Dark matter is an invisible substance estimated to make up about 85% of all matter in the Universe. Physicists still do not know what exactly forms it, although they have been searching for decades. One hypothesis links it to primordial black holes - objects that could have arisen not from the collapse of stars but directly from dense clumps of matter in the first moments after the Big Bang.
Of particular interest are the so-called "stupendously large" primordial black holes - each with a mass of at least 100 billion solar masses. That is larger than the biggest supermassive black holes at the centers of galaxies, including the Milky Way. If such objects existed in sufficient numbers, they could explain a significant share of dark matter.
However, new calculations by Nicholas Leister of the Institute for Theoretical Physics in Mainz and his colleagues show that even if these giants are real, they cannot account for a noticeable part of dark matter. The work is published in the journal Physical Review Letters.
The researchers tested the hypothesis from a new angle. If primordial black holes are scattered randomly through the cosmos, then in some regions there would by pure chance be more of them and in others fewer. At first the excess of black holes would be compensated by a shortage of radiation. But as the Universe expanded, the radiation thinned out and the balance was disrupted. As a result, the density of dark matter relative to radiation would become nonuniform - "patchy."
Having calculated the strength of such fluctuations, the team compared the prediction with the constraints provided by measurements of the cosmic microwave background - the faint microwave background left over from the Big Bang. This signal makes it possible to judge the distribution of galaxies on the scale of the entire Universe. The smaller and heavier the black holes, the more uneven their random distribution.
The analysis showed that a numerous population of stupendously large primordial black holes would make the ratio of dark matter to radiation too nonuniform. Such patchiness would have left characteristic traces in the cosmic microwave background, but there are none in the real signal. This also casts doubt on the idea that primordial black holes helped give rise to the unexpectedly massive early galaxies spotted by the James Webb telescope.
The authors do not completely rule out primordial black holes of that mass. The calculations assumed that all of them weigh the same, so variants with a wide spread of masses remain to be studied. Nevertheless, strict new constraints have been obtained on what share of dark matter such objects can make up.
Future experiments to study the cosmic microwave background, including LiteBIRD and the Simons Observatory, are forecast by the researchers to make these constraints even stricter. The scientists also hope to extend the analysis to heavier black holes by searching for gravitational waves that should have arisen during their formation. Together these efforts will help narrow the search for the true nature of dark matter.