New model aims to explain the lack of miniature black holes in the early universe

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The study shows how large amplitude fluctuations generated on small scales can amplify large-scale fluctuations observed in the cosmic microwave background. Credit: ESA/Planck Collaboration 2024, edited by Jason Kristiano CC-BY-ND

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The study shows how large amplitude fluctuations generated on small scales can amplify large-scale fluctuations observed in the cosmic microwave background. Credit: ESA/Planck Collaboration 2024, edited by Jason Kristiano CC-BY-ND

Researchers from the Research Center for the Early Universe (RESCEU) and the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) of the University of Tokyo have applied the well-understood and highly verified quantum field theory, mostly applied to the study from the very small, to a new goal, the early universe.

Their research led to the conclusion that there should be far fewer miniature black holes than most models suggest, although observations confirming this should be possible soon. The specific type of black hole in question could be a competitor for dark matter. Their work has been published in Physical Assessment Letters And Physical examination D.

The study of the universe can be extremely challenging, so let’s make sure we’re all on the same page. Although the details are vague, the general consensus among physicists is that the universe is about 13.8 billion years old, started with a bang, expanded rapidly in a period called inflation, and moved somewhere along the line from homogeneous to detail and structure went.

Most of the universe is empty, yet it appears to be significantly more massive than can be explained by what we can see. We call this discrepancy dark matter, and no one knows what it might be, but evidence is mounting that it might be. black holes are, especially old ones.

“We call them primordial black holes (PBH), and many researchers believe they are a strong candidate for dark matter, but there should be enough of them to satisfy that theory,” says graduate student Jason Kristiano.

‘They are also interesting for other reasons, as since the recent innovation of gravitational wave astronomy there have been discoveries of binary black hole mergers, which can be explained if PBHs occur in large numbers. But despite these strong reasons for their expected abundance, we haven’t seen any directly, and now we have a model that should explain why this is the case.”

Kristiano and his supervisor, Professor Jun’ichi Yokoyama, currently director of Kavli IPMU and RESCEU, have extensively investigated the different models for PBH formation, but found that the main contenders do not fit actual observations of the cosmic microwave background (CMB). , which looks a bit like a leftover fingerprint from the Big Bang explosion that marked the beginning of the universe. And if something doesn’t match solid observations, it may either not be true, or it may only paint part of the picture at best.

In this case, the team used a novel approach to correct the leading model of PBH formation by cosmic inflation so that it better fits current observations and could be further verified with future observations from terrestrial 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 expanded rapidly by 25 orders of magnitude. At the time, waves traveling through this small space could have had relatively large amplitudes, but very short wavelengths. What we have discovered is that these small but strong waves can translate into an otherwise inexplicable amplification of much longer waves that we see in today’s CMB, Yokoyama said.

“We believe 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 describe everyday phenomena such as photons or electrons. While individual short waves would be relatively powerless, coherent groups would have the power to reshape waves much larger than themselves. This is a rare example where a theory of something at one extreme scale seems to explain something at the other end of the scale.

If, as Kristiano and Yokoyama suggest, early small-scale fluctuations in the universe influence some of the larger fluctuations we see in the CMB, this could change the standard explanation of gross structures in the universe. But since we can use measurements of wavelengths in the CMB to effectively constrain the size of corresponding wavelengths in the early universe, this necessarily constrains any other phenomena that might depend on these shorter, stronger wavelengths. And this is where the PBHs come back.

“It is widely believed that the collapse of short but strong wavelengths in the early universe caused the formation of primordial black holes,” says Kristiano. “Our study suggests that there should be far fewer PBHs than would be necessary if they are indeed a strong candidate for dark matter or gravitational wave events.”

At the time of writing this article, the world’s gravitational wave observatories, LIGO in the US, Virgo in Italy and KAGRA in Japan, are in the middle of an observation mission that aims to observe the first small black holes, probably PBHs. In any case, the results should provide the team with solid evidence to further refine their theory.

More information:
Jason Kristiano, Jun’ichi Yokoyama, Limiting the Formation of Primordial Black Holes to Inflation in a Single Field, Physical Assessment Letters (2024).

Jason Kristiano, Jun’ichi Yokoyama, Note on the bispectrum and one-loop corrections in single-field inflation with primordial black hole formation, Physical examination D (2024).

Magazine information:
Physical Assessment Letters

Physical assessment D

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