arXiv subjects

G. S. Sharov

Publications and source records attributed to G. S. Sharov.

At least 19 recordsLinked to original sources

arXiv2026

Dark Energy in Ghost-free non-local Gravity

Ghost-free non-local gravity is investigated with regards to its late-time dynamics. Viable solutions in this model are confronted with the observational data including the Pantheon+ catalogue of Type Ia supernovae, the Dark Energy Spectroscopic Instrument, the measurements of baryon acoustic oscillations and the Hubble parameter estimations $H(z)$. The ghost-free non-local gravity is found to be successful in these tests in comparison to the $Λ$CDM model and can be also comparable with the generalized exponential $F(R)$ gravity scenario. However the model encounters difficulties when the data from the above observations and the cosmic microwave background radiation data are combined. In tests with the whole set of Pantheon+, DESI, $H(z)$ and CMB data, the generalized exponential $F(R)$ model is essentially more successful. This success is related with the dynamical behavior of its effective dark energy equation of state evolving from a phantom to a quintessence phase during the late-time epoch, whereas the ghost-free non-local model demonstrates only a quintessence behavior. Hence the ghost-free non-local gravity scenario is successful only when the Pantheon+, DESI and $H(z)$ data are considered. The generalized exponential $F(R)$ model satisfies the viability conditions and in tests with all observational data including CMB surpasses the $Λ$CDM model in $χ^2$ statistics and also with information criteria.

gr-qc↗

arXiv2026

Viable f(R) Scenarios Unifying Inflation with Realistic Dynamical Dark Energy

Two $F(R)$ gravity models are tested on the basis of their viability during all stages of cosmological evolution. It is shown that these models can describe both the early-time inflationary epoch and the dark energy epoch. The models are confronted with the latest observational data, including the Pantheon+ catalogue with Type Ia supernovae, the Dark Energy Spectroscopic Instrument measurements of baryon acoustic oscillations, the Hubble parameter estimations and data from cosmic microwave background radiation. Investigation of the viability conditions for these models, in particular, the condition $\frac{dF}{dR}>0$ required a deep analysis. Both models appeared to be viable during the early-time era, but for the late-time evolution the viability conditions are not fulfilled in definite domains in the parameter spaces of these models. However the best fitted parameters, determined in confrontation with the mentioned observational data, lie far from the forbidden domains for both models. These $F(R)$ gravity models describe the observations with the large advantage over the $Λ$-Cold-Dark-Matter model, not only in $χ^2$ statistics, but also with Akaike and Bayesian information criteria. This success of the two $F(R)$ gravity scenarios is connected with their capability to mimic dynamical dark energy, similarly to models with variable equation of state, that is necessary for describing the latest Pantheon+ and DESI observational data.

gr-qc↗

arXiv2025

Dynamical Dark Energy from $F(R)$ Gravity Models Unifying Inflation with Dark Energy: Confronting the Latest Observational Data

A class of viable $F(R)$ gravity models which can provide a unified description of inflation with the dark energy era is confronted with the latest observational data on the dark energy era. These models have the unique characteristic that the de Sitter scalaron mass in the Einstein frame counterpart theory is a monotonic function of the curvature, which renders them viable descriptions for both the inflationary and the late-time acceleration eras. We also compare these models with other well-known viable $F(R)$ gravity models and with the $Λ$-Cold-Dark-Matter model. As we show, the most phenomenologically successful models are those which deviate significantly from the $Λ$-Cold-Dark-Matter model. Also some of the models presented, provide a statistically favorable description of the dark energy eras, compared with the exponential $F(R)$ gravity model and of course compared with the $Λ$-Cold-Dark-Matter model. All the models we present in this article are confronted with the observational data from the Planck collaboration, the Pantheon plus data from Type Ia supernovae, the two rounds of observations of the Dark Energy Spectroscopic Instrument, data from baryon acoustic oscillations and the Hubble constant measurements by SH0ES group. As we show, two of the models are statistically favorable by the data.

gr-qc↗

arXiv2020

Cosmological models, observational data and tension in Hubble constant

We analyze how predictions of cosmological models depend on a choice of described observational data, restrictions on flatness, and how this choice can alleviate the $H_0$ tension. These effects are demonstrated in the $w$CDM model in comparison with the standard $Λ$CDM model. We describe the Pantheon sample observations of Type Ia supernovae, 31 Hubble parameter data points $H(z)$ from cosmic chronometers, the extended sample with 57 $H(z)$ data points and observational manifestations of cosmic microwave background radiation (CMB). For the $w$CDM and $Λ$CDM models in the flat case and with spatial curvature we calculate $χ^2$ functions for all observed data in different combinations, estimate optimal values of model parameters and their expected intervals. For both considered models the results essentially depend on a choice of data sets. In particular, for the $w$CDM model with $H(z)$ data, supernovae and CMB the $1σ$ estimations may vary from $H_0=67.52^{+0.96}_{-0.95}$ km\,/(s$\cdot$Mpc) (for all $N_H=57$ Hubble parameter data points) up to $H_0=70.87^{+1.63}_{-1.62}$ km\,/(s$\cdot$Mpc) for the flat case ($k=0$) and $N_H=31$. These results might be a hint how to alleviate the problem of $H_0$ tension: different estimates of the Hubble constant may be connected with filters and a choice of observational data.

