Mainz physicists find galaxy-scale black holes unlikely to be dark matter

A new analysis challenges the idea that the universe's missing mass is hidden inside black holes so large they outweigh entire galaxies. A team of theoretical physicists concluded that these hypothetical objects, which would exceed 100 billion solar masses, cannot make up a major share of dark matter. The finding narrows the search for the substance that holds galaxies together through gravity.

Dark matter is estimated to constitute approximately five-sixths of all matter in the cosmos. While its exact nature remains unknown, it was proposed decades ago to explain why galaxies rotate without flying apart. Black holes have been considered a potential candidate because of their high mass and low visibility. The researchers focused specifically on objects with more than 100 billion times the mass of the sun. Those in the range of 1 million to 10 million billion solar masses would be heavier than most galaxies. No such object has been detected yet, with the largest known black hole weighing roughly 36 billion suns.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, noted that dark matter is typically visualized as a fine dust spread throughout space. He explained that if the mass were instead concentrated in a few gigantic objects, the universe would appear grainy on very large scales. To test this, the team examined the cosmic microwave background, which is light released from the early universe. This radiation dates to a period 380,000 years after the Big Bang when the universe cooled enough for atoms to form and light to travel freely. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, stated that the background's temperature varies by about one part in 100,000 from place to place.

The team found that the cosmic microwave background lacks the specific graininess that would be produced by stupendously large black holes. Nicholas Leister, a colleague at the Mainz Institute for Theoretical Physics, said the data indicates these massive objects cannot be a significant part of the dark matter. The researchers clarified that the result does not prove such black holes do not exist. It only suggests they are not a major component of the universe's invisible mass.

The study leaves open the possibility that smaller black holes, roughly the size of asteroids, could still contribute to dark matter. The team is currently investigating whether those objects can also be ruled out as a major constituent. The findings are scheduled for publication in the journal Physical Review Letters on October 26.