How fast the universe has been expanding since the Big Bang is something of a puzzling question. It wasn’t that long enough that we didn’t know it was accelerating at all, and a new study finds the acceleration of the expansion of the universe might not be as fast as thought.
On Apr. 10/2015 UA team of astronomers led by Peter A. Milne, discovered that type Ia supernovae, which have been considered so uniform that cosmologists have used them as cosmic “beacons” to plumb the depths of the universe, actually fall into different populations. This finding have implications for our understanding of how fast the universe has been expanding since the Big Bang.
“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. “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.”
The Nobel laureates discovered independently 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.”
The authors said that the acceleration of the universe can be explained by color difference between the two groups of supernova, leaving less acceleration than previously reported and hence this will require less dark energy than assumed.
Milne said “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.”
The authors pointed out that more information have to be collected prior to scientists can fully grasp the effect on existing measures of dark energy.