arXiv subjects

Alexander F. Zakharov

Publications and source records attributed to Alexander F. Zakharov.

At least 19 recordsLinked to original sources

arXiv2026

A way to constrain a graviton mass from astronomical observations

Along with a whole range of alternative theories of gravity, variants of the massive theory of gravity, i.e. the theory of gravity, in which the graviton has mass, have been actively discussed in recent years. Theorists have proposed versions of massive gravity theories that address the shortcomings of early versions of such theories. Astrophysicists and experimental physicists have been discussing limitations on the graviton mass from various astronomical observations. In particular, in the first LIGO paper, where the discovery of gravitational waves from binary black holes was reported, a limit on the mass of the graviton was obtained from the analysis of the profile of the gravitational wave signal. In the paper graviton mass constraints were obtained by analyzing the trajectory of a bright star in the vicinity of the center of our Galaxy, using observations from the GRAVITY and Keck groups. Briefly other astronomical ways to limit a graviton mass were discussed.

gr-qc↗

arXiv2025

Nearly forgotten results in development of physical cosmology

It would be reasonable to recall some critical issues in physical cosmology development. GR was created by A. Einstein in 1915. In 1917 Einstein proposed the first (static) cosmological model. Soon after the A. Eddington proved that the model is unstable therefore it can not be realizable in nature. In 1922 and 1924 A. A. Friedmann found non-stationary solutions for cosmological equations written in the framework of GR. In 1927 G. Lemaitre obtained very similar results and, in addition, he derived the Hubble law (E. Hubble obtained this law from observations). Unfortunately, G. Lemaitre published his paper in not very popular Belgium journal. In 1931 Lemaitre proposed the first version of hot Universe model (he called it hypothesis of the primeval atom). In his book Lemaitre predicted even a background radiation as a signature of his model. One of the important property of the Lemaitre -- Gamow model was a prediction of CMB radiation with a temperature around a few K. It was recalled that the discovery of CMB radiation was done by T. Shmaonov in 1956 and his paper was published in 1957 (several years before Penzias and Wilson). In 1965, 1970 E. B. Gliner proposed vacuum like equation of matter which could correspond to exponential explosion of the Universe which was later called inflation. For decades in USSR, Friedmann's cosmological non-stationary models were treated as purely mathematical results without cosmologocal applications. On September 16, 1925 passed away untimely and it would be reasonable to remind today his great contribution in physical cosmology since the authors of book on Friedmann wrote that "similarly to Copernicus who forced the Earth to move Friedmann forced the Universe to expand".

physics.hist-ph↗

arXiv2025

Shadow in the Galactic Center: Theoretical Concept -- Prediction -- Realization

General Relativity (GR) was created in November 1915 and since its creation and up to now this theory has undergone many tests. The first realistic cosmological models were proposed in the works of Friedman, written in the 1920s. For a long time Friedman's cosmological works were actually banned in Soviet Union due to philosophical reasons, since the models where the birth and evolution of the Universe occurs were considered ideologically unacceptable. Due to great achievements in relativity and cosmology and due to increasing interest to these branches of of science in last decades we recall a development of relativistic astrophysics and contribution of Russian researchers in these studies. Since one of the world leaders in physical cosmology A. A. Friedman passed away in September 1925, it is reasonable to outline the main achievements of physical cosmology over the past 100 years. We discuss also observational and theoretical achievements in confirmations of relativistic observational predictions for black holes, including the closest supermassive black hole in our Galactic Center. We outline an evolution of black hole shadow from the purely theoretical concept to observable quantities for supermassive black holes in Sgr A* and M87*.

physics.hist-ph↗

arXiv2024

Comment on the article "Non-local gravitational corrections in black hole shadow images" by S. O. Alexeyev et al

Recently Alexeyev et al. published paper (J. Theor. Exper. Phys. v. 165, N 4, p. 508 in Russian; arXiv:2404.16079 [gr-qc], the reference is given also in [1]). In the paper the authors discussed an opportunity of estimating spins from the analysis of the shadow reconstruction of black holes, theoretically considered using the nonlocal gravity model proposed earlier for the description of "quantum" black holes. However, in essence, this paper considered circular photon orbits, and the fact that the corresponding motion parameters determine the shape and size of shadows, similarly to Kerr black holes, remained unproven. It is also remained unproven the statement that for an equatorial observer the shadow size in the direction of rotation of quantum black holes remains independent of spin. A long time ago the shadow property was established for the Kerr black hole case.

