Cosmic data sets 100 billion solar mass limit for dark matter black holes

Black holes weighing more than 100 billion times the mass of the sun are now ruled out as a primary component of dark matter. This finding, detailed in a study set for release in Physical Review Letters on October 26, places a hard ceiling on the size of these invisible objects if they are to account for the majority of the universe's missing mass. Dark matter is estimated to comprise approximately five-sixths of all matter in the cosmos, a substance proposed decades ago to explain the rotational speeds of galaxies.

The concept of dark matter emerged from the observation that galaxies rotate too quickly to be held together by visible matter alone. Theorists have long considered black holes a plausible candidate because they possess immense mass and are difficult to detect. However, no black hole larger than roughly 36 billion solar masses has been identified to date. The research team specifically targeted "stupendously large black holes" with masses ranging from 100 billion to 10 million billion solar masses. Objects in this upper range would outweigh entire galaxies, yet none have been found despite extensive surveys.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, noted that dark matter is typically visualized as a fine dust distributed evenly. He explained that if dark matter were instead concentrated in a few gigantic objects, the universe would appear "grainy on very large scales." To test this hypothesis, the team analyzed the cosmic microwave background. This radiation is the light released when the universe was 380,000 years old, a period when hot plasma cooled and allowed photons to travel freely. Gerlach described this light as a snapshot of the infant universe covering the entire sky.

Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, pointed out that the temperature of this background light varies by about one part in 100,000 from place to place. These minute fluctuations provide a map of matter distribution from the early universe. The analysis revealed that the background lacks the specific graininess expected if stupendously large black holes were prevalent. Nicholas Leister, also of the Mainz Institute, concluded that "these black holes can't be a significant part of the dark matter." The result does not disprove the existence of such massive black holes, only that they do not make up the bulk of the unseen matter.

The authors noted that smaller black holes, roughly the size of asteroids, remain a viable candidate for dark matter. The team is currently investigating whether these smaller objects can also be ruled out as a major constituent. The study suggests that the universe’s structure is smoother on large scales than the presence of galaxy-mass black holes would imply. The paper will appear in Physical Review Letters on October 26, providing a definitive upper mass limit for black hole dark matter candidates based on current observational data.