From the time of Isaac Newton to the late 1990s, the defining feature of gravity was its attractive nature. Gravity keeps us grounded. It slows the ascent of baseballs and holds the moon in orbit around the earth. Gravity prevents our solar system from flying apart and binds together enormous clusters of galaxies. Although Einstein's general theory of relativity allows for gravity to push as well as pull, most physicists regarded this as a purely theoretical possibility, irrelevant to the universe today. Until recently, astronomers fully expected to see gravity slowing down the expansion of the cosmos.

In 1998, however, researchers discovered the repulsive side of gravity. By carefully observing distant supernovae--stellar explosions that for a brief time shine as brightly as 10 billion suns--astronomers found that they were fainter than expected. The most plausible explanation for the discrepancy is that the light from the supernovae, which exploded billions of years ago, traveled a greater distance than theorists had predicted. And this explanation, in turn, led to the conclusion that the expansion of the universe is actually speeding up, not slowing down. This was such a radical finding that some cosmologists suggested that the falloff in supernova brightness was the result of other effects, such as intergalactic dust dimming the light. In the past few years, though, astronomers have solidified the case for cosmic acceleration by studying ever more remote supernovae.

This article was originally published with the title From Slowdown to Speedup.

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