gr-qc↗

arXiv2024

Constraints on graviton mass from Schwarzschild precession in the orbits of S-stars around the Galactic Center

In this paper we use a modification of the Newtonian gravitational potential with a non-linear Yukawa-like correction, as it was proposed by C. Will earlier to obtain new bounds on graviton mass from the observed orbits of S-stars around the Galactic Center (GC). This phenomenological potential differs from the gravitational potential obtained in the weak field limit of Yukawa gravity, which we used in our previous studies. We also assumed that the orbital precession of S-stars is close to the prediction of General Relativity (GR) for Schwarzschild precession, but with a possible small discrepancy from it. This assumption is motivated by the fact that the GRAVITY Collaboration in 2020 and in 2022 detected Schwarzschild precession in the S2 star orbit around the Supermassive Black Hole (SMBH) at the GC. Using this approach, we were able to constrain parameter $λ$ of the potential and, assuming that it represents the graviton Compton wavelength, we also found the corresponding upper bound of graviton mass. The obtained results were then compared with our previous estimates, as well as with the estimates of other authors.

gr-qc↗

arXiv2024

Improvement of graviton mass constraints using GRAVITY's detection of Schwarzschild precession in the orbit of S2 star around the Galactic Center

Here we study possible improvements of the existing constraints on the upper bound of graviton mass by the analysis of the stellar orbits around the SMBH at the GC in the framework of Yukawa gravity. A motivation for this study is a recent detection of Schwarzschild precession in the orbit of S2 star around the SMBH at the GC by the GRAVITY Collaboration. The authors indicated that the orbital precession of the S2 star is close to the General Relativity (GR) prediction, but with possible small deviation from it, and parametrized this effect by introducing an ad hoc factor in the parametrized PPN equations of motion. Here we use the value of this factor presented by GRAVITY in order to perform two-body simulations of the stellar orbits in massive gravity using equations of motion in the modified PPN formalism, as well as to constrain the range of massive interaction $Λ$. From the obtained values of $Λ$, and assuming that it corresponds to the Compton wavelength of graviton, we then calculated new estimates for the upper bound of graviton mass which are found to be independent, but consistent with the LIGO's estimate of graviton mass from the first GW signal GW150914 (later this graviton mass estimation was significantly improved with consequent observations of GW events). We also performed calculations including numerical simulations in order to constrain the bounds on graviton mass in the case of a small deviation of the stellar orbits from the corresponding GR predictions and showed that our method could further improve previous estimates for upper bounds on the graviton mass. It is also demonstrated that such analysis of the observed orbits of S-stars around the GC in the frame of the Yukawa gravity represents a tool for constraining the upper bound for the graviton mass, as well as for probing the predictions of GR or other gravity theories.

astro-ph.GA↗

arXiv2023

Shadows near supermassive black holes: from a theoretical concept to GR test

General relativity (GR) passed many astronomical tests but in majority of them GR predictions have been tested in a weak gravitational field approximation. Around 50 years ago a shadow has been introduced by J. Bardeen as a purely theoretical concept but due to an enormous progress in observational and computational facilities this theoretical prediction has been confirmed and the most solid argument for an existence of supermassive black holes in Sgr A* and M87* has been obtained.

