The search for extinct, miniature black holes left over from the Big Bang may be heating up.
Just when the trail of such tiny black holes seemed to have gone cold, an international team of scientists discovered clues in quantum physics that could reopen the case. One reason the search for these so-called primordial black holes is so pressing is that they have been proposed as possible candidates for dark matter.
Dark matter makes up 85% of the mass in the universe, but it doesn’t interact with light like everyday matter does. It 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. Perfect for space detective work.
If Big Bang black holes do 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. Yet, like their more massive counterparts, stellar-mass black holes, which can have a mass of 10 to 100 times that of the sun, and supermassive black holes, which can have a mass of millions or even billions times that of the sun, small black holes from the dawn of time will be 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 may be small enough to go unnoticed, but strong enough to impact a space.
Connected: Small black holes left over from 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 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 what is called quantum field theory (QFT).
Applying QFT to the infant 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 suspected entirely of dark matter.
“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 Christiano said in a statement. “They are also interesting for other reasons, as since the recent innovation in gravitational wave astronomy there have been discoveries of merging binary black holes, which can be explained if 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 look for primordial black holes
The most favored models of 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 cool enough to allow electrons and protons to bond and form the first atoms. Then the element hydrogen was born. Furthermore, before this cooling occurred, light could not travel through space. This is because electrons endlessly scatter photons, which are particles of light. Thus, during these literal dark ages, the universe was essentially opaque.
However, once the free electrons were able to bond with the protons and stop bouncing around, light could finally travel freely. After this event, called the “final dispersal,” and during the next period known as the “epoch of reionization,” the universe instantly became transparent to light. The first light that shone through the universe at that time can still be seen today as a mostly uniform field of radiation, a universal “fossil” called the “cosmic microwave background” or “CMB.”
Meanwhile, the hydrogen atoms created formed the first stars, the first galaxies, and the first galaxy clusters. And of course, some galaxies appear to have more mass than their visible constituents can account for, with that 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 grow from the 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 and dense early universe were such that smaller blobs of matter could have collapsed under their own gravity to give birth to these miniature black holes – with an event horizon of no more than wider than a dime or perhaps even smaller than a proton, depending on their mass.
The team behind this research had previously looked at models of primordial black holes in the early universe, but those models failed to match the CMB observations. To correct this, scientists applied corrections to the leading theory of primordial black hole formation. Corrections informed by QFT.
“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation quickly expanded by 25 orders of magnitude,” Kavli IPMU and RESCEU director Junichi Yokoyama said in the 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 strong 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 the early short waves, which can be explained using QFT.
“While individual short waves would be relatively powerless, coherent groups would have the power to alter waves much larger than themselves,” Yokoyama said. “It’s a rare case where a theory about something at one extreme seems to explain something at the opposite end of the scale.”
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 affect how structures grow in space. Measuring CMB fluctuations can help constrain the size of the initial 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 waves in the early universe is what creates primordial black holes,” Christiano said. “Our study suggests that there should be far fewer primordial black holes than would be necessary if they were indeed a strong candidate for dark matter or gravitational waves.”
Primordial black holes are firmly hypothetical at this time. This is because the light-trapping nature of stellar-mass black holes makes even these much larger objects difficult to see, 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 “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 spacetime from colliding primordial black holes, future projects, such as the Laser Interferometer Space Antenna (LISA), will carry the detection of gravitational waves into space. This could help confirm or disprove the team’s theory, bringing scientists closer to confirming whether primordial black holes can explain dark matter.
The team’s research was published Wednesday (May 29) in the journal Physical Review Letters.