sciencetech-blog

comments

Most physicists believe the universe is being pulled apart at an increasingly rapid rate by a mysterious force called dark energy.

But this growth may not be as fast as thought, after astronomers found certain types of supernovae are remarkably diverse.

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

The growth of the universe may not be as fast as expected, after astronomers found types of supernovae are more diverse than previously thought. This Swift image on the left shows galaxy M82 before a supernova appeared.  Swift's UVOT captured the new supernova (circled right) in three exposures taken on Jan 22, 2014

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.

By comparing optical and ultraviolet images, a team at the University of Arizona found that older exploding stars aren't behaving in the same way as newer ones closer to our own star.

This means ancient supernovae aren't necessarily as distant as believed - and if they aren't, the universe isn't growing as quickly.

These exploding stars have previously been considered so uniform that cosmologists have used them as cosmic 'beacons' to plumb the depths of the universe.

Scientists say this means there is less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed. Pictured on the right is an optical image of galaxy M101. On the right is that same galaxy in a Nasa Swift image, with bars indicating the location of supernova SN 2011fe

The scientists say the findings are analogous to sampling a selection of 100-watt light bulbs at the DIY shop 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,' said astronomer Peter Milne.

'There are different populations out there, and they have not been recognised. 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 existing view of the universe expanding at a faster and faster rate is based on observations that resulted in the 2011 Nobel Prize for Physics awarded to three scientists, including UA 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.

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.

'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.

For their study, the team combined observations made by the Hubble Space Telescope with those made by Nasa's Swift satellite.

The data collected with Swift were crucial because the differences between the populations — slight shifts toward the red or the blue spectrum - are subtle in visible light.

'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,' said Milne.

The scientists say this means there is less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed.

'To be clear, this research does not suggest that there is no acceleration,' Milne said, 'just that there might be less of it.'

'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,' he said.