This is what might (probably) be on the other side of a black hole.

Perhaps the most surprising scientific discovery of the past decade is that the universe is teeming with black holes.

These black holes have been observed in a variety of astonishing sizes: some with masses slightly larger than the Sun, others billions of times larger. They have also been observed in a variety of ways: by radio emissions from matter falling into the hole; by their effect on stars orbiting it; by the gravitational waves emitted as they merge; and by the very strange distortion of light they cause (remember the Einstein ring, seen in images of Sagittarius A*, the supermassive black hole at the center of the Milky Way, which graced the front pages of the world’s newspapers not long ago).

The space we live in is not smooth, but rather full of holes in the sky, like a colander. Einstein’s theory of general relativity predicted the physical properties of all black holes, and the theory described them well.

Everything we know about these strange objects is in perfect agreement with Einstein’s theory so far. But there are two major questions that Einstein’s theory does not answer.

The first question is: Where does matter go when it enters a black hole? The second question is: How do black holes end? Compelling theoretical arguments, first understood by Stephen Hawking several decades ago, suggest that in the distant future, after a lifetime depending on its size, a black hole will shrink (or, as physicists say, “evaporate”) by emitting hot radiation now known as Hawking radiation.

This causes the hole to get smaller and smaller, until it is very small. But what happens after that? The reason these two questions are not yet answered, and Einstein’s theory does not provide an answer, is that they both involve quantum aspects of spacetime.

This means that both involve quantum gravity, but we don’t have a solid theory of quantum gravity yet.

Attempt to answer

But there is hope, because we have tentative theories. These theories have not yet been proven, because they have not yet been supported by experiments or observations.

But they are sophisticated enough to provide us with tentative answers to these two important questions. So we can use these theories to make an educated guess about what’s going on.

Unknown

Perhaps the most detailed and advanced theory in the field of quantum spacetime is loop quantum gravity, or LQG – an experimental quantum gravity theory that has been steadily developing since the late 1980s.

Thanks to this theory, an interesting answer to these questions has emerged. This answer is illustrated in the following scenario. The interior of the black hole evolves until it reaches a stage where quantum effects begin to dominate.

This creates a strong 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 the theory of quantum gravity, the spacetime inside the hole again obeys Einstein’s theory, except that the black hole is now expanding rather than contracting.

The possibility of a black hole expanding was actually predicted by Einstein’s theory, in the same way that black holes are predicted. It has been known for decades; in fact, this corresponding region of spacetime has a name: 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. We can think of it in the same way that a bouncing ball follows an upward path that is the opposite of the downward path it took when that ball fell.

A white hole is a spacetime structure similar to a black hole but with time reversed. Inside a black hole, things fall in; inside a white hole, things move out. Nothing can get out of a black hole; likewise, nothing can get in a white hole.

Looking at it from the outside, what happens is that at the end of the evaporation process, the black hole, which is now small because most of its mass has evaporated, turns into a small white hole. LQG suggests that such structures become nearly stable by quantum effects, and can therefore 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, where electrons jump from one atomic orbit to another when they change energy.

Quantum jumps cause atoms to emit photons, which is what causes the light that allows us to see things. But quantum gravity theory predicts how big these tiny remnants are. And from there comes a remarkable physical result: the quantization of geometry. In particular, quantum gravity theory predicts that the area of ​​any surface can only have certain discrete values.

The area of ​​the white hole remnant horizon must be determined 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 the two questions posed earlier. What happens at the end of the evaporation process is that the quantum black hole jumps into a small, long-lived white hole. Matter falling into a black hole can later escape from this white hole.

Most of the matter’s energy will have already been released by Hawking radiation – low-energy radiation emitted by the black hole due to quantum effects that cause it to evaporate. What emerges from the white hole is not the energy of the matter that fell in, but the low-energy radiation that remains, which nevertheless carries all the remaining information about the matter that fell in.

One intriguing possibility this scenario opens up is that the mysterious dark matter that astronomers see traces of in the sky may actually have been formed, in whole or in part, from tiny white holes that were born from ancient, evaporating black holes. These holes may have been created in the early stages of the universe, perhaps in the period before the Big Bang that quantum gravity also seems to predict.

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 solely on general relativity and quantum mechanics, two well-established aspects of nature. It also does not add random field particles or new dynamical equations, as most alternative experimental hypotheses about dark matter do.

Next steps

So can we detect white holes? Observing white holes directly would be difficult because these tiny objects interact with space and the matter around them almost uniquely through gravity, and it is extremely weak.

It is not easy to detect a hair using its gravity alone. But it may not remain impossible as technology advances. Ideas have already been proposed about how to do this using detectors based on quantum technology.

If dark matter is made up of white hole remnants, a simple estimate suggests that a few of these objects could fly through an area the size of a large room every day. For now, we have to study this scenario and see how it fits with what we know about the universe, until technology helps us detect these objects directly.

But surprisingly, this scenario has not been considered before. The reason can be traced back to a hypothesis that many theorists with a background in string theory have embraced: 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 idea mentioned above. The hypothesis is based on the idea of ​​eternal black holes: technically, 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 the black hole is uniquely characterized by what can be seen from the outside.

But quantum gravitational phenomena disrupt the horizon when it becomes small, preventing it from being eternal. So the black hole horizon cannot be an “event” horizon. The information it contains may 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 were ignored, the eternal horizon was seen as their defining feature. Now that we understand black holes as real objects in the sky, and have investigated their quantum properties, we realize that the idea that their horizon should be eternal was just an ideal.

The reality is more nuanced. Perhaps nothing is eternal, not even the horizon of a black hole.

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