By Colin Stuart

Published: Wednesday, 08 December 2021 at 12:00 am

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Out there among the stars lies a hidden swarm of black holes. They have been around ever since the dawn of time, quietly and not so subtly influencing the evolution of the Universe. Without them there would be no stars, no planets and no life to marvel at the Universe’s wonders. Now, for the very first time, we may finally have the tools to find them.

Black holes are one of astronomy’s most famous objects. Their gravity is so extreme that escape is impossible if you venture too close. There are different sizes of black holes, but they are normally gargantuan monsters considerably more massive than our own Sun.

They usually remain hidden from view because no light can escape to reveal them to us. Yet we have seen them, thanks to gravitational wave detectors like LIGO and VIRGO that have detected signals from colliding black holes. We’ve never been more confident that these cosmic trapdoors exist.

However, there is one type of black hole that still remains theoretical, one that could solve a long-standing cosmological conundrum to boot.

Immediately after the Big Bang there were small fluctuations in the new Universe’s density – regions that had slightly more or less mass than the average. Where the mass was above the norm, material could have collapsed to form mini black holes. As they’ve been around for pretty much as long as the Universe itself, these black holes are called ‘primordial’ black holes.

According to theoretical models, primordial black holes can have a wide range of masses. They could be lighter than an eyelash or heavier than a star. So far we’ve been able to rule out some masses through observations, opening up two possible mass ranges or ‘windows’ for primordial black holes.

Read more about primordial black holes:

• Newly discovered black hole could have formed “before the first stars and galaxies”
• Gravitational waves point to primordial black holes

“The two windows are less than one lunar mass [one lunar mass = mass of our Moon] and a few tens of solar masses [one solar mass = mass of our Sun],” says Prof Sohrab Rahvar, an astrophysicist and cosmologist from Sharif University of Technology in Iran.

If this hidden population of miniature black holes exists, then they could account for some or all of the dark matter – the invisible glue that astronomers think helps hold galaxies like our own Milky Way together. It’s an idea that fell out of favour, but is now gaining traction again – particularly as traditional notions of what dark matter is made of continue to draw a blank.

With one eye on dark matter’s true identity, PhD student Gabriele Franciolini, from the University of Geneva, Switzerland, has been re-modelling the production of primordial black holes after the Big Bang in more detail.

The upshot? “There could be hundreds of times more primordial black holes out there,” he says. “You can explain all of the dark matter through primordial black holes.” That’s if they have a mass in the lower window, below that of the Moon, which would make them less than one-tenth of a millimetre in diameter – about the width of a human hair.

In this case, each primordial black hole would be tiny, but together they could provide enough gravity to keep a galaxy from flying apart. If galaxies like our Milky Way are chock full of teeny adhesive black holes then they should be everywhere. Amir Siraj, a theoretical astrophysicist from Harvard University, believes there’s even a chance that one is lurking in the outskirts of our Solar System.

Is there a black hole in our Solar System?

For a decade or so, astronomers have been puzzled by the orbits of small objects beyond Pluto. Their paths around the Sun should be pretty random, but they appear organised. It’s as if something is shepherding them onto similar trajectories. But what?

“It’s most likely a planet,” says Siraj. Dubbed Planet Nine, it would be the first newly discovered planet since Neptune was added to the register in 1846 (Pluto was called a planet upon discovery in 1930, but later demoted to dwarf planet status in 2006). However, extensive ongoing searches have failed to find any visible sign of such a world.

“If direct searches continue to fail, then a primordial black hole becomes an option,” Siraj says. After all, as Arthur Conan Doyle’s fictional detective Sherlock Holmes famously said, once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.

We know how much gravity we need to account for the clustered orbits of the objects beyond Pluto. If that gravity is being provided by a primordial black hole, then it is merely the diameter of a grapefruit. Yet despite its small size, it would still be approximately 10 times heavier than the Earth. If this is really what’s going on then it’s no surprise that telescopic trawls of the outer Solar System have so far come up empty.

Siraj thinks there is a way we could see it, though. It’s an idea that came to him during the first COVID lockdown while he was staying in his childhood bedroom at his parents’ house.

“The black hole should produce the occasional accretion flare,” he explains. In other words, we should see a flash if the grapefruit-sized object gobbles up a passing comet. Such an event would release a few per cent of the energy of the atomic bombs dropped on Japan at the end of WWII.

The flash may be intense, but it is happening a long way away in the backwaters of the Solar System. That makes the light pretty faint by the time it reaches Earth. Still, there is a new telescope about to come online that should be up to the job. “The flares are right on the detection limit for the Vera Rubin Observatory,” Siraj says. “It’s the perfect tool for ruling out a primordial black hole in the outer Solar System.” The observatory should start proper observations towards the end of 2023.

Should a primordial black hole turn out to be there, it likely entered the Solar System due to the fact that the Sun is dragging us through the minefield of the Milky Way as it orbits around the galactic centre. For some reason this one got stuck, but could a primordial black hole go further and enter the inner Solar System? Rahvar certainly thinks so. He’s calculated the chances of a primordial black hole passing through the Earth during our planet’s 4.54-billion-year history.

