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Were The First Stars Dark?
Study: Dark Matter in Newborn Universe Doused Earliest Stars
12/04/2007
Press Release
(Additional comments below)
"Dark star crashes, pouring its light
into ashes" - The Grateful Dead, 1967.
Dec. 3, 2007 - Perhaps the first stars in the newborn universe did
not shine, but instead were invisible "dark stars" 400 to 200,000
times wider than the sun and powered by the annihilation of
mysterious dark matter, a University of Utah study concludes.
The study - to be published next month in the journal Physical
Review Letters - calculated how the birth of the first stars almost
13 billion years ago might have been influenced by the presence of
dark matter - the unseen, yet-unidentified stuff that scientists
believe makes up most matter in the universe.
The findings "drastically alter the current theoretical framework
for the formation of the first stars," says study author and
astrophysicist Paolo Gondolo, associate professor of physics at the
University of Utah.
It is conceivable that gigantic dark stars may exist today, and
although they do not emit visible light, they could be detected
because they should spew gamma rays, neutrinos and antimatter and be
associated with clouds of cold, molecular hydrogen gas that normally
wouldn't harbor such energetic particles, he adds.
"Without detailed simulations, we cannot pinpoint the further
evolution of dark stars," Gondolo says. "They could last months.
They could last 600 million years. Or they could last billions of
years and still be around. We have to search for them."
He conducted the study with astrophysicist Katherine Freese of the
University of Michigan, Ann Arbor, and graduate student Douglas
Spolyar of the University of California, Santa Cruz.
Gondolo says he wanted to call the new, theoretical kind of
invisible star a "brown giant" - similar to the dim but smaller,
Jupiter-sized stars known as "brown dwarfs." But he says his
co-authors insisted on calling them "dark stars," after the song
"Dark Star" first played in 1967 by the revered rock band The
Grateful Dead.
"It's catchier," Gondolo acknowledges.
Dark Matter, the Big Bang and the First Stars
Gondolo says some studies have considered the role of dark matter in
the evolution of the early universe, but until now, not in the
formation of the first stars.
Scientists know dark matter exists because galaxies rotate faster
than can be explained by the visible matter within them. Also,
observations by satellites, balloons and telescopes have led to the
estimate that all the visible matter represents only 4 percent of
the universe, which also is made of 23 percent dark matter and 73
percent "dark energy" - a yet-unknown force helping the universe
expand, Gondolo says.
WIMPS - or weakly interacting massive particles - are among the main
candidates for dark matter. Gondolo says "neutralinos" are a type of
WIMP that must exist under particle physics theories that seek to
explain the origin of mass in the universe.
Scientists generally believe that the universe came into being 13
billion years ago in a sudden expansion or "inflation" of time and
space known as the "big bang."
The afterglow of that explosion - cosmic microwave background
radiation - developed small fluctuations in temperature that caused
some of the earliest matter to begin clumping together, a process
accelerated by gravity and that produced the first stars and
galaxies. The matter was mostly dark matter but also included normal
matter in the form of hydrogen and helium gas.
The conventional theory of how the first stars were born holds that
as hydrogen and helium atoms clumped and swirled together in
proto-stellar clouds, they began to cool, making the cloud shrink
and become denser. The cooling and shrinking of the embryonic star
continues until the fusion of hydrogen into helium begins, igniting
the fusion engine that burns in our sun and other stars.
How 'Twinkle, Twinkle Little Star' Got Snuffed
For the new study, the astrophysicists calculated how dark matter
would have affected the temperature and density of gas that clumped
together to form the first stars.
The findings suggest that dark matter neutralinos interacted so they
"annihilated" each other, producing subatomic particles called
quarks and their antimatter counterparts, antiquarks. That generated
heat. As a proto-stellar cloud of hydrogen and helium tried to cool
and shrink, the dark matter would keep it hot and large, preventing
fusion from igniting the star.
"The heating can counteract the cooling, and so the star stops
contracting for a while, forming a dark star," some 80 million to
100 million years after the big bang, says Gondolo. "This is our
main result."
Dark stars would contain mostly normal matter - mostly in the form
of hydrogen molecules and helium - but they would be vastly larger
and "fluffier" than the sun and other stars, he adds. They would
have glowed infrared, which is heat.
"They are much bigger than the sun," Gondolo says, with diameters
ranging from about 4 astronomical units (372 million miles, or four
times the average distance between the sun and Earth) to 2,000
astronomical units - big enough to swallow 15,000 solar systems like
our own.
The quarks and antiquarks produced within the dark star would, in
turn, generate descendant particles including gamma rays, neutrinos
and antimatter such as positrons and antiprotons, Gondolo says.
"With your bare eyes, you can't see a dark star. But the radiation
would fry you."
Implications of Dark Stars
Gondolo says dark stars have some important implications for
astrophysics:
-- They represent a new phase in the evolution of stars.
-- Their possible existence could aid the search to find and
identify dark matter. Gamma rays, neutrinos and antimatter have
characteristic energy signatures if they come from dark matter.
-- They could improve understanding of how heavy elements formed.
The first stars supposedly were the cradle of elements as heavy or
heavier than carbon, producing them via nuclear fusion. But if dark
stars existed and did not later evolve into normal stars, they
didn't make carbon. "Maybe carbon came from other stars" - perhaps
conventional stars that formed where there was no dark matter
nearby, Gondolo says.
-- Dark stars may explain why black holes - collapsed stars so dense
that not even light escapes - formed much faster than expected.
Gondolo says black holes existed only a few hundred million years
after the big bang, yet current theories say they took longer to
form. "These dark stars may help. They could collapse into black
holes very early because they are very short-lived and formed when
the universe was young, at least in one scenario."
Another possibility is that dark stars lasted quite a while but
eventually turned into conventional stars. Gondolo and colleagues,
however, argue the gas cooling and dark matter heating within a dark
star can remain in balance, allowing dark stars to survive, but that
depends on certain assumptions about the mass of neutralinos.
"We don't know how long they last, so we speculate. It depends very
sensitively on the parameters of the model."
The study was funded by the National Science Foundation, U.S.
Department of Energy and the University of Michigan.
This news release and
high-resolution art of a dark star may be downloaded from:
http://www.unews.utah.edu/p/?r=112707-2 by clicking on "Click to
view." Or download the art directly from:
http://www.unews.utah.edu/showImage.php?image=2364&size=400&resizeOn=w&q
uality=h
__________________________
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