Perhaps the most surprising scientific discovery of the last decade is that the Universe is full of black holes.
They have been discovered in a surprising variety of sizes: some with masses only slightly larger than the Sun, others that are billions of times larger. And they have been detected in a variety of ways: from radio emissions from matter falling toward the hole; from their influence on the stars revolving around them; from gravitational waves emitted as they merge; and from the extremely peculiar distortion of light they cause (think back to the ‘Einstein ring’ seen in pictures of Sagittarius A*, the supermassive black hole at the center of the Milky Way, which graced the front pages of the world’s newspapers a long time ago).
The space we inhabit is not smooth—it is gouged, like a colander, by these holes in the sky. The physical properties 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 matches Einstein’s theory so far. But there are two key questions that Einstein’s theory does not answer.
First: when matter enters the hole, where does it go next? Second: how do black holes end? Compelling theoretical arguments, first understood by Stephen Hawking several decades ago, show that in the distant future, after a lifetime that depends on its size, a black hole shrinks (or as physicists say, ‘evaporates’ ‘), emitting the already familiar hot radiation. like Hawking radiation.
This results in the hole getting smaller and smaller until it is tiny. But what happens next? The reason these two questions are still unanswered, and that Einstein’s theory does not provide an answer, is that they both involve quantum aspects of spacetime.
That is, both involve quantum gravity. And we don’t yet have a settled theory of quantum gravity.
An attempt at an answer
However, there is hope, because we have proof theory. These theories have not yet been established because, to date, they have not been supported by experiments or observations.
But they have developed sufficiently to give us preliminary answers to these two important questions. And so we can use these theories to make an educated guess about what’s going on.
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Perhaps the most detailed and developed theory of quantum space is loop quantum gravity, or LQG—a tentative theory of quantum gravity that has been steadily developed since the late 1980s.
Thanks to this theory, an interesting answer to these questions has emerged. This answer is provided by the following scenario. The interior of a black hole evolves until it reaches a stage where quantum effects begin to dominate.
This creates a powerful repulsive force that reverses the dynamics of the collapsing black hole’s interior, causing it to ‘recoil’. After this quantum phase, described by LQG, the space-time inside the hole is once again governed by Einstein’s theory, except now the black hole is expanding instead of contracting.
The possibility of an expanding hole is indeed predicted by Einstein’s theory, in the same way that black holes were predicted. It’s a possibility that’s been known for decades; so long, in fact, that this corresponding region of space even has a name: it’s called a ‘white hole’.
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Same idea, but opposite
The name reflects the idea that a white hole is, in a sense, the opposite of a black hole. It can be thought of in the same way that a ball that bounces up follows an upward trajectory that is the opposite of the downward trajectory taken when that ball fell.
A white hole is a space-time structure that is similar to a black hole, but with reversed time. Inside a black hole, things fall; inside a white hole, however, things move outward. Nothing can come out of 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 small because it has evaporated most of its mass, changes into a small white hole. LQG shows that such structures are made quasi-stable by quantum effects, so they can live for a long time.
White holes are sometimes called ‘leftovers’ because they are what remains after a black hole evaporates. The transition from a black hole to a white hole can be thought of as a ‘quantum jump’ This is similar to Danish physicist Niels Bohr’s concept of quantum jumps, in which electrons jump from one atomic orbit to another when they change energy.
Quantum jumps cause atoms to emit photons and are what cause the emission of light that allows us to see objects. But LQG predicts the size of these small remnants. From this follows a characteristic physical consequence: the quantization of geometry. In particular, LQG predicts that the area of any surface can have only a few discrete values.
The horizon area of the white hole remnant must be given by the smallest non-vanishing value. This corresponds to a white hole with a mass of a fraction of a microgram: roughly the weight of a human hair.
This scenario answers both of the questions posed earlier. What happens at the end of evaporation is that a quantum black hole jumps into a small long-lived white hole. And the matter that falls into a black hole can later come out of this white hole.
Most of the matter’s energy will have already been radiated by Hawking radiation – the low-energy radiation emitted by the black hole due to quantum effects that cause it to evaporate. What comes out of the white hole is not the energy of the matter that fell into it, but the low-energy residual radiation, which nonetheless carries all the remaining information about the matter that fell into it.
One intriguing possibility opened up by this scenario is that the mysterious dark matter whose effects astronomers see in the sky may be formed, in whole or in part, by tiny white holes created by ancient vaporized black holes. These could have been produced in the early stages of the Universe, perhaps in the pre-Big Bang phase that seems to have been predicted by LQG as well.
This is an attractive potential solution to the mystery of the nature of dark matter because it provides an understanding of dark matter that relies only on General Relativity and quantum mechanics, both well-defined aspects of nature. It also does not add ad hoc particle fields, or new dynamical equations, as most alternative dark matter hypotheses 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 is not easy to detect a strand of hair using only its gravitational pull. But it probably won’t remain impossible as technology advances. Ideas on how to do this using detectors based on quantum technology have already been proposed.
If dark matter consists of the remnants of white holes, a simple estimate shows that several of these objects can fly every day through a space the size of a large room. For now, we have to study this scenario and its compatibility with what we know about the Universe, waiting for the technology to help us detect these objects directly.
It is surprising that this scenario was not considered before, however. One reason can be traced to a hypothesis espoused by many theorists 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 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, what happens inside is effectively lost forever, and a black hole is uniquely characterized 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.
Surprisingly, 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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