A Type Ia supernova, SN1994D, is shown exploding in lower left corner
of the image at the top of the page of the galaxy NGC 4526 taken by the
Hubble Space Telescope. (High-Z Supernova Search Team, HST, NASA)
Excerpt from dailygalaxy.com
Certain types of supernovae, or exploding
stars, are more diverse than previously thought, a University of
Arizona-led team of astronomers has discovered. The results, reported in
two papers published in the Astrophysical Journal, 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 UA 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,” said 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. 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 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 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 faster and faster.
“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. Once people knew why, they started using them
as mileposts for the far side of the universe.
“The faraway supernovae should be like the ones nearby because they
look like them, but because they’re fainter than expected, it led people
to conclude they’re farther away than expected, and this in turn has
led to the conclusion that the universe is expanding faster than it did
in the past.”
Milne and his co-authors — Ryan J. Foley of the University of Illinois at Urbana-Champaign, Peter J. Brown at Texas A&M University and Gautham Narayan of the National Optical Astronomy Observatory,
or NOAO, in Tucson — observed a large sample of type Ia supernovae in
ultraviolet and visible light. For their study, they 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, which had been used to detect
type Ia supernovae previously, but became obvious only through Swift’s
dedicated follow-up observations in the ultraviolet.
“These are great results,” said Neil Gehrels, principal investigator
of the Swift satellite, who co-authored the first paper. “I am delighted
that Swift has provided such important observations, which have been
made toward a science goal that is completely independent of the primary
mission. It demonstrates the flexibility of our satellite to respond to
new phenomena swiftly.”
“The realization that there were two groups of type Ia supernovae
started with Swift data,” Milne said. “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.
“As you’re going back in time, we see a change in the supernovae
population,” he added. “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.”
The authors conclude that 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.
That same galaxy in a NASA Swift image below is shown, with bars
indicating the location of supernova SN 2011fe. The Swift image is a
false-color image with UV emission blue and optical emission red.
“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,” 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.”
The authors pointed out that more data have to be collected before
scientists can understand the impact on current measures of dark energy.
Scientists and instruments in Arizona will play important roles in
these studies, according to Milne. These include projects led by NOAO;
the Large Synoptic Survey Telescope, or LSST, whose primary mirror was
produced at the UA; and a camera built by the UA’s Imaging Technology
Lab for the Super-LOTIS telescope on Kitt Peak southwest of Tucson.
Super-LOTIS is a robotic telescope that will use the new camera to
follow up on gamma-ray bursts — the “muzzle flash” of a supernova —
detected by Swift.
Source Article from http://feedproxy.google.com/~r/AscensionEarth2012/~3/k8PRxA7KQWQ/new-light-on-our-accelerating-universe.html
New Light on Our Accelerating Universe --"Not as Fast as We Thought"
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