gr-qc↗

arXiv2023

Constraints on black hole charges in M87* and Sgr A* with the EHT observations

In May 2022 ICRANet organized the Workshop dedicated to the 80th anniversary of Professor Ruffini. This paper is based on the talk delivered at the meeting. Professor Ruffini was well known for Soviet scientific community not only due to his publications in leading journals but also due Russian translations of his books where he was an author or a contributor in collection of articles. But only in 1988 I had an opportunity to watch and listen professor R. Ruffini at the Conference dedicated to the century since the birthday of Alexander Alexandrovich Friedmann. This conference was organized in Leningrad (Soviet Union) in June during a short magic period when there are white nights there. In June 2023 we celebrate the 135th anniversary of Friedmann's birth. Friedmann and his closed friend V. K. Frederics were the founders of Soviet school of general relativity and George Gamow was one of the brilliant representative of the school and he was the author of the hot Universe model which is the most popular now. In the USSR a development of general relativity and relativistic cosmology was not smooth and only in sixties of the last century these branches of science freed from the total control of representatives of the ideology of Marxism -- Leninism. I also discussed a Soviet contribution in a discovery of cosmic microwave background radiation done by T. Shmaonov in 1957 and reasons why his supervisors did not connect these results with the hot Universe models discussed by G. Gamow. Author's results about observational features of supemassive black holes (including the black hole in our Galactic Center) are also briefly discussed, it was considered an opportunity to evaluate a (tidal) charge of Reissner -- Nordström black hole from observational estimates of shadow size in the Galactic Center and M87* done by the EHT Collaboration based its observations in April 2017.

gr-qc↗

arXiv2023

Shadows around Sgr A* and M87* as a tool to test gravity theories

In the framework of Randall -- Sundrum theory with extra dimension Reissner -- Nordström black hole solutions with a tidal charge have been found. The shadow around the supermassive black hole in M87 was reconstructed in 2019 based on observations with the Event Horizon Telescope (EHT) in April 2017. In May 2022 the EHT Collaboration presented results of a shadow reconstruction for our Galactic Center. Earlier, for Reissner -- Nordström metric we derived analytical expressions for shadow size as a function of charge and later generalized these results for a tidal charge case. We discuss opportunities to evaluate parameters of alternative theories of gravity with shadow size estimates done by the EHT Collaboration, in particular, a tidal charge could be estimated from these observations.

astro-ph.GA↗

arXiv2022

Constraints on a tidal charge of the supermassive black hole in M87* with the EHT observations in April 2017

More than two years ago the Event Horizon Telescope collaboration presented the first image reconstruction around the shadow for the supermassive black hole in M87*. It gives an opportunity to evaluate the shadow size. Recently, the Event Horizon Telescope collaboration constrained parameters ("charges") of spherical symmetrical metrics of black holes from an estimated allowed interval for shadow radius from observations of M87* in 2017. Earlier, analytical expressions for the shadow radius as a function of charge (including a tidal one) in the case of Reissner--Nordström metric have been obtained. Some time ago, Bin-Nun proposed to apply a Reissner--Nordström metric with a tidal charge as an alternative to the Schwarzschild metric in Sgr~A*. If we assume that a Reissner--Nordström black hole with a tidal charge exists in M87*, therefore, based on results of the shadow size evaluation for M87* done by the Event Horizon Telescope collaboration we constrain a tidal charge. Similarly, we evaluate a tidal charge from shadow size estimates for Sgr~A*.

gr-qc↗

arXiv2021

Testing the Galactic Centre potential with S-stars

Two groups of astronomers used the large telescopes Keck and VLT for decades to observe trajectories of bright stars near the Galactic Centre. Based on results of their observations the astronomers concluded that trajectories of the stars are roughly elliptical and foci of the orbits are approximately coincide with the Galactic Centre position. In a last few years a self-gravitating dark matter core--halo distribution was suggested by Ruffini, Arguelles, Rueda (RAR) and this model was actively used in consequent studies. In particular, recently it has been claimed that the RAR-model provides a better fit of trajectories of bright stars in comparison to the conventional model with a supermassive black hole. The dark matter distribution with a dense core having a constant density as it was suggested in the RAR-model leaves trajectories of stars elliptical like in Kepler's two-body problem. However, in this case not the foci of the ellipses coincide with the Galactic Center but their centers while the orbital periods do not depend on semi-major axes. These properties are not consistent with the observational data for trajectories of bright stars.

