It is generally accepted that all objects in the Universe can rotate, be it a star, planet or galaxy. For example, many of the compact non-luminous objects, which are usually identified with black holes, must spin around their axis like tops. Now scientists are actively discussing the possibility of interpreting some of these objects as non-luminous stars, consisting of the so-called dark matter. In a study by Russian scientists, it was shown that such “dark” stars cannot rotate. And that’s a way to tell them apart from black holes.
Dark matter is a substance that we cannot observe, but we can judge its presence, since we see the gravitational influence that it has on the observed matter.
Now physicists and astronomers believe that the Universe consists mainly of dark matter particles and the properties of these particles are still unknown. So, if the mass of such particles is small, then they are quite capable of forming a superfluid liquid. Under the action of self-gravity, the liquid will form balls – Bose stars. The appearance of such objects was predicted at once in several models of light dark matter. For example, it is known that if dark matter particles have a minimum mass of the order of 10 ^ {- 22} electron-volts, then a giant Bose star should appear in each galaxy, occupying its entire center with a size of several hundred parsecs. In other models, Bose stars can be small and have a mass of the order of the sun, that is, resemble astrophysical black holes.
The study of the properties of such stars is the subject of an article by theoretical physicists of the Institute of Nuclear Research of the Russian Academy of Sciences Igor Tkachev, Alexander Panin and Dmitry Levkov and their co-authors, accepted for publication in the authoritative journal Physical Review D. Scientists argue that Bose stars do not rotate, since they consist of condensate Bose-Einstein – a superfluid liquid (“superfluid”), which rotates extremely reluctantly, and if it does, it does it in an unusual way.
A Bose-Einstein quantum condensate can move without experiencing viscosity, that is, without losing energy due to the friction of adjacent liquid layers against each other. Such a condensate is formed by quantum particles — bosons, cooled to a sufficiently low temperature. To understand how the rotating condensate behaves, it is enough to twist the water in the lake with a stick. A “vortex” is formed in the water – a column of air, elongated along the vertical axis. Water revolves around this column. In ordinary liquids, the vortex movement requires a constant flow of energy, which is spent on friction. But vortices in superfluids can exist for a long time and appear whenever rotation occurs.
The most famous example of “super-liquid” is helium-4, which was discovered to be superfluid in 1937 by Peter Kapitsa in the laboratory.
Can a Bose-Einstein condensate exist in space? Of course, it is impossible to imagine that someone deliberately filled regions of the Universe with purified helium-4. Dark matter Bose stars present another intriguing possibility.
Getting Bose Stars spinning isn’t easy. To do this, you need to “drill a hole” through them, that is, add a vortex, which requires energy expenditure, and the resulting rotating Bose star takes the shape of a donut. But in their article, scientists have demonstrated that such a “donut” is unstable, with a short lifespan. Immediately after formation, it begins to eject particles from itself, which carry away the angular momentum to the periphery. After the disintegration of such a star, a configuration similar to Saturn appears – a non-rotating Bose star plus a cloud of dark matter particles rotating around it. In this article, physicists have proved that rotating Bose stars are destroyed in a similar way in all models of dark matter. The only exception is exotic models, where the quantum condensate of dark matter has a large positive internal pressure, which stabilizes Bose stars.
The instability discovered by scientists may be important for cosmological and astronomical observations.
First, don’t look for rotating, donut-shaped Bose stars of dark matter. Such objects cannot replace rotating black holes. Even if they appeared at some stage in the development of the Universe, they immediately collapsed.
Secondly, scenarios where many black holes in our Universe are formed from dark matter objects are now being actively discussed. Merging black holes are now being seen by experiments to search for gravitational waves. So, if some of these objects were formed as a result of the collapse of Bose stars of dark matter, then we can say with confidence that their angular momentum is zero. This prediction can be verified experimentally.
“One of the motivations for our research was interest in the mysterious phenomenon of fast radio flares. The mechanism of the origin of radio flares is not clear; there is still no generally accepted astrophysical explanation. On the other hand, a very similar phenomenon occurs in models with “new physics,” during the explosions of axion stars. This is a special case of Bose stars. Rotation would facilitate the conditions under which the axion star explodes, ”said Igor Tkachev, Academician of the Russian Academy of Sciences, Chief Researcher at the Institute for Nuclear Research of the Russian Academy of Sciences.
The difference between a black hole and a Bose star is that a black hole attracts all objects to itself with such a force that any particle, even a photon of light, crossing the horizon of this hole, cannot return back. At the same time, the gravitational field of the Bose star is not as strong. We could, in principle, observe the radiation emitted by it, but it does not glow.
“Black holes usually form when stars collapse. Such black holes must rotate rapidly. Let me explain. Everyone saw the skater, who, spinning on the ice, first spreads her arms, and then raises them above her head and begins to rotate even more, this is a consequence of the law of conservation of angular momentum. Likewise, a star collapsing, it is compressed, so it will rotate even faster, turning into a black hole. For example, supermassive black holes located in the centers of galaxies generally rotate with a maximum speed that approaches the speed of light, “added Igor Tkachev.
As we can see, sometimes the correct solution to a problem is counterintuitive. After all, it seems that all objects can rotate in the universe. Except for Bose stars.
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