Perhaps the most astonishing scientific discovery of the past decade is that the universe is full of black holes.
A surprising variety of sizes has been discovered. SunSome are billions of times larger. And they’re detected in a variety of ways: by radio emissions from material falling into the holes, by their effects on stars orbiting them, by gravitational waves emitted when they merge, and by the very strange distortions of light they cause (think of the “Einstein rings” seen in the photo). Sagittarius A*(The supermassive black hole at the center of the Milky Way, which until recently was making headlines in newspapers around the world.)
The universe we live in is not flat, but empty with holes like a sieve. The physical properties of all black holes are predicted by, and well explained by, Einstein’s theory of general relativity.
So far, everything we know about these strange objects matches Einstein’s theory almost perfectly. But there are two important questions that Einstein’s theory can’t answer.
First, once matter enters a black hole, where does it go next? Second, how does the black hole end? A compelling theoretical argument, first understood decades ago by Stephen Hawking, suggests that in the distant future, after a lifetime proportional to its size, a black hole will shrink (or “evaporate” as physicists call it) by emitting high-temperature radiation now known as Hawking radiation.
As a result, the hole gets smaller and smaller until it becomes very small. But what happens after that? The reason these two questions are still unanswered, and Einstein’s theory doesn’t provide the answer, is because they both have to do with quantum aspects of space-time.
So they both have to do with quantum gravity, which we don’t yet have a theory of.
An attempt at an answer
However, there is hope as tentative theories exist. These theories have not yet been established as they have not been supported by experiments or observations to date.
But they are developed enough to give us tentative answers to these two important questions, and so we can use these theories to make inferences about what might be going on.
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Perhaps the most detailed and developed theory of quantum space-time is loop quantum gravity (LQG), a tentative theory of quantum gravity that has been steadily developing since the late 1980s.
Thanks to this theory, an intriguing answer to these questions has emerged, which is given in the following scenario: 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 internal dynamics of the collapsing black hole, causing it to “bounce back”. After this quantum phase described by LQG, space-time inside the black hole is again governed by Einstein’s theory, but this time the black hole is expanding instead of contracting.
The possibility of expanding holes is actually predicted by Einstein’s theory, just as black holes were predicted. It’s a possibility that has been known for decades, in fact, this corresponding region of space-time even has a name: it’s called a “white hole.”
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Same idea but in reverse
White holes reflect the idea that, in some sense, they are the inverse of black holes: you can think of them in the same way that when a ball bounces upwards, it follows an upward trajectory that is the reverse of the downward trajectory the ball followed when it fell.
A white hole is a space-time structure that is similar to a black hole, but in which time is reversed. In a black hole, objects fall in, but in a white hole, objects fall out. Nothing can leave a black hole, and nothing can enter a white hole.
From the outside, at the end of the evaporation, the black hole, which has lost most of its mass, turns into a small white hole. LQG shows that such structures can become metastable due to quantum effects and can survive for long periods of time.
White holes are sometimes called “remnants” because they are what’s left over 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 a quantum jump, where an electron changes energy and jumps from one atomic orbital to another.
Quantum jumps cause atoms to emit photons, which trigger the emission of light that makes objects visible. But LQG predicts the size of these tiny remnants. This has distinctive physical consequences: the quantization of geometry. In particular, LQG predicts that the area of any surface can only take on certain discrete values.
The horizon area of a white hole remnant needs to be expressed as the smallest non-zero value, which corresponds to a white hole with a mass of a fraction of a microgram, or about the weight of a human hair.
This scenario answers the two questions posed above: what happens at the end of the evaporation is that the black hole makes a quantum jump and jumps into a much smaller, long-lived white hole, from which the matter that fell into the black hole can later escape.
Most of the matter’s energy has already been radiated away by Hawking radiation, the low-energy radiation emitted when a black hole evaporates due to quantum effects. What a white hole emits is not the energy of the matter that fell into it, but residual low-energy radiation. But this residual low-energy radiation contains all the residual information about the matter that fell into it.
An interesting possibility opened up by this scenario is that Dark matter The phenomena astronomers see in the sky may actually be formed in whole or in part by tiny white holes generated by ancient evaporating black holes. These could have been generated early in the universe, perhaps even before the universe began.big bang The phase that would also be predicted by the LQG.
This is an attractive solution to the mystery of the nature of dark matter because it provides an understanding of dark matter that relies solely on established aspects of nature: general relativity and quantum mechanics, and does not add ad hoc particles or new dynamical equations to the fields, as do most other tentative hypotheses for dark matter.
Next steps
So can white holes be detected? Detecting white holes directly would be difficult because these tiny objects interact almost exclusively with the space and matter around them through the very weak force of gravity.
Detecting hair using gravity alone is not easy, but it may become possible as technology improves. The idea of using quantum-based detectors to detect hair has already been proposed.
If dark matter is made up of the remnants of white holes, a simple estimate is that these objects could be whizzing around in a space the size of a large room every day. For now, we need to study this scenario and whether it fits with what we know about the universe. The UniverseWe are waiting for the arrival of technology that can help us detect these objects directly.
However, it is surprising that this scenario has not been considered before. The reason for this can be traced back to an assumption adopted 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. This hypothesis is based on the idea of an eternal black hole. In technical terms, the horizon of a black hole is necessarily an “event” horizon (which is, by definition, an eternal horizon). If the horizon is eternal, then anything that happens inside is effectively lost forever, and black holes have unique characteristics depending on what is visible from the outside.
However, quantum gravity phenomena disturb the horizon when it becomes small, preventing it from being eternal, so the horizon of a black hole cannot be an “event” horizon: even if the horizon is small, the information it contains can be large 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 thought to be its defining property. Now, as we understand black holes as real objects in the sky and are investigating their quantum properties, we see that the idea that their horizon must be eternal was just an idealization.
The reality is more nuanced: perhaps nothing is eternal, not even the horizon of a black hole.
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