Researchers Use Cosmic Microwave Background to Limit Massive Dark Matter Black Holes
Analysis of the cosmic microwave background rules out galaxy-mass black holes as the primary source of the universe's missing mass.
By Tunde Bailey ·
Black holes with more than 100 billion times the mass of the sun cannot make up a large portion of the universe's dark matter, according to a new study. The research narrows the search for the invisible substance that holds galaxies together. It does not rule out black holes entirely, but it eliminates the most massive candidates.
Dark matter was proposed to explain why galaxies rotate so quickly without flying apart. It is estimated to account for about five-sixths of all matter in the cosmos. Physicists have long considered black holes a potential candidate because they are massive and difficult to detect. The new study focuses on objects heavier than 100 billion solar masses, a category researchers call stupendously large black holes.
No black hole of this size has been observed yet. The largest known black hole weighs about 36 billion solar masses. Objects exceeding roughly 1 million to 10 million billion solar masses would outweigh entire galaxies. Since surveys have not found such objects, the team set out to test if they could still constitute a 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 pictured as a fine dust spread everywhere. If it were instead concentrated in a few gigantic objects, he said the universe becomes grainy on very large scales. To check for this graininess, the team examined the cosmic microwave background.
This light was released 380,000 years after the Big Bang. It provides a snapshot of the infant universe covering the whole sky. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi in the United Arab Emirates, 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 time.
The analysis showed the background lacks the 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 can't be a significant part of the dark matter. The authors stressed that the result does not mean such enormous black holes cannot exist, only that they are not a major component of dark matter.
The findings are due to be published on October 26 in the journal Physical Review Letters. The team is now examining whether relatively tiny black holes about the size of asteroids might still be possible as a major constituent of dark matter.
