A term in Einstein's field equations representing the energy density of empty space, originally introduced to allow for a static universe but now associated with dark energy driving cosmic acceleration. It represents a constant energy density that fills space homogeneously.
Introduced by Albert Einstein in 1917 as the Greek letter lambda (Λ) in his field equations to counteract gravitational collapse and maintain a static universe. After Hubble's discovery of expansion, Einstein called it his 'greatest mistake,' but it was later revived to explain accelerating expansion.
Einstein's 'greatest mistake' turned out to be his greatest triumph! The cosmological constant he reluctantly added to his equations is now our best explanation for why the universe's expansion is speeding up, making up about 68% of everything that exists.
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