astro-ph.GA↗

arXiv2019

Tests of gravity theories with Galactic Center observations

An active stage of relativistic astrophysics started in 1963 since in this year, quasars were discovered, Kerr solution has been found and the first Texas Symposium on Relativistic Astrophysics was organized in Dallas. Five years later, in 1967--1968 pulsars were discovered and their model as rotating neutron stars has been proposed, meanwhile J. A. Wheeler claimed that Kerr and Schwarzschild vacuum solutions of Einstein equations provide an efficient approach for astronomical objects with different masses. Wheeler suggested to call these objects black holes. Neutron stars were observed in different spectral band of electromagnetic radiation. In addition, a neutrino signal has been found for SN1987A. Therefore, multi-messenger astronomy demonstrated its efficiency for decades even before observations of the first gravitational radiation sources. However, usually, one has only manifestations of black holes in a weak gravitational field limit and sometimes a model with a black hole could be substituted with an alternative approach which very often looks much less natural, however, it is necessary to find observational evidences to reject such an alternative model. After two observational runs the LIGO-- Virgo collaboration provided a confirmation for an presence of mergers for ten binary black holes and one binary neutron star system where gravitational wave signals were found. In addition, in last years a remarkable progress has been reached in a development of observational facilities to investigate a gravitational potential, for instance, a number of telescopes operating in the Event Horizon Telescope network is increasing and accuracy of a shadow reconstruction near the Galactic Center is improving, meanwhile largest VLT, Keck telescopes with adaptive optics and especially, GRAVITY facilities observe bright IR stars at the Galactic Center with a perfecting accuracy.

gr-qc↗

arXiv2018

Constraints on tidal charge of the supermassive black hole at the Galactic Center with trajectories of bright stars

As it was pointed out recently in Hees et al. (2017), observations of stars near the Galactic Center with current and future facilities provide an unique tool to test general relativity (GR) and alternative theories of gravity in a strong gravitational field regime. In particular, the authors showed that the Yukawa gravity could be constrained with Keck and TMT observations. Some time ago, Dadhich et al. (2001) showed that the Reissner -- Nordström metric with a tidal charge is naturally appeared in the framework of Randall -- Sundrum model with an extra dimension ($Q^2$ is called tidal charge and it could be negative in such an approach). Astrophysical consequences of of presence of black holes with a tidal charge are considerered, in particular, geodesics and shadows in Kerr -- Newman braneworld metric are analyzed in (Schee and Stuchlik, 2009a), while profiles of emission lines generated by rings orbiting braneworld Kerr black hole are considered in (Schee and Stuchlik, 2009b). Possible observational signatures of gravitational lensing in a presence of the Reissner -- Nordström black hole with a tidal charge at the Galactic Center are discussed in papers by Bin-Nun (2010a, 2010b, 2011). Here we are following such an approach and we obtain analytical expressions for orbital precession for Reissner -- Nordstrom -- de-Sitter solution in post-Newtonian approximation and discuss opportunities to constrain parameters of the metric from observations of bright stars with current and future astrometric observational facilities such as VLT, Keck, GRAVITY, E-ELT and TMT.

gr-qc↗

arXiv2018

The black hole at the Galactic Center: observations and models

One of the most interesting astronomical objects is the Galactic Center. We concentrate our discussion on a theoretical analysis of observational data of bright stars in the IR-band obtained with large telescopes. We also discuss the importance of VLBI observations of bright structures which could characterize the shadow at the Galactic Center. There are attempts to describe the Galactic Center with alternative theories of gravity and in this case one can constrain parameters of such theories with observational data for the Galactic Center. In particular, theories of massive gravity are intensively developing and theorists have overcome pathologies presented in initial versions of these theories. In theories of massive gravity, a graviton is massive in contrast with GR where a graviton is massless. Now these theories are considered as an alternative to GR. For example, the LIGO-Virgo collaboration obtained the graviton mass constraint of about $1.2 \times 10^{-22}$ eV in their first publication about the discovery of the first gravitational wave detection event that resulted of the merger of two massive black holes. Surprisingly, one could obtain a consistent and comparable constraint of graviton mass at a level around $m_{g} < 2.9 \times 10^{-21}$ eV from analysis of observational data on the trajectory of the star S2 near the Galactic Center. Therefore, observations of bright stars with existing and forthcoming telescopes such as the European Extremely Large Telescope (E-ELT) and the Thirty Meter Telescope (TMT) are extremely useful for investigating the structure of the Galactic Center in the framework of GR, but these observations also give a tool to confirm, rule out or constrain alternative theories of gravity.

