Why some black holes are missing explained by quantum theory

Physicists have applied quantum field theory to the early universe, helping resolve a mystery around so-called “missing miniature black holes.”

This is a complex physics story but stick with me as the results are interesting.

Quantum field theory (QFT) is one of the crowning achievements of science in the 20th century. It combines classical field theory, special relativity and quantum mechanics to describe the relationship between subatomic particles and the fundamental forces of nature.

As a framework, QFT is generally used to explain tiny phenomena.

A tiny theory in a big universe

But new studies published in Physical Review Letters and Physical Review D show how QFT can be used to create a model of the early universe which helps explain missing black holes which have been bugging physicists for years.

The universe began about 13.8 billion years ago with the Big Bang. At the beginning, physicists theorise it was tiny – smaller than an atom. It then expanded rapidly in a period known as “inflation.”

Somewhere along the line, the universe developed detail and structure. This led to the coalescence of the matter which made up the first atoms, gases and eventually stars and galaxies.

But that’s not all in our universe – there’s dark matter too.

Are ancient black holes dark matter?

This invisible substance should, according to observations of gravitational effects around galaxies and other massive structures in the cosmos, be about 5 times as plentiful as visible matter.

One theory to explain dark matter is that the elusive substance is made up of old black holes.

“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,” says first author on both papers Jason Kristiano, a graduate student at the University of Tokyo.

Kristiano adds that PBHs would help explain the recent discovery of merging binary black holes.

“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.”

The oldest observation we have of the universe is the cosmic microwave background (CMB) which dates back to when the universe was only about 300,000 years old. CMB is a kind of left over fingerprint of the Big Bang.

Studies of the details in the CMB do not reveal any PBH candidates which align with models.

Making waves

Kristiano’s supervisor, Professor Jun’ichi Yokoyama, says that during inflation, waves traveling through the early universe had large amplitudes, but short wavelengths.

Wave circles arrows
The study finds how large amplitude fluctuations generated on small scales can amplify large-scale fluctuations observed in the cosmic microwave background. Credit: ©2024 ESA/Planck Collaboration, modified by Jason Kristiano (CC-BY-ND).

“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,” Yokoyama explains.

QFT can explain these observations.

If the team is correct that tiny fluctuations in the early universe affected the larger-scale fluctuations in the CMB, then it could rewrite how structure developed in the universe. Because there are certain wavelengths in the CMB that we can observe, it means there are only certain corresponding wavelengths from the early universe that they might relate to, helping explain why we’re seeing fewer PMHs than theory says we should.

“It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes,” Kristiano adds. “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.”

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