Black holes are elegant and simple celestial bodies, but in the public's imagination, they sometimes appear very terrible. In many ways, they are like an ideal black body (which can absorb all external electromagnetic radiation without any reflected and transmitted objects), making it difficult for us to estimate how many black holes exist in the universe and their sizes. Therefore, from September 2065438 to September 2005, when the detector of the Laser Interferometer Gravitational Wave Observatory (LIGO) first detected gravitational waves, it really brought surprises to the physics community. Prior to this, the largest stellar black hole-the black hole formed after the gravitational collapse of a massive star-was about 20 times the mass of the sun; These newly discovered black holes are all about 30 times the mass of the sun, which is not incredible, but very strange. In addition, when LIGO started and immediately began to hear the signal of the fusion of such objects, astrophysicists realized that there must be more black holes lurking there, perhaps far beyond their imagination.
The discovery of these strange black holes has injected new vitality into an old view that has been gradually marginalized in recent years. We know that dying stars will produce black holes, but black holes may also be born in BIGBANG. These "primitive" black holes may be hidden and constitute dark matter. After all, despite decades of exploration, researchers have not detected dark matter particles. Maybe we can boldly assume that what will happen if the black hole is always under our noses?
Mark Camio, a cosmologist at Johns Hopkins University in the United States, said that this is indeed a crazy idea, but it is not necessarily crazier than other viewpoints. In fact, many papers have been exploring this possibility. On 20 16, Kamiokusky's research group also published a remarkable paper.
Unfortunately, new york University astrophysicist Yassin? Yacine Ali-Haimoud published a paper on 20 17, and studied how this type of black hole would affect the detection rate of LIGO. Then the relationship between dark matter and primitive black holes began to be questioned. Ali-Haymond calculated that if there are enough black holes in the new universe to explain dark matter, then over time, these black holes will form a double black hole system, which will surround each other and get closer and closer, and the merging speed will even be thousands of times higher than the merger event observed by LIGO. He called on other researchers to continue to study the idea in other ways. But many people lost hope. Camio Kosky pointed out that Ali-Haymond's argument was so compelling that his own interest in this hypothesis was doused.
Now, however, with the publication of a series of recent papers, the view of primitive black holes seems to be revived. Not long ago, Carleston Dacik, a cosmologist at Montpellier University in France, published the latest research report in the Journal of Cosmology and Astrophysics, in which he explained how a large number of primitive black holes caused collisions that were completely consistent with LIGO observations. "If his result is correct-it seems that he has calculated quite carefully-it will stop our calculation," Ali Haymond said. "This will mean that they may actually be dark matter." In his subsequent papers, he also continued to study the viewpoint of primitive black holes.
Christian Byrnes, a cosmologist at the University of Sussex in the United Kingdom, said that this result is very exciting. "He has gone further than anyone before." Burns helped Dachik put forward some ideas.
The original idea of this argument can be traced back to the work of Stephen Hawking and Bernard Carr in the 1970s. They concluded that in the first few seconds of the universe, small fluctuations in density may give some areas too much mass. Each such region will collapse into a black hole, and the size of the black hole will be determined by the horizon of the region. The so-called horizon is the space around any point that can be reached at the speed of light. Anything in the horizon will feel the gravity of the black hole and fall in. Hawking's rough calculations show that if black holes are bigger than smaller asteroids, they may still be lurking in today's universe.
Great progress was made in the 1990s. At that time, theoretical physicists also put forward the theory of cosmic inflation, arguing that the universe experienced an extreme expansion after the Big Bang. The inflation theory can explain where the initial density fluctuation came from. On the basis of density fluctuation, physicists also considered a key transition along the collapse direction.
When the universe was just formed, all its matter and energy were boiling in unimaginable high-temperature plasma. After the first one hundred thousandth of a second, the universe cooled down slightly, and the loose quarks and gluons in the plasma combined to form heavier particles. When some lightning-fast moving particles are bound together, the pressure will also drop. This may help more regions collapse into black holes.
However, in the 1990s, no one knew enough about the physics of quark and gluon fluids, so it was impossible to accurately predict how this transformation would affect the formation of black holes. Theoretical physicists still don't know how big the mass of a primitive black hole should be, or how much it should be.
Besides, cosmologists don't really seem to need primitive black holes. The sky survey scanned a small area of the sky, hoping to find a large number of dark objects as dense as black holes floating around the galaxy, but it didn't yield much. On the contrary, most cosmologists begin to believe that dark matter is composed of extremely "unsociable" massive weakly interacting particles (WIMP). This is a kind of particle that is still in the theoretical stage. It only interacts with weak nuclear force and gravity, and basically does not interact with ordinary matter. The purpose-built WIMP detector and the upcoming Large Hadron Collider may soon find conclusive evidence of their existence. I hope so.
The problem of dark matter seems to be about to break through, and no other options have been observed, so the primitive black hole has become an academic backwater. "A senior cosmologist seems to be laughing at me for studying this," said Jean Dachick. "So I stopped because I needed a permanent position." His interest in this field can be traced back to the 1990s.
Of course, in the decades after that, scientists did not find WIMP or any new particles (except the Higgs boson, which was predicted long ago). The solution to the mystery of dark matter is still far away.
