April 28, 2013
Did Dark Stars Once Roam The Universe?
The search for dark matter continues to be one of astronomy’s great puzzles. In the last year, several experiments, such as the Fermi Gamma-ray Space Telescope and the Alpha Magnetic Spectrometer (AMS), have reported hints of dark matter collisions emanating from our galactic center.
Most references to dark matter in the popular media point to its role in galactic rotation curves, or perhaps the way in which it affects clusters of galaxies. But given how it dominates our Universe, we are probably only scratching the surface of how dark matter influenced the formation of our universe and how it continues to guide its evolution today.
In fact, some are wondering if dark matter can be used to explain a mystery that has been frustrating astronomers for the last decade; the origin of supermassive black holes. These behemoths reside in the center of most galaxies, influencing the structure and evolution of the host in which they reside.
Normal stars are limited in the size that they can achieve, as they must maintain a balance between the outward radiation pressuring pushing and the immense gravitational force attempting to collapse the star into a black hole. Once stars made of normal (baryonic) matter reach about 150 solar masses the gravitational force begins to overcome the radiation pressure and the star will collapse in a brilliant supernova.
While such stars will form black holes that are many times the mass of our sun, they are simply dwarfed by supermassive black holes that can reach millions, or even billions of solar masses. So, how do these monsters form if they do not originate from normal stars?
There are several theories that have been developed, such as the build up of mass over time. (Essentially the black hole would have started out much smaller then accumulated mass over time to reach its current size.) Others believe that as the galaxies that form the inner region of the galactic cloud become too dense, too quickly, that the stellar stage is passed all together leading directly to a black hole that quickly consumes much of the forming galaxies mass.
These models have had some success in standing up to available data, but we lack any observational data of such a supermassive black hole forming, so there is considerable uncertainty in the field. But, at least one group of researchers is considering the possibility that dark matter may be the key to the mystery.
In the early Universe, stars, as we conventionally discuss them, would not have been able to exist. However, regions of dark matter would have been able to pull together to form dark stars. These objects would also have contained normal (baryonic) matter, but would have been dominated by dark matter.
Such stars would not have relied on core fusion to sustain hydrostatic equilibrium, but rather would be driven by dark matter collisions beneath the surface. These events would release significant energy, providing the outward pressure to stave off gravitational collapse. In fact, the outward force would be so great that these objects could conceivably reach masses millions of times that of the sun, possibly more.
So, could it be that the supermassive black holes that occupy the centers of most galaxies are due not to conventional gravitational collapse, but are the remnants of the dark stars of the early universe? It is not clear. Specifically, dark stars would not readily form in the modern age of the universe, and those that would have evolved billions of years ago would not likely still be around, at least not in such a form. As a result, we can’t be certain that they ever existed at all.
However, dark stars provide a tempting alternative to some other supermassive black hole models, and as we continue to characterize and understand dark matter, we may find that dark stars may have once roamed the Universe.
Image Credit: Igor Zh. / Shutterstock