Two papers recently published in the Astrophysical Journal suggest that the universe may not be expanding at the rate that textbooks claim that it is. That conclusion would also imply that the amount of dark energy in the universe is less than current estimates claim it is.
The team reached this conclusion by studying Ia supernovae, which are thought to be uniform enough to be used as beacons to measure distances in the cosmos.
The team found that the supernovae were not, in fact, uniform but fell into different populations. “ The findings are analogous to sampling a selection of 100-watt light bulbs at the hardware store and discovering that they vary in brightness,” according to a statement.
If their findings are correct, it means that a great deal of the math which astronomers use to measure the universe needs to be re-done. Among other things it would mean that many of the measured distances to objects, the rate at which the universe is expanding and the amount of dark energy involved are currently wrong.
“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. There are different populations out there, and they have not been recognized. The big assumption has been that as you go from near to far, type Ia supernovae are the same. That doesn’t appear to be the case,” said Milne, an associate astronomer with the UA’s Department of Astronomy and Steward Observatory.
The current view of the universe is that it is continuing to expand at an ever increasing rate, pulled apart by dark energy. This view resulted in the Nobel Prize for Physics for Brian Schmidt, Saul Perlmutter and Adam Riess in 2011.
The three researchers independently arrived at the conclusion that many supernovae appeared to be fainter than predicted because they had moved farther away than they should have given the accepted rate of universal expansion.
“The idea behind this reasoning. 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,” explained Milne.
Milne along with 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. Used date from NASA’s Swift satellite as well as the Hubble Space Telescope to study Ia supernovae in ultraviolet and visible light.
Using visible light, the differences between the type Ia supernovae were subtle but the differences, in the red and blue spectrum, became much more pronounced using the Swift satellite’s ultraviolet scans.
“These are great results. I am delighted that Swift has provided such important observations, which have been made toward a science goal that is completely independent of the primary mission. It demonstrates the flexibility of our satellite to respond to new phenomena swiftly,” said Neil Gehrels, principal investigator of the Swift satellite, who co-authored the first paper.
“The realization that there were two groups of type Ia supernovae started with Swift data. 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. 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,” said Milne.
“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,” he added.
The researchers concluded that at least some of the reported acceleration of the universe can be explained by misconceptions about the differences between the two groups of supernovae. That means that the universe isn’t accelerating as rapidly as previously thought and that less dark energy is involved.
“We’re proposing that our data suggest there might be less dark energy than textbook knowledge, but we can’t put a number on it. Until our paper, the two populations of supernovae were treated as the same population. To get that final answer, you need to do all that work again, separately for the red and for the blue population,” Milne said.
The researchers concede that these two papers are just the starting point and that a great deal of additional data will have to be collected before any final determinations can be made.