A black hole may have passed through Earth, but we just haven't seen it yet

Do you think black holes are far away from our lives? But for some physicists, the lingering unawareness of dark matter makes them prefer to believe that black holes have passed through the earth, but we just haven't discovered them.

Generally, black holes are formed when stars exhaust their nuclear fuel. The mass of black holes produced by this process is generally greater than that of the sun. If a black hole heavier than the sun passes through the earth, not only the earth, but also the orbit of the entire solar system will be messed up by it. The impact is comparable to the "light particles" in "The Three-Body Problem". It is impossible for humans not to notice it. If they don't discover it, it can only be that human civilization has been destroyed by it before discovering such a black hole.

Black hole imagination (Image source: pixabay)

Primordial black hole

But a recent paper published in Physics of the Dark Universe has turned its attention to another type of black hole, the primordial black hole. This type of black hole is a small black hole created by local density fluctuations during the Big Bang. The mass of such a black hole may be only equivalent to that of an asteroid and the size of an atom. We cannot observe such a black hole, but it will have a gravitational effect on the outside and is also a candidate component of dark matter.

The paper believes that if primordial black holes really exist in the universe, they may give birth to some hollow asteroids in the universe, or they may leave straight and thin tunnels in the ancient rocks of the earth. We can use these phenomena to search for primordial black holes.

Schematic diagram of the primordial black hole (Image source: NASA)

Atomic-sized primordial black holes may be captured by asteroids. If the core of the asteroid is composed of lava, then a situation may arise: the lava in the center of the asteroid is swallowed by the primordial black hole in the core, and the outer rock shell is solid, and it resists the tendency of gravity to collapse inward with its own strength. In the end, the asteroid will become a hollow asteroid. The paper calculated that for common materials in the universe, such as granite or iron, as long as their size does not exceed 1/10 of the radius of the earth, the strength of the material can resist the stretching of gravity. Therefore, if we find an asteroid with a very low density and a size not exceeding 1/10 of the radius of the earth in the universe, it may be a hollow asteroid caused by a primordial black hole.

So far, things seem to be within the scope of conventional astronomy, but the paper is not satisfied with looking for primordial black holes in space. They even want to look for traces of primordial black holes on Earth.

Primordial black holes are likely to travel quickly in the universe. If a primordial black hole with a mass of 10²² grams passes through the earth, it may leave a tunnel with a diameter of about 0.1 microns in a rigid object. Don't be afraid, the probability of such a primordial black hole passing through the human body is very low. Even if it does pass through the human body, due to the high relative speed, the damage caused is very small. Such a tunnel is too thin and will not have any impact on our body.

But if such a thin tunnel is really observed, it means that primordial black holes may really exist. Researchers calculated that, on average, 0.000001 such tunnels can be left on a cross-sectional area of ​​10 square meters every billion years. Researchers said that such a probability is acceptable for the search for dark matter. Perhaps we can place some large-area metal plates and use a microscope to find tunnels with a diameter of about 1 micron on them, and use this method to look for the possibility of dark matter - primordial black holes.

Crazy ideas, helpless reality

Whether they believe that the black hole has passed through the earth, or that such a low probability is acceptable, is it the distortion of human nature or the decline of morality that makes physicists come up with such crazy ideas? In fact, it is all driven by dark matter.

In the eyes of astronomers, dark matter has long been an indispensable part of explaining the rotation speed of "nearby" galaxies or the evolution of the entire universe. We cannot observe this matter through electromagnetic waves, but we can observe its gravitational influence almost everywhere. Therefore, what exactly is this invisible but gravitationally exerting matter? This is one of the most concerned issues for physicists.

Until recent years, physicists have focused their exploration only on weakly interacting massive particles (WIMPs). This is a hypothetical class of particles that are basically compatible with the current standard model of particle physics and can naturally explain the proportion of dark matter in the universe. Physicists have designed a large number of experiments to search for possible WIMPs, such as my country's PandaX experiment and Italy's XENON experiment, both of which use liquid xenon to search for dark matter. If dark matter passes through liquid xenon, it may induce liquid xenon flashes. Scientists can infer the probability of interaction between dark matter and xenon atoms based on the number and frequency of liquid xenon flashes, which is called the collision cross section in physics and reflects some basic properties of dark matter particles.

But the key problem is that, although the precision of dark matter detection experiments is getting higher and higher, we still can't find any WIMP. In fact, because the current experimental constraints on WIMP collision cross sections are too precise, some classical WIMP candidates have been ruled out.

In November this year, my country's PandaX experiment and Italy's XENON experiment published papers in Physical Review Letters, indicating that the two experiments may have detected the solar neutrino background. Neutrinos emitted by the sun also have a collision cross section, which will affect the accuracy of the experiment. Although the experimental confidence is not high, the PandaX experiment is 2.64σ and the XENON experiment is 2.73σ, which does not meet the 5σ standard, but it is in line with the researchers' predictions and they are full of confidence in this.

The PandaX and XENON experiments may have observed the solar neutrino background (Image credit: APS/Alan Stonebraker)

But the key point is that scientists observed the solar neutrino background before finding dark matter, which shows that the signal of dark matter is weaker than that of solar neutrinos. If the observation accuracy is further improved to find dark matter, the signal of solar neutrinos will become a significant interference, further increasing the difficulty of finding dark matter.

Therefore, physicists have to consider the possibility of dark matter other than WIMP. Recently, various new ideas for searching for dark matter have emerged, such as axions, primordial black holes, and even the Modified Newtonian Theory of Dynamics (MOND) that abandons dark matter. The primordial black hole mentioned in this article is also one of their ideas.

References

[1]https://www.eurekalert.org/news-releases/1066694

[2]https://www.sciencedirect.com/science/article/abs/pii/S2212686424002449?via%3Dihub

[3]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.191001

[4]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.191002

[5]https://physics.aps.org/articles/v17/161

[6] “The Next Treasure Map of Dark Matter”, Global Science, October 2024

Planning and production

Source: Global Science (ID: huanqiukexue)

Author: Wang Yu

Editor: Wang Mengru

Proofread by Xu Lailinlin