Cosmic Microwave Background: Limits on Supermassive Dark Matter Holes

New research indicates that black holes with masses exceeding 100 billion suns cannot form a major portion of dark matter. The study, led by theoretical physicist Christopher Gerlach of the Mainz Institute for Theoretical Physics in Germany, relies on data from the cosmic microwave background. Dark matter is estimated to comprise approximately five-sixths of all matter in the cosmos. It was proposed decades ago to explain why galaxies rotate fast enough to fly apart but remain intact due to gravitational forces.

The team focused on objects they described as stupendously large black holes. These hypothetical bodies would weigh more than entire galaxies, ranging from one million to ten million billion solar masses. No such object has been detected to date. The largest known black hole weighs roughly 36 billion suns. Gerlach noted that dark matter is typically imagined as fine dust spread across space. If it were instead concentrated in a few gigantic objects, the universe would appear grainy on very large scales.

To test this hypothesis, the researchers analyzed the cosmic microwave background. This is the light released from the early universe after the first 380,000 years. During that period, the cosmos was filled with hot plasma that trapped light. As expansion cooled the universe, the plasma dissipated, allowing light to travel freely. Gerlach described this radiation as a snapshot of the infant universe covering the whole sky.

Co-author Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, stated that the background 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 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 the dark matter.

The authors clarified that the result does not rule out the existence of such enormous black holes. It only suggests they are not a major component of the missing mass. The team added that another candidate, relatively tiny black holes about the size of asteroids, might still be possible. They are now examining whether those smaller objects can also be ruled out as a major constituent of dark matter.

The findings are due to be published in the journal Physical Review Letters on October 26. The study provides a new constraint on the nature of the universe's invisible mass.