Ancient Light Excludes Galaxy-Sized Black Holes From Dark Matter Budget

Physicists have established a hard upper limit of 100 billion solar masses for black holes that can constitute a major portion of dark matter. This finding eliminates the possibility that supermassive objects weighing more than entire galaxies make up a significant share of the universe's invisible mass. The study relies on data from the cosmic microwave background, the oldest light in the cosmos.

Dark matter is estimated to account for approximately five-sixths of all matter in the universe. It was proposed to explain why galaxies maintain their structure despite high rotation speeds. Black holes have long been considered a candidate for this unseen mass due to their high density and difficulty of detection. However, no black hole exceeding 36 billion solar masses has been detected to date, despite extensive surveys.

The research team analyzed the temperature fluctuations in the cosmic microwave background. Co-author Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, noted that these variations amount to roughly one part in 100,000. This pattern provides a snapshot of matter distribution when the universe was 380,000 years old. If dark matter were concentrated in objects larger than 100 billion solar masses, the background would exhibit a distinct grainy texture on large scales.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, explained that such massive objects would make the universe grainy on very large scales. The team checked for this specific signature and found it absent. Nicholas Leister, also of the Mainz Institute, stated that the lack of this graininess indicates these stupendously large black holes cannot be a significant component of dark matter.

The authors clarified that the result does not rule out the existence of such massive black holes, only their dominance in the dark matter inventory. They noted that smaller black holes, roughly the size of asteroids, remain a viable candidate. The team is currently analyzing whether these smaller objects can also be excluded as a major constituent of the universe's mass.

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