There may be far fewer miniature black holes than theorized, and that’s a problem
The hunt for missing tiny black holes left behind by the Big Bang could be on the verge of a turning point, but leaving big doubts.
Just when the trail of these tiny 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 pressing is that they have been suggested as possible candidates for dark matter, but matter that is missing to explain the universe.
Dark matter comprises 85% of the universe's mass, but it does not interact with light like ordinary matter does. This is the matter made up of atoms that make up stars, planets, moons and our bodies. However, dark matter interacts with gravity and this influence can affect 'ordinary matter' and light. But no one can see it
If Big Bang-induced black holes really exist, they would be absolutely tiny – some could even be as small as a dime – and therefore have masses equal to those of asteroids or planets. However, like their larger counterparts, stellar-mass black holes, which can have masses 10 to 100 times that of the sun, and supermassive black holes, which can have masses millions or even billions of times that of the Sun, tiny black holes since the dawn of time are bounded by a light-trapping surface called the “ 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 could be small enough to go unnoticed, but strong enough to impact space.
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 to the early universe that combines classical field theory, Einstein's theory of special relativity, and quantum mechanics. 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 many fewer hypothetical primordial black holes in the universe than many models estimate. If this is the case, it could completely rule out primordial black holes as dark matter suspects.
“We call them primordial black holes, and many researchers believe they are a strong candidate for dark matter, but there would have to be many of them to satisfy this theory,” University of Tokyo graduate student Jason Kristiano said in a statement. They are interesting for other reasons too, since after the recent innovation of gravitational wave astronomy, binary black hole mergers have been discovered, which can be explained if primordial black holes exist in large numbers.”
“But despite these strong reasons for their predicted abundance, we haven't seen any directly, and now we have a model that should explain why this happens.”
Returning to the birth of the Universe.
After the first particles emerged into the universe during this initial expansion, space became cold enough for electrons and protons to bond and form the first atoms. It was then that the element hydrogen was born. Furthermore, before cooling occurred, light could not travel through the cosmos. This is because electrons infinitely scatter photons, which are particles of light. So, during these literally dark ages, the universe was essentially opaque.
The current theory of the history of the universe
However, once the free electrons were able to bind to the protons and stop bouncing all over the place, light could finally travel freely. After this event, called the “last dispersion,” and during the subsequent period known as the “reionization epoch,” the universe instantly became transparent to light. The first light that passed through the universe during this time can still be seen today as a mostly uniform radiation field, a universal “fossil” called the “cosmic microwave background” or “CMB.”
Meanwhile, the hydrogen atoms created went on to form the first stars, the first galaxies and the first galaxy clusters. And, sure enough, some galaxies appeared to have greater mass than their visible constituents can account for, with this excess attributed to none other than dark matter.
While stellar-mass black holes form from the collapse and death of massive stars, and supermassive black holes develop from successive mergers of smaller black holes, primordial black holes predate stars – therefore, they must have a unique origin .
Some scientists believe that conditions in the hot, dense early universe were such that tiny bits of matter could collapse under their own gravity to give rise to these tiny black holes – with event horizons no wider than a dime, or perhaps even smaller than a proton, depending on their mass.
The team behind this research previously examined models of primordial black holes in the early universe, but these models did not align with the CMB observations. To correct this problem, scientists have applied corrections to the main theory of the formation of primordial black holes.
“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded it by 25 orders of magnitude,” Kavli IPMU and RESCEU director Jun'ichi Yokoyama said in the statement. “At that time, the waves traveling through this tiny space could have relatively large amplitudes, but very short wavelengths.”
The team found that these tiny but strong waves can undergo amplification to become the much larger and longer waves that astronomers see in the current CMB. The team believes that this amplification is the result of coherence between the early short waves, which can be explained using QFT.
“While individual short waves would be relatively underpowered, coherent groups would have the power to reshape waves much larger than themselves,” Yokoyama said. “This is a rare case where a theory of something at one extreme scale appears to explain something at the opposite end of the scale.”
An image of the CMB taken by the Planck telescope shows tiny variations that can be revealing to cosmologists (Image credit: ESA and Planck Collaboration)
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, this will impact how structures grew in the cosmos. Measuring CMB fluctuations could help constrain the size of the original fluctuations in the early universe. This, in turn, places constraints on phenomena that rely on shorter fluctuations, such as primordial black holes.
“It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes,” Kristiano said. “Our study suggests that primordial black holes should be much fewer in number than would be necessary if they are indeed a strong candidate for dark matter or gravitational wave events.”
Primordial black holes are currently decidedly hypothetical . That's because the light-trapping nature of stellar-mass black holes makes it difficult to see even these much larger objects, so imagine how difficult it would be to spot a black hole with an event horizon the size of a dime.
The key to spotting primordial black holes may not lie in “traditional astronomy,” but rather in measuring tiny ripples in space, called gravitational waves. While current gravitational wave detectors are not sensitive enough to detect ripples in spacetime due to collisions between primordial black holes, future projects, such as the Laser Interferometer Space Antenna (LISA) , will bring gravitational wave detection to space. This could help confirm or reject the team's theory, bringing scientists closer to confirming whether primordial black holes could explain dark matter.
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The article There may be far fewer miniature black holes than theorized, and that's a problem comes from Economic Scenarios .
This is a machine translation of a post published on Scenari Economici at the URL https://scenarieconomici.it/i-buchi-neriin-miniatura-potrebbero-essere-molti-meno-di-quanto-teorizzato-e-questo-e-un-problema/ on Sun, 02 Jun 2024 13:00:57 +0000.