Instead, this time he made what he called a "slight modification" to his theory....

Instead, this time he made what he called a "slight modification" to his theory. To keep the matter in the universe from imploding, Einstein added a "repulsive" force: a little addition to his general relativity equations to counterbalance gravity in the overall scheme. In his revised equations, this modification was signified by the Greek letter lambda, , which he used to multiply his metric tensor gμv in a way that produced a stable, static universe. In his 1917 paper, he was almost apologetic: "We admittedly had to introduce an extension of the field equations that is not justified by our actual knowledge of gravitation." He dubbed the new element the "cosmological term" or the "cosmological constant" {kosmologische Glied was the phrase he used}. Later,* when it was discovered that the universe was in fact expanding, Einstein would call it his "biggest blunder." But even today, in light of evidence that the expansion of the universe is accelerating, it is considered a useful concept, indeed a necessary one after all.14 During

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