Type Ia supernovae, which are bright beacons used as distance markers both for studying the expansion rate of the Universe, and for measuring cosmological distances from about 3.26 million to 3 billion light-years away, actually fall into two distinct groups of supernovae, says a team of astronomers led by Dr Peter Milne of the University of Arizona’s Steward Observatory.
This image shows the remnant of Tycho’s supernova, or SN 1572, a Type Ia supernova in the constellation Cassiopeia. Image credit: X-ray – NASA / CXC / SAO; infrared – NASA / JPL-Caltech; optical – MPIA / Calar Alto / O.Krause et al.
“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,” said Dr Milne, the first author of the study published in the Astrophysical Journal (arXiv.org preprint).
This discovery sheds new light on the currently accepted view of the Universe expanding at a faster and faster rate, pulled apart by dark energy.
Dr Milne and his colleagues observed a sample of 23 Type Ia supernovae in ultraviolet and visible light.
They combined observations made by the NASA/ESA Hubble Space Telescope with those made by NASA’s Swift satellite.
The Swift data were crucial because the differences between the two groups of type Ia supernovae – slight shifts toward the red or the blue spectrum – are subtle in optical wavelengths, but became obvious only through Swift’s observations in the ultraviolet.
“Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at ten of them nearby, those ten are going to be redder on average than a sample of ten faraway supernovae,” Dr Milne said.
Dr Milne and co-authors said that there is the possibility that the acceleration of the expansion of the Universe might be lower than previously thought.
“Some of the reported acceleration of the Universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed,” they 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,” Dr Milne said.
“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.”
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Peter A. Milne et al. 2015. The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift. ApJ 803, 20; doi: 10.1088/0004-637X/803/1/20
