The well-known and extensively verified quantum field theory, which is typically used to study the very small, has been extended to a new target: the early universe. This work was done by researchers at the University of Tokyo’s Research Centre for the Early Universe (RESCEU) and Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI). After much investigation, they concluded that there should be a lot fewer tiny black holes than most models predict, however, it might soon be feasible to make measurements to support this. This particular type of black hole may be a candidate for dark matter.
It might be intimidating to study the cosmos, so let’s make sure we’re all on the same page. Physicists generally agree that the universe is around 13.8 billion years old, started with a bang, expanded quickly during a period known as inflation, and at some point along the way changed from being homogeneous to having structure and detail, even if specifics are still unclear. The universe looks to be far heavier than what can be explained by what we can see, even if the majority of it is empty. This disparity is known as dark matter, and while its nature is unknown, there is growing evidence that it may contain black holes, particularly ancient ones.
We call them primordial black holes (PBH), and many researchers feel they are a strong candidate for dark matter, but there would need to be plenty of them to satisfy that theory,
They are interesting for other reasons too, as since the recent innovation of gravitational wave astronomy, there have been discoveries of binary black hole mergers, which can be explained if PBHs exist in large numbers. But despite these strong reasons for their expected abundance, we have not seen any directly, and now we have a model which should explain why this is the case.
Jason Kristiano
The leading candidates for PBH formation have not matched actual observations of the cosmic microwave background (CMB), which is essentially a remnant of the Big Bang explosion that signifies the beginning of the universe, according to Kristiano and his supervisor, Professor Jun’ichi Yokoyama, who is currently the director of Kavli IPMU and RESCEU. Furthermore, if anything contradicts reliable observations, it can either not be genuine or, at most, just provide a partial picture. In this instance, the scientists corrected the dominant model of PBH creation from cosmic inflation using a unique technique, which improved its alignment with current results and may allow for future verification using observations from terrestrial gravitational wave detectors.
At the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded that by 25 orders of magnitude. At that time, waves traveling through this tiny space could have had relatively large amplitudes but very short wavelengths. What we have found is that these tiny but strong waves can translate to otherwise inexplicable amplification of much longer waves we see in the present CMB,
We believe this is due to occasional instances 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 instance of where a theory of something at one extreme scale seems to explain something at the opposite end of the scale.
We believe this is due to occasional instances 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 instance of where a theory of something at one extreme scale seems to explain something at the opposite end of the scale.
Jun’ichi Yokoyama
The usual explanation of coarse patterns in the cosmos may change if, as Kristiano and Yokoyama argue, some of the larger-scale fluctuations we witness in the CMB are influenced by early small-scale fluctuations in the universe. Furthermore, any additional phenomena that could depend on these shorter, stronger wavelengths are inherently constrained, as we can effectively restrict the extent of equivalent wavelengths in the early cosmos by using measurements of wavelengths in the CMB. And here’s where the PBHs reappear.
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It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes,
Our study suggests there should be far fewer PBHs than would be needed if they are indeed a strong candidate for dark matter or gravitational wave events.
Jason Kristiano
The world’s gravitational wave observatories, Virgo in Italy, KAGRA in Japan, and LIGO in the United States are now conducting an observation mission to see the first tiny black holes, most likely PBHs. In any event, the outcomes need to provide the group with substantial proof to aid in the continuing development of their hypothesis.
Source: University of Tokyo Press Releases
Journal Reference:
- Kristiano, Jason, and Jun’ichi Yokoyama. “Constraining Primordial Black Hole Formation from Single-Field Inflation.” Physical Review Letters 132.22 (2024): 221003. http://doi.org/10.1103/PhysRevLett.132.221003.
- Kristiano, Jason, and Jun’ichi Yokoyama. “Note on the bispectrum and one-loop corrections in single-field inflation with primordial black hole formation.” Physical Review D 109.10 (2024): 103541. http://doi.org/10.1103/PhysRevD.109.103541.
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