According to a team of astronomers acceleration of the expansion of the universe is not as fast as they previously believed.

A team led by the University of Arizona found that certain types of the exploding stars or supernova are more diverse than they previously believed and this findings has led them to big cosmological questions, such as how fast is the universe expanding since the Big Bang.

Researchers found that type Ia supernova, 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.

The findings are similar to sampling 100-watt electric bulb and finding out that their brightness varies.

Peter A. Milne of the University of Arizona said, “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.”
The discovery throws new light on the currently accepted view of the universe expanding at a faster and faster rate pulled apart by an unknown force called dark energy this observation resulted in 2011 Nobel Prize for Physics.

Milne said, “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.”

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.