- A dense cluster of galaxies some 8 billion
light-years away from us is adding to the growing body of evidence that
we live in a lightweight universe that will never collapse on itself, but
instead will expand forever.
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- The cluster, known as MS1054-0321, is
surprisingly dense, which paradoxically means the universe isn't, say astronomers.
It's so far away that the image we see today is what it looked like eons
ago.
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- "It's what I like to call cosmic
archaeology," says astronomer John Stocke at the University of Colorado
in Boulder, who was an author on the paper that appeared in the Astrophysical
Journal. "Because of the amount of time that it takes light to get
from there to here, we're seeing this object at something like half the
age of the universe."
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- At that point in time, according to theories
that assume our universe is relatively dense, there shouldn't be big clusters.
A dense universe would have enough gravity to form clusters slowly and
continuously over time.
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- "If clusters have indeed been growing
all along, we're looking back to a time where there shouldn't have been
any massive clusters at all, and we're finding some," says Megan Donahue,
an astronomer with the Space Science Telescope Institute and lead author
of the paper.
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- "Our conclusion," says Donohue,
"is that clusters of galaxies have slowed their growth rate significantly
over the last 5 billion years or so, and that slower growth rate implies
that the universe isn't dense enough to stop expanding." As the universe
gets less dense, the growth rate of clusters slows down.
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- So given the number of clusters you see
now," she says, "you should see far fewer of them in the past.
Since this is way off in the past, we'd expect it not to be very massive."
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- But the team found that the cluster weighs
as much as one quadrillion suns -- quite massive indeed. It is also one
of the hottest clusters ever discovered at 300 million degrees Fahrenheit.
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- The density of the universe is a key
component for a variety of astrophysics theories, not the least of which
is whether or not the universe will expand -- as it is doing now -- eternally,
or eventually come to a halt, reverse direction, and fall in on itself
in a reverse of the Big Bang.
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- Such hot, huge clusters are typical in
our own galactic neighborhood. "What a cluster looked like half a
billion years ago is surprisingly like what it looks like today,"
says Stocke. "That means that the evolution of the universe must have
begun a long time prior to when we are viewing this cluster, and that can
occur only in a universe with a very low density."
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