All of the dark matter in the universe may be primordial black holes – formed from the collapse of children’s universes shortly after the Big Bang

Small universes branching out of our universe shortly after the Big Bang appear to us as black holes. Credit: Kavli IPMU

The Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) is home to several interdisciplinary projects that take advantage of the synergy of the wide range of expertise available at the institute. One of these projects is the study of black holes that could have formed in the early universe, before the birth of stars and galaxies.

These primordial black holes (PBHs) can represent all or part of dark matter, and are responsible for some of the observed elements. Gravitational waves The signals, and the seeds of supermassive black holes at the center of our galaxy and other galaxies. It can also play a role in the synthesis of heavy elements when they collide with and destroy neutron stars, thus releasing a substance rich in neutrons.

In particular, there is an exciting possibility that the mysterious dark matter, which represents most of the matter in the universe, was made of primordial black holes. The 2020 Nobel Prize in Physics was awarded to theorist Roger Penrose and two astronomers, Reinhard Gensel and Andrea Gies, for their discoveries that confirmed the existence of black holes. Because black holes exist in nature, they offer a very attractive dark matter filter.

Recent advances in fundamental theory, astrophysics, and astronomical observations in the search for surface holes 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 Takestov.

To learn more about primordial black holes, the research team looked into the early universe for clues. The early universe was so dense that any positive fluctuation in density of more than 50 percent would create a Black hole. However, cosmic disturbances in classed galaxies are known to be much smaller. However, a number of processes in the early universe could have created the conditions for black holes to form.

The Hyper Suprime-Cam (HSC) is a giant digital camera on the Subaru Telescope. Credit: HSC / NAOJ Project

One interesting possibility is that primordial black holes could form from „small universes“ that arose during inflation, a period of rapid expansion believed to be responsible for implanting the structures we observe today, such as galaxies and galaxy clusters. During inflation, children’s universes can branch out from our universe. The tiny universe (or „daughter“) will eventually collapse, but the large amount of energy released at the tiny size causes a black hole to form.

An even more strange fate awaits being an older child. If it is greater than some critical size, Einstein’s theory of gravity allows the infant universe to exist in a state that appears different to the observer from the inside and outside. An inner observer sees it as an expanding universe, while an external observer (like us) sees it as a black hole. Either way, we view universes large and small as primitive black holes, hiding the basic structure of multiple universes behind „horizons of events.“ The event horizon is the boundary below which everything, even light, is trapped and cannot escape from a black hole.

The star in Andromeda becomes temporarily brighter if a primordial black hole passes in front of the star, focusing its light according to the theory of gravity. Credit: Kavli IPMU / HSC Collaboration

In their paper, the team described a new scenario for PBH formation and showed that black holes from the „multiverse“ scenario could be found using the Hyper Suprime-Cam (HSC) from the 8.2-meter Subaru Telescope, a gigantic digital camera – the management in which Kavli played IPMU An important role – near the summit of a mountain of 4200 meters. Mauna Kea in Hawaii. Their work is an exciting extension of HSC’s research on PBH that Masahiro Takada, principal investigator at Kavli IPMU, is pursuing, and his team. The HSC team recently reported a major limitation in the presence of PBHs in Niikura, Takada et. The. Natural Astronomy 3, 524-534 (2019)

Why was HSC indispensable in this research? HSC has the unique ability to photograph the entire Andromeda galaxy every few minutes. If a black hole passes through the line of sight to a star, 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 brightness tells astronomers about the mass of the black hole. Using HSC observations, one can simultaneously observe a hundred million stars, casting a vast network of primordial black holes that might cross a line of sight.

The first HSC observations have already reported a very interesting filter event corresponding to the PBH of the „multiverse“, with a black hole similar to the mass of the moon. Encouraged by this first sign, and guided by a new theoretical understanding, the team is conducting a new round of observations to broaden the research and provide a final test of whether black holes in the multiverse scenario can explain all of dark matter.

Reference: “Exploring Primordial Black Holes from the Multiverse Using Optical Telescopes” by Alexander Kusenko, Mizao Sasaki, Sunao Sugiyama, Masahiro Takada, Volodymyr Takeistov and Eduardo Vitaliano, October 30, 2020 Physical review letters.
DOI: 10.1103 / PhysRevLett.125.181304