source COSMOS+
by Carolyn Barry
14 September 2007
SYDNEY: The nature of the universe's earliest stars is providing
insights into the mysterious properties of dark matter, the shadowy
material that makes up most of the universe, report cosmologists behind
a new study.
Dark matter is so named because it doesn't emit radiation or light
and we understand little about it. The only certain thing about dark
matter is that it exists, said Tom Theuns of Durham University in
England, lead author of the report in the U.S. journal Science, today. "We known very well how much there is, but not about what it is."
Missing mass
Measurements of the velocity and rotational speed of galaxies in the
1970s found they possessed insufficient mass to hold themselves
together. In fact, the visible matter - in the form of stars and gas -
only accounted for around 30 percent of the apparent mass and
gravitational influence of the galaxies. As a result, cosmologists
surmised that the missing mass was the 'dark matter' first described by
astronomer Fritz Zwicky in 1933.
The prevailing theory of early star formation centres on the
assumption that dark matter is 'cold'. In other words, particles of
dark matter were big and slow moving, and as such, formed clumps called
'mini haloes' at the dawn of the universe. Early stars that formed this
way would be at least a hundred times more massive than the Sun, and
have short life spans.
However, using a computer simulation of the formation of early
structures in the universe, Theuns and fellow Durham University
cosmologist Liang Gao, have now come up with a totally different
scenario.
The pair proposes that, instead, the universe is made of
faster-moving 'warm' dark matter that is too speedy to form clumps.
Instead, they postulate, warm dark matter caused the primordial gases
in the infant universe to collapse into giant filaments, or strings,
about one quarter of the size of the Milky Way.
"What we think happens with this curious string is that it starts
forming stars in a big burst," said Theuns. "This could be quite
spectacular."
Indirect evidence
Stars formed from these fragmented filaments would have less mass
and longer lives than if they were formed from cold dark matter, argues
Theuns. There would also be clusters of stars along the filament,
rather than individual stars in isolation.
Because dark matter can't be seen directly, scientists must look for
indirect evidence to study its nature. The evidence to prove this new
theory would be the discovery of a low-mass star made of the basic
primordial material, hydrogen and helium, said the researchers.
"It's hitting cosmologists on the head, that when it comes to the
first stars, the details of the cold dark matter is vitally important,"
said Volker Bromm of the University of Texas who wrote an accompanying
commentary in Science. Looking at the composition of stars to
determine the formation of the universe the way Theuns and Gao have
represents an "unusually remarkable connection between large scale and
small scale physics," he said.
The new theory is a "significant piece of research," commented
cosmologist Andrew Bunker of the Anglo-Australian Observatory in
Epping, New South Wales. Bunker said the theory "gets around a
long-standing problem in cosmology [that] the large number of
mini-haloes predicted is not really observed ... It's nice to see a
simulation paper make some definitive predictions about the universe."
source COSMOS+
related:
NASA finds further proof of dark matter
Dark matter exists: scientists finally find proof
Big Bang seems wrong, computers to blame
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