Accelerating universe? Not so fast

Certain types of supernovae or exploding stars are more diverse than previously thought, astronomers from the University of Arizona have discovered.

The results have implications for big cosmological questions, such as how fast the universe has been expanding since the Big Bang.

Most importantly, 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, led by astronomer 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.

The findings are analogous to sampling a selection of 100-watt light bulbs at the hardware store and discovering that they vary in brightness.

"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," explained 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 Brian P Schmidt from University of Arizona.

The Nobel laureates discovered independently that many supernovae appeared fainter than predicted because they had moved farther away from the 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 authors conclude that 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.

This would, in turn, require less dark energy than currently assumed.

The findings were reported in two papers published in the Astrophysical Journal,