Einstein regarded this as the greatest mistake of his life. When Albert Einstein developed his equation of general relativity, he found that the equations predicted an expanding universe, something that was not known to astronomers back then. Einstein also believed that the universe was static, and he didn't want his equations to predict an expanding universe. So, he added a small number, the cosmological constant, which acted like a repulsive force that balanced out the attractive force of gravity. However, in 1929, Edwin Hubble discovered that the universe is actually expanding. This meant that Einstein's cosmological constant was unnecessary, and he later called it his "biggest blunder.” But in recent years, scientists have revived the cosmological constant to explain a mysterious force called dark energy, that astronomers think is responsible for the accelerated expansion of the universe.
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