Cosmic Microwave Background Excludes Massive Black Holes From Dark Matter Composition

Physicists have established a strict upper limit on the size of black holes that could make up a large share of the universe's dark matter. The new analysis rules out objects heavier than 100 billion solar masses as a primary component of the invisible substance that holds galaxies together. This finding narrows the search for the material estimated to account for five-sixths of all matter in the cosmos.

Dark matter remains a central mystery in astrophysics because it does not emit light but exerts gravitational force. Previous assumptions suggested that giant, unseen black holes might explain this missing mass. The team tested this hypothesis by looking for specific patterns in the oldest light in the universe. If dark matter consisted of a few enormous objects rather than a fine dust, the early universe would have looked grainy on large scales.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, explained that checking for this graininess is possible because of the cosmic microwave background. This light was released 380,000 years after the Big Bang when the universe cooled enough for atoms to form. It acts as a snapshot of the infant universe covering the entire sky.

Antonio Iovino, a co-author and theoretical physicist at New York University Abu Dhabi, noted that the temperature of this background varies by about one part in 100,000 across the sky. These tiny fluctuations reveal how matter was distributed in the early universe. The researchers analyzed these variations to see if they matched the gravitational signature of stupendously large black holes.

The study found no evidence of the graininess that would be produced by black holes with more than 100 billion times the mass of the sun. Nicholas Leister, also from the Mainz Institute for Theoretical Physics, stated that the background lacks the specific patterns these objects would create. Consequently, these massive black holes cannot be a significant part of the dark matter inventory.

The result does not mean such black holes cannot exist, only that they are not a major constituent of the universe's matter. The largest known black hole currently weighs about 36 billion solar masses, well below the new limit. The authors are now examining whether smaller black holes, roughly the size of asteroids, might still account for a large fraction of dark matter.

The findings are scheduled for publication in the journal Physical Review Letters on October 26. The study provides a new constraint on models of the early universe and the nature of dark matter. It shifts the focus away from supermassive objects toward other potential candidates for the invisible mass.