Perhaps the most surprising scientific discovery of the last decade is that the universe is full of black holes.
They have been detected in a surprisingly wide range of sizes: some with masses barely larger than the Sun, others billions of times larger. And they have been detected in a variety of ways: by radio emissions from matter falling toward the hole; by their effect on the stars orbiting them; by gravitational waves emitted when they merge; and by the extremely peculiar distortion of light they cause (think of the Einstein ring, visible in photos of Sagittarius A*, the supermassive black hole at the center of the Milky Way, which made headlines around the world not long ago).
The space we live in is not smooth: it is riddled, like a sieve, with holes in the sky. The physical characteristics of all black holes were predicted by Einstein’s theory of general relativity and are well described by that theory.
So far, everything we know about these strange objects fits perfectly with Einstein’s theory. But there are two essential 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 up? Compelling theoretical arguments, first understood by Stephen Hawking 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”), emitting hot radiation now called Hawking radiation.
The hole then gets smaller and smaller, until it becomes tiny. But what happens next? The reason why these two questions are still unanswered, and why Einstein’s theory does not provide an answer, is that they both involve quantum aspects of spacetime.
In other words, they both involve quantum gravity. Now, we do not yet have an established theory of quantum gravity.
An attempt at an answer
There is hope, however, because we have some tentative theories. These theories are not yet established, because they have not yet been confirmed by experiments or observations.
But they are developed enough to give us some answers to these two important questions. So we can use these theories to make an informed hypothesis about what is going on.
indefinite
Perhaps the most detailed and well-developed theory of quantum spacetime is loop quantum gravity, or LQG – a tentative theory of quantum gravity that has been steadily developing since the late 1980s.
Thanks to this theory, an interesting answer to these questions has emerged. This 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 collapsing black hole’s interior, causing it to “bounce back.” After this quantum phase, described by quantum magnitude theory, spacetime inside the hole is again governed by Einstein’s theory, except now the black hole is expanding rather than contracting.
Einstein’s theory actually predicts the possibility of an expanding hole, in the same way that black holes do. It’s a possibility that’s been known for decades, so long in fact that the corresponding region of spacetime 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 reverse 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 reverse of the downward trajectory taken when the ball fell.
A white hole is a space-time structure similar to a black hole, but with time reversed. Inside a black hole, objects fall in; inside a white hole, on the other hand, objects come out. Nothing can come out of a black hole; likewise, nothing can go into a white hole.
From the outside, what happens is that at the end of its evaporation, a black hole, now tiny because it has lost most of its mass, transforms into a tiny white hole. LQG indicates that such structures are made quasi-stable by quantum effects, which allows them to live a long time.
White holes are sometimes called “leftovers” because they are what is left 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 leaps, in which electrons jump from one atomic orbit to another as they change energy.
Quantum jumps cause atoms to emit photons, which in turn cause the light to be emitted that allows us to see objects. But quantum quantization theory predicts how small these tiny remnants are. This results in a characteristic physical consequence: the quantization of geometry. In particular, quantum quantization theory predicts that the area of any surface can only have certain discrete values.
The area of the horizon of the remainder of the white hole must be given by the smallest non-zero value. This corresponds to a white hole whose mass is a fraction of a microgram, or about the weight of a human hair.
This scenario answers the two questions posed previously. What happens at the end of evaporation is that a black hole makes a quantum jump into a tiny, long-lived white hole. And matter that falls into a black hole can then flow out of that white hole.
Most of the energy of the matter will have already been removed by Hawking radiation, a 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 a residual low-energy radiation, which nevertheless carries all the residual information about the matter that fell into it.
An interesting possibility opened by this scenario is that the mysterious dark matter whose effects astronomers observe 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 the earliest phases of the Universe, perhaps in the pre-Big Bang phase that also seems to be predicted by LQG.
This is a possible and interesting solution to the mystery of the nature of dark matter, because it allows an understanding of dark matter that relies solely on general relativity and quantum mechanics, two well-established aspects of nature. It also does not add ad hoc field particles or new dynamical equations, as most alternative hypotheses about dark matter do.
Next steps
So, can we detect white holes? Directly detecting a white hole would be difficult because these tiny objects interact with space and the matter around them almost entirely through gravity, which is very weak.
It is not easy to detect a hair based on its gravitational pull alone. But it may not remain impossible as technology advances. Ideas have already been proposed to achieve this using detectors based on quantum technology.
If dark matter is made up of the remains of white holes, a simple estimate suggests that a few of these objects could pass through a space the size of a large room every day. For now, we have to study this scenario and how it fits with what we know about the Universe, while we wait for technology to help us detect these objects directly.
It is surprising, however, that this scenario has not been considered before. The reason is an assumption adopted by many theorists with a background in string theory: a strengthened version of the so-called “holographic” hypothesis.
According to this hypothesis, the information contained in a small black hole is necessarily small, which contradicts the previous 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 characterized only by what can be seen from the outside.
But quantum gravitational phenomena disturb the horizon when it has become small, preventing it from being eternal. The horizon of a black hole is therefore not 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.
Interestingly, 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 study 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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