The Kavli Institute for the Physics and Mathematics of the Universe (Kavli
IPMU) is home to many interdisciplinary projects which 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) 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.
The recent progress in fundamental theory, astrophysics, and astronomical
observations in search of PBHs has 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 primordial black holes, the research team looked at the
early universe for clues. The early universe was so dense that any positive
density fluctuation of more than 50 percent would create a black hole.
However, cosmological perturbations that seeded galaxies are known to be
much smaller. Nevertheless, a number of processes in the early universe
could have created the right conditions for the black holes to form.
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 (or "daughter") universe would
eventually collapse, but the large amount of energy released in the small
volume causes a black hole to form.
An even more peculiar fate awaits a bigger baby universe. If it is bigger
than some critical size, Einstein's theory of gravity allows the baby
universe to exist in a state that appears different to an observer on the
inside and the outside. An internal observer sees it as an expanding
universe, while an outside observer (such as us) sees it as a black hole. In
either case, the big and the small baby universes are seen by us as
primordial black holes, which conceal the underlying structure of multiple
universes behind their "event horizons." The event horizon is a boundary
below which everything, even light, is trapped and cannot escape the black
hole.
In their paper, the team described a novel 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 gigantic
digital camera -- the management of which Kavli IPMU has played a crucial
role -- near the 4,200 meter summit of Mt. Mauna Kea in Hawaii. Their work
is an exciting extension of the HSC search of PBH that Masahiro Takada, a
Principal Investigator at the Kavli IPMU, and his team are pursuing. The HSC
team has recently reported leading constraints on the existence of PBHs in
Niikura, Takada et. al. (Nature Astronomy 3, 524-534 (2019))
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.
Reference:
Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada, Volodymyr
Takhistov, Edoardo Vitagliano. Exploring Primordial Black Holes from the
Multiverse with Optical Telescopes. Physical Review Letters, 2020; 125 (18)
DOI:
10.1103/PhysRevLett.125.181304