Discussion in 'Politics' started by aphexcoil, Jan 3, 2004.
Chandra X-ray Observatory Center, CFA, Cambridge, MA
David Buote, (949) 824-6280, firstname.lastname@example.org
Dark Matter Reality Check: Chandra Casts Cloud On Alternative
New evidence from NASA's Chandra X-ray Observatory challenges
an alternative theory of gravity that eliminates the need for
dark matter. The observation also narrows the field for
competing forms of dark matter, the elusive material thought
to be the dominant form of matter in the universe.
An observation of the galaxy NGC 720 shows it is enveloped
in a slightly flattened, or ellipsoidal cloud of hot gas
that has an orientation different from that of the optical
image of the galaxy. The flattening is too large to be
explained by theories in which stars and gas are assumed
to contain most of the mass in the galaxy.
"The shape and orientation of the hot gas cloud require it
to be confined by an egg-shaped dark matter halo," said
David Buote of the University of California, Irvine, and
lead author of a report on this research in the 2002
September 20 issue of The Astrophysical Journal. "This
means that dark matter is not just an illusion due to a
shortcoming of the standard theory of gravity -- it is
According to the generally accepted standard theory of
gravity, the hot X-ray cloud would need an additional
source of gravity -- a halo of dark matter -- to keep
the hot gas from expanding away. The mass of dark matter
required would be about five to ten times the mass of
the stars in the galaxy. If the dark matter tracked
the optical light from the stars in the galaxy, the hot
X-ray cloud would be more round than it is. The
flattened shape of the hot gas cloud requires a flattened
dark matter halo.
An alternative theory of gravity called MOND, for
Modified Newtonian Dynamics, was proposed in 1983 by
Mordecai Milgrom of the Weizmann Institute in Israel,
and has remained viable over the years. MOND does away
with the need for dark matter by modifying the theory
where the acceleration produced by gravity is very small,
such as the outskirts of galaxies. However, MOND cannot
explain the observations of NGC 720. This is apparently
the first dynamical evidence that has successfully
distinguished dark matter from MOND.
The researchers also found that the Chandra data fit
predictions of the cold dark matter theories, according
to which dark matter consists of slowly moving particles,
which interact with each other and "normal" matter only
through gravity. Other forms of dark matter, such as
self-interacting dark matter, and cold molecular dark
matter, are not consistent with the observation in that
they require a dark matter halo that is too round or too
"Chandra's ability to precisely identify and locate the
point-like sources contaminating the diffuse emission in
the X-ray image was absolutely essential," said Buote.
"Only then could we make accurate measurements of the
shape and orientation of the X-ray image contours."
The conclusion from the Chandra data that NGC 720
possesses a dark matter halo assumes that the hot gas
cloud has not been unduly disturbed by collisions or
mergers with other galaxies in the last 100 million years.
The lack of evidence of such activity indicates that this
assumption is valid.
Chandra observed NGC 720, which is about 80 million light
years from Earth, for 11 hours with the Advanced CCD
Imaging Spectrometer (ACIS). Other members of the team
includes Tesla Jeltema and Claude Canizares of
Massachusetts Institute of Technology (MIT) in Cambridge,
and Gordon Garmire of Pennsylvania State University in
University Park. Penn State and MIT developed the
instrument for NASA.
NASA's Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program, and TRW, Inc., Redondo Beach,
Calif., is the prime contractor for the spacecraft. The
Smithsonian's Chandra X-ray Center controls science and
flight operations from Cambridge, Mass.
Images associated with this release are available on the
World Wide Web at:
[NOTE: An image supporting this release is available at
When I have the time, I have to read these articles.
Ditto - great articles. I'm interested in any more you find...
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