gr-qc↗

arXiv2018

How predictions of cosmological models depend on Hubble parameter data sets

We explore recent estimations of the Hubble parameter $H$ depending on redshift $z$, which include 31 $H(z)$ data points measured from differential ages of galaxies and 26 data points, obtained with other methods. We describe these data together with Union 2.1 observations of Type Ia supernovae and observed parameters of baryon acoustic oscillations with 2 cosmological models: the standard cold dark matter model with the $Λ$ term ($Λ$CDM) and the model with generalized Chaplygin gas (GCG). For these models with different sets of $H(z)$ data we calculate two-parameter and one-parameter distributions of $χ^2$ functions for all observed effects, estimate optimal values of model parameters and their $1σ$ errors. For both considered models the results appeared to be strongly depending on a choice of Hubble parameter data sets if we use all 57 $H(z)$ data points or only 31 data points from differential ages. This strong dependence can be explained in connection with 4 $H(z)$ data points with high redshifts $z>2$.

gr-qc↗

arXiv2016

Observational constraints on cosmological models with Chaplygin gas and quadratic equation of state

Observational manifestations of accelerated expansion of the universe, in particular, recent data for Type Ia supernovae, baryon acoustic oscillations, for the Hubble parameter $H(z)$ and cosmic microwave background constraints are described with different cosmological models. We compare the $Λ$CDM, the models with generalized and modified Chaplygin gas and the model with quadratic equation of state. For these models we estimate optimal model parameters and their permissible errors with different approaches to calculation of sound horizon scale $r_s(z_d)$. Among the considered models the best value of $χ^2$ is achieved for the model with quadratic equation of state, but it has 2 additional parameters in comparison with the $Λ$CDM and therefore is not favored by the Akaike information criterion.

gr-qc↗

arXiv2014

Parameters of cosmological models and recent astronomical observations

For different gravitational models we consider limitations on their parameters coming from recent observational data for type Ia supernovae, baryon acoustic oscillations, and from 34 data points for the Hubble parameter $H(z)$ depending on redshift. We calculate parameters of 3 models describing accelerated expansion of the universe: the $Λ$CDM model, the model with generalized Chaplygin gas (GCG) and the multidimensional model of I. Pahwa, D.~Choudhury and T.R.~Seshadri. In particular, for the $Λ$CDM model $1σ$ estimates of parameters are: $H_0=70.262\pm0.319$ km\,c${}^{-1}$Mpc${}^{-1}$, $Ω_m=0.276_{-0.008}^{+0.009}$, $Ω_Λ=0.769\pm0.029$, $Ω_k=-0.045\pm0.032$. The GCG model under restriction $α\ge0$ is reduced to the $Λ$CDM model. Predictions of the multidimensional model essentially depend on 3 data points for $H(z)$ with $z\ge2.3$.

gr-qc↗

arXiv2014

Multidimensional gravitational model with anisotropic pressure

We consider the gravitational model with additional spatial dimensions and anisotropic pressure which is nonzero only in these dimensions. Cosmological solutions in this model include accelerated expansion of the Universe at late age of its evolution and dynamical compactification of extra dimensions. This model describes observational data for Type Ia supernovae on the level or better than the $Λ$CDM model. We analyze two equations of state resulting in different predictions for further evolution, but in both variants the acceleration epoch is finite.

gr-qc↗

arXiv2013

String Models, Stability and Regge Trajectories for Hadron States

Various string models of mesons and baryons include a string carrying 2 or 3 massive points (quarks or antiquarks). Rotational states (planar uniform rotations) of these systems generate quasilinear Regge trajectories and may be used for describing excited hadron states on these trajectories. For different string models of baryon we are to solve the problem of choice between them and the stability problem for their rotational states. An unexpected result is that for the Y string baryon model these rotations are unstable with respect to small disturbances on the classical level. This instability has specific feature, disturbances grow linearly, whereas for the linear string baryon model they grow exponentially and may increase predictions for baryon's width $Γ$. The classical instability of rotational states and nonstandard Regge slope are the arguments in favor of the stable simplest model of string with massive ends both for baryons and mesons. Rotational states of this model with two types of spin-orbit correction are used to describe Regge trajectories for light, strange, charmed, bottom mesons and for $N$, $Δ$, $Σ$, $Λ$ and $Λ_c$ baryons.

hep-ph↗

arXiv2010

Contribution to Hadrons' Width from Classical Instability of Y Configuration and Other String Hadron Models

