Our universe is not accelerating as fast as thought

Providing clues to understanding cosmological questions like how fast the universe has been expanding since the Big Bang, a team of astronomers, including an Indian American, has identified two distinct types of supernovaes.

A University of Arizona-led team of astronomers has discovered that certain types of supernovae, or exploding stars, are more diverse than previously thought.

The findings hint at the possibility that the acceleration of the expansion of the universe might not be quite as fast as textbooks say.

The team 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.

"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 Peter A. Milne, astronomer at University of Arizona.

"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," pointed out 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 view is based on observations that resulted in the 2011 Nobel Prize for Physics awarded to three scientists, including University of Arizona alumnus Brian P. Schmidt.

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.

Milne and his co-authors, including Indian-American 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.

Some of the reported acceleration of the universe can be explained by colour differences between the two groups of supernovae, leaving less acceleration than initially reported, concluded the team.

The results appeared in the Astrophysical Journal.