“The biggest blunder of my life,” as Einstein later called it, the cosmological constant represents a hypothetical force of repulsion. When Einstein developed his general theory of relativity in 1915, he realized that the original equations required the universe to be in motion. He accepted the belief of his time, however, that the universe was static. So he invented the cosmological constant to balance the force of gravity, allowing galaxies to remain at fixed distances. In retrospect, it seems surprising that no one suspected the universe was expanding. But belief that the universe was static—neither expanding nor contracting—apparently had as powerful a grip on scientist’s minds at the time as did the idea among the Greeks, centuries earlier, that planets must move in circular paths.
Einstein didn’t perceive the cosmological constant as a “blunder” until Edwin Hubble showed that all the galaxies are receding from one another, a result Einstein might have predicted had he believed his original equations. Once the genie was out of the bottle, however, it’s been difficult to put back in. Many physicists continue to use the constant because it allows more freedom in building models of the universe.