The Cosmological ConstantAlbert 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 constantthat 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, the vacuum energy discussed above. 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".
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