New Data Limits Dark Matter Black Holes to 100 Billion Solar Masses

For observers tracking the composition of the universe, the share of mass attributed to black holes has just shrunk. A new study determines that black holes with more than 100 billion times the mass of the sun cannot constitute a large portion of dark matter. This finding narrows the search for the invisible substance that physicists believe holds galaxies together through gravity.

Dark matter was proposed to explain why galaxies rotate without flying apart. It is estimated to make up about five-sixths of all matter in the cosmos. While its nature remains unknown, black holes have long been considered a possible candidate due to their mass and invisibility. The researchers specifically tested objects they described as stupendously large black holes. These hypothetical bodies would exceed 1 million billion solar masses, a weight that would cause them to outweigh entire galaxies. No such object has been detected yet, with the largest known black hole weighing approximately 36 billion solar masses.

To test if these massive objects exist in sufficient numbers, the team analyzed the cosmic microwave background. This is light released from the early universe, specifically from the period before the first 380,000 years after the Big Bang. Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, noted that if dark matter were concentrated in a few gigantic objects rather than spread out, the universe would appear grainy on large scales. The team checked for this specific pattern in the data.

Antonio Iovino, a co-author and theoretical physicist at New York University Abu Dhabi, pointed out that the temperature of this background light varies by about one part in 100,000 across the sky. This variation acts as a map of how matter was distributed in the infant universe. Nicholas Leister, also at the Mainz Institute for Theoretical Physics, stated that the analysis showed no such graininess. Consequently, the data indicates these extremely massive black holes cannot be a significant part of the dark matter.

The authors clarified that this result does not prove such black holes do not exist. It only confirms they are not a major component of the universe's mass. They noted that smaller black holes, roughly the size of asteroids, remain a viable candidate. The team is currently examining whether those smaller objects can also be ruled out as a primary constituent. The findings are scheduled for publication in the journal Physical Review Letters on October 26.