We consider various hadron models with a string carrying $n=3$ massive points (quarks): Y configuration, linear baryon model $q$-$q$-$q$ and the closed string. For these models classical rotational states (planar uniform rotations) are tested for stability with respect to small disturbances. It is shown that rotations of all mentioned models are unstable, but nature of this instability is different. For the model Y the instability results from existence of multiple real frequencies in the spectrum of small disturbances, but for the linear model and the closed string the similar spectra contain complex frequencies, corresponding to exponentially growing modes of disturbances. This classical rotational instability is important for describing excited hadrons, in particular, for the linear model and the closed string it results in additional contribution in width of hadron states.

hep-ph↗

arXiv2008

Unstable Rotational States of Closed String with Massive Points

For the closed string carrying 2 or 3 point-like masses the stability problem for central and linear rotational states is considered. This problem is important for applications of these model to describing baryons, glueballs or other exotic hadrons. The linear rotational state correspond to an uniform rotation of the system with rectilinear string segments, connecting massive points. The state is named ``central'' one, if there is a massive point at the rotational center. It is shown that the linear rotational states with 2 massive points are stable with respect to small disturbances. But the central rotational states with 3 masses are not stable, if the central mass it less than energy of the string with other massive points. This effect may change properties of excited hadron states, in particular, increase their width.

hep-ph↗

arXiv2007

Closed String with Masses in Models of Baryons and Glueballs

The closed string carrying $n$ point-like masses is considered as the model of a baryon ($n=3$), a glueball ($n=2$ or 3) or another exotic hadron. For this system the rotational states are obtained and classified. They correspond to exact solutions of dynamical equations, describing an uniform rotation of the string with massive points. These rotational states result in a set of quasilinear Regge trajectories with different behavior. The stability problem for the so called central rotational states (with a mass at the rotational center) is solved with using the analysis of small disturbances. These states turned out to be unstable, if the central mass is less than some critical value.

hep-ph↗

arXiv2006

String Models of Glueball and Regge Trajectories

The closed relativistic string carrying two point-like masses is considered as the model of a glueball with two constituent gluons. Here the gluon-gluon interaction is simulated by a pair of strings. For this system exact solutions of classical equations of motions are obtained. They describe rotational states of the string resulting in the set of quasilinear Regge trajectories with different behavior.

hep-ph↗

arXiv2006

Unstable States for Closed String with Massive Point

The stability problem for the hypocycloidal rotational states of the closed relativistic string with a point-like mass is solved with the help of analysis of small disturbances of these states. Both analytical and numerical investigations showed an unexpected result: the mentioned states turned out to be unstable. This conclusion is based upon the presence of roots with positive imaginary parts (increments) in the spectrum of frequencies of small disturbances. But these increments were small enough, so this instability had not been detected in previous numerical experiments. For the linear rotational states (the particular case of hypocycloidal states) the stability was confirmed. These results are important for applications of this model in hadron spectroscopy.

hep-th↗

arXiv2005

Stability and Symmetry Breaking for Closed String with Massive Point

The closed relativistic string carrying a point-like mass in the space with nontrivial geometry is considered. For rotational states of this system (resulting in non-trivial Regge trajectories) the stability problem is solved. It was shown that rotations of the folded string with the massive point placed at the rotational center are stable (with respect to small disturbances) if the mass exceeds some critical value: $m>m_{cr}$. But these rotational states are unstable in the opposite case $m<m_{cr}$. We can treat this effect as the spontaneous symmetry breaking for the string state. Other classes of rotational motions of this system have appeared to be stable. These results were obtained both in numerical experiments and the analytical investigation of small disturbances for the rotational states.

hep-th↗

arXiv2004

Closed Relativistic String Carrying Pointlike Mass

For the closed relativistic string carrying a pointlike mass the exact solutions of the dynamical equations are obtained and studied. These solutions describe states of the mentioned system moving in Minkowski space and also in the space that is the direct product of Minkowski space and a compact manifold (torus).

hep-th↗

arXiv2003

Excited States of Rotating Relativistic String with Massive Ends

For the relativistic string with massive ends arbitrary small disturbances of the uniform rotation of the rectilinear string are investigated. There are two classes of these oscillations with different spectra of frequencies. They are stationary waves oscillating in the rotational plane and in the orthogonal direction. These states are presented in the form of Fourier series that helps to quantize string excitations in the linear vicinity of the classical rotation. These string oscillations may be applied to describing daughter Regge trajectories in the framework of the considered model

hep-ph↗

arXiv2002

Quasirotational Disturbances of the Relativistic String with Massive Ends and Higher Radial Excitations of Hadrons

For the relativistic string with massive ends the small disturbances of its rotational motion (quasirotational states) are investigated. They are presented in the form of Fourier series with the two types of oscillatory modes. They have the form of stationary waves correspondently in the rotational plane and in the orthogonal direction. The calculated values of the energy and angular momentum of these states give us possibility to describe higher radial excitations for hadrons with the help of the planar quasirotational oscillatory modes of the string with massive ends.

hep-ph↗