麻豆传媒

IN THE beginning, there was light. Then, perhaps, a point of darkness. More dark spots appeared, the light circling them before falling in like water down a drain. These would have been our universe鈥檚 first inhabitants, strange baby black holes gorging on the radiation that flooded out of the big bang. As the cosmos expanded and cooled, their feasting slowed.

Millions of years passed, some of the radiation that filled the cosmos giving way to matter, which eventually clumped together to form the first stars, planets and galaxies. Over time, some stars grew so large that when they ran out of fuel and collapsed, they turned into black holes themselves. But what happened to their distant ancestors from the dawn of time? Maybe those very first, primordial black holes faded away or perhaps they were big enough to survive to the present. Either way, they could help solve some of the biggest problems in cosmology. If they were ever there.

The concept of black holes, objects so enormously dense that not even light can escape their gravitational pull, has haunted cosmology for decades. Until recently, we had no direct evidence they existed. That changed in 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the aftershock of a pair of black holes colliding 1.3 billion light years away.

Experiments such as LIGO could be our best shot at finding evidence of primordial black holes too. In fact, some people think we have already spotted them. That would be a monumental discovery because these cosmic ancients wouldn鈥檛 only be our universe鈥檚 first black holes, but also its most interesting.

The universe鈥檚 earliest moments saw a period of rapid expansion called inflation. Areas of space-time that once sat side by side shot apart faster than the speed of light, never to come into contact again. In some models of this process, tiny fluctuations in the universe鈥檚 density prior to inflation would have resulted in small areas of extreme density immediately afterwards. In those moments, a fraction of a second after the big bang, these areas could have pulled in beams of ambient light before collapsing into black holes. Any that formed from this primeval radiation would rank as primordial black holes.

In 1975, the possible existence of these cosmic old-timers sparked widespread interest when published a landmark paper on the properties of black holes. He calculated that black holes should emit particles in a process called Hawking radiation, causing them to slowly shrink and 鈥� eventually 鈥� evaporate.

Hawking radiation would be a particularly significant factor for primordial black holes. Regular, or astrophysical, black holes form from the collapse of dense stars, with a minimum mass of about 1.4 times that of our sun. But primordial black holes are created via the direct collapse of lots of radiation, so they can be as small as you like. That is bad news for their longevity. 鈥淗ow long a black hole lives depends on its mass: the smaller it is, the shorter it lives,鈥� says 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 Hawking radiation.

More than 40 years on, no such traces have been spotted. And as primordial black holes remained equally invisible, interest in them waned. Lately, though, that is changing. 鈥淭he really exciting thing about primordial black holes is that there are so many mysteries that in principle they could explain,鈥� says , University of London, who worked on these objects with Hawking.

Cosmic clean-up

For one thing, they might make up the mysterious substance we call dark matter. For decades, galaxies have been observed rotating faster than they should given all the visible stuff within them. That has led cosmologists to believe that an invisible 鈥渄ark鈥� matter lurks within these galaxies too, giving them the gravitational heft they need to spin at the speeds we see without flying apart. 鈥淲e鈥檙e pretty sure dark matter exists, but we have no idea what it is,鈥� says at the University of Nottingham in the UK.

The preferred candidate 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. 鈥淭hat鈥檚 why primordial black holes are getting more interest lately,鈥� says Green.

If WIMPs don鈥檛 make up dark matter, there are a host of rivals waiting to take their place. On the other end of the spectrum, appropriately enough, are MACHOs: massive compact halo objects. These are large objects that float freely through space and emit little if any radiation, which would explain why we haven鈥檛 seen them. Neutron stars and starless planets have been proposed as MACHOs, as have primordial black holes.

鈥淧rimordial black holes are my favourite explanation for dark matter,鈥� 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.

鈥淚f 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,鈥� says , head of the theoretical astrophysics group at Fermilab in Illinois.

Another, more intriguing, option is that the primordial black holes could be creating the dark matter particles themselves through the medium of Hawking radiation. Calculations predict 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.

鈥淭he kinds of particles that are generated by Hawking radiation don鈥檛 depend on the stuff that falls into a black hole,鈥� says Hooper. 鈥淭he black hole doesn鈥檛 care what kind of particle you are, you鈥檙e just as likely to be made. That includes dark matter and everything else.鈥� 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.

