Dark Stars
Friday, May 23, 2014
Science Update - A look at CfA discoveries from recent journals

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.

Reference(s): 

"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.