Astronomers: 'Dark
matter' exists
Tuesday
22 August 2006, 5:39 Makka Time, 2:39 GMT
Astronomers
have said they have found the best evidence to date of "dark matter"
the mysterious invisible substance that is believed to account for the bulk of
the universe's mass.
Using
a host of telescopes, researchers focused on the collision between two galactic
clusters. They found that most of the gravitational pull from the aftermath of
the encounter comes from a relatively empty looking patch of sky, a strong
suggestion that there is something more there than meets the eye.
Doug Clowe, a research
astronomer at the University of Arizona, said: "This provides the first
direct proof that dark matter must exist."
His colleagues used
NASA's Chandra X-ray observatory, the Hubble Space Telescope and several
ground-based observatories to examine the "bullet cluster", a clump
of galaxies that formed over the last 100million years from the violent
collision of two smaller galactic clusters. The object gets its name from a
bullet-shaped cloud of super hot gas on one of its sides.
Bullet cluster
Most of the visible mass
in the bullet cluster is concentrated in that cloud and another near it. But
using a technique known as gravitational lensing, Clowe and his colleagues show
that the force of gravity is actually stronger in a part of the cluster that
appears to be emptier.
They will publish their
results in a future issue of Astrophysical Journal Letters.
Sean Carroll, a
university of Chicago physicist, said: "This is really exciting",
adding that the observations demonstrate the existence of dark matter
"beyond a reasonable doubt".
Astronomers have used
dark matter for 70 years to explain various observations about the universe's
behaviour. They have shown that rotating spiral galaxies would fly apart if it
were not for the gravitational pull of undetectable matter in addition to their
stars.
Other observations show
that the expansion of the universe is being held back by a force greater than
the gravitational pull of visible matter alone.
Alternative
theories
Though dark matter
clearly provides the best explanation for such observations, Clowe said,
"astronomers have long been in the slightly embarrassing position" of
having to appeal to some mysterious, unobservable material in order to make
things fit together.
Some physicists have even
proposed that it is not the amount and type of matter in the universe that
needs to be adjusted, it is the law of gravity itself. They have suggested
alternative theories that boost the strength of gravity on galactic and
intergalactic scales in order to do away with the need for dark matter.
Carroll said: "It's
always possible that there's some modification of gravity going on as well.
"No matter what you
do you're going to have dark matter."
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Updated 09/06