However, we now know more about the environment that may produce primitive black holes. Physicists have been able to calculate how quark-gluon plasma evolved pressure and density at the beginning of the universe. Burns said that it took decades for physics to achieve these results. Using this information, theoretical physicists such as Burns and Juan García-Bellido of the Autonomous University of Madrid have published a series of papers in the past few years, predicting that black holes in the early universe may have not only one size, but a series of different sizes.
At first, quarks and gluons combined to form protons and neutrons. This leads to a drop in pressure and may produce a group of primitive black holes. As the universe continues to cool, particles such as π mesons form, leading to another pressure drop and possible black hole explosion.
Between these two periods, space itself is also expanding. The original black hole can absorb something about the mass of the sun from the horizon around it. The second round may inhale about 30 times the mass of the sun, just like the strange object that LIGO first detected. "Gravitational waves saved us," Garcia Beredo said.
In 20 16, the original black hole hypothesis was revived within a few weeks after the LIGO team announced the detection of the first gravitational wave. But the next year, Ali Haymond put forward his view that primitive black holes would collide too frequently, which brought great challenges to the supporters of the primitive black hole hypothesis.
Let Dachek accept the challenge. During a long vacation in Costa Rica, he carefully studied Ali Haymond's statement. Ali-Haymond analyzed this problem through equations, but when Dachik was asked to simulate the same problem numerically, he found a turning point.
The original black hole did form a double black hole system, but let Dachik conclude that in a universe full of black holes, the third black hole usually approaches the first pair of black holes and exchanges places with one of them. This process will be repeated again and again.
With the passage of time, this transformation from a pair of black holes to another pair of black holes will make the orbit of the double black hole system almost circular. These black hole pairs will collide very slowly. Even if there are a large number of primitive black holes, they will not merge so frequently. This makes the whole hypothesis still conform to the merger rate observed by LIGO.
In June 2020, Jean Dachik published his research results on the Internet and answered questions raised by external experts such as Ali Haymond. He said: "It is very important to do everything possible to convince the academic community that you are not talking nonsense."
He also predicted that the original black hole will be located in a dark cluster, and its diameter is about the same as the distance between the sun and the nearest star. Each cluster may contain about 1000 crowded black holes. A giant black hole with the mass equivalent to 30 suns will be located in the center; More ordinary smaller black holes fill the remaining space. These clusters of galaxies will lurk wherever astronomers think dark matter exists. Just like a star in a galaxy, or a planet orbiting the sun, the orbital motion of each black hole will prevent it from swallowing another black hole-unless something unusual happens.
In the second paper, let Dachek accurately calculate the rarity of these merger events. He calculated the big black hole observed by LIGO and the small black hole not observed (the small black hole will send out a weak and sharp signal, which can only be detected very close to it). "Of course, I was shocked when I got the correct values of the merger rate one by one," said Jean Dachek.
Supporters of the original black hole hypothesis still need to do a lot of work to be more convincing. Most physicists still believe that dark matter is composed of some elementary particles, which are extremely difficult to detect. In addition, if the black holes detected by LIGO come from ordinary stars, they are not much different from the black holes we expected. "To some extent, this fills a hole in the theory that does not actually exist," said Carl Rodriguez, an astrophysicist at Carnegie Mellon University in the United States. "Some LIGO light sources are strange, but we can explain everything we have seen so far through the normal process of star evolution."
Selma, an astrophysicist at Harvard University? De? Selma Mink said even more bluntly, "I think astronomers can laugh it off." He once put forward the theory of how the stars observed by LIGO form a large double black hole system alone.
According to the original black hole hypothesis, it is found that sub-solar black holes should be common, and this black hole cannot be formed by stars. If this view is correct, it will change the whole debate. In every future observation, with the improvement of LIGO sensitivity, LIGO will eventually find these small black holes, or strictly limit the number of possible black holes. "This assumption is different from string theory. Ten or thirty years later, we may still be discussing whether string theory is correct, "Burns said.
Meanwhile, other astrophysicists are exploring different aspects of this theory. For example, the strongest restriction on primitive black holes may come from gravitational microlens search. Gravitational microlens was put forward in 1960s, which described the phenomena of gravitational microlens in star-level celestial bodies, and the research on these phenomena began in 1990s. Astronomers monitor bright but distant light sources through these surveys, waiting for dark objects to pass in front of them. Long-term research has ruled out the possibility of uniformly distributed small black holes.
But Garcí a-Beredo said that if primitive black holes exist in a series of masses of different sizes, and if they are compressed into dense massive clusters, these results may not be as important as researchers think. The next observation may finally solve this problem. The European Space Agency recently agreed to provide a key additional function for NASA's upcoming Rome Space Telescope (formerly known as the large-field infrared survey telescope), which will enable it to conduct groundbreaking gravitational microlens research.
This function was introduced under the guidance of Gunther Hasinger, scientific director of the European Space Agency, who proposed that primitive black holes could explain many mysteries. In Hasinger's view, this is a very attractive idea, because no new particles or new physical theories are introduced, but the old elements are reused. "I believe that maybe some unsolved mysteries can actually be solved by yourself, as long as you look at them differently," he said. (Ren Tian)