Astrophysics officially classifies black holes into three categories, according to their mass:stellar black holes, three intermediate black holes and supermassive black holes. But theoretical cosmology offers additional hypotheses such as micro black holes or primordial black holes (PBH). The latter would have formed in the early Universe long before the birth of the first stars. Some theories suggest that PBHs could constitute part of dark matter. Recently, a team of researchers from the Kavli Institute proposed a scenario for the formation of PBHs involving the inflationary multiverse. This work has already led to a new series of observations aimed at determining whether, by this means, primordial black holes could indeed constitute dark matter.
Primordial black holes could represent some of the dark matter, be responsible for some of the observed gravitational wave signals, and be the source of the supermassive black holes found at the centers of 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 a possibility that the mysterious dark matter, which accounts for most of the matter in the Universe, is partially 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.
To learn more about primordial black holes, the research team peered into the early Universe for clues. The early Universe was so dense that any positive density fluctuation of more than 50% would create a black hole. However, the cosmological disturbances that seeded the galaxies are known to be much smaller. Nevertheless, a number of processes in the early Universe could have created the right conditions for the formation of black holes. The study was published in the (very serious) journal Physical Review Letters .
One exciting possibility is that primordial black holes could form from the "baby universes" created during inflation, a period of rapid expansion that would be responsible for forming the structures we observe today, such as galaxies. and clusters of galaxies. During inflation, baby universes can detach from our universe. A small baby universe would eventually collapse, but the large amount of energy released in the small volume would then cause a black hole to form.
An even more peculiar destiny awaits a larger baby universe. If larger than a certain critical size, Einstein's theory of general relativity allows the baby universe to exist in a state that looks different to inside and outside observers. An internal observer sees it as an expanding universe, while an external observer sees it as a black hole.
In both cases, both the large and the small baby universes are seen by outside observers as primordial black holes, which conceal the underlying structure of multiple universes behind their event horizons (the event horizon is a boundary below from which everything, even light, is trapped and cannot escape the black hole).
In their paper, the team described a new scenario for the formation of PBHs and showed that black holes in the scenario involving a multiverse can be found using the Hyper Suprime-Cam (HSC) of the 8.2 m Subaru Telescope, the equivalent of a gigantic digital camera located near the summit (at 4200 meters) of Mount Mauna Kea in Hawaii. The HSC team has recently pointed out major constraints on the existence of PBHs.
Why was HSC essential in this research? The HSC has a unique ability to image the Andromeda Galaxy every few minutes. If a black hole passes through the line of sight of one of the stars, the black hole's gravity bends the light rays and causes the star to appear brighter for a short time. The duration of star brightening tells astronomers the mass of the black hole. With HSC observations, one hundred million stars can be observed simultaneously, a wide window for primordial black holes that can cross one of the lines of sight.
Early HSC observations have already reported a very intriguing candidate event consistent with a "multiverse" PBH, with a black hole mass comparable to the mass of the Moon. Encouraged by this first sign and guided by their new theory, the team is conducting a new series of observations to extend the research and provide a definitive test of whether PBHs in the multiverse scenario can account for all dark matter.
