Dark energy is a little-understood form of energy that apparently makes the universe expand more and more rapidly. In 1917, the German-born physicist Albert Einstein discovered that such energy might exist. He made his discovery when he applied general relativity—his theory of space, time, and gravity—to the universe. The theory originally indicated that the universe could not remain at a constant size due to gravitational attraction between the objects in it. However, astronomers had found no evidence that the universe had ever been a different size. So Einstein added a cosmological constant to the theory. That term represented a repulsion (pushing away) of every point in space by the surrounding points, acting against gravitational attraction.
But in 1929, the American astronomer Edwin P. Hubble found that the universe is expanding. As a result, Einstein and the Dutch astronomer Willem de Sitter rejected the cosmological constant and theorized that the universe has critical density. Density is mass (the amount of matter) divided by volume. The universe has critical density if there is just enough matter in it to decrease its rate of expansion. According to this idea, the universe has been expanding since an explosive beginning called the big bang—but more and more slowly.
By the late 1990’s, astronomers had confirmed that the universe has critical density, with matter accounting for about 30 percent of it. However, they had also found that, several billion years ago, the rate of expansion began to speed up. According to relativity theory, energy also contributes to density. Dark energy—perhaps simply the cosmological constant—can account for the increase in the expansion rate if there is enough of it to account for the remaining 70 percent of critical density.
See also Big bang ; Cosmology ; Dark matter ; De Sitter, Willem ; Einstein, Albert ; Gravitation ; Hubble, Edwin Powell ; Relativity ; Universe .