gr-qc↗

arXiv2017

Different ways to estimate graviton mass

An experimental detection of graviton is extremely hard problem, however, there are different ways to evaluate a graviton mass if it is non-vanishing. Theories of massive gravity or theories with non-vanishing graviton mass initially have a number of pathologies such as discontinuities, ghosts etc. In last years theorists found ways to overcome weaknesses of such theories meanwhile observational features are also discussed. In the first publication reporting about the discovery of gravitational waves from the binary black hole system the LIGO-Virgo collaboration obtained the graviton mass constraint around $1.2 \times 10^{-22}$ eV (later the estimate was improved with new data). A comparable and consistent graviton mass constraint around $2.9 \times 10^{-21}$ eV has been obtained from analysis of the bright star S2 trajectory near the Galactic Center.

gr-qc↗

arXiv2016

Constraining the range of Yukawa gravity interaction from S2 star orbits II: Bounds on graviton mass

Recently LIGO collaboration discovered gravitational waves \cite{Abbott_16} predicted 100 years ago by A. Einstein. Moreover, in the key paper reporting about the discovery, the joint LIGO \& VIRGO team presented an upper limit on graviton mass such as $m_g < 1.2 \times 10^{-22} eV$ (Abbott et al. (LIGO collaboration) PRL 116 (2016) 061102). Since the graviton mass limit is so small the authors concluded that their observational data do not show violations of classical general relativity. We consider another opportunity to evaluate a graviton mass from phenomenological consequences of massive gravity and show that an analysis of bright star trajectories could bound graviton mass with a comparable accuracy with accuracies reached with gravitational wave interferometers and expected with forthcoming pulsar timing observations for gravitational wave detection. It gives an opportunity to treat observations of bright stars near the Galactic Center as a wonderful tool not only for an evaluation specific parameters of the black hole but also to obtain constraints on the fundamental gravity law such as a modifications of Newton gravity law in a weak field approximation. In particular, we obtain bounds on a graviton mass based on a potential reconstruction at the Galactic Center.

gr-qc↗

arXiv2015

Origin of masses in the Early Universe

New model is suggested, where the Casimir mechanism is the source of masses and conformal symmetry breaking at the Planck epoch in the beginning of the Universe. The mechanism is the Casimir energy and associated condensate, which are resulted from the vacuum postulate and normal ordering of the conformal invariant Hamiltonian with respect to the quantum elementary field operators. It is shown that the Casimir top-quark condensate specifies the value of the Higgs particle mass without involving the Higgs tachyon mass, which is put equal to zero. The Casimir mechanism yields another value of the coupling constant for the self-interaction of scalar field than the standard model does.

gr-qc↗

arXiv2014

Constraints on a charge in the Reissner--Nordström metric for the black hole at the Galactic Center

Using an algebraic condition of vanishing discriminant for multiple roots of fourth degree polynomials we derive an analytical expression of a shadow size as a function of a charge in the Reissner -- Nordström (RN) metric \cite{Reissner_16,Nordstrom_18}. We consider shadows for negative tidal charges and charges corresponding to naked singularities $q=\mathcal{Q}^2/M^2 > 1$, where $\mathcal{Q}$ and $M$ are black hole charge and mass, respectively, with the derived expression. An introduction of a negative tidal charge $q$ can describe black hole solutions in theories with extra dimensions, so following the approach we consider an opportunity to extend RN metric to negative $\mathcal{Q}^2$, while for the standard RN metric $\mathcal{Q}^2$ is always non-negative. We found that for $q > 9/8$ black hole shadows disappear. Significant tidal charges $q=-6.4$ (suggested by Bin-Nun (2010)) are not consistent with observations of a minimal spot size at the Galactic Center observed in mm-band, moreover, these observations demonstrate that a Reissner -- Nordström black hole with a significant charge $q \approx 1$ provides a better fit of recent observational data for the black hole at the Galactic Center in comparison with the Schwarzschild black hole.

gr-qc↗