According to IKBFU scientists, neuron stars are remarkably small-sized objects (with a diameter of no more than 30 kilometres) with a mass at least twice that of the Sun. They are 90 percent neutrons, and the density of matter at the centre of such a star can exceed the density of water by a quadrillion (1015) times.
These objects are particularly important for astrophysics, as the observed stars of this type serve as a natural laboratory for verification of our knowledge about gravity, physics of the atomic nucleus, and elementary particles at very high densities, the scientists explained.
One of the key questions of neutron star physics is what their maximum possible mass is. According to the IKBFU scientists, until recently the heaviest known neutron star was an object with a mass of 2.04 solar masses.
However, in 2019, the LIGO observatory detected gravitational waves (so-called GW190814 - gravitational-wave event) from the fusion of a massive black hole of about 23 solar masses with an object estimated to have a mass of a little more than 2.5 solar masses. Researching the issue, the IKBFU scientists as part of a scientific group have developed a new model of neutron star structure for gravity theories that differ from the General Theory of Relativity, proposed by Albert Einstein.
"The discussion within the scientific community has shown that there is no consensus regarding the nature of a small object in GW190814. Some astrophysicists believe that it is a heavy neutron star. However, the problem is that most realistic equations show that the maximum mass of neutron stars is significantly less than 2.5 solar masses. We assume that at very high densities and in high fields, the gravitational processes go beyond those proposed by the General Theory of Relativity", Artem Astashenok, head of the Laboratory of Astrophysics and Cosmology at IKBFU, said.
The General Theory of Relativity is a generally recognised theory of gravity describing it as a manifestation of space-time geometry. As the IKBFU specialists explained, the study of paradoxical objects that do not fit into the accepted models is of paramount importance for the refinement and modification of Einstein`s theory.
"It turns out that if Einstein’s gravitational equations are slightly amended, the maximum mass of neutron stars may be larger than the General Theory of Relativity allows. In the long run, if the hypothesis is confirmed, it will not just open up new opportunities for describing heavy neutron stars, but may also change our view of the accelerated expansion of the universe issue", Artem Astashenok explained.
The IKBFU scientists believe that if it is confirmed that the smaller object in the GW190814 event is a neutron star of "non-standard" mass, this fact will be indirect evidence of their modified gravity model.
"The results will also help in answering another question that astrophysicists have long been focused on. Observations suggest that there is a mismatch between the masses of neutron stars and black holes. According to the modified theory of relativity, there is no mismatch in masses, and the maximum possible mass of a neutron star is close to the minimum possible mass of a black hole", Artem Astashenok noted.
Physicists from Greece, Italy, and Spain took part in the study as well. The IKBFU specialists are currently studying the maximum baryonic mass limit of neutron stars using simple models of gravity. According to them, the magnitude of this limit may be the lower threshold for the mass of black holes.
The article was published in the Physics Letters journal.