Cosmic Background Data: Massive Black Holes Ruled Out as Primary Dark Matter

Researchers have determined that black holes with masses greater than 100 billion times that of the sun cannot constitute a large portion of dark matter. This finding narrows the search for the substance that makes up approximately five-sixths of all matter in the universe.

The study, led by theorists Christopher Gerlach of the Mainz Institute for Theoretical Physics in Germany and Antonio Iovino of New York University Abu Dhabi, focused on what the team terms "stupendously large black holes." These objects would have masses ranging from 1 million to 10 million billion solar masses, a weight sufficient to outweigh entire galaxies. While no such object has been detected, the known largest black hole has a mass of about 36 billion suns.

Gerlach noted that dark matter is typically conceptualized as a fine dust distributed evenly throughout space. He argued that if dark matter were instead concentrated in a few gigantic objects, the universe would appear "grainy on very large scales." To test this hypothesis, the team analyzed the cosmic microwave background, the light released from the early universe.

This light dates to the first 380,000 years after the Big Bang, when the cosmos was a hot plasma. As the universe expanded and cooled, the plasma dissipated, allowing light to travel freely. Iovino explained that the temperature of this background varies by about one part in 100,000 from place to place, a pattern that maps the distribution of matter in the infant universe.

The analysis revealed 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 results indicate such black holes "can't be a significant part of the dark matter."

The authors clarified that the finding does not rule out the existence of these massive black holes, only their status as a major component of dark matter. They are now investigating whether smaller black holes, roughly the size of asteroids, might also be excluded as a primary constituent of the universe's hidden mass.

The findings are scheduled for publication in the journal Physical Review Letters on Oct. 26.