Recent research by a team of astronomers led by the University of Arizona’s Peter A. Milne won’t overturn the Nobel-Prize-winning discovery that the expansion of the universe is accelerating.
It does suggest, however, that acceleration and the “dark energy” that fuels it — a discovery that totally changed our view of the cosmos — needs a mathematical adjustment.
Milne and colleagues used data from a space telescope observing in the ultraviolet spectrum of light to identify two different varieties of Type 1a supernovae — the “standard candles” used to map that acceleration.
The team, which published its results in Astrophysical Journal, includes Gautham Narayan of the National Optical Astronomy Observatory in Tucson, Peter J. Brown of Texas A&M University and Ryan J. Foley of the University of Illinois at Urbana-Champaign.
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“This group has shown that we have been making a fundamental mistake in our use of supernovae,” said Nicholas Suntzeff, co-founder of the High-z Supernova team that announced its discovery of an accelerating universe in 1998.
Supernovae — dying stars that explode with the light of an entire galaxy — are cosmic mileposts for astronomers, in particular a class of such phenomena known as Type 1a Supernovae. Their consistent brightness makes it possible to gauge distance in the cosmos.
Two members of Suntzeff’s High-z team — Brian Schmidt and Adam Riess — shared the 2011 Nobel Prize in Physics with Saul Perlmutter, who led a team at Lawrence Berkeley National Laboratory that made a simultaneous discovery of acceleration, also using Type 1a supernovae as guideposts.
Suntzeff, who heads the astronomy group at Texas A&M, said he had been waiting years for some correction of the team’s original findings.
“I love it. It’s really boring in science if everybody ends up getting the same answer. It’s much more interesting when people poke little holes in your argument.”
He said he was especially pleased that Brown, who works in his department, was a member of the team.
Brown works with data from NASA’s Swift space telescope, which took multiple measurements of about 30 nearby supernovae in the ultraviolet and in visible wavelengths. It compared them with similar archived data for more distant supernovae gathered by ground-based and space telescopes.
In optical, the differences in brightness were subtle, but they corresponded with more pronounced differences in ultraviolet. There were two distinct types of Type 1a supernovae. Nearby, the majority were “redder” and far away the majority were “bluer” or brighter.
Milne said earlier researchers had found it impossible to find “order” in observations of supernovae done in ultraviolet wavelengths. In an email, the UA professor clarified how this study is different:
“Swift has observed so many SNe (supernovae) with so many observations per SNe that we think that we see order, in that we can separate the normal SNe into two groups. Once separated, the disorder is much less, making the UV more useful than it was thought to be.”
Milne said the information gathered in UV wavelengths was then applied to the more subtle variations in the optical/visible colors. “Is there anything else in optical light that 100-percent matches up to what we’re seeing in the ultraviolet?”
The study concluded that Type 1a supernovae can be divided into two distinct categories, with one brighter than the other.
Milne said he was fully aware that his results would meet with skepticism. “It made me very nervous,” he said.
He said he and his fellow researchers contacted members of the Nobel-Prize-winning teams. “We sent the draft out to those we actually knew and worked with, and they had comments. Bob Kirshner and Adam Riess were very skeptical. Bob thinks there could be alternate explanations.”
Kirshner, who was also on the High-z team, is an authority on supernovae and the Clowes Professor of Science at the Harvard-Smithsonian Center for Astrophysics.
“The UV is tricky,” said Kirshner. “It’s interesting, but it’s mostly telling you how these stars explode. The study shows the UV light from supernovae varies quite a bit in ways we need to understand. My approach is not to observe in the UV.”
“We’ll have to look at it carefully but I’m certain those (original) measurements are quite good.”
Suntzeff said the recent study doesn’t challenge the existence of acceleration or the dark energy driving it, but it could lead to more precise measurement of it.
“Their paper is really solid science,” he said. This is the way science progresses. No result is absolutely right.”
