In search of gravastars
In the modern scientific community, it is generally accepted that the general theory of relativity provides a complete description of the properties of rotating (as well as stationary) black holes. But it remains unclear how the properties of magnetic fields, interstellar gas and X-rays change close to such a powerful source of gravity. The first object, which was considered a black hole in the place of a former star, was discovered in the constellation Cygnus in 1964 and was called “Cyg X-1“(“Cygnus” means “swan” in Latin). This is a binary system, which includes a black hole with a mass of about 14.9 solar and a blue supergiant. They revolve around a common center of mass, and the stellar matter of the supergiant gradually flows into the black hole.
However, it is in the theory of relativity that many observations are not easily reconciled with the theory. Already in 1973, Kip Thorne and Igor Novikov described exactly how stellar gas should flow into a black hole. The potential gravitational energy of this gas would be converted into heat and radiation, the temperature of which reaches millions of degrees. That is, a ring of fire forms around the event horizon and jets burst out, whose temperatures reach millions of degrees. However, this picture is in no way consistent with the quantum nature of the event horizon and with the disappearance of information in a black hole (the so-called “information paradox of black holes”, which I already mentioned above and which is discussed in detail Here). Below we will consider other contradictions associated with black holes, as well as a non-trivial model of “gravitational vacuum stars” (gravastars), proposed at the beginning of this century to potentially clarify the nature of black holes.
Gravastar as a remedy for paradoxes
In 2001, Emil Mottola of the New Mexico National Laboratory and Paul Mazur of South Carolina State University formulated a model of a celestial body that would appear to an external observer as a black hole, but would have no event horizon at the outer boundary and a singularity at the center. They called such an object a “black star” or a “gravitational vacuum star,” or “gravastar” for short.
The Gravastar should be as compact as a black hole, and the gravity on its surface should be so strong that we would consider this surface an “event horizon.” However, there is no event horizon or singularity in a gravastar. The inner layers of the gravastar must consist of exotic matter (equivalent to dark energy), which affects the outer layers of the object negative pressure, which ensures the stability of the gravastar and does not allow it to collapse into a singularity under the influence of ordinary (for example, stellar) matter coming from outside. In this case, the gravastar can be covered with the thinnest but integral crust of ordinary matter.
It is extremely doubtful whether such objects can, in principle, be formed and, once formed, remain stable. It is difficult to explain why exotic matter with negative energy would concentrate in the form of gravastars and not leak out beyond them. In 2018, Raul Carballo-Rubio from the International Institute for Advanced Study in Trieste, Italy proposed a mechanismwhich could explain the existence of black stars and gravastars.
He investigated the phenomenon quantum vacuum polarization, associated with the boiling of quantum foam. As is known, according to the Heisenberg uncertainty principle, it is impossible to simultaneously know the position of a particle and its momentum. Under such conditions, the vacuum is never completely empty; pairs of virtual particles constantly appear and annihilate in it. At one time, I also translated an article about the fact that no virtual particles exist, but for now I propose to abstract from this hypothesis.
If virtual particles exist, then they are formed from the energy filling the vacuum, and during the collapse of a giant star, such energy should be released in a huge volume. Then, according to Carballo-Rubio, virtual particles can be arranged in layers, gravitating towards the north or south magnetic pole. In this case, the layered structure shown above could arise, where the individual layers do not collapse, but repel each other.
According to the theory of relativity, all matter and energy bend space, generating strong gravitational fields, bending light and causing blue shift. Therefore, from the outside, a gravastar can attract matter exactly as a black hole does, and in the optical range it can look like a black hole, from time to time dumping excess energy in the form of bursts of X-ray radiation.
Gravastars and Bose-Einstein condensates
In 1925, Einstein also predicted the existence of an exotic state of matter, today called “Bose-Einstein condensate” Indian physicist Shatyendranath Bose (1894-1974) suggested that in the minimum energy state (at a temperature close to absolute zero) a gas could be formed consisting not of atoms, but of elementary particles, which were later called “bosons” in honor of Bose. Einstein, in turn, believed that in a “bosonic gas” quantum effects should manifest themselves on a macroscopic scale. In 1995, Eric Cornell and Carl Wyman were able to get Bose-Einstein condensate, cooling about 2000 rubidium atoms to tens of nanokelvins. In the 21st century, other types of such “gas” were obtained, including those consisting only of photons. If such an aggregative state of matter can be formed in nature, then it will be characterized by very high homogeneity (low entropy), while a traditional black hole, on the contrary, should have maximum possible entropy.
If we assume that a gravastar is the next stage in the development of a neutron star, then as a result of further collapse, neutrons should decay into individual bosons, as a result of which the gravastar approaches the properties of a giant atom. According to the model of Mazur and Mottola, in the depths of the gravastar there may exist de Sitter spacetime, in which positive vacuum energy and negative pressure are established. Negative pressure prevents the gravastar from developing into a singularity. In this case, the outer layer of the gravastar may consist of neutrons and thus appear to be an “event horizon.” But when it gets behind this layer, matter coming from outside decays into bosons and disappears for an external observer, without increasing the entropy of the internal layers of the gravastar.
Finally, already in February 2024, physicists Daniel Yampolsky and Luciano Rezzolla from Goethe University in Frankfurt am Main proposed a new solution that is consistent with the general theory of relativity and allows for the existence of several gravastars nested inside each other like a nesting doll. They called such a star “nestar” from the English “nested”. Their study published in the magazine “Classical and Quantum Gravity“
Conclusion
The described calculations are strictly hypothetical, but they still resolve some of the accumulated contradictions and paradoxes associated with black holes. In principle, black holes and gravastars may well coexist in the Universe, remaining similar only to an external observer. We still have no idea what state matter can be in in the depths of a black hole, but we can well study the Bose-Einstein condensate; Moreover, recent experiments on the ISS have shown that in weightlessness matter remains in this state of aggregation more than a second, and it is possible to cool it down to fractions of a nanokelvin. Considering how desirable the production of matter with negative energy is for modern physics (in particular, to create warp bubbles), the gravastar hypothesis deserves further study, or at least a confident refutation. Probably, further work of the LIGO collaboration can help in this case. Gravitational waves from collisions between gravastars, black holes, and neutron stars may well differ in intensity and decay rate due to distinct differences in the composition and degree of entropy of such objects. Simulation of such differences at the Goethe University in Frankfurt am Main has been in progress since 2016but so far no candidates for gravastars have been discovered.
