Southern Federal University has proposed a new model of compact stars, paving the way for the exploration of new physics.

For centuries, the stars overhead have stirred the consciousness of humanity, serving as a source of inspiration and scientific curiosity. Observations of these celestial bodies and the laws of physics have enabled scientists to construct a theory of stellar evolution, which describes various stages of their life cycle. One of the key stages is when the material of a star compresses to densities characteristic of atomic nuclei. Such states provide unique opportunities to study matter under extreme conditions, allowing scientists to move beyond standard models of fundamental forces of nature.

Precision data from modern cosmology confirm that hidden mass and dark energy dominate our Universe. Describing them necessitates a revision of current physical models. As noted by the chief researcher at the Institute of Physics at Southern Federal University, Doctor of Physical and Mathematical Sciences, Professor Maxim Khlopov: “Such a revision is essential for describing the initial conditions of the Universe's evolution, which involves inflation and baryogenesis, mechanisms that cannot be solely based on known physical laws. Thus, the currently standard cosmological model draws on physics beyond the standard models of fundamental interactions. Research in this area is based on a combination of experimental physics and astrophysical studies. In recent analyses, the natural conditions of matter in compact stars hold significant importance.”

During their evolution, stars pass through stages where their matter is compressed to extremely high densities. This leads to a significant reduction in the size of stars, making them increasingly compact. Such compression of massive stars results in their collapse into black holes. For stars with masses less than 2–3 solar masses, the result of their evolution is the formation of neutron stars, whose matter is compressed to the density of atomic nuclei. However, the macroscopic description of matter at such densities can differ significantly from the structure of atomic nuclei.

It is typically believed that at these densities, the matter of neutron stars consists of stable neutrons that do not decay, unlike free neutrons. However, researchers at Southern Federal University have proposed an alternative explanation in their study — at these densities, a transition to a so-called “color-superconducting” state may occur. In this state, protons and neutrons, which make up atomic nuclei, can break down into quarks, forming a giant “superfluid quark droplet.” Interestingly, this state involves not only light quarks but also heavier “strange” quarks, which prove to be stable under such conditions.

The term “color-superconducting state” may seem unusual, but it carries profound physical significance. In quantum chromodynamics — the theory describing the strong interaction between quarks — special characteristics of particles are introduced, which physicists metaphorically refer to as “color” and “flavor.”

The “color” of a quark is a specific quantum charge that takes one of three values, conventionally named red, green, and blue (anti-colors also exist). It is important to note that these “colors” have nothing to do with visible colors. This is merely an analogy: just as the combination of all colors in the visible spectrum gives white light, so too does the combination of the three quark “colors” create a “colorless” state that can exist in nature, as in protons and neutrons.

When physicists speak of the “color-superconducting” state of matter in superdense stars, they refer to a special quantum state in which quarks of different “flavors” and “colors” form complex structures. This state is fundamentally different from ordinary nuclear matter and can lead to entirely new properties of matter.

The aim of the research, the results of which are published in the Chinese Journal of Physics, was to investigate the possibility of such a color-superconducting quark state in compact stars.

“Based on solutions to the equations of general relativity, we obtained a description of the quark state of matter and the structure of compact stars. We concluded that this description is not only realistic but also leads to new insights about the possible properties of compact stars,” noted Maxim Khlopov.

The results obtained by the researchers open new horizons for astronomical studies. The developed model allows for the prediction of stars with masses significantly exceeding the limits accepted for neutron stars. This discovery may lead to a reassessment of observational data regarding such objects and their interpretation.

“New solutions for describing the structure of stars lead to predictions of new types of celestial objects and interpretations of both astronomical observation data and processes of compact star mergers, accessible through multi-channel astronomy methods,” emphasizes the scientist.

As part of this work, the scientists created graphs describing the dependence of mass and radius of stars on their density, allowing for more accurate modeling of potential properties of compact objects.

This research at Southern Federal University was conducted with financial support from the Ministry of Science and Higher Education of Russia. The results of the work may prove significant for interpreting gravitational wave signals from mergers of compact stars and for analyzing the processes occurring in such systems. According to Maxim Khlopov, collaboration with Indian colleagues, which has been ongoing for many years, provides excellent opportunities for further research.

This pertains not only to the study of new forms of matter but also their evolution and astrophysical manifestations. The importance of this research lies in its expansion of methods for exploring physics beyond standard models of fundamental interactions, offering new approaches to mastering this domain. The experience of the group of Indian scientists led by Professor Saibal Ray, Deputy Director of the Center for Cosmology, Astrophysics, and Space Science (CCASS) at GLA University (Mathura, Uttar Pradesh), plays a crucial role in solving equations in both standard (GR) and modified theories of gravity.

Thus, the proposed model of compact stars opens wide prospects for studying new physical phenomena and helps refine current understandings of the structure and evolution of the Universe.