Early Universe Light Rules Out Massive Black Holes as Dark Matter Source

Black holes with masses exceeding 100 billion solar masses do not make up a large portion of dark matter, according to a new analysis of early universe light. The study limits the role of these hypothetical objects in the composition of the cosmos. Dark matter constitutes roughly five-sixths of all matter in the universe.

Scientists proposed dark matter decades ago to explain galactic rotation speeds. Without this invisible mass, galaxies would fly apart due to their rapid spin. While much remains unknown about the substance, black holes have been a candidate because they are massive and difficult to detect.

The research team focused on stupendously large black holes. These objects would weigh between 1 million billion and 10 million billion solar masses. Such sizes would outweigh entire galaxies. No such object has been detected yet. The largest known black hole weighs approximately 36 billion suns.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, noted that dark matter is often imagined as a fine dust spread everywhere. He explained that if the mass were concentrated in a few gigantic objects, the universe would appear grainy on very large scales. The team could check for this graininess by examining the cosmic microwave background.

The cosmic microwave background is 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, atoms formed and light began traveling freely. Gerlach described this light as a snapshot of the infant universe covering the whole sky.

Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, stated that the background temperature varies by about one part in 100,000 from place to place. This pattern reveals how matter was distributed in the early cosmos. The analysis showed the background lacks the graininess that stupendously large black holes would have produced.

Nicholas Leister, also at the Mainz Institute for Theoretical Physics, concluded that these black holes cannot be a significant part of 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. The team is now examining whether tiny black holes, about the size of asteroids, can be ruled out as a major constituent. The findings are scheduled for publication on Oct. 26 in the journal Physical Review Letters.