Astronomers Find Direct Evidence of Dark Matter
A team of astronomers has seen direct evidence for the exisitence of
dark matter in the collision of two large clusters of galaxies. Using
NASA's Chandra X-ray Observatory, the team observed dark matter and
normal matter being wrenched apart by this
"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.
A composite image of the various observations of the galaxy cluster 1E
0657-66. The galaxies making up the cluster are from optical images
taken by the Hubble Space Telescope and Magellen Telescope. On either side of
the center of the cluster, the pink clumps show the hot gas detected
by Chandra. Just outside this gas are regions where
the bulk of the matter resides (shown in blue). This matter, detected via
gravitational lensing, shows that dark matter makes up most of the
mass of the cluster.
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.
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
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.
Gravitational lensing can be used to determine the location of mass in
a galaxy cluster. Gravity from mass in the galaxy cluster distorts
light from background galaxies. In the idealized case shown here, two
distorted images of one background galaxy are seen above and below the
real location of the galaxy. By looking at the shapes of many
different background galaxies, it is possible to make a map showing
where the gravity and therefore the mass in the cluster is
located. This technique can show where dark matter resides.
(Illustration Credit : NASA/CXC/M.Weiss)
"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.