Perhaps the most surprising scientific discovery of the last decade is that the universe is teeming with black holes.
They have been detected in a surprising variety of sizes: some with masses only slightly larger than the Sun, others that are billions of times larger. And they were detected in many different ways: by radio emissions from matter falling toward the hole; by their effect on the stars which revolve around them; by the gravitational waves emitted when they merge; and the extremely strange distortions of light they cause (think of the “Einstein Ring” seen in photographs of Sagittarius A*, the supermassive black hole at the center 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 is littered like a colander with 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.
So far, everything we know about these strange objects fits Einstein’s theory pretty perfectly. However, 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 several decades ago by Stephen Hawking, suggest that in the distant future, after a life that depends on its size, a black hole shrinks (or, as physicists say, “evaporates”), emitting hot radiation, which is known today. like Hawking radiation.
This results in the hole getting smaller and smaller until it is tiny. But what’s next? The reason these two questions have not yet been answered, and that Einstein’s theory does not provide an answer, is that they both involve quantum aspects of spacetime.
This means that both involve quantum gravity. And we don’t have an established theory of quantum gravity yet.
Attempt to answer
However, there is hope as we have tentative theories. These theories have not yet been established because they have not yet been supported by experiments or observations.
However, they are sufficiently developed 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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Probably the most detailed and developed theory of quantum spacetime is loop quantum gravity, or LQG – a preliminary theory of quantum gravity that has been in constant development since the late 1980s.
Thanks to this theory, an interesting answer to these questions appeared. The following scenario provides this answer. The interior of the black hole evolves until it reaches a stage where quantum effects start to dominate.
This creates a strong repulsive force that reverses the dynamics of the interior of the collapsing black hole so that it “bounces”. 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 expands rather than contracts.
Indeed, the possibility of an expanding hole is predicted by Einstein’s theory in the same way that black holes were predicted. It’s a possibility that’s been known about for decades; so long that this corresponding space-time region even has a name: it’s called a “white hole”.
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Same idea but in reverse
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 bouncing upwards follows an upward trajectory that is the opposite of the downward trajectory it took when the 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 in; inside a white hole, however, things move outward. Nothing can leave a black hole; likewise, nothing can enter a white hole.
Seen from the outside, what happens is that at the end of its evaporation, the black hole, which is now small because it has evaporated most of its mass, mutates into a small white hole. LQG suggests that such structures are quasi-stable due to quantum effects, so they can live for long periods of 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 considered a “quantum jump”. This is similar to Danish physicist Niels Bohr’s concept of quantum hopping, in which electrons jump from one atomic orbital to another as 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 tiny remnants. This leads to a characteristic physical consequence: the quantization 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 rest of the white hole must be given by the smallest non-vanishing value. This corresponds to a white hole weighing a fraction of a microgram: roughly the weight of a human hair.
This scenario answers both questions above. At the end of the evaporation, what happens is that the black hole quantum jumps into a long-lived small white hole. And matter that falls into a black hole can later come out of that white hole.
Most of the matter’s energy has already been emitted by Hawking radiation—low-energy radiation emitted by a black hole as a result of quantum effects that cause it to evaporate. What leaves the white hole is not the energy of the matter that fell into it, but the residual low-energy radiation that nevertheless carries all the residual information about the matter that fell into it.
An interesting possibility raised by this scenario is that the mysterious dark matter whose effects astronomers observe in the sky could actually be made up, in whole or in part, of tiny white holes generated 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 LQG also seems to predict.
This is an attractive possible solution to the mystery of the nature of dark matter, as it provides an understanding of dark matter that relies only on general relativity and quantum mechanics, both well-established aspects of nature. It also does not add ad hoc particle fields or new dynamical equations, as most alternative tentative 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 exclusively through gravity, which is very weak.
It is not easy to detect a hair using only its gravitational attraction. But it may not remain impossible as technology advances. Ideas to achieve 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 through a space the size of a large room every day. For now, we have to study this scenario and its compatibility with what we know about the universe, and wait for the technology to help us detect these objects directly.
It is surprising that this scenario was not considered earlier. The reason can be traced to a hypothesis 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 inside a small black hole is necessarily small, which contradicts the above idea. The hypothesis is based on the idea of ​​eternal black holes: from a technical point of view, it is 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 as it shrinks, preventing it from being eternal. So the horizon of a black hole cannot be an “event” horizon. The information it contains can be large, even if 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 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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