Study Rules Out Galaxy-Outweighing Black Holes as a Major Part of Dark Matter

A new study concludes that black holes heavier than entire galaxies cannot account for a significant share of dark matter, the mysterious substance believed to make up most of the matter in the universe.

Dark matter was proposed decades ago to explain why galaxies do not fly apart given how fast they rotate. Its gravitational pull is thought to hold them together, and it is estimated to account for about five-sixths of all matter in the cosmos. Much about it is still unknown, including whether it exists at all. Black holes are an appealing candidate because they are very massive and hard to see in the darkness of space.

The researchers focused on what they call "stupendously large black holes," objects with more than 100 billion times the mass of the sun. Those exceeding roughly 1 million billion to 10 million billion solar masses would outweigh whole galaxies. No such object has been detected so far; the biggest black hole known may weigh about 36 billion suns. Because none has turned up despite surveys of many galaxies, the team noted that such objects cannot make up all dark matter, but they wanted to test whether they might still be a substantial fraction.

Study co-author Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, said dark matter is often pictured "as a fine dust spread everywhere." If it were instead concentrated in a few gigantic objects, he said, "the universe becomes grainy on very large scales. And we can check that."

To do so, the team examined the cosmic microwave background, the light released from the early universe. For the first 380,000 years after the Big Bang, the cosmos was filled with hot plasma that trapped light. As expansion cooled the universe, the plasma largely dissipated and atoms formed, allowing light to travel freely. "That light is still arriving today," Gerlach explained, describing it as "a snapshot of the infant universe covering the whole sky."

Co-author Antonio Iovino, a theoretical physicist at New York University Abu Dhabi in the United Arab Emirates, 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 that stupendously large black holes would have produced. "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 stressed that the result does not mean such enormous black holes cannot exist, only that they are not a major component of dark matter. They added that another candidate, relatively tiny black holes about the size of asteroids, might still be possible, and they are now examining whether those too can be ruled out as a major constituent.

The findings are due to be published Oct. 26 in the journal Physical Review Letters.