|NASA Swift image of galaxy M101, with bars indicating the location of supernova SN 2011fe. The Swift image is a false-color image with UV emission blue and optical emission red. (Image: NASA/Swift)|
The universe may be expanding at a slower rate since the Big Bang than previously thought. That's because a team of astronomers, led by University of Arizona astronomer Peter A. Milne, discovered that type Ia supernovae, which have been considered so uniform that cosmologists have used them as cosmic "beacons" to measure the depths of the universe, actually fall into different populations.
"We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances — and thus when the universe was younger," said Milne.
The discovery casts new light on the currently accepted view of the universe expanding at a faster and faster rate, pulled apart by a poorly understood force called dark energy. This current view is based on observations that many supernovae appeared fainter than predicted because they had moved farther away from Earth than they should have done if the universe expanded at the same rate. This indicated that the rate at which stars and galaxies move away from each other is increasing; in other words, something has been pushing the universe apart faster and faster.
"The idea behind this reasoning," Milne explained, "is that type Ia supernovae happen to be the same brightness — they all end up pretty similar when they explode. Once people knew why, they started using them as mileposts for the far side of the universe."
"The faraway supernovae should be like the ones nearby because they look like them, but because they're fainter than expected, it led people to conclude they're farther away than expected, and this in turn has led to the conclusion that the universe is expanding faster than it did in the past."
Milne and his co-authors — Ryan J. Foley of the University of Illinois at Urbana-Champaign, Peter J. Brown at Texas A&M University and Gautham Narayan of the National Optical Astronomy Observatory, or NOAO, in Tucson — observed a large sample of type Ia supernovae in ultraviolet and visible light. For their study, they combined observations made by the Hubble Space Telescope with those made by NASA's Swift satellite.
Researchers say that the data collected with Swift were crucial because the differences between the populations — slight shifts toward the red or the blue spectrum — are subtle in visible light, which had been used to detect type Ia supernovae previously, but became obvious only through Swift's dedicated follow-up observations in the ultraviolet.
"The realization that there were two groups of type Ia supernovae started with Swift data," Milne said. "Then we went through other datasets to see if we see the same. And we found the trend to be present in all the other datasets.
"As you're going back in time, we see a change in the supernovae population," he added. "The explosion has something different about it, something that doesn't jump out at you when you look at it in optical light, but we see it in the ultraviolet."
"Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at 10 of them nearby, those 10 are going to be redder on average than a sample of 10 faraway supernovae."
The authors conclude that some of the reported acceleration of the universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed.