Ancient Light Limits Dark Matter Black Holes to 100 Billion Suns

Black holes weighing more than 100 billion times the mass of the sun cannot make up a large share of the universe's dark matter.

Dark matter was proposed decades ago to explain why galaxies do not fly apart despite their rapid rotation. It is estimated to account for roughly five-sixths of all matter in the cosmos. Black holes are a candidate because they are massive and difficult to detect. However, researchers have now tested whether these objects could be the primary source of this missing mass.

The team examined what they term "stupendously large black holes." These objects would exceed 100 billion solar masses. If they were heavy enough, some would outweigh entire galaxies. No such object has been detected yet. The largest known black hole is estimated at 36 billion solar masses. The researchers wanted to determine if these unseen giants could still form a substantial fraction of dark matter.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, explained that dark matter is often imagined as a fine dust spread everywhere. He noted that if the mass were concentrated in a few gigantic objects, the universe would appear grainy on large scales. This graininess can be measured.

The team analyzed the cosmic microwave background, the light released from the early universe. For the first 380,000 years after the Big Bang, the cosmos was filled with hot plasma that trapped light. As the universe expanded and cooled, the plasma dissolved, allowing light to travel freely. Antonio Iovino, a co-author at New York University Abu Dhabi, stated that the background's temperature varies by about one part in 100,000. This pattern reveals how matter was distributed at that time.

The analysis showed that the background lacks the graininess that stupendously large black holes would have produced. Nicholas Leister, a theoretical physicist at the Mainz Institute for Theoretical Physics, said these black holes cannot be a significant part of the dark matter. The authors stressed that the result does not mean such enormous black holes cannot exist. It only means they are not a major component of the dark matter.

The findings are due to be published on Oct. 26 in the journal Physical Review Letters. The team is now examining whether smaller black holes, about the size of asteroids, can also be ruled out as a major constituent of dark matter.