Einstein originally added a cosmological constant to his equations to prevent a presumably static universe from either contracting or expanding. Modern cosmologists have reintroduced it to their equations, where as the source of dark energy it causes the universe to expand ever faster.

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Einstein added the cosmological constant to his equations to achieve a static universe model, but it was later found that the universe is expanding. Modern cosmologists now associate the cosmological constant with dark energy, which is responsible for the accelerated expansion of the universe.

Einstein Originally Added a Cosmological Constant to His Equations to Prevent