Einstein's Cosmological Constant
Albert Einstein, unquestionably one of the greatest minds in history, wasn't immune to error either. His equations describing how gravity works in his general theory of relativity, published in 1916, were a tour de force, though he did make a significant mistake.
Among the terms in the equations was one Einstein called the cosmological constant, which he introduced because he thought the universe was static. The cosmological constant achieved a static universe by counteracting the inward pull of gravity. Later, when astronomers discovered the universe is actually expanding, Einstein regretted including the constant and removed it from his equations.
Legend has it Einstein called the creation of the cosmological constant his "greatest blunder" (though Livio thinks he never actually used the term). But in fact, Einstein's real mistake was taking the constant out, Livio said.
In 1998, after Einstein's death, it was discovered that not only is the universe expanding, but this expansion is accelerating over time. To explain why that's happening, scientists have reintroduced the cosmological constant to the general relativity equations.
"His real blunder was to take it out, not to keep it in," Livio said. "The theory allowed him to put it in. We've since learned that everything the theory allows appears to be compulsory."
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