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The hunt for missing mini black holes left behind by the Big Bang may be about to intensify.
Just as the trajectory of such diminutive black holes appears to have gone cold, an international team of scientists has found evidence in quantum physics that could reopen the case. One reason the search for so-called primordial black holes is urgent is that they have been proposed as potential candidates for dark matter.
Dark matter makes up 85% of the universe’s mass, but it does not interact with light the way everyday matter does. This is the stuff of atoms that makes up stars, planets, moons, and our bodies. However, dark matter interacts with gravity, and this effect can occur impact “Ordinary matter” and light. Perfect for cosmic detective work.
If black holes from the Big Bang did exist, they would be very small – some 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 be 10 to 100 times the mass of the Sun, supermassive black holes, which can be millions or even billions of times the mass of the Sun, and mini-black holes of the Sun . The dawn of time will be surrounded 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 strong candidates for dark matter. They may be small enough that no one will notice, but powerful enough to affect space.
Related: Small black holes left behind by the Big Bang may be prime suspects in 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 that combines classical field theory and Einstein’s own theory. 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 nascent universe led the team to believe that there are far fewer hypothetical primordial black holes in the universe than many models currently estimate. If this were the case, it would rule out the existence of primordial black holes as dark matter altogether.
“We call them primordial black holes, and many researchers feel they are strong candidates for dark matter, but there would have to be a lot of them to satisfy this theory,” Jason Christiano, a graduate student at the University of Tokyo, said. He said in a statement. “It’s interesting for other reasons too. Since the recent innovation in gravitational wave astronomy, there have been discoveries of binary black hole mergers, which could be explained if primordial black holes exist in large numbers.
“But despite these powerful reasons for their predicted abundance, we haven’t seen any of them directly, and now we have a model that should explain why this happens.”
Go back to the Big Bang to search for primordial black holes
The most favored models in cosmology 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 cold enough to allow electrons and protons to bond and form the first atoms. That’s when the element hydrogen was born. In addition, before this cooling occurred, light was not able to travel through the universe. This is because electrons endlessly scatter photons, which are particles of light. Thus, during these Dark Ages, the universe was essentially dark.
However, once the free electrons could bind to the protons and stop bouncing around, light was finally able to travel freely. After this event, called the “last scattering,” and during the following period known as the “era of reionization,” the universe immediately became transparent to light. The first light that shone across the universe at this time can still be seen today as a mostly uniform radiation field, a global “fossil” called the cosmic microwave background, or CMB.
Meanwhile, the hydrogen atoms that were created went on to form the first stars, the first galaxies, and the first galaxy clusters. Certainly, some galaxies appear to have more mass than their visible components can account for, and this excess is attributed solely to dark matter.
While stellar-mass black holes form from the collapse and death of massive stars, and supermassive black holes grow from successive mergers of smaller black holes, primordial black holes predate stars, so they must have a unique origin.
Some scientists believe that conditions in the hot, dense early universe were such that smaller patches of matter could collapse under their own gravity to give birth to these tiny black holes, with an event horizon no wider than a dime, or perhaps smaller than a proton, depending. Block them.
The team behind this research had previously looked at models of primordial black holes in the early universe, but these models failed to fit with CMB observations. To correct this, scientists applied corrections to the leading theory of primordial black hole formation. Corrections reported by QFT.
“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded by 25 orders of magnitude,” Kavli IPMU and RESCEU Director Jun’ichi Yokoyama said in the statement. “At that time, waves traveling through such a small space would have had relatively large amplitudes but very short wavelengths.”
The team found that these small but powerful waves can undergo amplification to become the much larger, longer waves seen by astronomers in the current CMB. The team believes this amplification is the result of coherence between early short waves, which can be explained using QFT.
“While individual short waves would be relatively powerless, cohesive groups would have the ability to re-form waves much larger than themselves,” Yokoyama said. “This is a rare example where a theory of something on one extreme scale seems to explain something on the other end of the scale.”
If the team’s theory that early, small-scale fluctuations in the universe could grow and influence large-scale fluctuations in the CMB is correct, this would affect how structures in the universe grow. Measuring CMB fluctuations can help constrain the magnitude 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 powerful wavelengths in the early universe is what led to the creation of primordial black holes,” Christiano said. “Our study suggests that there should be far fewer primordial black holes than would be needed if they were indeed strong candidates for dark matter or gravitational wave events.”
Primordial black holes are currently confirmed hypotheses. This is because the light-trapping nature of stellar-mass black holes makes it difficult to see these larger objects, so just imagine how difficult it would be to spot a black hole with an event horizon the size of a dime.
The key to discovering primordial black holes may not lie in “conventional astronomy,” but in measuring tiny ripples in space-time called gravitational waves. While current gravitational wave detectors are not sensitive enough to detect ripples in spacetime caused by colliding primordial black holes, future projects, such as the Laser Interferometer Space Antenna (LISA), will take gravitational wave detection into space. This could help confirm or reject the team’s theory, bringing scientists closer to confirming whether primordial black holes could be responsible for dark matter.
The team’s research was published on Wednesday (May 29) in the journal Physical Review Letters.
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