arXiv subjects

German S. Sharov

Publications and source records attributed to German S. Sharov.

15 recordsLinked to original sources

arXiv2025

Einstein-Gauss-Bonnet Cosmology Confronted with Observations

Several models within the framework of Einstein-Gauss-Bonnet gravities are considered with regard their late-time phenomenological viability. The models contain a non-minimally coupled scalar field and satisfy a constraint on the scalar field Gauss-Bonnet coupling, that guarantees that the speed of the tensor perturbations is equal to the speed of light. The late-time cosmological evolution of these Einstein-Gauss-Bonnet models is confronted with the observational data including the Pantheon plus Type Ia supernovae catalogue, the Hubble parameter measurements (cosmic chronometers), data from cosmic microwave background radiation (CMB) and baryon acoustic oscillations (BAO) including the latest measurements from Dark Energy Spectroscopic Instrument (DESI). Among the considered class of models some of them do not fit the CMB and BAO data. However, there exists some models that generate a viable Einstein-Gauss-Bonnet scenario with well-behaved late-time cosmological evolution that fits the observational data essentially better in comparison to the standard $Λ$-Cold-Dark-Matter model.

gr-qc↗

arXiv2024

Modified gravity/Dynamical Dark Energy vs $Λ$CDM: is the game over?

Over the last decades, tests on the standard model of cosmology, the so-called $Λ$CDM model, have been widely analysed and compared with many different models for describing dark energy. Modified gravities have played an important role in this sense as an alternative to $Λ$CDM model. Previous observational data has been always favouring $Λ$CDM model in comparison to any other model. While statistically speaking, alternative models have shown tehir power, fitting in some cases the observational data slightly better than $Λ$CDM, the significance and goodness of the fits were not significantly relevant to exclude the standard model of cosmology. In this paper, a generalisation of exponential $F(R)$ gravity is considered and compared with $Λ$CDM model by using the latest observational data. Also some well-known model independent parameterisations for the equation of state (EoS) of dark energy are explored. These scenarios are confronted with the renewed observational data involving the Pantheon plus datasets of supernovae type Ia, the Hubble parameter estimations, data from the cosmic microwave background and baryon acoustic oscillations, where the latter includes the data provided by Dark Energy Spectroscopic Instrument collaboration. Results of this analysis suggest that standard exponential $F(R)$ models provide much better fits than $Λ$CDM model, which is excluded at 4$σ$. Moreover, the parameterisations of the equation of state suggest a non-constant EoS parameter for dark energy, where $Λ$CDM model is also excluded at 4$σ$.

gr-qc↗

arXiv2024

Exponential gravity with logarithmic corrections in the presence of axion dark matter

An exponential modified gravity with additional logarithmic corrections is considered with the presence of an axion-like scalar field in the role of dark matter. Axion fields are thought to become important at late-times when the axion-like scalar field oscillates around its vacuum expectation value, mimicking dark matter behaviour. The model is compared with the usual pressureless fluid description of dark matter. Both models are tested with observational data including some of the latest sources, providing similar fits in comparison with the $Λ$CDM model. Despite results are not statistically relevant to rule out any model, the number of free parameters still favours $Λ$CDM model, as shown by computing the goodness of the fits.

gr-qc↗

arXiv2023

Exponential F(R) gravity with axion dark matter

The cosmological evolution within the framework of exponential $F(R)$ gravity is analysed by assuming two forms for dark matter: (a) a standard dust-like fluid and (b) an axion scalar field. As shown in previous literature, an axion-like field oscillates during the cosmological evolution but can play the role of dark matter when approaching the minimum of its potential. Both scenarios are confronted with recent observational data including the Pantheon Type Ia supernovae, Hubble parameter estimations (Cosmic Chronometers), Baryon Acoustic Oscillations and Cosmic Microwave Background distances. The models show great possibilities in describing these observations when compared with the $Λ$CDM model, supporting the viability of exponential $F(R)$ gravity. The differences between both descriptions of dark matter is analysed.

