Giant Black Holes Cannot Fill Dark Matter Void

Black holes heavier than entire galaxies cannot account for a major share of the universe's dark matter, according to a study by physicists in Germany and the United Arab Emirates. The work is scheduled for publication on Oct. 26 in Physical Review Letters.

Dark matter was proposed decades ago to explain why galaxies hold together despite their rotation speeds, and it is estimated to make up about five-sixths of all matter in the cosmos. Black holes are an attractive candidate because they are massive and hard to detect. The team focused on objects exceeding 100 billion times the sun's mass, which they call stupendously large black holes. Objects above roughly 1 million billion to 10 million billion solar masses would outweigh entire galaxies. The largest black hole known weighs about 36 billion suns, and no heavier object has been found in surveys of many galaxies, though the researchers wanted to test whether such objects might still constitute a substantial fraction of dark matter.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany and a study co-author, said dark matter is often imagined as fine dust spread everywhere. If it were instead packed into a few gigantic objects, he said, the universe becomes grainy on very large scales, and that graininess can be checked.

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 expanding universe cooled, atoms formed and light began to travel freely. Gerlach described it as a snapshot of the infant universe covering the whole sky. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi and a study co-author, 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 showed the background lacks the graininess stupendously large black holes would have produced, according to 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 objects, only their role as a major component of dark matter. They added that asteroid-sized black holes remain a candidate, and they are examining whether those can also be excluded as a major constituent.