“Ancient black holes would give us access to physics we would never otherwise be able to do,” wrote Dan Hooper, head of the theoretical astrophysics group at Fermilab, in an email to The Daily Galaxy. If primordial black holes are real, they’d have potential to solve a whole host of the biggest problems in cosmology, not the least being the mystery of dark matter, considered to be the backbone to the structure of the universe.
The First Second after the Big Bang
“If these black holes were initially lighter than a million kilograms or so,” Hooper added, “they would have evaporated in the first second after the Big Bang. In the process of this evaporation, they could have created any number of exotic forms of matter and energy, including dark matter.”
“But it is also undeniable that we are profoundly puzzled, especially when it comes to the first fraction of a second that followed the Big Bang,” Hooper adds. “I have no doubt that these earliest moments hold incredible secrets, but our universe holds its secrets closely. It is up to us to coax those secrets from its grip, transforming them from mystery into discovery.”
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, reports the Kavli Institute for the Physics and Mathematics of the Universe.
Intriguing PBH Candidate Event
The first observations of the Hyper Suprime-Cam (HSC) of the 8.2m Subaru Telescope, a gigantic digital camera near the 4,200 meter summit of Mt. Mauna Kea in Hawaii, 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.
PBH’s could also play a role in the synthesis of heavy elements when they collide with neutron stars and destroy them, releasing neutron-rich material, reports the Kavli Institute. “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.”
Existence of Black holes Confirmed –Enter PHBs
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, contradicting Albert Einstein who believed that they “did not exist in the real world,” nor that a fathomless dark creation existed at the very real, violent center of our home galaxy.
In January 1965, ten years after Einstein’s death, Penrose proved that black holes really can form and described them in detail; at their heart, black holes hide a singularity in which all the known laws of nature cease. His groundbreaking theory is still regarded as the most important contribution to the general theory of relativity since Einstein.
Their existence proven, says the Kavli Institute, PHBs 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.
PHBs Form Baby Universes
One exciting possibility, suggests the Kavli Institute, 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 (image at the top of the page). 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.
Small baby universes are seen by us as primordial black holes, which conceal the underlying structure of multiple universes behind their “event horizons
“There is good reason to think that everything we can see in our sky represents only the smallest tip of the cosmic iceberg, writes Hooper, who was not involved in the Kavli study, in The Edge of Time. “During inflation, countless pieces of space were stretched into newly formed universes, populating a greater multiverse of disconnected worlds. And despite the fact that we have no way to observe this panoply of universes, there is every reason to suspect that it does, in fact, exist.”
A Diversity of Physical Laws
It is also possible, Hooper, adds, “different regions of the multiverse could be dictated by a diversity of physical laws. New forces and new forms of matter may rule many of these realms of existence. In some, there might be more—or fewer—than three dimensions of space. Many worlds may be utterly unlike anything we can imagine.”
The Event Horizon Boundary
An even more peculiar fate awaits a bigger baby universe, suggests the Kavli team. 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 versus 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 HSC. 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)
The Indispensable Hyper Suprime-Cam
The Hyper Suprime-Cam was indispensable, says the Kavli Institute, through its 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.
Source: Alexander Kusenko et al, Exploring Primordial Black Holes from the Multiverse with Optical Telescopes, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.181304
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. via Dan Hooper, At the Edge of Time: Exploring the Mysteries of Our Universe’s First Seconds (Kindle Edition) and The Kavli Institute
Image credit: Leiden Institute of Physics