gr-qc↗

arXiv2023

Early Dark Energy with Power-law F(R) Gravity

We study a power-law $F(R)$ gravity with an early dark energy term, that can describe both the early-time and the late-time acceleration of the Universe. We confront this scenario with recent observational data including the Pantheon Type Ia supernovae, measurements of the Hubble parameter $H(z)$ (Cosmic Chronometers), data from Baryon Acoustic Oscillations and standard rulers data from the Cosmic Microwave Background (CMB) radiation. The model demonstrates some achievements in confronting with these observations and can be compared with the $Λ$-Cold-Dark-Matter model. In particular, in both models we obtain very close estimates for the Hubble constant $H_0$, but it is not true for $Ω_m^0$. The early dark energy term supports viability of the considered $F(R)$ gravity model.

gr-qc↗

arXiv2022

Testing viable extensions of Einstein-Gauss-Bonnet gravity

Some models within the framework of Gauss-Bonnet gravities are considered in the presence of a non-minimally coupled scalar field. By imposing a particular constraint on the scalar field coupling, an extension of the called Einstein-Gauss-Bonnet gravity that keeps the correct speed of propagation for gravitational waves, is considered. The cosmological evolution for this viable class of models is studied and compared with observational data (BAO, CMB, Sne Ia,..), where we obtain the corresponding bounds for these theories and show that such theories fit well the data and provide a well-behaved cosmological evolution in comparison to the standard model of cosmology. Some statistical parameters show that the goodness of the fits are slightly better than those for $Λ$CDM model.

gr-qc↗

arXiv2022

Modified Equations of State for Dark Energy and Observational Limitations

Cosmological models with variable and modified equations of state for dark energy are confronted with observational data, including Type Ia supernovae, Hubble parameter data $H(z)$ from different sources, and observational manifestations of cosmic microwave background radiation (CMB). We consider scenarios generalizing the $Λ$CDM, $w$CDM, and Chevallier--Polarski--Linder (CPL) models with nonzero curvature and compare their predictions. The most successful model with the dark energy equation of state $w= w_0+ w_1(1-a^2)/2$ was studied in detail. These models are interesting in possibly alleviating the Hubble constant $H_0$ tension, but they achieved a modest success in this direction with the considered observational data.

gr-qc↗

arXiv2021

Modelling and testing the equation of state for (Early) dark energy

A general equation of state is considered for analysing the possible behaviors for (Early) dark energy that alleviates the Hubble parameter tension problem. By departing from the possible evolution for the (Early) dark energy density and the corresponding dynamical equations, the equation of state is obtained, which allow us to analyze qualitatively the cosmological evolution and the dominance of each term in the equation of state along the cosmic expansion, which show some interesting consequences as the occurrence of (past) future singularities. Then, by considering two general models, their free parameters are fit with different sources of data, showing the goodness of the fits in comparison to more standard models. Results might be considered as a promising starting point to get a better understanding of the cosmological evolution as a whole.

gr-qc↗

arXiv2021

Analyzing the $H_0$ tension in $F(R)$ gravity models

The Hubble constant tension problem is analysed in the framework of a class of modified gravity, the so-called $F(R)$ gravity. To do so, we explore two models: an exponential and a power-law $F(R)$ gravities, which includes an early dark energy (EDE) term in the latter. These models can describe both an early time inflationary epoch and the late time accelerating expansion of the universe. We confront both models with recent observational data including the Pantheon Type Ia supernovae sample, the latest measurements of the Hubble parameter $H(z)$ from differential ages of galaxies (cosmic chronometers) and separately from baryon acoustic oscillations. Standard rulers data set from the Cosmic Microwave Background radiation are also included in our analysis. The estimations of the Hubble constant appear to be essentially depending on the set of observational data and vary in the range from 68 to 70{.}3 km\,/(s$\cdot$Mpc). The fits of the other free parameters of the models are also obtained, revealing interesting consequences.

gr-qc↗

arXiv2020

Field theoretic interpretations of interacting dark energy scenarios and recent observations

