NASA Finds Direct Proof of Dark Matter
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Erica Hupp
Headquarters, Washington
202-358-1237
Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
256-544-6535
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
RELEASE: 06-297
NASA FINDS DIRECT PROOF OF DARK MATTER
Dark matter and normal matter have been wrenched apart by the
tremendous collision of two large clusters of galaxies. The
discovery, using NASA's Chandra X-ray Observatory and other
telescopes, gives direct evidence for the existence of dark matter.
"This is the most energetic cosmic event, besides the Big Bang, which
we know about," said team member Maxim Markevitch of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
These observations provide the strongest evidence yet that most of the
matter in the universe is dark. Despite considerable evidence for
dark matter, some scientists have proposed alternative theories for
gravity where it is stronger on intergalactic scales than predicted
by Newton and Einstein, removing the need for dark matter. However,
such theories cannot explain the observed effects of this collision.
"A universe that's dominated by dark stuff seems preposterous, so we
wanted to test whether there were any basic flaws in our thinking,"
said Doug Clowe of the University of Arizona at Tucson, and leader of
the study. "These results are direct proof that dark matter exists."
In galaxy clusters, the normal matter, like the atoms that make up the
stars, planets, and everything on Earth, is primarily in the form of
hot gas and stars. The mass of the hot gas between the galaxies is
far greater than the mass of the stars in all of the galaxies. This
normal matter is bound in the cluster by the gravity of an even
greater mass of dark matter. Without dark matter, which is invisible
and can only be detected through its gravity, the fast-moving
galaxies and the hot gas would quickly fly apart.
The team was granted more than 100 hours on the Chandra telescope to
observe the galaxy cluster 1E0657-56. The cluster is also known as
the bullet cluster, because it contains a spectacular bullet-shaped
cloud of hundred-million-degree gas. The X-ray image shows the bullet
shape is due to a wind produced by the high-speed collision of a
smaller cluster with a larger one.
In addition to the Chandra observation, the Hubble Space Telescope,
the European Southern Observatory's Very Large Telescope and the
Magellan optical telescopes were used to determine the location of
the mass in the clusters. This was done by measuring the effect of
gravitational lensing, where gravity from the clusters distorts light
from background galaxies as predicted by Einstein's theory of general
relativity.
The hot gas in this collision was slowed by a drag force, similar to
air resistance. In contrast, the dark matter was not slowed by the
impact, because it does not interact directly with itself or the gas
except through gravity. This produced the separation of the dark and
normal matter seen in the data. If hot gas was the most massive
component in the clusters, as proposed by alternative gravity
theories, such a separation would not have been seen. Instead, dark
matter is required.
"This is the type of result that future theories will have to take
into account," said Sean Carroll, a cosmologist at the University of
Chicago, who was not involved with the study. "As we move forward to
understand the true nature of dark matter, this new result will be
impossible to ignore."
This result also gives scientists more confidence that the Newtonian
gravity familiar on Earth and in the solar system also works on the
huge scales of galaxy clusters.
"We've closed this loophole about gravity, and we've come closer than
ever to seeing this invisible matter," Clowe said.
These results are being published in an upcoming issue of The
Astrophysical Journal Letters. NASA's Marshall Space Flight Center,
Huntsville, Ala., manages the Chandra program. The Smithsonian
Astrophysical Observatory controls science and flight operations from
the Chandra X-ray Center, Cambridge, Mass. For additional information
and images, visit:
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