New data caps dark matter black holes at 100 billion solar masses

A team of physicists has determined that black holes exceeding 100 billion times the mass of the sun are too rare to make up a large share of dark matter. The study, led by researchers at the Mainz Institute for Theoretical Physics in Germany, relies on observations of the earliest light in the universe to place this limit on the mass of these hypothetical objects.

Dark matter is estimated to account for roughly five-sixths of all matter in the cosmos. It was first proposed to explain why galaxies do not disintegrate despite their rapid rotation. While its exact nature remains unknown, black holes are a frequent candidate because their immense gravity and lack of visible light make them difficult to detect. The researchers focused specifically on stupendously large black holes, a category that includes objects weighing between 1 million billion and 10 million billion solar masses.

No such object has been observed yet. The largest known black hole weighs approximately 36 billion solar masses. Christopher Gerlach, a theoretical physicist at the Mainz Institute, noted that dark matter is often visualized as a fine dust. However, if the mass were concentrated in a few gigantic objects, the universe would appear grainy on large scales. The team tested this possibility by examining the cosmic microwave background.

This background radiation is the light released 380,000 years after the Big Bang. Antonio Iovino, a co-author at New York University Abu Dhabi, explained that the temperature of this light varies by about one part in 100,000 across the sky. These variations provide a snapshot of how matter was distributed in the infant universe. The team looked for the specific graininess that would result if huge black holes were prevalent.

The analysis revealed no such graininess in the background light. Nicholas Leister, a theoretical physicist at the Mainz Institute, stated that this lack of variation means these black holes cannot be a significant part of dark matter. The findings do not rule out the existence of these massive objects, only their role as a primary component of the universe's hidden mass.

The researchers noted that smaller black holes, roughly the size of asteroids, remain a possible candidate. They are currently investigating whether those objects can also be ruled out. The findings are scheduled for publication in the journal Physical Review Letters on October 26.