鈥淎ncient black holes would give us access to physics we would never otherwise be able to do鈥�

That includes massive dark matter particles that are too big for us to create in the Large Hadron Collider or any experiments planned on Earth. 鈥淚f 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鈥檇 never otherwise be able to do,鈥� says at the University of North Florida. If those massive particles do exist, they could turn the standard model on its head.

Primordial black holes could clear up other cosmological conundrums too. One of the biggest puzzles in the universe today is the mystery surrounding its present rate of expansion. At the moment, we have two ways of measuring it: one involves flashing a speed gun at nearby objects to detect their acceleration away from us, the other extrapolates data from ancient light to work out a current value.

The trouble is, the two methods produce conflicting results 鈥� they are in tension. 鈥淥ne thing that is known to relax this tension is if there was some extra 鈥榙ark鈥� radiation earlier in the universe鈥檚 history,鈥� says Hooper. So-called dark radiation 鈥� particles that move close to the speed of light and can travel straight through matter without stopping 鈥� could be emitted by primordial black holes as they evaporate away. If enough of these black holes existed in the early universe, that extra radiation could have given a little oomph to its expansion, explaining the discrepancy in our cosmological measurements.

That isn鈥檛 all. Most large galaxies have supermassive black holes at their centres that are up to tens of billions of times the mass of the sun. Based on our understanding of how standard black holes form, these enormous objects are impossible 鈥� there just hasn鈥檛 been enough time for a star to grow big enough to collapse into a black hole that can get anywhere near that large. But if supermassive black holes have been there since moments after the big bang, there鈥檚 no problem. 鈥淚f you start out with a million-solar-mass black hole in the early universe, it鈥檚 easy to get to a billion,鈥� says Carr.

Where are they then?

For all the excitement surrounding black holes, until recently, the evidence for their existence was pretty thin. Then LIGO switched on. An enormous international collaboration, LIGO searches for ripples in the fabric of space-time caused by the movement of massive objects. Since it spotted its first black hole collisions in 2015, the experiment has turned up evidence for about 30 more, ranging in size from just a few times the mass of the sun to more than 50 times bigger.

鈥淚 definitely think that primordial black holes are out there. I am convinced that we will find one鈥�

In the intervening years, some physicists have suggested that LIGO may actually have detected primordial black holes colliding, rather than standard stellar black holes. The idea isn鈥檛 widely accepted by astrophysicists, but remains tantalisingly plausible.

One reason to suspect the LIGO black holes may actually be primordial is that most don鈥檛 . 鈥淚f 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,鈥� says Carr. 鈥淏ut primordial black holes born in the early universe don鈥檛 tend to have spin.鈥�

Another hint comes from 鈥� 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. 鈥淲e predicted before the LIGO detections that black holes of this size should have formed in the early universe,鈥� says at the Autonomous University of Madrid, Spain. 鈥淢ost astronomers did not expect LIGO鈥檚 first black holes to be this massive, but they were.鈥�

The easiest way to catch primordial black holes red-handed is probably with LIGO. We could alternatively look for radiation emitted by matter falling into them, or use gravitational lensing, a phenomenon whereby massive objects stretch and distort the light passing near them. Some researchers, such as Carr and Garc铆a-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 鈥� or, indeed, supermassive. Looking at how much energy they emit over time could help, says MacGibbon. 鈥淲ith 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,鈥� she says.

We could also look extraordinarily far away. By doing so, we would be peering back billions of years, into the first few hundred million years of the universe. 鈥淎 pretty definitive way you could know you鈥檙e 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,鈥� says Adam Coogan at the University of Amsterdam in the Netherlands, as such systems with non-primordial black holes wouldn鈥檛 have been possible then.

Beyond finding a small, evaporating black hole or spotting one in the early universe, though, there are very few ways to prove that an observed black hole is primordial. And the lack of evidence even has those who study them unsure. Carr, who has devoted his career to these black holes, puts the probability that they are real at between 20 and 50 per cent.

The promise of these early universe relics to explain so many cosmic phenomena is a powerful motivation to keep up the hunt. 鈥淚 definitely think that primordial black holes are out there,鈥� says Garc铆a-Bellido. 鈥淚 am convinced that we will find one.鈥� Carr says the search must go on. 鈥淲e 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鈥檛 be too surprised if we have to wait another 50 years to find them.鈥� If he is right, then the wait will have been worth it.