CMB Data Caps Dark Matter Black Holes at 100 Billion Solar Masses

Physicists at the Mainz Institute for Theoretical Physics and New York University Abu Dhabi have determined that black holes exceeding 100 billion solar masses cannot make up a large portion of dark matter. The study uses measurements of the cosmic microwave background to place a hard upper limit on the mass of these invisible objects. The findings challenge the idea that the universe’s missing mass is concentrated in a few gigantic, galaxy-scale holes.

Dark matter was hypothesized decades ago to explain the rapid rotation of galaxies. It is estimated to comprise roughly five-sixths of all matter in the cosmos, yet its physical nature remains undefined. Black holes have been considered a plausible candidate because of their immense mass and low visibility. However, no object heavier than 36 billion solar masses has been detected in surveys of galaxies to date.

The research team analyzed light released 380,000 years after the Big Bang. This radiation, known as the cosmic microwave background, serves as a snapshot of the early universe. As the cosmos expanded and cooled, atoms formed and allowed this light to travel freely. The pattern of this light preserves information about how matter was distributed in the infant universe.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics, explained that if dark matter were clumped into a few massive objects, the universe would appear grainy on large scales. "The universe becomes grainy on very large scales. And we can check that," Gerlach said. The team looked for this specific texture in the background radiation.

Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, noted that the background temperature varies by about one part in 100,000 from place to place. These minute fluctuations map the density of matter in the early universe. The analysis revealed that the expected graininess from stupendously large black holes is absent from the data.

Nicholas Leister, also of the Mainz Institute for Theoretical Physics, stated, "So these black holes can't be a significant part of the dark matter." The authors clarified that the result does not rule out the existence of such massive holes, only their role as a primary component of the universe's invisible mass. They noted that smaller black holes, roughly the size of asteroids, remain a possible candidate.

The team is currently examining whether these asteroid-sized black holes can be ruled out as a major constituent of dark matter. The full findings are scheduled for publication in the journal Physical Review Letters on October 26.