Ancient light rules out giant black holes as main dark matter
Data from the cosmic microwave background shows that black holes exceeding 100 billion solar masses cannot make up a large fraction of the universe's missing mass.
By David Almasi ·
Scientists have ruled out the possibility that enormous black holes account for a large portion of dark matter. The new analysis shows that if these objects made up a significant share of the unseen mass, the early universe would look different than it does today. For ordinary observers, the finding narrows the search for the substance that holds galaxies together.
Dark matter is estimated to constitute roughly five-sixths of all matter in the cosmos. It was proposed to explain why rotating galaxies do not fly apart. While black holes are massive and difficult to see, the study focused on a specific class of these objects. The team examined black holes with more than 100 billion times the mass of the sun. Objects exceeding 1 million billion to 10 million billion solar masses would outweigh entire galaxies. No such object has been detected yet, and the largest known black hole weighs about 36 billion solar masses.
Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, co-authored the study. He noted that dark matter is typically viewed as a fine dust spread throughout space. If it were instead concentrated in a few gigantic objects, the universe would appear grainy on very large scales. Gerlach said the team could test for this graininess by looking at specific data.
The researchers analyzed the cosmic microwave background, which 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, the plasma dissipated, allowing light to travel freely. Gerlach described this light as a snapshot of the infant universe that covers the whole sky. It is still arriving at Earth today.
Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, co-authored the study. He explained that the temperature of this background varies from place to place by about one part in 100,000. This pattern reveals how matter was distributed in the early universe. The analysis showed that 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, said 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 cannot exist. It only indicates they are not a major component of the missing mass. The team noted that smaller black holes, about the size of asteroids, might still be possible. They are now examining whether those objects can also be ruled out as a major constituent.
The findings are scheduled for publication in the journal Physical Review Letters on Oct. 26.
