Perhaps the most surprising scientific discovery of the last decade is that the Universe is teeming with black holes.
They’ve been detected in a surprising variety of sizes: some with masses only a bit larger than the Sun, others that are billions of times larger. And they’ve been detected in a variety of different ways: by radio emissions from the matter falling towards the hole; by their effect on the stars orbiting them; by the gravitational waves emitted as they merge; and by the extremely peculiar distortion of light they cause (think back to the ‘Einstein ring’, seen in the photos of Sagittarius A*, the supermassive black hole at the centre of the Milky Way, that graced the front pages of the world’s newspapers not so long ago).
The space we inhabit is not smooth – it’s pitted, like a colander, by these holes in the sky. The physical features of all black holes were predicted by Einstein’s theory of General Relativity and are well described by the theory.
Everything we know about these strange objects fits with Einstein’s theory rather perfectly, so far. But there are two key questions that Einstein’s theory doesn’t answer.
The first: when matter enters the hole where does it go next? The second: how do black holes end? Convincing theoretical arguments, first understood by Stephen Hawking several decades ago, indicate that in the distant future, after a life that depends on its size, a black hole shrinks (or as physicists say, ‘evaporates’), by emitting hot radiation now known as Hawking radiation.
This results in the hole becoming smaller and smaller, until it’s tiny. But, what happens next? The reason these two questions are yet to be answered, and that Einstein’s theory doesn’t provide an answer, is that they both involve quantum aspects of spacetime.
That is, they both involve quantum gravity. And we don’t have an established theory of quantum gravity yet.
An attempt at an answer
There is hope, however, because we do have tentative theories. These theories aren’t established yet because, to date, they haven’t been supported by experiments or observations.
But they are developed enough to give us tentative answers to these two important questions. And so we can use these theories to make an educated guess about what is occurring.
undefinedArguably the most detailed and developed theory of quantum spacetime is loop quantum gravity, or LQG – a tentative quantum gravity theory that has been steadily developing since the late 1980s.
Thanks to this theory, an interesting answer to these questions has emerged. That answer is given by the following scenario. The interior of a black hole evolves until it reaches a phase where quantum effects begin to dominate.
This creates a powerful repulsive force that reverses the dynamics of the interior of the collapsing black hole, making it ‘bounce back’. After this quantum phase, described by LQG, the spacetime inside the hole is once again governed by Einstein’s theory, except that now the black hole is expanding rather than contracting.
The possibility of an expanding hole is indeed predicted by Einstein’s theory, in the same manner in which black holes were predicted. It’s a possibility that has been known about for decades; so long, in fact, that this corresponding spacetime region even has a name: it’s called a ‘white hole’.
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The same idea, but in reverse
The name reflects the idea that a white hole is, in a sense, the reverse of a black hole. It can be thought of in the same way that a ball bouncing upwards follows an upward trajectory that’s the reverse of the downward trajectory taken when that ball fell.
A white hole is a spacetime structure that’s similar to a black hole but with time reversed. Inside a black hole, things fall in; inside a white hole, however, things move out. Nothing can exit a black hole; likewise, nothing can enter a white hole.
Seen from the outside, what happens is that, at the end of its evaporation, a black hole, which is now tiny because it has evaporated away most of its mass, mutates into a tiny white hole. LQG indicates that such structures are rendered quasi-stable by quantum effects, so they can live for a long time.
White holes are sometimes called ‘remnants’ because they’re what remains after the evaporation of a black hole. The transition from black to white hole can be thought of as a ‘quantum leap’ This is akin to the Danish physicist Niels Bohr’s concept of quantum leaps, in which electrons jump from one atomic orbit to another when they change energy.
Quantum leaps cause atoms to emit photons and are what causes the emission of light that allows us to see objects. But LQG predicts the size of these tiny remnants. From this follows a characteristic physical consequence: the quantisation of geometry. In particular, LQG predicts that the area of any surface can only have certain discrete values.
The area of the horizon of the white hole remnant must be given by the smallest non-vanishing value. This corresponds to a white hole with the mass of a fraction of a microgram: roughly the weight of a human hair.
This scenario answers both questions posed earlier. What happens at the end of the evaporation is that a black hole quantum leaps into a long-living tiny white hole. And the matter that falls into a black hole can later exit from this white hole.
Most of the energy of the matter will have already been radiated away by Hawking radiation – low-energy radiation emitted by the black hole due to quantum effects that cause it to evaporate. What exits the white hole isn’t the energy of the matter that fell in it, but residual low-energy radiation, which nevertheless carries all the residual information about the matter that fell in.
An intriguing possibility opened by this scenario is that the mysterious dark matter that astronomers see the effects of in the sky could actually be formed, entirely or in part, by tiny white holes generated by ancient evaporated black holes. These could have been produced in early phases of the Universe, possibly in the pre-Big Bang phase that appears to be also predicted by LQG.
This is an attractive possible solution to the mystery of the nature of dark matter, because it provides an understanding of dark matter that relies solely on General Relativity and quantum mechanics, both well-established aspects of nature. It also doesn’t add ad hoc particles of fields, or new dynamical equations, as most of the alternative tentative hypotheses about dark matter do.
Next steps
So, can we detect white holes? Direct detection of a white hole would be difficult because these tiny objects interact with the space and matter around them almost uniquely through gravity, which is very weak.
It’s not easy to detect a hair using only its gravitational attraction. But perhaps it won’t remain impossible as technology advances. Ideas on how to do so using detectors based on quantum technology have already been proposed.
If dark matter is composed of white hole remnants, a simple estimate shows that a few of these objects might fly through a space the size of a large room every day. For now, we must study this scenario and its compatibility with what we know about the Universe, waiting for technology to help us detect these objects directly.
It’s surprising that this scenario wasn’t considered previously, though. A reason can be traced to a hypothesis adopted by many theoreticians with a background in string theory: a strong version of the so-called ‘holographic’ hypothesis.
According to this hypothesis, the information inside a small black hole is necessarily small, contradicting the above idea. The hypothesis is grounded on the idea of eternal black holes: in technical terms, the idea that the horizon of a black hole is necessarily an ‘event’ horizon (an ‘event’ horizon is by definition an eternal horizon). If the horizon is eternal, what happens inside is effectively lost forever, and a black hole is uniquely characterised by what can be seen from the outside.
But quantum gravitational phenomena disrupt the horizon when it has become small, preventing it from being eternal. So the horizon of a black hole fails to be an ‘event’ horizon. The information it contains can be large, even when the horizon is small, and can be recovered after the black hole phase, during the white hole phase.
Curiously, when black holes were studied theoretically and their quantum properties disregarded, the eternal horizon was considered their defining property. Now that we understand black holes as real objects in the sky, and investigate their quantum properties, we realise that the idea that their horizon must be eternal was just an idealisation.
Reality is subtler. Perhaps nothing is eternal, not even the horizon of a black hole.
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