How Dark Matter Might Have Snuffed Out the First Stars
May 8th, 2007

What role did dark matter play in the early Universe? Since it makes
up the majority of matter, it must have some effect. A team of
researchers is proposing that massive quantities of dark matter
formed dark stars in the early Universe, preventing the first
generations of stars from entering their main sequence stage. Instead
of burning with hydrogen fusion, these "dark stars" were heated by
the annihilation of dark matter.

And these dark stars might still be out there.

Just a few hundred thousand years after the Big Bang, the Universe
cooled enough for first matter to coalesce out of a superheated cloud
of ionized gas. Gravity took hold and this early matter came together
to form the first stars. But these weren't stars as we know them
today. They contained almost entirely hydrogen and helium, grew to
tremendous masses, and then detonated as supernovae. Each successive
generation of supernovae seeded the Universe with heavier elements,
created through the nuclear fusion of these early stars.

Dark matter dominated the early Universe too, hovering around normal
matter in great halos, concentrating it together with its gravity. As
the first stars gathered together inside these halos of dark matter,
a process known as molecular hydrogen cooling helped them collapse
down into stars.

Or, that's what astronomers commonly believe.

But a team of researchers from the US think that dark matter wasn't
just interacting through its gravity, it was right there in the thick
of things. Their research is published in the paper "Dark matter and
the first stars: a new phase of stellar evolution". Particles of dark
matter compressed together began to annihilate, generating massive
amounts of heat, and overwhelming this molecular hydrogen cooling
mechanism. Hydrogen fusion was halted, and a new stellar phase - a
"dark star" - began. Massive balls of hydrogen and helium powered by
dark matter annihilation, instead of nuclear fusion.

If these dark stars are stable enough, it's possible that they could
still exist today. That would mean that an early population of stars
never reached the Main Sequence stage, and still live in this aborted
process, sustained by the annihilation of dark matter. As the dark
matter is consumed in the reaction, additional dark matter from
surrounding regions could flow in to keep the core heated, and
hydrogen fusion might never get a chance to take over.

Dark stars might not be so long lasting, however. The fusion from
regular matter might eventually overwhelm the dark matter
annihilation reaction. Its evolution into a regular star wouldn't be
halted, only delayed.

How could astronomers search for these dark stars?

They would be very large, with a core radius larger than 1 AU (the
distance from the Earth to the Sun), so they might be candidates for
gravitational lensing experiments. These observations use the gravity
from nearby galaxies to serve as an artificial telescope to focus the
light from a more distant object. This is the best technique
astronomers have to find the most distant objects.

They could also be detectable by the annihilation products of the
dark matter. If the nature of dark matter matches the Weakly
Interacting Massive Particles theory, its annihilation would give off
very specific radiation and particles in large quantities.
Astronomers could look for gamma-rays, neutrinos, and antimatter.

A third way to detect them would be to search for a delay in the
transition to the Main Sequence stage for the early stars. The dark
stars could have interrupted this stage for millions of years,
leading to an unusual gap in stellar evolution.

Perhaps these dark stars will give astronomers the evidence they need
to finally know what dark matter really is.

Original Source: Dark matter and the first stars: a new phase of
stellar evolution.
http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.0521v1.pdf