Dear Colleagues,

The Galactic Centre (GC) is usually identified with the radio source Sgr A*. It is the closest galactic center, and it is generally accepted that the closest supermassive black hole is located there. Therefore, the object is one of the most attractive for observations in a wide band of electromagnetic radiation.

There have previously been attempts to detect gravitational radiation and neutrinos from GC. In contrast to physics in astronomy, opportunities to conduct experiments are very limited since we can only observe objects in space. As is well known, gravitational potential can be evaluated from analysis of trajectories of probe bodies (stars act as test bodies in this case).

For decades, two groups of astronomers have monitored the trajectories of bright stars at the GC to test the gravitational field there. The first group, led by A. Ghez in the US, has been using the Keck telescope, while the second group, led by R. Genzel in Europe, has been using VLT telescopes for these observations. Recently, VLT telescopes have been combined in the GRAVITY interferometer. Analyzing the results of these observations, astronomers established that the bounded orbits of bright stars are fitted by quasi-elliptical orbits, and the foci of the trajectories coincide with the Sgr A* position, which means that the Newtonian potential of point-like mass is a rather good approximation for the GC.

Analyzing these observations, the Keck and GRAVITY (VLT) teams discovered that the gravitational redshift of the S2 star near its pericenter passage in May 2018 fitted nicely with general relativity in the first post-Newtonian approximation, so general relativity passed one important test. Moreover, in 2020, the GRAVITY (VLT) collaboration identified the presence of the Schwarzschild precession for the S2 star trajectory. In May 2022, the EHT collaboration reconstructed the shadow around the black hole at the GC (using photons as test bodies in this case), a discovery which provides an important confirmation for the conventional model, where the existence of a supermassive black hole at the Galactic Centre is declared.

In recent studies, it has been shown that GC observations provide opportunities to constrain parameters of alternative models for this object. Therefore, GC is a unique astronomical laboratory where physical laws (including gravity law) may be tested, and current and forthcoming observations offer an opportunity to test general relativity, constrain alternative models of gravity, and clarify theoretical models for GC and its environment.

I invite authors to contribute to the development of theoretical models for GC and its observations.

Prof. Dr. Alexander F. Zakharov
Guest Editor

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• the Galactic Centre
• supermassive black hole
• black holes
• general relativity
• accretion
• interferometry
• VLBI
• tests of general relativity
• alternative theories of gravity
• variations of fundamental constants