If the Big Bang created miniature black holes, where are they?

The hunt for missing, tiny black holes left over from the Big Bang may be about to heat up.

Just when the trail for such small black holes seemed to have gone cold, an international team of scientists has found clues in quantum physics that could reopen the case. One of the reasons why the hunt for these so-called primordial black holes is so urgent is that they have been suggested as possible candidates for dark matter.

Dark matter comprises 85% of the mass in the universe, but does not interact with light the way regular matter does. That is the matter consisting of atoms that make up stars, planets, moons and our bodies. However, dark matter does interact with gravity, and this influence may be as well to influence “ordinary matter” and light. Perfect for cosmic detective work.

If Big Bang-induced black holes did indeed exist, they would be absolutely tiny – some could even be as small as a dime – and therefore have a mass equal to that of asteroids or planets. Yet, like their larger counterparts, stellar-mass black holes, which can have masses of 10 to 100 times that of the Sun, and supermassive black holes, which can have masses of millions or even billions of times that of the Sun, are small black holes from the sun. The dawn of time would be bounded by a light-catching surface called an “event horizon.” The event horizon prevents black holes from emitting or reflecting light, making small primordial black holes a solid candidate for dark matter. They may be small enough to go unnoticed, but strong enough to impact the room.

Related: Small black holes left over from the Big Bang could be the prime suspects of dark matter

The team of scientists – from the Research Center for the Early Universe (RESCEU) and the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) at the University of Tokyo – applied a theoretical framework combining classical field theory , Einstein’s special theory of relativity and quantum mechanics to the early universe. The latter explains the behavior of particles such as electrons and quarks and gives rise to the so-called quantum field theory (QFT).

Applying QFT to the early cosmos led the team to believe that there are far fewer hypothetical primordial black holes in the universe than many models currently estimate. If this is the case, this could rule out primordial black holes, as dark matter already suspects.

“We call them primordial black holes, and many researchers believe they are a strong candidate for dark matter, but there should be enough of them to satisfy that theory,” said Jason Kristiano, a graduate student at the University of Tokyo in a statement. ‘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 large numbers of primordial black holes exist.

“But despite these strong reasons for their expected abundance, we haven’t seen any directly yet, and now we have a model that should explain why this is the case.”

Back to the Big Bang to hunt for original black holes

The most favored cosmological models suggest that the universe began about 13.8 billion years ago during an initial period of rapid inflation: the Big Bang.

After the first particles appeared in the universe during this initial expansion, space eventually became cool enough to allow electrons and protons to bond and form the first atoms. Then the element hydrogen was born. Furthermore, before that cooling occurred, light could not travel through the cosmos. This is because electrons endlessly scatter photons, which are light particles. So during these literal dark ages, the universe was essentially opaque.

A diagram of the expanding universe, showing when the first stars formed and when the Earth was formed.

But once free electrons could bond with protons and stop bouncing around everywhere, light could finally travel freely. After this event, called the “last scattering,” and during the subsequent period known as “the epoch of reionization,” the universe immediately became transparent to light. The first light that shone through the universe at that time can still be seen today as a largely uniform radiation field, a universal “fossil” called the “cosmic microwave background” or “CMB.”

Meanwhile, the hydrogen atoms that formed formed the first stars, the first galaxies and the first clusters of galaxies. Sure enough, some galaxies appeared to have more mass than their visible constituents could explain, with this excess attributed to none other than dark matter.

An oval shape with yellow and blue spots in it.

While stellar-mass black holes are formed by the collapse and death of massive stars, and supermassive black holes are formed by the successive mergers of smaller black holes, the primordial black holes predate the stars. So they must have a unique origin.

Some scientists think that conditions in the hot and dense early universe were such that smaller pieces of matter could collapse under their own gravity and produce these tiny black holes – with an event horizon no wider than a dime, or perhaps even smaller than a proton, depending on the circumstances. their mass.

The team behind this study has previously looked at models of primordial black holes in the early universe, but these models did not match observations from the CMB. To rectify this, the scientists applied corrections to the leading theory of primordial black hole formation. Corrections informed by QFT.

Four circles represent black holes of different sizes.

“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation quickly expanded that by 25 orders of magnitude,” Kavli IPMU and RESCEU director Jun’ichi Yokoyama said in the statement. “Back then, waves traveling through this small space could have relatively large amplitudes but very short wavelengths.”

The team found that these small but strong waves can be amplified to become much larger and longer waves that astronomers see in today’s CMB. The team believes that this enhancement is the result of the coherence between the early short waves, which can be explained using QFT.

“Although individual short waves would be relatively powerless, coherent groups would have the power to reshape waves much larger than themselves,” Yokoyama said. “This is a rare example where a theory of something at one extreme scale appears to explain something at the other end of the scale.”

A diagram of a wave with colors from the other CMB diagram. Black circles indicate where black holes can form in these fluctuations.

If the team’s theory that early, small-scale fluctuations in the universe can grow and influence large-scale fluctuations in the CMB is correct, it will influence how structures in the cosmos grew. Measuring fluctuations of the CMB could help constrain the magnitude of the original fluctuations in the early universe. This in turn places limits on phenomena that depend on shorter fluctuations, such as primordial black holes.

“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 primordial black holes than necessary if they are indeed a strong candidate for dark matter or gravitational wave events.”

Back to the Big Bang to hunt for original black holes

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