Such primordial black holes (PBHs) could account for all or part of dark matter, be responsible for some of the observed gravitational waves signals, and seed supermassive black holes found in the center of our Galaxy and other galaxies. They could also play a role in the synthesis of heavy elements when they collide with neutron stars and destroy them, releasing neutron-rich material.
In particular, there is an exciting possibility that the mysterious dark matter, which accounts for most of the matter in the universe, is composed of primordial black holes. The 2020 Nobel Prize in physics was awarded to a theorist, Roger Penrose, and two astronomers, Reinhard Genzel and Andrea Ghez, for their discoveries that confirmed the existence of black holes. Since black holes are known to exist in nature, they make a very appealing candidate for dark matter.
Primordial black holes
Primordial black holes (PBHs) have been a source of interest for nearly 50 years even though there is still no evidence for them. One reason for this interest is that only PBHs could be small enough for Hawking radiation to be important. This discovery has not yet been confirmed experimentally, and there remain major conceptual puzzles associated with the process.
Nevertheless, it is generally recognized as one of the key developments in twentieth-century physics because it beautifully unifies general relativity, quantum mechanics, and thermodynamics. The fact that Hawking reached this discovery only through contemplating the properties of PBHs illustrates that it can be useful to study something even if it does not exist. But, of course, the situation is much more interesting if PBHs do exist.
Dark matter from the multiverse
As with other CDM candidates, there is still no compelling evidence that PBHs provide the dark matter. However, there have been claims of evidence from dynamical and lensing effects. In particular, there was a flurry of excitement in 1997, when the MACHO microlensing results suggested that the dark matter might comprise compact objects of mass 0.5M⊙.
Alternative microlensing candidates could be excluded, and PBHs of this mass might naturally form at the quark–hadron phase transition at 10−5 s. Subsequently, however, it was shown that such objects could account for only 20% of the dark matter, and indeed, the entire mass range of 10−7 to 10M⊙ was later excluded from providing all of it. In recent decades, attention has focused on other mass ranges in which PBHs could have a significant density, and numerous constraints allow only three possibilities: the asteroid mass range (1016–1017 g), the sublunar mass range (1020–1026 g), and the intermediate mass range (10–103M⊙).
One exciting possibility is that primordial black holes could form from the “baby universes” created during inflation, a period of rapid expansion that is believed to be responsible for seeding the structures we observe today, such as galaxies and clusters of galaxies. During inflation, baby universes can branch off of our universe. A small baby universe would eventually collapse, but the large amount of energy released in the small volume causes a black hole to form.
Why was the HSC indispensable in this research? The HSC has a unique capability to image the entire Andromeda galaxy every few minutes. If a black hole passes through the line of sight to one of the stars, the black hole’s gravity bends the light rays and makes the star appear brighter than before for a short period of time. The duration of the star’s brightening tells the astronomers the mass of the black hole. With HSC observations, one can simultaneously observe one hundred million stars, casting a wide net for primordial black holes that may be crossing one of the lines of sight.
The first HSC observations have already reported a very intriguing candidate event consistent with a PBH from the “multiverse,” with a black hole mass comparable to the mass of the Moon. Encouraged by this first sign, and guided by the new theoretical understanding, the team is conducting a new round of observations to extend the search and to provide a definitive test of whether PBHs from the multiverse scenario can account for all dark matter.