“The really exciting thing about primordial black holes is that there are so many mysteries that in principle they could explain,” says Stephen Hawking’s colleague, physicist Bernard Carr. “Not the least of them being the existence of dark matter and dark energy.”“One exciting possibility is that a population of primordial black holes may have created dark matter in the early universe,” replied Dan Hooper, head of the theoretical astrophysics group at Fermilab to an email from The Daily Galaxy, asking Hooper what new physics could be revealed by the discovery of these elusive relics. “If these black holes were initially lighter than a million kilograms or so,” Hooper added, “they would have evaporated in the first second after the Big Bang. In the process of this evaporation, they could have created any number of exotic forms of matter and energy, including dark matter.”
Primordial black holes —Planck-mass relics of evaporating black holes– created via the direct collapse of primordial radiation, a fraction of a second after the big bang, could have pulled in beams of ambient light before collapsing into black holes writes Leah Crane for New Scientist. If primordial black holes are real, they’d have potential to solve a whole host of the biggest problems in cosmology. And also they’d be extremely awesome, she Tweeted.
“By the late 1990s,” writes Hooper and Gianfranco Bertone in A History of Dark Matter, “it had become clear that baryonic dark matter does not constitute a large fraction of the Universe’s dark matter. Although these results seem to imply that the dark matter must consist of one or more new particle species, there remains a caveat to this conclusion: the dark matter might instead consist of black holes that formed before the epoch of Big Bang nucleosynthesis and with masses below the sensitivity range of microlensing surveys.”
“How long a black hole lives depends on its mass: the smaller it is, the shorter it lives,” says Francesca Vidotto at the University of the Basque Country in Spain. Accelerated evaporation of primordial black holes makes them an obvious place to look for traces of radiation, proposed by Stephen Hawking and Bernard Carr in 1974, causing them to slowly shrink, and eventually, evaporate, flickering lights in the cosmic dawn.
Although dark matter is considered the backbone to the structure of the universe, scientists know little about its nature, as the particles have so far evaded detection. More than 40 years after Hawking’s radiation theory, no such traces have been spotted. Indeed, says astrophysicist Tommi Tenkanen at the Johns Hopkins University, “We do not know what dark matter is, but if it has anything to do with any scalar particles, it may be older than the Big Bang.”
WIMPS vs MACHOS
Scientists are confident that dark matter exists because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe. WIMPs, or weakly interacting massive particles, which are among the leading candidates for dark matter are thought to interact with other matter only on very rare occasions, which is why they have yet to be detected directly.
In 1970, physicist Ken Freeman reached a striking conclusion. He found that “if [the data] are correct, then there must be in these galaxies additional matter which is undetected, either optically or at 21 cm. Its mass must be at least as large as the mass of the detected galaxy, and its distribution must be quite different from the exponential distribution which holds for the optical galaxy.”
Since Freeman, galaxies have been observed rotating faster than they should given all the visible matter within them, which has led cosmologists to believe that an invisible “dark” matter lurks within these galaxies too, giving them the gravitational heft they need to spin at the speeds we see without flying apart.
“Primordial black holes (PBHs) remain hypothetical objects for the moment,” following direct observations of gravitational waves by the VIRGO and LIGO detectors in 2016, says Alvise Raccanelli at CERN. “Initially proposed by Stephen Hawking in 1971, they have come back to the fore in recent years as possible candidates for explaining dark matter. Looking for evidence of the existence of PBHs, or excluding their existence, provides us with information on the physics of the primordial universe.
Perhaps the most intriguing option is that the primordial black holes could be creating the dark matter particles themselves through Hawking radiation. Hwking predicted that the bigger one of these black holes is, the lower its temperature, meaning it emits fewer and lighter particles. As it shrinks, it heats up, radiating more and more energy. That means small primordial black holes can spew more massive, complex particles.
“The kinds of particles that are generated by Hawking radiation don’t depend on the stuff that falls into a black hole,” says Hooper. “The black hole doesn’t care what kind of particle you are, you’re just as likely to be made. That includes dark matter and everything else.” Whatever particles exist, whether they are predicted by the standard model of particle physics or not, should be emitted by primordial black holes as they evaporate. Ancient black holes would give us access to physics we would never otherwise be able to do”
The preferred candidate, writes Leah Green, “has long been vast numbers of tiny particles, each possessing mass but lacking the capacity to interact with ordinary matter. Yet although these weakly interacting massive particles, or WIMPs, remain the theoretical front runners, they have yet to show up in experiments, which is why physicists are now looking back at primordial black holes as a possible answer”
If WIMPs are found not to make up dark matter, MACHOs: massive compact halo objects, are ready to take their place. It has been theorized that dark matter could be made of these are large objects that float freely through space and emit little if any radiation, which would explain why we haven’t seen them. Neutron stars and starless planets have been proposed as MACHOs, as have primordial black holes.
