Perhaps the most surprising scientific discovery of the past decade is that the universe is teeming with black holes.
They have been discovered in a surprising variety of sizes, some with masses only slightly greater than the Sun, others billions of times greater. And they have been discovered in a variety of ways: by radio emissions from the matter falling into the hole; by their effect on the stars orbiting them; by the gravitational waves emitted when they merge; and by the extremely peculiar distortion of light they cause (think back to the “Einstein ring” seen in images of Sagittarius A*, the supermassive black hole at the center of the Milky Way, which recently made headlines around the world).
The space we live in is not smooth – it is pitted, like a sieve, by these holes in the air. The physical properties of all black holes were predicted by Einstein’s general theory of relativity and are well described by the theory.
Everything we know about these strange objects so far fits Einstein’s theory almost perfectly. But there are two important questions that Einstein’s theory does not answer.
The first: where does matter go once the hole enters? The second: how do black holes end? Compelling theoretical arguments, first understood by Stephen Hawking a few decades ago, indicate that in the distant future, after a lifespan that depends on its size, a black hole shrinks (or as physicists say, “evaporates”), emitting hot radiation now known as Hawking radiation.
This results in the hole getting smaller and smaller, until it is tiny. But what happens then? The reason that these two questions are not yet answered, and that Einstein’s theory does not provide an answer, is that they both concern 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 are not yet established, because they have not been supported by experiments or observations to date.
But they are developed enough to give us preliminary answers to these two important questions. And so we can use these theories to make an educated guess about what is happening.
not defined
The most detailed and developed theory of quantum spacetime is undoubtedly loop quantum gravity, or LQG – a preliminary 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 stage where quantum effects start to dominate.
This creates a powerful repulsive force that reverses the dynamics of the interior of the collapsing black hole, causing it to “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 instead of shrinking.
The possibility of an expanding hole is indeed predicted by Einstein’s theory, in the same way that black holes were predicted. It is a possibility that has been known for decades; so long, in fact, that this corresponding region of spacetime even has a name: it is called a “white hole.”
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Same idea, but the other way around
The name reflects the idea that a white hole is in some sense the inverse of a black hole. It can be thought of in the same way that a ball bouncing upward follows an upward trajectory that is the reverse of the downward trajectory it took when it fell.
A white hole is a spacetime structure similar to a black hole, but with time reversed. Inside a black hole, things fall inward; inside a white hole, things move outward. Nothing can leave a black hole; likewise, nothing can enter a white hole.
Seen from the outside, what happens at the end of the evaporation is that a black hole, now tiny because it has evaporated most of its mass, mutates into a tiny white hole. LQG indicates that such structures become quasi-stable due to quantum effects, so that they can live for a long time.
White holes are sometimes called “leftovers” because they are 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 similar to 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 the source of the emission of light that allows us to see objects. But LQG predicts the size of these small remnants. This has a distinctive physical consequence: the quantization of geometry. In particular, LQG predicts that the area of a surface can only have certain discrete values.
The area of the horizon of the remnant of the white hole must be given by the smallest non-vanishing value. This corresponds to a white hole with the mass of a fraction of a microgram: about the weight of a human hair.
This scenario answers both of the questions posed earlier. What happens at the end of the evaporation is that a black hole makes a quantum leap into a long-lived small white hole. And the matter that falls into a black hole can later escape 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 leaves the white hole is not the energy of the matter that fell in, but residual low-energy radiation, which nevertheless carries with it all the residual information about the matter that fell in.
One intriguing possibility that this scenario opens up is that the mysterious dark matter whose effects astronomers see in the sky could in fact be formed entirely or in part by small white holes generated by old evaporating black holes. These could have been produced in early phases of the universe, possibly in the pre-Big Bang phase that LQG also seems to predict.
This is an attractive potential solution to the mystery of the nature of dark matter, because it offers an understanding of dark matter that relies solely on general relativity and quantum mechanics, both well-established aspects of nature. It also does not add ad hoc particles or fields, or new dynamical equations, as most 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 small objects interact almost uniquely with the space and matter around them via gravity, which is very weak.
It is not easy to detect a hair using gravity alone. But perhaps it will not remain impossible as technology advances. Ideas on how to do this with detectors based on quantum technology have already been proposed.
If dark matter is the remnants of white holes, then a simple estimate shows that a few of these objects could fly through an area 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, while we wait for technology to help us detect these objects directly.
It is surprising that this scenario has not been considered before. One reason for this can be traced to a hypothesis that has been accepted by many theorists with a background in string theory: a strong version of the so-called ‘holographic’ hypothesis.
According to this hypothesis, the information in a small black hole is necessarily small, which contradicts the idea above. The hypothesis is based 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, then what happens inside is effectively lost forever, and a black hole is uniquely characterized by what can be seen from the outside.
But quantum gravity phenomena disturb the horizon when it has become small, so it cannot be eternal. So the horizon of a black hole is not an ‘event’ horizon. The information it contains can be large, even when the horizon is small, and can be retrieved after the black hole phase, during the white hole phase.
Oddly enough, when black holes were studied theoretically and their quantum properties were ignored, 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 realize that the idea that their horizon must be eternal was just an idealization.
The reality is more subtle. Perhaps nothing is eternal, not even the horizon of a black hole.
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