If primordial black holes exist they will be very small (Image: Stuart Daly)
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.
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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 , 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 “primordial” black holes 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.
“If primordial black holes exist, they will be very small – from microscopic to the size of a fridge”
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.”
Experiments have already put some limits on how much primordial black holes of different sizes can contribute to dark matter. Dark objects passing in front of stars would boost, or “microlens”, the starlight as it travels towards us, making the star appear briefly brighter. Various probes have looked for this effect and seen very little evidence for it. Primordial black holes in the range from about one-millionth the mass of the sun – about a third the mass of Earth’s moon – up to 10 solar masses can make up 10 per cent of dark matter at most.
A further limit comes from the prediction that, over time, black holes evaporate away, giving out so-called Hawking radiation. Primordial black holes with less than the mass of a typical asteroid, about one-hundred-millionth the mass of the sun, would have evaporated by now. Since astronomers see no sign of the flashes of high-energy gamma rays expected to accompany this process, these holes can be ruled out as a big component of dark matter.
That still leaves unexplored a range of masses for primordial black holes between asteroid and moon size. Some of these might have exploded as bombs – and so have left behind proof of their existence. The turbulent conditions of the big bang would make it likely that these tiny primordial holes would be born spinning, says Loeb. “It is hard to imagine a collapse symmetric enough that it would have spawned a black hole without a spin.”
Now all we need to make a bomb is a mirror. No problem, the big bang fireball would have provided that, too. Although overwhelmingly made of photons – about 10 billion for every particle of matter – it also contained a plasma of electrons sloshing back and forth with a natural frequency. As long as this frequency is greater than the frequency of any photons that encounter it, the photons will be reflected back in the direction from which they came. The electron plasma forms an amplifying mirror for the photons around a black hole.
Crucially, the electrons’ sloshing frequency depends on their density. In the frenetically expanding early universe, this density was constantly falling. At some point, the electron frequency would have fallen below the critical threshold needed to contain the photons. They would have been released in searingly powerful bursts of heat and light. Voila – black hole bombs.
Visible signatures
That has observational consequences. “Crucially, a black hole bomb injects heat into its immediate surroundings,” says Loeb. With his colleague Paolo Pani of the Technical University of Lisbon, Portugal, he has shown that this heat should still be visible as deviations from the perfect “black body” spectrum expected of the cosmic microwave background, the relic radiation of the big bang that suffuses the cosmos today at a frigid temperature of 2.7 kelvin ().
“A black hole bomb injects heat into its surroundings that might be visible in the cosmic background”
It is an idea worth investigating, says , a physicist at the NASA Goddard Space Flight Center in Greenbelt, Maryland, and co-winner of the 2006 Nobel prize in physics for his investigations of the cosmic microwave background. “I have no idea why nobody noticed it before. It’s pretty interesting,” he says.
at the Free University of Brussels in Belgium agrees. “This is an important piece of work. Such a situation has not been considered in any previous analysis,” he says. But he sounds a note of caution. “All of the work is based on the assumption that primordial black holes acquire a spin when they are created,” he says. “Nobody knows if this is true.”
So let’s take a look at the cosmic background radiation to find out. Loeb and Pani have already looked for evidence of distortions caused by black hole bombs in data taken by NASA’s in 1993. Their analysis more or less rules out larger primordial black holes making up more than about 1 per cent of the missing dark-matter mass today, but still leave open the possibility that smaller ones might make up all of it. Experiments are on the drawing board to study the cosmic microwave background in even finer detail. They should either see something, or narrow things down still further.
The interest in black hole bombs is not just confined to dark matter. Observations tell us that stellar-mass and supermassive black holes can also spin. Theoretically, they too could kick off a lasing effect – not with photons, but with other, so far undiscovered particles similar to photons that might not have anything to do with dark matter. Any evidence of such bombs – or lack of it – could put stringent limits on the existence of such particles. “We haven’t done the test yet, but it is eminently possible,” says Loeb.
But it is undoubtedly those smallest primordial black holes, potentially forged in the earliest moments of the big bang, that would be the most exciting catch. One process that could have caused the density within a given space in the early universe to be higher than average, and so have created them, is a change of state akin to the transition from steam to liquid water that liberates a lot of heat energy. Evidence for such changes of state is eagerly sought, as they would take us beyond the standard model of particle physics to a new and deeper level of understanding. “Evidence of primordial black holes at a particular mass will tell us about physical processes operating at a very particular time,” says Loeb. “It could provide a unique window on exotic events going on in the first moments of the universe’s existence.”
Now that really would be a bombshell.
Read more: “Dark stars: Our best black hole matter“
This article appeared in print under the headline “Cosmic bombshells”
