Cosmic Microwave Background Data Rules Out Giant Black Holes as Primary Dark Matter

Black holes massive enough to outweigh entire galaxies have been ruled out as a primary component of dark matter, according to a new study published in Physical Review Letters. The research team used data from the cosmic microwave background, the oldest light in the universe, to place a strict upper limit on the size of these hypothetical objects. The findings suggest that dark matter is not composed of a small number of gigantic, heavy objects, a theory that had remained a viable possibility for years.

Dark matter is an invisible substance that makes up approximately five-sixths of all matter in the cosmos. Scientists proposed it decades ago to explain why galaxies do not disintegrate despite their rapid rotation. While the exact nature of dark matter remains unknown, black holes have often been considered candidates because of their immense mass and difficulty to detect. The study specifically targeted objects with masses greater than 100 billion solar masses, a category the researchers termed stupendously large black holes.

Christopher Gerlach, a theoretical physicist at the Mainz Institute for Theoretical Physics in Germany, explained the logic behind the investigation. He noted that if dark matter existed as a few massive objects rather than a fine dust spread throughout space, the universe would appear grainy on large scales. "We can check that," Gerlach said. The team looked for this specific graininess in the temperature fluctuations of the cosmic microwave background.

The cosmic microwave background is light released 380,000 years after the Big Bang, when the universe cooled enough for atoms to form. Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, stated that the background's temperature varies from place to place by about one part in 100,000. This pattern provides a snapshot of how matter was distributed in the infant universe. The analysis revealed that this ancient light lacks the specific graininess that stupendously large black holes would have created.

Nicholas Leister, also of the Mainz Institute, concluded that the absence of these patterns means such black holes cannot be a significant part of dark matter. The result does not prove that these enormous black holes do not exist; it only indicates they are not a major constituent of the universe's missing mass. The largest black hole currently known has a mass of about 36 billion suns, well below the 100 billion solar mass threshold established by the new study.

The researchers noted that they are now turning their attention to a different candidate: relatively tiny black holes about the size of asteroids. These smaller objects might still account for a portion of dark matter, and the team is examining whether they can be ruled out as well. The findings are scheduled for publication on October 26 in the journal Physical Review Letters, marking the first time cosmic background data has been used to limit the mass of dark matter candidates to this specific degree.