All dark matter in the universe could be primary black holes – formed by the collapse of baby universes shortly after the Big Bang

The Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) is home to many interdisciplinary projects that benefit from the synergy of a wide range of expertise available at the institute. One such project is the study of black holes that could have formed in the early Universe before stars and galaxies were born.

Such primordial black holes (PBHs) may be responsible for some or all of the dark matter, being responsible for some of the observed gravitational waves signals and seed supermassive black holes found in the center of our Milky Way and other galaxies. They can also play a role in the synthesis of heavy elements when they collide with and destroy neutron stars, releasing neutron-rich material.

In particular, there is an exciting possibility that the mysterious dark matter, which is responsible for most of the matter in the universe, is primal 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 confirming the existence of black holes. Because black holes are known to occur in nature, they are a very attractive candidate for dark matter.

Recent advances in fundamental theory, astrophysics, and astronomical observations in the search for PBHs have been made by an international team of particle physicists, cosmologists and astronomers, including Kavli IPMU members Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada, and Volodymyr Takhistov.

To learn more about primal black holes, the research team searched for clues in the early universe. The early universe was so dense that any positive variation in density of more than 50 percent was one black hole. However, cosmological perturbations that seeded galaxies have 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.

An exciting possibility is that primordial black holes may emerge from the “baby universes” created during inflation, a period of rapid expansion believed to be responsible for seeding the structures we observe today, such as galaxies. and galaxy clusters. During the inflation, baby universes can branch off from our universe. A small baby (or “daughter”) universe would eventually collapse, but the large amount of energy released into the small volume creates a black hole.

An even stranger fate awaits a larger baby universe. If it is larger than some critical dimension, Einstein’s theory of gravity allows the baby universe to exist in a state that looks different from an observer inside and out. An internal observer sees it as an expanding universe, while an external observer (like us) sees it as a black hole. In either case, the large and small baby universes are seen by us as primal black holes, hiding the underlying structure of multiple universes behind their “event horizon.” The event horizon is a boundary under which everything, even light, is trapped and cannot escape the black hole.

In their paper, the team described a new scenario for PBH formation and showed that the black holes from the ‘multiverse’ scenario can be found using the Hyper Suprime-Cam (HSC) of the 8.2m Subaru telescope , a giant digital camera – the management of which Kavli IPMU has played a critical role – near the 4,200-foot summit of Mt. Mauna Kea in Hawaii. Their work is an exciting extension of PBH’s HSC quest pursued by Masahiro Takada, a principal investigator at Kavli IPMU, and his team. The HSC team recently reported significant limitations to the existence of PBHs in Niikura, Takada et. al. Nature Astronomy 3, 524-534 (2019)

Why was the HSC indispensable in this study? The HSC has the unique ability to image the entire Andromeda galaxy every few minutes. When a black hole passes through the line of sight to one of the stars, the black hole’s gravity deflects the light rays and makes the star appear brighter than before for a short time. The duration of the star’s lighting up tells astronomers the mass of the black hole. With HSC observations, one can observe a hundred million stars simultaneously and cast a wide net for primal black holes that may intersect one of the sight lines.

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 the new theoretical understanding, the team is conducting another round of observation to expand the search and provide a definitive test of whether PBHs from the multiverse scenario can explain all dark matter.

Reference: “Exploration of Primordial Black Holes from the Multiverse with Optical Telescopes” by Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada, Volodymyr Takhistov and Edoardo Vitagliano, October 30, 2020, Physical Review Letters.
DOI: 10.1103 / PhysRevLett.125.181304