Ancient light disqualifies giant black holes as the missing mass of the universe

A new study establishes a hard upper limit on the mass of black holes that could constitute a major portion of dark matter. Researchers determined that these objects must be smaller than 100 billion times the mass of the sun to fit within the observed structure of the universe. This finding removes the largest proposed candidates for the mysterious substance that makes up most of the matter in the cosmos.

Dark matter remains one of the biggest puzzles in physics. It was originally proposed to explain why galaxies rotate fast enough to fly apart yet remain intact. Current estimates suggest that dark matter accounts for roughly five-sixths of all matter. While scientists have not yet detected it directly, black holes are a plausible candidate because they are massive and difficult to see against the backdrop of space.

The team, led by Christopher Gerlach at the Mainz Institute for Theoretical Physics in Germany and Antonio Iovino at New York University Abu Dhabi, focused on what they describe as stupendously large black holes. These hypothetical objects would weigh more than 100 billion solar masses. Some would be massive enough to outweigh entire galaxies. No such objects have been detected yet, and the largest known black hole currently weighs about 36 billion solar masses.

To test the hypothesis, the researchers analyzed the cosmic microwave background, the light released 380,000 years after the Big Bang. At that time, the universe was hot and dense, trapping light in a plasma. As the universe expanded and cooled, the plasma dissipated, allowing light to travel freely. Gerlach described this light as a snapshot of the infant universe covering the whole sky. Iovino noted that the temperature of this background varies by about one part in 100,000, a pattern that reflects how matter was distributed in the early cosmos.

If dark matter were concentrated in a few gigantic black holes, the universe would appear grainy on very large scales. The analysis showed that the background lacks this expected graininess. Nicholas Leister, also at the Mainz Institute for Theoretical Physics, stated that the data indicates these massive black holes cannot be a significant part of the dark matter.

The authors clarified that the result does not prove such enormous black holes do not exist. It only indicates they are not a major component of the invisible mass. The team is now investigating whether smaller black holes, roughly the size of asteroids, could still make up a large fraction of dark matter. The findings are scheduled for publication in the journal Physical Review Letters on Oct. 26.