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Albert Einstein completed his theory of General Relativity in 1915. When he applied his theory to the spacetime of the universe, he found that gravity would not permit the universe to be static. Over a decade before Hubble's discovery of an expanding universe, Einsten made the reasonable assumption that the universe is static and unchanging (the perfect cosmological principle). He introduced a term called the cosmological constant that would act as a repulsive form of gravity to balance the attractive nature of gravity. The cosmological constant is an exotic form of energy filling empty space, a vacuum energy. The vacuum energy creates a repulsive gravitational force that does not depend on position or time; it truly is a constant. When Einstein learned of Hubble's discovery, he realized that he should have had more faith in his original General Relativity. He discarded the cosmological constant as the ``biggest blunder of his life''.
Recent observations are indicating that the cosmological constant should be brought back. Astronomers are finding that even when they include the maximum amount of dark matter allowed by the observations, there is not enough matter (luminous or dark) to flatten the universe---the universe is open with negative curvature if the cosmological constant is zero. The inflation theory predicts that the universe should be flat to very high precision. The extra vacuum energy can bend space as matter does. Perhaps the combined efforts of matter and vacuum energy could flatten space as much as that predicted by inflation theory.
Another set of observations of very distant (``high-Z'') Type I supernovae show that the expansion rate is slower than expected from a flat universe. Type I supernovae are very luminous and can be used as standard candles to measure very large distances because they form from the collapse of a stellar core of a particular mass (1.4 solar masses). By measuring very large distances, astronomers can determine the geometry of the universe. The supernovae are fainter than expected. After exploring ordinary possibilities like intergalactic dust, gravitational lensing effects, and metallicity effects, astronomers are forced to conclude that either the universe has negative curvature (is open) or that the supernovae are farther away than the Hubble Law says they are—their redshifts are ``too small'' because the universe expanded more slowly in the past than expected. What is surprising about the supernova observations is that they may indicate that the expansion is accelerating! Accelerating expansion is impossible without a repulsive cosmological constant to overcome the slowing down effect of gravity. Higher resolution observations of the microwave background by the MAP mission and further observations of supernovae with better detectors and new larger space telescopes in the future should tell us if Einstein's greatest blunder was saying that he made a blunder!