PureInsight | October 1, 2006
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."
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This composite
image shows the galaxy cluster 1E 0657-56, also known as the "bullet
cluster." This cluster was formed after the collision of two large
clusters of galaxies, the most energetic event known in the universe
since the Big Bang.
Hot gas detected by Chandra in X-rays is seen as two pink clumps in the
image and contains most of the "normal," or baryonic, matter in the two
clusters. The bullet-shaped clump on the right is the hot gas from one
cluster, which passed through the hot gas from the other larger cluster
during the collision. An optical image from Magellan and the Hubble
Space Telescope shows the galaxies in orange and white. The blue areas
in this image show where astronomers find most of the mass in the
clusters. The concentration of mass is determined using the effect of
so-called gravitational lensing, where light from the distant objects
is distorted by intervening matter. Most of the matter in the clusters
(blue) is clearly separate from the normal matter (pink), giving direct
evidence that nearly all of the matter in the clusters is dark.
The hot gas in each cluster was slowed by a drag force, similar to air
resistance, during the collision. 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. Therefore, during the collision the dark matter
clumps from the two clusters moved ahead of the hot gas, producing the
separation of the dark and normal matter seen in the image. If hot gas
was the most massive component in the clusters, as proposed by
alternative theories of gravity, such an effect would not be seen.
Instead, this result shows that dark matter is required.
Credit: X-ray:
NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI;
Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI;
Magellan/U.Arizona/D.Clowe et al.
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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.
Sources:
http://www.nasa.gov/centers/marshall/news/news/releases/2006/06-096.html
http://chandra.harvard.edu/photo/2006/1e0657/index.html
