The Missing Black Holes

A swarm of tiny black holes could flood the cosmos - and explain one of the deepest mysteries of modern physics.

We can’t, of course, see a black hole. They are, by definition, the darkest possible objects in the universe. They emit no light, no heat or sound; nothing, indeed, that would give any hint of their presence. Even in the first photograph – captured to worldwide acclaim two years ago – the black hole itself is invisible, an object marked only by darkness; by nothingness where there should instead be something.

The image shows a glowing ring, slightly brighter in the lower half, slightly darker in the upper right. That, more or less, is all it shows. The rest of the photograph is black: the inky void of space, and a dark central spot that might – one could believe – mark the location of a black hole.

In reality the image is somewhat of an illusion. The glowing ring of orange comes not from the black hole, but instead from a cloud of superheated gas swirling around its edge. The dark spot is a silhouette; the consequence of light warping and twisting around an extreme point in space.

Nonetheless, the picture serves as proof, almost, of their existence. Proof, indeed, that one of the strangest and most controversial predictions of Einstein’s theory of relativity does actually exist, a fact that even Einstein could scarcely bring himself to believe. Black holes were an aberration, in his eyes, a thing so extreme and unnatural that some law of nature must surely forbid their creation. 

In that Einstein was wrong. No greater power prohibits black holes; nothing can prevent a star big enough from collapsing into one. Such a fate awaits the giants of our galaxies: stars at least three times bigger than our own Sun. The black holes they will one day become are destined to stick around for eternity, slowly growing in stature over the aeons.

Another type of black hole exists too: the supermassive beasts at the heart of many galaxies. This, indeed, is what we photographed. The black hole that lies at the core of M87, a massive galaxy fifty million light years away, is one of the biggest known: it contains the combined mass of several billion stars like our Sun. Nobody is quite certain how it, or the others scattered across the cosmos, reached their gigantic size - but we know, for sure, that they did.

Some astronomers suspect a third, more sinister, kind of black hole may be lurking in the cosmos. These - tiny black holes as small as an atom or as big as a planet - could even be the most common form of matter in the universe; forming invisible chains and halos stretching across the heavens.

The idea of a tiny black hole may seem odd - after all, they are famed for their intense gravity, something that small black holes surely lack. And indeed, they are strange; so strange that they could only form in the most extreme event in the history of the universe: the Big Bang itself.

In order for a black hole to form, a huge amount of matter or energy must be concentrated in a tiny volume. Normally that is difficult. In the current universe such conditions are only found when a giant star implodes, placing the core of the star under enormous pressure. In the first moments of the Big Bang, however, random energy fluctuations could, briefly, have created the right conditions.

The result would have been a stream of tiny black holes popping frantically into existence. Some would have been incredibly small, little bigger than an atom. Cosmologists calculate that others would have been larger: the size of a planet, of a star, or even – at the upper end of the scale – the size of a solar system.

One early proponent of the idea was Stephen Hawking. His calculations showed that their existence was possible – but that there was also a limit on how small they could be. Over time, thanks to a phenomenon known as Hawking Radiation, black holes gradually shrink in size: literally evaporating away.

This radiation, Hawking showed, would have destroyed the smallest black holes made in the Big Bang. An atom sized black hole should last no more than a fraction of a second; one weighing a thousand tons would last barely a minute. Bigger ones, the size of a small asteroid say, should have survived until the present. Larger ones still, the size of Jupiter, for example, should stick around for several billion years longer.

If black holes were really created in the Big Bang, therefore, many of them should still be lurking in the cosmos today. The problem, of course, is finding them. Black holes, by their very nature, do not shine or give out heat. They cannot be directly seen - which means astronomers instead hunt for signs of their influence on surrounding objects.

For supermassive black holes this is relatively easy. They are disruptive beasts, ones that destroy stars and suck in vast quantities of matter. That leaves a distinctive trace, like the orange halo seen in our black hole photograph. Smaller ones are harder to spot, though astronomers can sometime pick out energetic bursts of radiation coming from in-falling debris, or see signs of space and matter warping around their presence.

But finding one the size of an asteroid? A black hole that small would still suck in matter, true, but it would do so very slowly, barely emitting any tell-tale radiation at all. Its effects on space and time would also be limited, little more than any other asteroid or planet. One could even be lurking at the edge of the Solar System, and we would have almost no way to know.

Some astronomers, however, have suggested they might be hiding in plain sight. The properties of these primordial black holes are surprisingly close to that of dark matter. If enough black holes had formed in the Big Bang, and enough had survived to the present day, they could be a perfect match for the invisible, dark and mysterious matter that seems to fill the cosmos.

The idea gained force in the 2000s, until, in 2017, a paper seemingly disproved the possibility. According to the author, Yacine Ali-Haimoud, these early black holes should have ended up orbiting each other, gradually spiralling inwards until they collided and merged.

Ali-Haimoud predicted that these collisions should have produced gravitational waves. Since there seemed to be so many tiny black holes – in order to explain dark matter – a lot of these collisions should be happening. That means our gravitational wave detectors, like LIGO, should already have picked up traces of them. Nothing, however, has been found.

Not everyone agrees with Ali-Haimoud’s conclusion. Some think that the collisions can be avoided, perhaps by the influence of other black holes pulling the spiralling pairs back from the brink of merging. But no other evidence for their presence has been found. Surveys of the edge of the Milky Way, looking for signs that large numbers of black holes cluster there, have come up empty handed.

Others have suggested looking for traces on the Moon; scouring the surface for craters formed by long ago collisions with black holes. Some think we could find examples hidden in our own Solar System. They suggest that some kind of gravitational anomaly exists beyond Pluto, a mystery that could be explained by a black hole – or, more prosaically, by an unknown planet.

The simplest solution, however, is to find a black hole just slightly too small to explain in any other way. Black holes formed in the heat of a supernova have a minimum possible size – roughly two and a half times more massive than the Sun. Anything smaller than that must have been created in some other way – and that, probably, means they were made in the Big Bang itself. 

So far no sign of such a small black hole has ever been found. But astronomers are busy scouring the skies, and one day they may pick up traces of one. If they do, the discovery would be ground breaking: a possible solution to the problem of dark matter and a glimpse into the very earliest moments of creation.

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