Galaxy-mass holes fail to explain dark matter

Physicists at the Mainz Institute for Theoretical Physics and New York University Abu Dhabi have determined that black holes exceeding 100 billion solar masses cannot constitute the majority of dark matter. The team analyzed data from the cosmic microwave background to test a hypothesis that this invisible mass might be concentrated in a small number of extremely massive objects rather than being spread evenly across the universe. Their results, scheduled for publication in Physical Review Letters on October 26, suggest that while these stupendously large black holes could exist, they do not hold enough gravitational weight to explain the bulk of the cosmos.

Dark matter accounts for approximately five-sixths of all matter in the universe. It was originally proposed to explain why galaxies rotate so fast that they should have flown apart without an additional gravitational force. Because black holes are massive and difficult to observe, they have long been considered a plausible candidate for this missing mass. The largest known black hole weighs about 36 billion times the mass of the sun, but the study focused on hypothetical objects ranging from 100 billion to 10 million billion solar masses, a range that would allow a single object to outweigh an entire galaxy.

Christopher Gerlach, a theoretical physicist at the Mainz Institute, noted that if dark matter were clumped into such gigantic objects, the universe would appear grainy on large scales. He explained that this texture would leave a specific imprint on the light released from the early universe. That light, known as the cosmic microwave background, was trapped in hot plasma for the first 380,000 years after the Big Bang. As the universe cooled and expanded, the plasma dissipated, allowing the light to travel freely. Gerlach described this radiation as a snapshot of the infant universe that is still arriving at telescopes today.

Antonio Iovino, a co-author from New York University Abu Dhabi, pointed out that the temperature of this background radiation varies by about one part in 100,000 across different areas of the sky. These variations provide a map of how matter was distributed in the early cosmos. The research team examined these temperature fluctuations to look for the graininess that would be caused by the gravitational influence of massive black holes. They found that the data lacks the specific pattern predicted by models containing stupendously large black holes.

Nicholas Leister, another theoretical physicist at the Mainz Institute, stated that the absence of this graininess means these objects cannot be a significant part of dark matter. The researchers clarified that their findings do not rule out the existence of such black holes entirely, only their dominance in the cosmic mass budget. They noted that smaller black holes, roughly the size of asteroids, remain a possible candidate and are currently under investigation to determine if they can also be excluded as a major component of the universe's invisible matter.

The study provides a concrete boundary for future models of dark matter. By establishing a hard upper limit of 100 billion solar masses for black holes that make up the majority of this substance, the team narrowed the search for the source of cosmic gravity. The results rely on the precision of the cosmic microwave background data and the statistical analysis of its temperature variations. This method allows scientists to test physical properties of the early universe that are otherwise inaccessible through direct observation.