Ancient Light Confirms Dark Matter Is Not Giant Black Holes

Dark matter, the invisible substance estimated to make up five-sixths of all matter in the universe, likely does not consist of black holes larger than 100 billion solar masses. A new study published in Physical Review Letters on Oct. 26 rules out these massive objects as a major component of the cosmic dark matter.

Scientists have long hypothesized that dark matter exists to explain why galaxies rotate so quickly without flying apart. While its gravitational pull holds these structures together, the substance remains elusive. Black holes are a logical candidate due to their mass and difficulty in detection. However, the largest known black hole weighs approximately 36 billion times the mass of the sun, far below the threshold tested in this new analysis.

Researchers focused on hypothetical black holes exceeding 100 billion solar masses. Objects in the range of 1 million to 10 million billion solar masses would outweigh entire galaxies. Although no such objects have been detected in galaxy surveys, the team sought to determine if they could still constitute a substantial fraction of the total dark matter.

The study utilized the cosmic microwave background, light released from the early universe. During the first 380,000 years after the Big Bang, the cosmos was a hot plasma that trapped light. As the universe expanded and cooled, atoms formed, allowing that light to travel freely. Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, described this radiation as a snapshot of the infant universe covering the whole sky.

If dark matter were concentrated in a few gigantic objects, Gerlach noted, the universe would appear grainy on very large scales. The team examined the temperature variations in the background, which differ by about one part in 100,000 from place to place. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, explained that these variations reveal how matter was distributed in the early cosmos.

The analysis found no such graininess in the data. Nicholas Leister, also of the Mainz Institute for Theoretical Physics, stated that the absence of this pattern means these 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, only that they are not a primary constituent. They are now investigating whether smaller, asteroid-sized black holes might account for the missing mass.

The findings provide a concrete upper limit for this class of dark matter candidates. By linking the smoothness of the ancient light to the distribution of mass, the study narrows the search for the universe's missing substance.