(ORDO NEWS) — Black holes excite the imagination. Perfect traps from which not even light can escape. Monsters capable of tearing apart a star and reshaping the fate of the galaxy. Nightmare of intuition, doing puzzling tricks with space and time.
This world is created by gravity. She wants to destroy him. The same force that fashioned galaxies, planets, and stars turns everything into black holes, if only it is allowed to. To understand the essence of this paradox, let’s talk in more detail first about creation, then about destruction.
Porridge with lumps
Imagine that all matter in the visible universe, including even dark matter, consists of hydrogen. With regard to ordinary (not dark) matter, this, by the way, is not far from the truth: it consists of 77% hydrogen. Let’s evenly distribute this matter in space. How much will it work?
The answer is shocking : six atoms per cubic meter. No laboratory in the world can create such a deep vacuum. By and large, the Universe is a great emptiness (Buddhists giggle into a fist at this point).
Immediately after the Big Bang, matter was spread over space almost evenly. There were only small random inhomogeneities.
But gravity came into play. Where the density of the matter turned out to be at least slightly higher than the background density, centers of attraction arose (recall that the force of gravity depends on mass).
This gravity attracted more and more portions of matter. The lump of matter gained mass, which means it became an even more powerful center of gravity, and the circle closed. Eventually matter gathered into galaxies, and within galaxies into stars and planets.
The process of clumping the universe has not gone too far. After all, 80 percent of the mass of ordinary (not dark) matter is still intergalactic gas, and half of the remaining 20 percent is interstellar.
And yet, it is precisely thanks to universal gravitation that at least something exists in the world other than a deserted abyss, in which lonely atoms occasionally meet.
But gravity does not know how to stop voluntarily. The closer two particles of matter, the stronger the attraction between them. Drawn by this attraction, they will become even closer, if only no other force interferes with this.
And then the gravity will increase again. Gravity is an insatiable monster that tends to compress any object into… what? The good old theory of Newton answers: to the point. Einstein’s general theory of relativity clarifies: into a black hole.
This, in essence, is the answer to the question of where black holes come from. They arise when gravity is stronger than any force that prevents the compression of matter. But with what kind of objects does such a nuisance occur, not to say – a tragedy?
When the stars die
The more massive the celestial body, the greater the mutual attraction of its constituent particles of matter and the more difficult it is to resist compression. The planet or brown dwarf copes with this simply due to the pressure of the compressible matter.
With the stars, this number no longer works. The stellar embryo contracts under the influence of gravity until its interior becomes dense and hot enough to ignite thermonuclear reactions. From this moment on, the pressure of radiation is added to the pressure of the substance.
This is the same light pressure discovered by the great Russian physicist P. N. Lebedev. Only on Earth did he need sensitive instruments, but in the depths of a star it is radiation, and not matter at all, that makes a decisive contribution to pressure.
From turning into a black hole, this stronghold of eternal darkness, the star is literally kept by the forces of light.
But thermonuclear fuel runs out sooner or later. True, by this time the star has already scattered a significant part of its mass in space.
But in its place remains a dense and still quite massive core – a stellar remnant . And gravity, which is no longer opposed by the pressure of radiation, rapidly compresses it. The compression continues until quiet, until …
For now? It depends on the mass of the remnant, which, of course, is determined by the mass of the original star. Suppose it was a luminary of moderate mass (up to ten suns). The process then stops when the electrons in the stellar remnant go into a special state: they become a degenerate electron gas.
It resists compression much more violently than ordinary matter. A stellar remnant that has stopped at this stage is called a white dwarf. A cubic centimeter of its substance can weigh a ton, or even a thousand tons! In connection with such a huge density, the white dwarf of the solar mass resembles in size … the Earth.
Do you think this is an attraction of unprecedented density? No matter how. If the original star is more massive than ten suns, the gravity in the stellar remnant is even stronger. Then the degenerate electron gas can no longer stop the compression.
As a result, electrons fuse with protons to form neutrons. It turns out a neutron star . Its radius at the solar mass is already measured in a few kilometers. A cubic centimeter of this substance weighs hundreds of millions of tons.
Well, if a star during its lifetime was more massive than thirty suns, even the pressure of neutron matter is not able to stop the compression. Then there is a “transition to the dark side” – the transformation into a black hole.
By the way, R. Penrose received the 2020 Nobel Prize in Physics for the theoretical description of this metamorphosis (sharing it with R. Genzel and A. Ghez, whom we will meet again).
According to theorists, the lower limit of the mass of a “stellar” black hole is about three solar. The upper limit, if we talk about the stars of our galaxy, is about 20 suns. In galaxies with a slightly different chemical composition, it can be even larger.
Talking about the size and density of a black hole is a thankless task, because it has no surface in the usual sense.
Usually, the event horizon is taken as the conditional surface of a black hole – that very fatal boundary, after crossing which nothing, not even light, can return back. For an “invisible” object with a mass of three Suns, the radius of the event horizon is only nine kilometers.
Cannibals and clashes
How do we know that stellar-mass black holes exist in reality, and not just in the calculations of theorists? First of all, we observe gravitational waves from their collisions. For the discovery of these waves, by the way, the Nobel Prize in Physics in 2017 was awarded.
This is decisive evidence, an official form with a signature and a seal. No other process can generate a gravitational signal of the same structure. The number of recorded space accidents is already approaching a hundred.
In addition, it happens that a black hole forms a close pair with a normal star. Proximity with a predator does not bode well for the luminary. With its powerful gravity, it sucks the substance out of the partner, engaging in real cannibalism.
A cloud of matter gradually falling onto it is spinning around the black hole – an accretion disk . The jets of gas in this disk are heated by friction to such an extent that they shine brightly in the X-ray range.
