[sac-forum] Case for Dark Matter
- From: Stan Gorodenski <stan_gorodenski@xxxxxxxxxxxxx>
- To: Sac Forum <sac-forum@xxxxxxxxxxxxx>
- Date: Sun, 08 Oct 2006 19:15:06 -0700
I just read this in an August issue of Science. I think it is really
interesting and so I wanted to pass it on.
Stan
Science 25 August 2006:
Vol. 313. no. 5790, p. 1033
DOI: 10.1126/science.313.5790.1033
Prev | Table of Contents | Next
News of the Week
ASTRONOMY:
Satellite's X-ray Vision Clinches the Case for Dark Matter
Tom Siegfried*
A fantastically energetic collision between clusters of galaxies has
demolished a challenge to the law of gravity, providing the clearest
evidence yet for the existence of intergalactic dark matter.
For decades, astronomers have inferred that unseen matter lurks within
and between galaxies. Luminous stars alone, they realized, don't exert
enough gravitational force to explain how individual galaxies spin and
clusters of galaxies clump together. Something invisible must be
pulling, too.
Some of the extra matter in galactic clusters is just hot gas. But even
more mass seems to exist in the form of "nonbaryonic" dark matter, made
of something other than ordinary atoms.
A few holdouts have insisted that the observations could be explained by
modifying the law of gravity at great distances. But a new result from
the Chandra X-ray Observatory satellite offers clear-cut evidence that
dark matter really does infuse galactic clusters. "It demonstrates
beyond a reasonable doubt that dark matter exists," says Sean Carroll, a
cosmologist at the University of Chicago, Illinois, not involved in the
study.
Speaking this week at a NASA briefing, astronomers reported on new
Chandra images of the "bullet cluster" of galaxies, 1E0657-56, created
by an energetic collision of smaller clusters. It is the most
explosively violent such merger ever observed, said astrophysicist Maxim
Markevitch of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts.
Figure 1 Dark evidence. A composite image depicting normal matter
(pink) and gravity (blue) shows dark matter's presence in the "bullet
cluster" of galaxies.
CREDITS: NASA/CXC/CFA/M. MARKEVITCH ET AL.
The shock wave from the cluster collision dragged the hot gas between
galaxies into its unusual shape but would not have affected dark matter,
which interacts only via gravity. Consequently, the explosive collision
stripped the ordinary gaseous matter away from the nonbaryonic dark matter.
"Because of this collision, for the first time, we're actually able to
see dark and ordinary matter separated in space. And this proves in a
simple and direct way that dark matter exists," Markevitch said at the
briefing.
With no dark matter, the gravity of the cluster would remain
concentrated on the gas, which vastly outweighs the galaxies it
surrounds. But in fact, the gravitational field of the cluster no longer
matches the location of the gas. Astronomers measured the cluster's
gravitational influence by tracking its effect on the light from more
distant "background" galaxies, a phenomenon known as gravitational
lensing. The results show a clear separation between the gas and the
gravity.
"In the bullet cluster, we've seen for the first time a large spatial
separation in the sky between where the majority of the normal matter is
found and where most of the gravity is found," said team leader Douglas
Clowe of the University of Arizona, Tucson. "This cannot be explained by
altered gravity for normal matter." A paper describing the results will
be published in Astrophysical Journal Letters.
While the new result specifically demonstrates the existence only of
intergalactic dark matter, it strengthens the case for dark matter
within galaxies as well. The same dark matter could explain both why
clusters of galaxies do not fly apart and why galaxies themselves rotate
as rapidly as they do, Carroll says. There is no need to invoke
modifications to Newtonian gravity.
Carroll pointed out that it remains possible that the laws of gravity
may need to be modified. But those modifications can no longer do away
with dark matter. "No matter what you do, you're going to have to
believe in dark matter," he said at the briefing. "It is once and for
all the case that dark matter does exist."
Tom Siegfried is a writer in Los Angeles, California.
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