Cosmic Background Light Rules Out Giant Black Holes as Dark Matter Source

Dark matter is the invisible stuff holding galaxies together as they spin, and for decades physicists have hunted for what it is made of. Now a team led by researchers at the Mainz Institute for Theoretical Physics says one popular candidate, black holes heavier than entire galaxies, does not fit the bill.

The researchers studied what they call stupendously large black holes, objects exceeding 100 billion times the mass of the sun. Black holes above roughly 1 million billion to 10 million billion solar masses would outweigh whole galaxies, but no such object has ever been detected. The largest known black hole may weigh about 36 billion suns. Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics, said dark matter is usually imagined as a fine dust spread everywhere, but if it were packed into a few gigantic objects, the universe would look grainy on very large scales. "And we can check that," he said.

To run that check, the team examined the cosmic microwave background, the light released from the early universe. For the first 380,000 years after the Big Bang, hot plasma trapped that light. As the universe expanded and cooled, atoms formed and the light began traveling freely. "That light is still arriving today," Gerlach said, calling it a snapshot of the infant universe covering the whole sky. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, said the background's temperature varies from place to place by about one part in 100,000, a pattern that reveals how matter was distributed at that time.

The analysis found no graininess in that pattern of the kind stupendously large black holes would have left behind. "So these black holes can't be a significant part of the dark matter," said Nicholas Leister, a theoretical physicist at the Mainz Institute for Theoretical Physics.

The authors said the result does not rule out the existence of such enormous objects, only their role as a major source of dark matter. They noted that tiny black holes roughly the size of asteroids remain a possible candidate and said they are now testing whether those can also be excluded. Dark matter is estimated to make up about five-sixths of all matter in the cosmos, and its existence itself has not been confirmed. The findings are due for publication Oct. 26 in Physical Review Letters.