Dark
stars, entirely speculative entities for now, are hypothesized to form
in the early cosmos when dark matter condenses and associated particle
annihilation reactions heat up the matter, perhaps disrupting the
formation of normal stars. The image shows one result from a new
simulation of the birth of the first stars: the left panel shows normal
stars forming when no annihilating dark matter is present and the right
panel shows one instance of what happens when dark stars form and
disrupt the process.Credit: A. Stacey
The
vast majority of matter is the universe, about eighty-five percent of
it, is so-called "dark matter." It consists not of ordinary atoms, but
of some still unknown kind of particle. Understanding this ubiquitous
yet mysterious substance is a prime goal of modern astrophysics. Dark
matter is detected via its gravitational influence on stars and other
normal matter, and some astronomers speculate that it might have another
other property besides gravity in common with ordinary matter: It
might come in two forms, matter and anti-matter, that annihilate on
contact emitting high energy radiation.
Several hundred million years after the big bang, the first stars
began to form as gravity gradually condensed the primordial material and
heated it to temperatures able to trigger nuclear burning. Scientists
have speculated that something roughly similar might also have occurred
to dark matter: Gravity condensed it into cores that ultimately ignite,
not with nuclear burning – dark matter is not atomic and has no (normal)
nuclei – but rather via annihilation radiation. These so-called "dark
stars" might have shone for some time as more and more dark matter
accreted onto them, powering ongoing annihilation. They may even have
heated up their environment in a way that inhibited the growth of the
first generation of normal stars.
CfA astronomer Avi Loeb and three of his colleagues used computer
simulations of dark matter in the early universe to investigate if and
how dark matter might influence the development of normal stars. The
details are complex, in part because the growing clumps of matter tend
to fragment into clusters within which they then scatter off one
another. The scientists tested their simulations under a variety of
assumptions, and found in one of the more sophisticated versions that
the annihilating dark matter was considerably less effective in forming a
dark star (or disrupting normal stars) than had been thought because of
the disruption from scattering. They conclude that dark stars may never
have existed, and hence that the formation of normal stars may not have
been retarded. They caution, however, that further research is needed
to refine these conclusions, not least to determine nature of the
mysterious dark matter itself. Astronomers are optimistic that some of
the first stars in the universe will actually be detected in this
decade; some proposed space missions (like the Japanese WISH mission)
set this as their primary goal. These new simulations provide a basis
for interpreting those detections.
"The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter," Athena Stacy, Andreas H. Pawlik, Volker Bromm and Abraham Loeb, MNRAS 441, 822, 2014.
