Tiny exploding black holes that formed from crinkled bits of the big bang might just explain where most of the universe's mass is hiding
A BOMB made out of a black hole is a rather unsettling thought. The idea of a monstrous entity consuming everything that ventures too close is sinister enough. You wouldn't want one exploding in front of you in a searing flash of heat and light.
Yet according to two astrophysicists in the US, something like that might actually have happened – not in today's cosmos, but way back in the fireball of the big bang. In fact, laser-powered black hole bombs may have been going off like firecrackers across the length and breadth of our infant universe.
If so, they would give us a valuable new window on the exotic physics that went on in our universe's obscure first moments, and perhaps also help to explain one of its biggest contemporary mysteries – where most of its mass is hiding. A crazy idea? Perhaps – but the best thing is, we can test it right now.
We know that two distinct classes of black hole exist. There are stellar-mass ones, weighing in at anything between a few times and several 10 times the mass of the sun. These form when very massive stars collapse, and are thought to pockmark galaxies including our own, the Milky Way. Then there are "supermassive" black holes, weighing in at anything up to 30 billion solar masses. These are thought to form in the heart of most galaxies – again, including our own – when a dense star cluster or gas cloud collapses into a black hole and begins to pull in, or accrete, more material from its surroundings.
What unites both sorts of black hole is an agglomeration of mass so great in so small a space – a "singularity" in space-time – that nothing can escape its gravitational pull. Everything that gets too close is simply sucked in, never to be seen again.
The idea that one of these monsters might make a bomb dates back four decades. In 1974, theorists William Press and Saul Teukolsky noted that if a black hole were spinning fast enough, light of a long-enough wavelength passing close by would scatter off it, rather than being sucked in. If this spinning black hole were to be surrounded by something like a mirror, the light could be reflected and scattered many times. Fuelled by energy from the rotation of the black hole, it would bounce back and forth and amplify itself in a runaway process rather like what happens in the mirrored cavity within a laser. If the surrounding mirror were removed or shattered, the light would instantaneously escape in a powerful burst of light and heat – a black hole bomb.
Little big crunch
It sounds like a theorist's pipe dream, but then physics is a strange business. Actually, exactly this sort of set-up could have existed in the universe's infancy, says Abraham Loeb, who is head of astronomy at Harvard University. That is, it could if a third class of black hole were to exist beyond the stellar-mass and supermassive types.
The basic premise of such is simple, and the idea has been around for a while. Our observable universe is surrounded by a horizon that marks the furthest points from which light can have travelled to us in the 13.8 billion years or so since the big bang. If the density of matter within this horizon is greater than a critical value, its gravitational attraction will at some point in the future cause everything to collapse down on itself in a "big crunch" – a kind of big bang in reverse.
The same would have been true of the universe in its earliest moments. Back then, the horizon seen from any point in space would have encompassed a much smaller region. If any particular region within the wider universe were to have a density greater than the critical density, it would begin to collapse. "All the stuff within it – essentially photons – would shrink down in a big crunch to form a primordial black hole," says Loeb.
As the universe grows, the potentially collapsible regions within it grow too. Because these regions contain more mass, the mass of the black holes that can form in this way also increases. At the same time, though, the likelihood of sustaining a large enough density fluctuation to start such a collapse decreases. This means that if primordial black holes exist, the bulk of them are likely to be very small; from microscopic ones with masses less than that of the moon to ones the size of a fridge with the mass of Jupiter.
Loeb thinks such black holes might provide an identity for dark matter. To make our standard cosmology work and keep galaxies and the like stuck together, this invisible stuff must outweigh visible stars and galaxies by a factor of about 5.5. Most physicists believe it takes the form of a soup of hitherto-unknown subatomic particles, but Loeb suggests they are barking up the wrong tree. "The subatomic particles are predicted by speculative theories of particle physics, whereas we have strong observational evidence that black holes exist, spanning a wide range of masses," he says. "It's really not such a leap of faith to imagine them as the dark matter."
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