The search for the missing, miniature black holes left over from the Big Bang may be heating up.
Just as the trail of such tiny black holes seemed to have cooled, an international team of scientists 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 makes up 85% of the matter in the universe, but it does not interact with light like everyday matter. 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 effect can affect “ordinary matter” and light. Perfect for space detective work.
If the Big Bang-induced black holes really exist, they would be absolutely tiny—some could be as small as a dime—and thus have masses equal to the masses of asteroids or planets. Still, 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, small black holes from the dawn of time would be bounded by a light-trapping surface called the “event horizon”. The event horizon prevents black holes from emitting or reflecting light—making tiny primordial black holes a solid candidate for dark matter. They can be small enough not to be noticed, but powerful enough to impact a space.
Related: Tiny black holes left over from the Big Bang may be prime suspects for dark matter
A 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 accounts for the behavior of particles such as electrons and quarks and leads to what is called quantum field theory (QFT).
Applying QFT to the newborn universe led the team to believe that there are far fewer hypothetical primordial black holes in the universe than many models currently estimate. If so, this may rule out primordial black holes, as dark matter is entirely suspected.
“We call them primordial black holes, and many researchers believe they are a strong candidate for dark matter, but there would have to be a lot of them to satisfy this theory,” University of Tokyo graduate student Jason Kristiano said in a statement. “They are interesting for other reasons as well, since the recent innovation of gravitational wave astronomy has seen the discovery of binary black hole mergers, which can be explained by the fact that primordial black holes exist in large numbers.
“But despite these strong reasons for their expected abundance, we haven’t seen any directly, and now we have a model that should explain why this is so.”
Back to the Big Bang to the hunt for primordial black holes
The most popular models of cosmology suggest that the universe began about 13.8 billion years ago during the initial period of rapid inflation: the Big Bang.
After the first particles appeared in the universe during this initial expansion, space eventually cooled enough to allow electrons and protons to combine to form the first atoms. That’s when the element hydrogen was born. Furthermore, before cooling occurred, light could not travel through space. This is because electrons are endlessly scattering photons, which are particles of light. So during these literal dark ages, the universe was essentially opaque.
But once the free electrons were able to bond with the protons and stop bouncing around all over the place, light could finally move freely. After this event, called the “last dispersion”, and during the subsequent period known as the “reionization epoch”, the universe immediately became transparent to light. mostly a uniform field of radiation, a universal “fossil” called the “cosmic microwave background” or “CMB”.
Meanwhile, the hydrogen atoms formed formed the first stars, the first galaxies, and the first clusters of galaxies. And indeed, some galaxies appeared to have more mass than their visible components could account for, the excess being attributed to none other than dark matter.
While stellar mass black holes are created by the collapse and death of massive stars, and supermassive black holes grow by the subsequent merger of smaller black holes, primordial black holes are older than stars – so they must have a unique origin.
Some scientists believe that conditions in the hot, dense early universe were such that smaller bits of matter could collapse under their own gravity, giving birth to these tiny black holes—with event horizons no wider than a dime or perhaps even smaller than a proton, depending on their weight.
The team behind this research had previously looked at models of primordial black holes in the early universe, but failed to match those models with CMB observations. To correct this, the scientists applied corrections to the leading theory of the formation of primordial black holes. Corrections reported by QFT.
“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation quickly expanded it by 25 orders of magnitude,” Kavli IPMU and RESCEU director Jun’ichi Yokoyama said in a statement. “At that time, waves traveling through this small space could have relatively large amplitudes but very short wavelengths.”
The team found that these small but powerful waves can undergo amplification to become the much larger and longer waves that astronomers see in today’s CMB. The team believes that this amplification is the result of coherence between early short waves, which can be explained by QFT.
“While 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 case where a theory of something at one extreme scale seems to explain something at the opposite end of the scale.”
If the team’s theory that early, small fluctuations in the universe can grow and affect large fluctuations in the CMB is correct, this will have implications for how structures grew in the universe. Measuring CMB fluctuations could help constrain the magnitude of the original fluctuations in the early universe. This in turn constrains 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 creates the primordial black holes,” Kristiano said. “Our study suggests that there should be far fewer primordial black holes than needed if they are indeed a strong candidate for dark matter or gravitational wave events.”
Primordial black holes are firmly hypothetical at this point. This is because the light-trapping nature of stellar mass black holes makes even these much larger objects difficult to see, so imagine how difficult it would be to spot a black hole with an event horizon the size of a dime.
The key to detecting primordial black holes may not lie in “traditional astronomy,” but rather in measuring tiny ripples in space-time called gravitational waves. While current gravitational wave detectors are not sensitive enough to detect ripples in space-time from primordial black hole collisions, future projects such as the Laser Interferometer Space Antenna (LISA) will take gravitational wave detection into space. This could help confirm or disprove the team’s theory, bringing scientists closer to confirming whether primordial black holes may be responsible for dark matter.
The team’s research was published Wednesday (May 29) in the journal Physical Review Letters.