Hubble's Exciting Universe: Discovering Dark Energy
Hubble played a central role is making one of the biggest discoveries in modern astrophysics: the fact that we live in a runaway universe. Day by day, the universe is expanding at an ever-faster rate, like a runaway train. The interpretation is that there is a fundamental property to the universe that can best be described as an anti-gravity pushing objects apart rather than pulling them together. For want of better term, it is simply called “dark energy.”
A key project for Hubble astronomers was to measure the amount of deceleration the universe is undergoing. The conventional wisdom was that gravity must be exerting a pull on the expansion of space after the big bang, like a ball rolling up an incline and gently slowing down under gravity.
The question persisted for decades as to whether there was enough mass, hence gravity, in the universe to halt the expansion of space. Astronomers, bolstered by the Hubble’s telescope’s ability to see faraway supernovas and precisely measure their distances, used the telescope to look ever deeper back in time to measure the expansion rate of the universe in its early years.
In 1998, two independent teams of astronomers presented evidence—to their surprise—that the universe's expansion rate is actually accelerating rather than slowing down. The evidence was based primarily on the faintness of distant supernovas compared to their expected brightness in a universe decelerating under gravity. The acceleration appears to be propelled by the repulsive property of space—sort of an "anti-gravity" that pushes rather than pulls—called "dark energy."
Previous Hubble observations of the most distant supernovas known revealed that the early universe was dominated by matter whose gravity was slowing down the universe's expansion rate. The newer observations showed that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy's repulsive force overtook gravity's attractive grip.
Making up about 68 percent of the universe’s contents, dark energy’s precise nature is the most profound puzzle facing physics today. The idea of a ghostly counterbalancing force to the universe goes back to Einstein's theories, where he invoked the idea of a cosmological constant that would keep the universe from imploding if gravity were the only force at work among the stars. It is not known if dark energy matches Einstein's prediction, or if it could be unstable over time, further altering the expansion rate of space. If dark energy is stable, space will just keep stretching and stretching forever. This means that distant galaxies will grow dimmer and dimmer as light it stretched to longer wavelengths. Eventually the universe will be expanding faster than the speed of light, and the light from billions of galaxies around us will have disappeared.
Astronomers will make additional observations with Hubble and future observatories, such as the Wide Field Infrared Survey Telescope, to attempt to characterize dark energy and explore possible explanations for its existence.