laureates in Physics

ADAM RIESS when: 1998-2001 what he did: Helped prove that Einstein was right in the first place: a mysterious antigravity force that acts like Einstein's cosmological constant is evidently quite real Innovators — Science on the Edge: Adventures in Antigravity

Photos: Einstein by AP; Hubble photo courtesy of Palomar Observatory; Zwicky by BETTMAN—CORBIS; Penzias and Wilson by TED THAI FOR TIME; Riess by JONATHAN SAUNDERS FOR TIME