Black holes may hint at the nature of dark matter
Research suggests that dark matter density is constant at the centers of galaxies.
Provided by the National Autonomous University of Mexico, Mexico City
March 23, 2010
About 23 percent of the universe is made up of mysterious dark matter, invisible material only detected through its gravitational influence on its surroundings. Now two astronomers based at the National Autonomous University of Mexico (UNAM) have found a hint of the way it behaves near black holes.
In the early universe, clumps of dark matter are thought to have attracted gas, which then coalesced into stars that eventually assembled the galaxies we see today. In their efforts to understand galaxy formation and evolution, astronomers have spent a good deal of time attempting to simulate the buildup of dark matter in stars.
The UNAM astronomers, Xavier Hernandez and William Lee, calculated the way in which the black holes found at the centers of galaxies absorb dark matter. These black holes have anything between millions and billions of times the mass of the Sun and draw in material at a high rate.
The researchers modeled the way in which black holes absorb dark matter and found that the rate at which this happens is very sensitive to the amount of dark matter found in a black hole's vicinity. If this concentration were larger than a critical density of 7 Suns of matter spread over each cubic light-year of space, the black hole mass would increase rapidly, engulfing such large amounts of dark matter that soon the entire galaxy would be altered beyond recognition.
"Over the billions of years since galaxies formed," Hernandez said, "such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe."
Their work therefore suggests that the density of dark matter in the centers of galaxies tends to be a constant value. By comparing their observations to what current models of the evolution of the universe predict, Hernandez and Lee conclude that it is probably necessary to change some of the assumptions that underpin these models — dark matter may not behave in the way scientists thought it did.