Observers are aware of several dozen bright X-ray objects that are too massive for neutron stars. Scientists, obliged to be meticulous to the point of tediousness, call them candidates for black holes. But in general, there is almost no doubt that these are black holes.
It happens that a black hole forms a pair with a normal star, but not so close that the relationship has reached cannibalism. In this case, the “clot of darkness” can be detected by noticing that the luminary revolves around an invisible cartridge.
Observers proceed from the principle “judging by the orbit of the satellite star, this thing is too massive for a neutron star, and if it were a normal star, we would have seen it.”
It’s a simple idea, but it’s only recently that observations have reached the required accuracy. So the number of black holes discovered in this way is still measured in units.
In fact, all this is a drop in the ocean. There must be hundreds of millions of stellar-mass black holes in the Milky Way alone. But what to do: they are really black, and it is very difficult to detect them.
The next class of black holes with which observers are very familiar is the supermassive. They have masses from millions to tens of billions of suns, and, of course, there can be no talk of any “stellar” origin of them.
Supermassive black holes form at the centers of galaxies. This is not surprising, because it is there that the density of matter is especially high. Matter flows to the center, attracted by the total gravity of the entire galaxy.
At some point, this cloud of dust and gas becomes so dense that it collapses into a black hole under its own gravity.
Researchers still do not have clarity on how exactly this happens, or rather, why it happens so quickly. Observations of the most distant galaxies show that supermassive black holes already existed when the universe was only 5% of its current age. Such a rapid emergence of these monsters is a mystery that has yet to be solved.
By the way, about observations. Supermassive black holes often have very impressive accretion disks, because there is more than enough material in the center of the galaxy.
Paradoxically, black holes themselves do not radiate anything, but the cloud of matter falling on them turns them into the brightest sources of radiation in the universe. Hundreds of thousands of such objects are known to observers.
Are astronomers sure that these are black holes, and not something else? Yes. First, in 2008, R. Genzel and A. Ghez measured the mass and radius of the central object of the Milky Way quite accurately.
It turned out that a body comparable in size to the solar system has a mass of four million suns (what do you know about efficient packaging!). Such an object can only be a black hole.
Secondly, in 2019, astronomers for the first time obtained an image of the “shadow” of a black hole in the M87 galaxy, which exactly coincided with the predictions of the theory.
Of course, there is no such detailed information about hundreds of thousands of other supermassive black holes, but the precedent has been created.
Members of the middle class
There are also black holes in the Universe, too massive for “stellar”, but not up to the honorary title of supermassive. They are called black holes of medium, or intermediate, mass.
This too broad class was clearly formulated according to the principle “and here we have a blank spot on the map.” It is clear that a hundred suns and a hundred thousand suns are very different masses, and behind them there must be equally different mechanisms of formation. But very little is known about either of them.
Gravitational wave detectors once detected the collision of two unusually large black holes. The mass of the first was 71-106 solar, and the mass of the second was 48-83 solar. When they collided, an object with a mass of 126-170 suns was formed, which certainly belongs to the “middle class”.
But the “participants in the accident” are too big for stellar remnants. Perhaps they themselves are the product of the collision and merger of stellar-mass black holes.
On the other side of the abyss are black holes weighing hundreds of thousands of suns. They could have formed in the same way as supermassive ones.
It’s just that their parent galaxies are small, so black holes are, so to speak, undernourished. In the centers of some dwarf galaxies, X-ray telescopes do detect what looks like a “sub-supermassive” black hole.
The number of such objects has already exceeded one hundred. And recently, a “predator” with a mass of about 90 thousand suns was found in the core of a dwarf system, once swallowed by the Andromeda galaxy.
It remains to tell about the primary black holes – the first cry of the newborn Universe. They must exist, but there is no way to find them.
We mentioned that the matter in the newborn cosmos was distributed almost uniformly. This is an important “almost”, because the primary inhomogeneities became points of growth, from which galaxies eventually formed.
But in some extremely rare points, the density of matter was so high from the very beginning that they immediately turned into black holes. This happened in the first fraction of a second after the Big Bang.
There were minutes before the formation of atomic nuclei, and hundreds of thousands of years before the appearance of the first atoms. These black holes are called primordial.
Cosmologists are convinced that primordial black holes exist. There is no way to avoid them without breaking the whole theory of the early universe. But observers shrug their shoulders: not a single black hole that could be confidently attributed to the primary ones has yet been discovered.
All data on the number of such objects are upper bounds. In other words, “there are definitely no more than so many, because if there were more, we would have noticed them by now.”
How is it possible to detect and identify this space fossil? First of all, by weight. At the time of birth, relic black holes had a very different mass, from a speck of dust to hundreds of thousands of suns.
And this is the only known mechanism for the formation of black holes with a mass significantly less than the sun. If we ever find such a crumb, it will become clear: here it is, a relic animal.
However, the smallest of the primordial black holes have long faded away due to Hawking radiation. To survive to this day, such an object needs to have a mass at least as large as a large asteroid (and at the same time it will be… about a proton in size).
Speaking of Hawking radiation. It consumes the mass of the black hole. But the smaller the mass, the stronger the radiation, so that the process goes with self-acceleration. When the black hole becomes lighter than the nucleus of a small comet, it disappears in a bright flash of gamma rays.
It literally looks like an explosion. Theoretically, some of the primordial black holes are exploding right now, this very second. Observers do not leave hope to see through gamma-ray telescopes, if not a single such event, then at least the background from many distant explosions.
There are other ways to look for primordial black holes, but so far none of them has been successful. If and when it finally happens, this discovery will certainly be worthy of another Nobel Prize.
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