Astronomers have confirmed one of the weirdest properties of the universe: Some mysterious force is pushing galaxies apart at a faster and faster rate.
Researchers base their new evidence on the intensity of light from the most distant exploding star ever observed. The intrinsic brightness of this supernova, as well as those of a slew of others closer to Earth, suggests that gravity on the cosmic scale has reversed its familiar role. Instead of pulling objects together, it now pushes them apart. Cosmologists attribute this flip side of gravity to a substance that would pervade the universe and that they call dark energy.
Dark energy would complete a picture of the cosmos in which more than 98 percent of the matter and energy is of some exotic, unseen form. As bizarre as that may seem, it fits remarkably well with a panoply of other findings about the universe.
The new results challenge the standard theory of cosmology, which holds that ever since the Big Bang, gravity’s tug has slowed the expansion of the universe. The new observations suggest that such a scenario did hold true, but only for the first several billion years of cosmic history. A different type of gravity then came to the fore, causing the expansion of the universe to accelerate.
At press time, Adam Riess of the Space Telescope Science Institute in Baltimore and his colleagues were scheduled to present their findings April 2 at a briefing in Washington, D.C.
Three years ago, Riess and his collaborators, as well as another team, presented their first evidence that the universe had revved up its expansion. As with the new findings, the researchers based that claim on a type of exploding star known as a type 1A supernova. Researchers refer to these supernovas as standard candles because they all have about the same intrinsic brightness, like light bulbs of similar wattage. That allows astronomers to use supernovas to measure several properties of the cosmos, including its age.
Astronomers observe these exploded stars as they appeared when the universe was several billion years younger than it is now. If gravity were continuously slowing cosmic expansion, the distance between Earth and those supernovas would be less than if the expansion had been constant. Likewise, because the supernovas would not lie as far away, they would appear brighter.
In 1998, the two teams startled astronomers by finding just the opposite. A group of moderately distant supernovas appeared about 20 percent dimmer than expected. This indicated that over the past several billion years, cosmic expansion had sped up and that the space between Earth and those supernovas had stretched out much more than anticipated (SN: 12/19 & 26/98, p. 392).
Amid the hoopla of the 1998 discoveries, astronomers worried that they might have been fooled. Maybe the supernovas looked dimmer because dust had absorbed some of their light. Or perhaps the stars that exploded as type 1A supernovas several billion years ago had a different composition that yielded explosions less luminous than relatively recent supernovas did.
Although researchers couldn’t build a convincing case that these confounding effects were actually present, neither could they rule them out. But one test beckoned. It relies on the relative strengths of dark energy and the density of matter in the distant past.
Dark energy, which would oppose ordinary gravity, is believed to have been nearly constant for most of the history of the cosmos. In contrast, the density of matter, which gives rise to the normal tug between objects, was much larger in the past because the universe was much smaller. This density would have overwhelmed dark energy’s ability to push objects apart. Some 5 billion to 10 billion years ago, ordinary gravity ruled.
During that early era, gravity would have acted as a brake on cosmic expansion. Supernovas that exploded at that time would appear brighter from Earth than ones in a universe expanding at a constant or accelerated rate.
Evidence that ancient supernovas were brighter than expected and that more recent ones are dimmer would be a clear sign that the universe had revved up its rate of expansion. Dust or compositional differences could only make the older supernovas appear dimmer and dimmer.
The characteristic brightening of very distant supernovas is just what Riess and his collaborators have now found.
The team bases its analysis on a serendipitous find by Ronald L. Gilliland of the Space Telescope Science Institute and his colleagues, who discovered the most distant supernova ever detected. Observations with the Hubble Space Telescope reveal that the supernova, dubbed 1997ff, lies 10 billion light-years from Earth and hails from a time when the universe was only about 4 billion years old.
Calculations show that the supernova is about twice as bright as would be expected if cosmic expansion hadn’t been slower in the past, Riess says. Preliminary analysis of four other supernovas that aren’t nearly as distant but also hail from the era when ordinary gravity reigned reveals that they are also brighter, says John L. Tonry of the University of Hawaii in Honolulu, one of Riess’ collaborators. The five supernovas “paint a consistent picture,” notes Tonry. “These supernovas are brighter, the way a [dark energy] explanation would imply.”
“This is really spectacular,” says cosmologist Michael S. Turner of the University of Chicago. “If there were any doubts about the universe accelerating, this should dispel them,” he asserts.
To firm up the findings, Tonry hopes to find distant supernovas with the Subaru Telescope on Hawaii’s Mauna Kea. “It’s a little premature to feel elated,” he says. “In 1998, I would have bet even money that we were right. Now I think I would bet odds at two to one.”