“Primordial black holes are my favorite explanation for dark matter,” says Vidotto. Astronomical observations, however, have concluded that they are unlikely to account for all of dark matter, which means there must be something else out there to pick up the slack. If WIMPs made up the other part, we would expect them to surround every primordial black hole, drawn in by its gravitational pull. That higher density of WIMPs would increase the probability of WIMP-WIMP collisions, generating a distinctive shower of gamma rays that has never been seen.
“If one day we discovered even a few primordial black holes, you just have to concede that whatever dark matter is, not all of it is made of WIMPs,” says Dan Hooper, head of the theoretical astrophysics group at Fermilab in Illinois.
“If we can find primordial black holes and observe them in their last few seconds as they get to those high temperatures, it gives us access to physics that we’d never otherwise be able to do,” says Jane MacGibbon at the University of North Florida. If those massive particles do exist, they could turn the standard model on its head.
“I definitely think that primordial black holes are out there. I am convinced that we will find one,” says Carr.
Detected by LIGO?
It’s been hypothesized that there could be black holes that formed in the very early universe before stars existed at all.” said Savvas Koushiappas, an associate professor of physics at Brown University and coauthor of a study with Avi Loeb from Harvard University.
“The idea is very simple,” Koushiappas said. “With future gravitational wave experiments, we’ll be able to look back to a time before the formation of the first stars. So if we see black hole merger events before stars existed, then we’ll know that those black holes are not of stellar origin.”
The study published in Physical Review Letters outlined how scientists could use LIGO gravitational wave experiments to test the existence of primordial black holes, gravity wells formed just moments after the Big Bang that some scientists have posited could be an explanation for dark matter.
Some physicists, Crane observers, have speculated that LIGO may actually have already detected primordial black holes colliding, rather than standard stellar black holes, an idea not widely accepted by astrophysicists, but remains plausible.
“If the black holes which are detected by LIGO come from stars, those stars are in binary systems so you tend to get black holes that form with some spin,” says Carr. “But primordial black holes born in the early universe don’t tend to have spin.”
Another hint comes from calculations of when primordial black holes were most likely to have formed – when the pressure in the universe dipped slightly and allowed for more intense gravitational collapse. When they formed can tell us what their masses would probably be today. One of these dips lines up with a primordial black hole mass about 30 times that of the sun, similar to the masses of most of the LIGO black holes.
“We predicted before the LIGO detections that black holes of this size should have formed in the early universe,” says Juan García-Bellido at the Autonomous University of Madrid, Spain. “Most astronomers did not expect LIGO’s first black holes to be this massive, but they were.”
Some researchers, such as Carr and García-Bellido, suspect we may already have seen primordial black holes acting as lenses, but other objects could have been responsible.
So how do we know for sure if we have spotted a primordial black hole? A small size is one obvious sign, but some could be just as big as regular black holes – or, indeed, supermassive. Looking at how much energy they emit over time could help, says MacGibbon. “With most objects in astrophysics, you see the energy decaying with time, whereas an evaporating black hole would be rising higher and higher in temperature and energy,” she says.
“A pretty definitive way you could know you’re looking at primordial black holes would be to see a black hole binary system really far away, at a very early time in the universe,” says Adam Coogan at the University of Amsterdam in the Netherlands, as such systems with non-primordial black holes wouldn’t have been possible then.
“I definitely think that primordial black holes are out there,” says Juan Garcia-Bellido, Professor of Physics, Universidad Autónoma de Madrid. “I am convinced that we will find one.”
“We had to wait 100 years after gravitational waves were predicted before we found them, for black holes we had to wait 50 years, and if primordial black holes exist, we shouldn’t be too surprised if we have to wait another 50 years to find them,” concludes Stephen Hawking’s collaborator, Bernard Carr, Emeritus Professor of Mathematics and Astronomy at Queen Mary, University of London.
Image at top of page: Hubble Space Telescope