Ancient light caps black hole mass for dark matter role

Physicists have placed a strict upper limit on the mass of black holes that could serve as the primary component of dark matter. The new study indicates that objects exceeding 100 billion solar masses are too large to account for the mysterious substance that holds galaxies together.

The research team, led by Christopher Gerlach of the Mainz Institute for Theoretical Physics in Germany and Antonio Iovino of New York University Abu Dhabi, analyzed the cosmic microwave background. This radiation is the light left over from the first 380,000 years of the universe, a period when the cosmos was a hot plasma that trapped photons until expansion allowed them to travel freely.

Gerlach described dark matter as typically being viewed like fine dust spread uniformly through space. He argued that if the mass were instead clumped into gigantic black holes, the universe would appear grainy on large scales. The team checked for this graininess by examining the temperature variations in the background light, which shift by about one part in 100,000 from place to place.

The data showed no evidence of the specific patterns that stupendously large black holes would have created. Nicholas Leister, a colleague at the Mainz Institute for Theoretical Physics, stated that these results mean such massive objects cannot be a major part of dark matter. The largest known black hole currently weighs about 36 billion suns, well below the 100 billion solar mass threshold established by the analysis.

Dark matter is estimated to comprise five-sixths of all matter in the universe. It was proposed to explain the rotational speeds of galaxies, which would otherwise fly apart without sufficient gravitational pull. Black holes have long been a candidate because they are massive and difficult to detect in the darkness of space.

The authors clarified that their findings do not rule out the existence of these enormous black holes entirely, only their dominance in the dark matter budget. They noted that smaller black holes, roughly the size of asteroids, remain a possible candidate. The team is currently testing whether those smaller objects can also be excluded as a primary constituent.

The findings are scheduled for publication in the journal Physical Review Letters on Oct. 26.