A new model aims to explain the absence of miniature black holes in the early universe

Researchers from the Research Center for Early Universe Studies (RESCEU) at the University of Tokyo and the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) have a well-understood and widely tested quantum field theory. Their study of very small celestial objects applies it to a new goal: the early universe. Their work led them to the conclusion that the number of miniature black holes should be much smaller than most models suggest, but observations that confirm this should be possible soon. The particular type of black hole in question could be a candidate for dark matter. Her work has been published in Physical Review Letters and Physical Review D. Studying the universe can be a tough job, so let’s make sure everyone is on the same page. The details are unclear, but the general consensus among physicists is that the universe began about 13.8 billion years ago, started explosively, expanded rapidly during a period called inflation, and eventually went from a homogeneous world to a world with detail and structure. . Most of the universe is empty, but it still seems to be much heavier than what we can explain with what we can see. We call this discrepancy dark matter, but no one knows what it is, but there is growing evidence to suggest that it could be black holes, and they are old. “We call them primordial black holes (PBHs), and many researchers think they are good candidates for dark matter, but to support this theory, there needs to be a lot of black holes,” says graduate student Jason Christiano. “They are interesting for another reason, too, because recent innovations in gravitational wave astronomy have found binary black hole mergers that can be explained if there are a lot of PBHs. But despite these strong reasons for the expected frequency, we don’t have a direct reason, and now we have a model that explains why that happens.”

Cristiano and his supervisor, Professor Junichi Yokoyama (currently Director of Kavli IPMU and RESCEU), have extensively studied different models of PBH formation but found that the leading candidates do not match actual observations of the cosmic microwave background (CMB), which is like a residual fingerprint of the Big Bang explosion that marked the beginning of the universe. And when something does not match solid observations, it may not be true or at best only paint part of the picture. In this case, the team used a new approach to revise the leading model of PBH formation by cosmic inflation, making it better match current observations and further verifiable with upcoming observations by Earth gravitational wave observatories around the world. “In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded its size by 25 orders of magnitude. Back then, the waves traveling through this tiny space could have been relatively large in amplitude but very short in wavelength. We found that these small but powerful waves could lead to the inexplicable amplification of the much longer waves we see in the CMB today,” Yokoyama said. “We think this is due to occasional cases of coherence between these early short waves, which can be explained using quantum field theory, the most robust theory we have to explain everyday phenomena such as photons and electrons. While individual short waves would be relatively anemic, a coherent group would have the power to reshape into waves much larger than itself. This is a rare case where a theory at one extreme scale appears to be able to explain something at the other end of the scale.”

If, as Cristiano and Yokoyama suspect, small-scale fluctuations in the early universe are indeed influencing some of the large-scale fluctuations seen in the CMB, this could change the standard explanation of the overall structure of the universe. But wavelength measurements at the CMB can also be used to effectively constrain the range of corresponding wavelengths in the early universe, which would inevitably constrain other phenomena that could be attributed to these shorter and more intense wavelengths. This is where PBHs come into play again. “It is widely believed that the collapse of short but intense wavelengths in the early universe is the formation of primordial black holes,” Cristiano says. “Our study suggests that the number of PBHs should be much smaller than would be required if PBHs were in fact strong candidates for dark matter and gravitational wave events.” At the time of writing, the world’s gravitational wave observatories, LIGO in the United States, Virgo in Italy, and KAGRA in Japan, are in the midst of observational missions aimed at observing the first small black holes, which are probably PBHs. In any case, the results should provide the team with solid evidence to further refine their theory.

source: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.221003