Chuck Bednar for redOrbit.com – @BednarChuck
Rumors of our universe’s rapid expansion may have been greatly exaggerated. According to new research led by University of Arizona astronomers, the type of supernova used to measure that growth are actually far more diverse than previously realized.
The findings, published last week in The Astrophysical Journal, reveal that these supernovae fall into distinct populations that had not been previously recognized. The authors believe that the revelation could have a significant impact on their understanding of just how rapidly the universe had been expanding since the Big Bang and other key cosmological issues.
Not all type Ia supernovae are created equal
The exploding stars at the heart of the matter, type Ia supernovae, have long been considered so uniform that researchers have used them as cosmic “beacons” through which they can measure and observe the depths of the universe. However, the UA team revealed that this is not necessarily the case, and that type Ia supernovae actually fall into different populations.
Lead author Peter A. Milne and his colleagues compare the discovery to purchasing different types of 100-watt light bulbs at the store, then taking them home and finding that, even though they are all supposed to be the same brightness, there are actually some variations.
“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, an associate astronomer with the UA’s Department of Astronomy and Steward Observatory.
“There are different populations out there, and they have not been recognized,” he added. “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,” and that could indicate that the acceleration of the expansion of the universe isn’t happening quite as quickly initially believed.
Subtle differences become more obvious in the UV spectrum
The currently accepted view is that the universe is expanding at an ever-increasing rate as it is being ripped apart by a mysterious force known as dark energy, the authors noted. The theory is based on the discovery that some supernovae appear fainter than predicted because they moved further away from Earth than expected based on a constant rate of universal expansion.
This seemed to indicate that the rate at which stars and galaxies move away from one another is increasing, and that some force has been tearing the universe apart faster and faster. The theory is based on the concept that type Ia supernovae are all the same brightness when they explode, and that the far-off supernovae are fainter than expected because they are farther away.
However, Milne and his colleagues observed a large sample of type Ia supernovae in ultraviolet and visible light using observations from both the Hubble telescope and the Swift satellite, and found that subtle differences between the populations in visible light were more obvious in their follow-up observations in the UV spectrum.
“The realization that there were two groups of type Ia supernovae started with Swift data. Then we went through other datasets to see if we see the same. And we found the trend to be present in all the other datasets,” Milne said. “As you’re going back in time, we see a change in the supernovae population. The explosion has something different about it, something that doesn’t jump out at you when you look at it in optical light, but we see it in the ultraviolet.”
“Since nobody realized that before, all these supernovae were thrown in the same barrel. But 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,” he added. “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.”
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