Dark matter thory in trouble?

Hello folks,


just saw an interesting article on NewScientist.com. The subject
matter is really quite a bit beyond me. However, I must admit I´ve
never quite liked the idea of there being 'dark matter.' But majority
of astro-scientists have thought it´s precense likelly in latter
years. But, perhaps these new findings do throw in a spanner.

The full text of the article below!


Cheers, Einar

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http://space.newscientist.com/articl...-theories.html

Cosmic 'train wreck' defies dark matter theories

* 20:07 16 August 2007
* NewScientist.com news service
* Stephen Battersby

Disturbing evidence has emerged from the wreckage of an intergalactic
pile-up suggesting that the already mysterious substance known as dark
matter may be even less well understood than astronomers thought.

The observations come from a massive galaxy cluster called Abell 520
that lies 3 billion light years away and results from a high-speed
collision between smaller galaxy clusters. Astronomers examined the
wreckage using a technique called weak lensing, which relies on the
fact that the gravity of any matter in the cluster bends the light of
background galaxies. This distorts their images and so reveals where
the cluster's matter lies.

Abell 520 turns out to hold a massive dark core, empty of bright
galaxies. Some of the core is made up of hot gas, which the team
detected from its emission of X-rays, but most of it has to be
something else - presumably the same dark matter that astronomers
detect elsewhere in the universe.

Except that dark matter and galaxies usually stick together. How have
they become separated here? One possibility is that the galaxies were
once in the core, along with the dark matter, but then close
encounters between the galaxies threw them out to the cluster's
fringes. Unfortunately, the team can't get that to happen in their
computer simulations, even if they tailor the initial conditions to
encourage these gravitational slingshots.

A more intriguing explanation is that when the original clusters
collided, their dark matter was stripped out. Astronomers expect that
to happen to gas clouds in colliding clusters, but dark matter is
supposed to be more slippery, barely interacting with other matter or
with itself. "We expect clouds of dark matter to flow right through
each other," says team member Arif Babul at the University of Victoria
in Canada.
Invisible substance

Indeed, recent observations of the Bullet Cluster, another violent
collision between clusters, seem to show that dark matter does exactly
that, following the galaxies and leaving only gas behind in the
middle.

Could there be two types of dark matter, the conventional slippery
form and another that interacts more strongly? Babul says it is
possible, but dislikes the idea of invoking yet another invisible
cosmic substance to explain these observations. "It would push us in
an uncomfortable direction. Maybe that is the way nature is driving
us, but I can imagine there would be a lot of resistance to that
idea."

Nor does he have encouraging words for supporters of an alternative
theory to dark matter called MOND, which relies on a modified theory
of gravity. Before the team wrote up their results, they tried and
failed to fit their data using MOND. "It didn't seem very promising,"
Babul told New Scientist.

David Spergel at Princeton University in New Jersey, US, who is not
part of the team, says the big question is whether this observation
rules out the standard model of cosmology, called lambda-CDM, which
assumes that dark matter only interacts very weakly.

He says it is much too early to make any such claim. "A number of
things need to happen next - more observations and more theoretical
studies, simulations of what lensing looks like for clusters in
collision," Spergel told New Scientist. "You need to make sure that
this is not a chance angle that somehow gives you the impression
there's more dark matter there."

The team now plans to train the Hubble Space Telescope on Abell 520 to
get a closer look at its puzzling dark core.

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