100 Billion Solar Mass: The Limit for Dark Matter Black Holes

Physicists have set a hard ceiling on the size of black holes that could make up the universe's invisible mass. New research indicates that black holes weighing more than 100 billion times the mass of the sun cannot account for a major portion of dark matter. This finding narrows the search for the substance that holds galaxies together. Dark matter is estimated to comprise about five-sixths of all matter in the cosmos. It was proposed decades ago to explain why galaxies do not fly apart despite their rapid rotation. While its gravitational pull is detected, its physical nature remains unknown.

The study focused on what researchers call stupendously large black holes. These hypothetical objects would exceed the mass of entire galaxies if they reached between one and ten million billion solar masses. No such object has been detected yet. The largest black hole currently known weighs approximately 36 billion suns. Because surveys of many galaxies have not found these massive objects, the team tested whether they might still exist as a smaller but substantial fraction of dark matter.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, noted that dark matter is often visualized as a fine dust spread throughout space. If the mass were instead concentrated in a few gigantic objects, the universe would appear grainy on very large scales. Gerlach stated that this graininess can be checked using existing data. The team analyzed the cosmic microwave background, which is the light released from the early universe. For the first 380,000 years after the Big Bang, the cosmos was filled with hot plasma that trapped light. As the universe expanded and cooled, the plasma dissipated, allowing light to travel freely.

Gerlach described this background light as a snapshot of the infant universe covering the whole sky. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, explained that the background's temperature varies from place to place by about one part in 100,000. This pattern reveals how matter was distributed at that early stage. The analysis showed that the background lacks the specific graininess that stupendously large black holes would have produced.

Nicholas Leister, a theoretical physicist at the Mainz Institute for Theoretical Physics, stated that these black holes cannot be a significant part of the dark matter. The authors clarified that the result does not mean such enormous black holes cannot exist. It only means they are not a major component of the invisible mass. The team is now examining whether relatively tiny black holes, about the size of asteroids, might still be a possible candidate.

The findings are scheduled to be published on October 26 in the journal Physical Review Letters.