Billions of years ago, the
universe was crowded with tight-knit clusters of galaxies. Then, a party
crasher got the upper hand. This mysterious force now called dark energy has
since been expanding the universe at an increasing pace.
New measurements of this accelerating
expansion, which drives galaxies away from one another on large scales but
so far shows no effects on small scales (such as within a galaxy), provide
details about the nature of the unseen and unknown dark energy that is at work.
The results, announced today
at a news conference organized by NASA, reveal a decrease in the mass of galaxy
clusters in more recent times, which would be a consequence of this hastening
and ripping force that some think could eventually
tear apart even star systems, planets and eventually the very molecules
we're made of.
"If there were any doubts
10 years after the initial discovery that the universe was speeding up, this
should really dispel them," said Michael Turner of the University of Chicago's Department of Astronomy and Astrophysics, who was not involved in the
current study.
In addition, the new results,
which relied on the Chandra X-ray Observatory, suggest dark
energy takes the form of what Einstein called the cosmological constant — a
term in Einstein's general relativity that represents the possibility of empty
space having a density and pressure associated with it.
If dark
energy is indeed some kind of repulsive force that is linked with
"nothing," and the density of dark energy stays the same over time,
astrophysicists say the expansion of the universe will continue to speed up. So
rather than galaxies mingling and merging, they will fly away from one another.
And
billions of years from now, the scientists say, local superclusters of galaxies
will also disintegrate and all other galaxies will ultimately disappear from
the Milky Way's view.
"We
don't really have a clue why the universe is speeding up. We have some ideas,
but we really don't understand it," said Turner, who is credited with
coming up with the term "dark energy." "And so having yet
another method to study how that speed-up happened can only help us, can only
make us more optimistic about eventually understanding what the dark energy
is."
Invisible
force
Dark energy was discovered in
1998 by two teams of astronomers, who measured light coming from exploding
stars called Type IA supernovae, known as "standard candles" for
their consistent brightness. The striking result was that distant supernovae
were dimmer (farther away) than they would be in a universe that was slowing
down. The result suggested the expansion of the universe was accelerating. And
the teams proposed something called dark energy could be driving this
acceleration. This acceleration, it is thought, began about 5 billion years
ago.
That was the first stand-alone
evidence to support the idea of dark
energy.
And whereas then the repulsive force could have been brushed off as the result of possible errors in the measurements, more and more independent detections have solidified dark energy's existence.
Astronomers estimate now that
out of the total mass-energy budget in the universe, about 74 to 76 percent is
dark energy, 20 to 22 percent is dark matter and 4 percent or so is normal
matter that makes stars, planets and everything we see. And they know that some
"force" is causing galaxies to fly away from one another, operating
like antigravity.
Rather than using Type IA supernovae, the new study is based on observations of clusters of galaxies at
different time points in the history of the universe. Scientists say the new
study marks the second stand-alone evidence for the existence of dark energy.
"This is surely the best
job that anyone has been able to do so far in using clusters to measure how the
universe has gotten clumpy over time," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Massachusetts. Kirshner, who was not involved in
the current study, was on one of the teams that first discovered dark energy.
Galactic growth
A team led by Alexey Vikhlinin
of the Smithsonian Astrophysical Observatory in Cambridge, Mass., used the
Chandra X-ray Observatory to measure the hot gas in dozens of galaxy clusters,
which are the largest collapsed objects in the universe.
Some of these clusters are
relatively nearby and others are more than halfway across the universe.
Basically, the team was looking at X-rays emitted from this hot gas as it fell
into areas chock-full
of dark matter, or the mysterious material thought to act as scaffolding
onto which galaxies mature. The X-rays can be converted into mass for a given
cluster at a given point in time (depending on the age of the cluster).
"It's like there's a
tug-of-war going on between the dark matter trying to slow things down and
clump things and the dark energy trying to speed things up and eventually
making it hard for the galaxies or the dark matter to cluster," Kirshner
told SPACE.com.
When astronomers look farther
across the cosmos, they are looking back in time. And in fact, the results show
an increase in the mass of the galaxy clusters further back in time,
which supports the idea that dark energy started to win out in the tug of war
at some point in the universe's history. Astronomers are not certain on the
timing of the change from an expanding universe to one whose expansion is
speeding up.
With dark energy taking over,
it would be more difficult for objects such as galaxies to get together and
form clusters as space is being stretched. So astronomers would expect to see a
slowdown of the growth of galaxy clusters in a dark-energy-dominated universe.
"This result could be
described as 'arrested development of the universe'," Vikhlinin said.
"Whatever is forcing the expansion of the universe to speed up is also
forcing its development to slow down."
Future of dark energy
While the new galaxy-cluster
results strengthen the case for an accelerating universe, scientists have a
long trek before cracking the case of what dark energy is.
"I don't see us solving
this in two or three years. We're going to have to bring to bear lots of
different techniques to figure out what dark energy is," Turner said
during a telephone interview. "This is a very big puzzle. This may be the
most profound problem in all of science."
Scientists can continue to
study the clustering of galaxies over time. Further study could show dark
energy doesn't take the form of the cosmological constant. For instance,
another idea is that general relativity falls apart on larger scales.
And perhaps dark energy is
stirring up trouble in another way, not just speeding up the expansion of the
universe. "One of the areas where we think we might get really lucky is
that maybe there will be some other manifestation [of dark energy], but we
don't have any yet," Turner said. "And there's so much of this stuff
in the universe it's hard to believe there isn't another manifestation. But
right now the only thing we know that dark energy is doing is causing the
universe to speed up."
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