New research shows that stupendously large primordial black holes cannot be the source of dark matter. Nicholas Leister and his team found that these massive objects would create uneven patterns in the cosmic microwave background. Since these patterns are not there, the theory that they form dark matter got ruled out.
Primordial black holes are hypothetical objects that could have formed directly from dense clumps of matter in the very early universe. If they exist, their dramatic origins would set them apart from the black holes we see today, which form when massive stars collapse. Some theories suggest that "stupendously large" primordial black holes could account for a significant share of the universe's elusive dark matter.
Yet new calculations by Nicholas Leister at the Mainz Institute for Theoretical Physics and colleagues show that even if these immense objects do exist, they can't make up any meaningful fraction of dark matter.
Their research has been published in Physical Review Letters.
Stupendously large
Dark matter is thought to make up around 85% of the matter in the universe—but despite decades of intensive searching, physicists still don't know what it is really made from. One proposed candidate is a population of primordial black holes, each weighing at least 100 billion times as much as the sun. Named "stupendously large" black holes, these bodies would be even more massive than the largest supermassive black holes found at the centers of galaxies including the Milky Way.
In their study, Leister's team tested this possibility from a new angle. If primordial black holes were scattered randomly through space, some regions would contain more of them than others, purely by chance.
At first, each surplus of black holes would be balanced by a slight shortage of radiation. But as the universe expanded and its radiation thinned out, this balance would break down. The result would be patchy variations in the amount of dark matter relative to radiation.
Searching the Big Bang's afterglow
After calculating the resulting strengths of these variations, the team compared their predictions with limits set by measurements of the cosmic microwave background (CMB). This subtle microwave signal is the faint afterglow of the Big Bang and can tell us how galaxies are spread across the largest scales of the universe. The fewer and heavier the black holes, the more uneven their random scattering becomes.
Through their analysis, Leister's team found that a substantial population of stupendously large primordial black holes would leave the balance between dark matter and radiation far too uneven.
This patchiness would imprint distinctive patterns on the CMB, but these patterns clearly aren't present in the signal. The result also challenges suggestions that primordial black holes helped seed the surprisingly massive early galaxies spotted by the James Webb Space Telescope.
Tightening dark matter theories
The study doesn't rule out primordial black holes in this mass range altogether. Since the team assumed that all the black holes share the same mass, populations with a wide range of masses still need to be explored before their calculations can be fully conclusive. All the same, their results place tight new limits on how much these objects could contribute to dark matter.
With upcoming CMB experiments, including LiteBIRD and the Simons Observatory, the researchers predict that these limits would be tightened even further. They also hope to extend their analysis to even heavier black holes by searching for the gravitational waves that would have been released as they formed. Together, these efforts could ultimately help to narrow the search for the true identity of dark matter.