“The probability is one passage per billion years,” he says. If he’s right, we’ve been struck by a primordial black hole on four separate occasions. Rahvar’s calculation assumes 100 per cent of dark matter is made of sub-lunar mass primordial black holes. Even if they make up just a quarter of dark matter, Earth has still encountered a primordial black hole once before and is likely to do so again.

Our planet getting hit by a black hole sounds apocalyptic, but it isn’t necessarily. We’re still here, after all. The worst outcome is that the primordial black hole settles in the Earth’s core. “Then the black hole starts to swallow all the matter of the Earth, grows and after a finite time the Earth will collapse into the black hole,” Rahvar explains.

Fortunately, Rahvar’s calculations reveal that the probability of trapping a black hole inside the Earth is almost zero. What is far more likely is that the black hole passes straight through the planet and emerges on the other side to continue its journey through space. If it happens to pass through you, however, the results aren’t pretty.

How can we tell if a black hole really has passed through the planet? “During its passage through the Earth it can heat the planet’s interior,” says Rahvar. This could show up as melting traces in rocks along the straight line that represents the black hole’s path. However, Rahvar cautions that as this has only occurred a maximum of four times in Earth’s entire history, it would be extremely difficult to discover these traces.

How gravitational waves could find primordial black holes

So how else could we prove that primordial black holes really exist? Franciolini thinks that the answer is the same way we proved that ordinary black holes exist: gravitational waves. Observed for the first time in 2015, gravitational waves are ripples in the fabric of the Universe itself, caused by events happening within it.

The gravitational waves that we’ve detected so far have come from collisions between compact objects like ordinary black holes and neutron stars. Or at least that’s the conventional wisdom. Franciolini isn’t quite so sure. “A significant fraction of the events could be of primordial origin,” he says.

Now we’re talking about primordial black holes in the upper mass window – those weighing in at tens of solar masses. We know from an effect called ‘gravitational lensing’ that primordial black holes in this mass range cannot make up more than 10 per cent of dark matter. Gravitational lensing is where a foreground object (say a black hole) magnifies the light of a background object (like a star). If there were lots of big primordial black holes around then we’d see more of these events than we do – hence the 10 per cent ceiling.

According to Franciolini, if large primordial black holes represent just 0.1 per cent of dark matter then they would merge with each other at the same rate as ordinary black holes do. If we’ve been able to see the latter, there’s a good chance we’ve also already seen the former. In other words, we’ve already spotted two large primordial black holes colliding and wrongly assumed it was two ordinary black holes instead. “You have to disentangle the two scenarios,” says Franciolini. “That’s the challenge.”

One recent gravitational wave event is particularly intriguing: GW190521. It takes its name from the fact that it was detected on 21 May 2019, and the discovery was subsequently announced in September 2020. This particular merger of two black holes produced a single black hole that tips the scales at 140 solar masses. That’s big enough to class it as an intermediate black hole.

According to Franciolini, it’s harder to explain the existence of such a monster merger with ordinary black holes. That opens the door to the possibility of a primordial origin instead. “The data supports the idea that primordial black holes are there,” Franciolini says, “but we need more data and more work on the theoretical side.”

We might not have to wait too long for that deluge of data, as we’re already building out our network of gravitational wave detectors. The Kamioka Gravitational Wave Detector (KAGRA) in Japan will soon join forces with LIGO and VIRGO after some setbacks due to the COVID pandemic and Hurricane Ida. Plans are underway for a new LIGO detector in India later this decade.

With four detectors spread around the globe, astronomers will be able to pinpoint gravitational wave events across the entire sky. Right now they are limited to just half of the heavens.

The new gravitational wave detectors should bring more black hole mergers events with more precise data. Researchers like Franciolini are eager to comb through that data for any signs of primordial black holes. Perhaps then we’ll know whether these ancient, invisible relics have been silently steering the evolution of the Universe, binding galaxies together and setting the stage for us to build gravitational wave detectors in the first place.

What would happen if you were hit by a primordial black hole?

Well, first off, you’d probably be the unluckiest person in human history. According to Prof Sohrab Rahvar, an astrophysicist and cosmologist from Sharif University of Technology in Iran, Earth has only been hit by a maximum of four primordial black holes in its entire 4.54-billion-year existence. For one to pass through the Earth during your lifetime is almost impossible. For it to pass through the narrow one-metre corridor that coincides with where you are is off-the-scale unlikely.

That said, you’re in for a torrid time should you be fortune’s fool. The primordial black hole will only be in your body for 10 microseconds, but will be travelling at 160km/s (0.05 per cent of the speed of light). Although the black hole is only 1,000 times wider than an atom, it comes with its famous brand of intense gravity.

According to Prof Avi Loeb, the esteemed theoretical physicist from Harvard University, it’s enough to shrink your body by a couple of inches. All of your internal organs will become distorted as the black hole warps the very fabric of space in which they reside. It would lead to an agonising death. Still, imagine the epitaph – ‘Here lies – [insert your name] – killed by a black hole’.

Read more about black holes:

• Here’s what could happen if you fell into a black hole
• Can life exist around a black hole?

• This article first appeared in issue 370 of BBC Science Focus Magazine – find out how to subscribe here

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