Cosmological models describing the non-gravitational interaction between dark matter and dark energy are based on some phenomenological choices of the interaction rates between dark matter and dark energy. There is no such guiding rule to select such rates of interaction. {\it In the present work we show that various phenomenological models of the interaction rates might have a strong field theoretical ground.} We explicitly derive several well known interaction functions between dark matter and dark energy under some special conditions and finally constrain them using the latest cosmic microwave background observations from final Planck legacy release together with baryon acoustic oscillations distance measurements. Our analyses report that one of the interacting functions is able to alleviate the $H_0$ tension. We also perform a Bayesian evidence analyses for all the models with reference to the $Λ$CDM model. From the Bayesian evidence analyses, although the reference scenario is preferred over the interacting scenarios, however, we found that two interacting models are close to the reference $Λ$CDM model.

astro-ph.CO↗

arXiv2020

Testing the equation of state for viscous dark energy

Some cosmological scenarios with bulk viscosity for the dark energy fluid are considered. Based on some considerations related to hydrodynamics, two different equations of state for dark energy are assumed, leading to power-law and logarithmic effective corrections to the pressure. The models are tested with the latest astronomical data from Type Ia supernovae (Pantheon sample), measurements of the Hubble parameter $H(z)$, Baryon Acoustic Oscillations and Cosmic Microwave Background radiation. In comparison with $Λ$CDM model, some different results are obtained and their viability is discussed. The power-law model shows some modest results, achieved under negative values of bulk viscosity, while the logarithmic scenario provide good fits in comparison to $Λ$CDM model.

gr-qc↗

arXiv2018

Testing logarithmic corrections on $R^2$-exponential gravity by observational data

This paper is devoted to the analysis of a class of $F(R)$ gravity, where additional logarithmic corrections are assumed. The gravitational action includes an exponential term and a $R^2$ inflationary term, both with logarithmic corrections. This model can unify an early time inflationary era and also the late time acceleration of the universe expansion. This model is deeply analysed, confronting with recent observational data coming from the largest Pantheon Type Ia supernovae sample, the latest measurements of the Hubble parameter $H(z)$, manifestations of Baryon Acoustic Oscillations and Cosmic Microwave Background radiation. The viability of the model is studied and the corresponding constraints on the free parameters are obtained, leading to an statistical analysis in comparison to $Λ$CDM model. The inflationary era is also analysed within this model and its compatibility with the latest observational data for the spectral index of primordial curvature perturbations and the scalar-to-tensor ratio. Finally, possible corrections on the Newton's law and constraints due to primordial nucleosynthesis are analysed.

gr-qc↗

arXiv2017

Is exponential gravity a viable description for the whole cosmological history?

Here we analysed a particular type of $F(R)$ gravity, the so-called exponential gravity which includes an exponential function of the Ricci scalar in the action. Such term represents a correction to the usual Hilbert-Einstein action. By using Supernovae Ia, Barionic Acoustic Oscillations, Cosmic Microwave Background and $H(z)$ data, the free parameters of the model are well constrained. The results show that such corrections to General Relativity become important at cosmological scales and at late-times, providing an alternative to the dark energy problem. In addition, the fits do not determine any significant difference statistically with respect to the $Λ$CDM model. Finally, such model is extended to include the inflationary epoch in the same gravitational Lagrangian. As shown in the paper, the additional terms can reproduce the inflationary epoch and satisfy the constraints from Planck data.

gr-qc↗

arXiv2017

A model with interaction of dark components and recent observational data

In the proposed model with interaction between dark energy and dark matter, we consider cosmological scenarios with different equations of state ($w_d$) for dark energy. For both constant and variable equation of state, we analyze solutions for dark energy and dark matter in seven variants of the model. We investigate exact analytic solutions for $w_d={}$ constant equation of state, and several variants of the model for variable $w_d$. These scenarios are tested with the current astronomical data from Type Ia Supernovae, baryon acoustic oscillations, Hubble parameter $H (z)$ and the cosmic microwave background radiation. Finally, we make a statistical comparison of our interacting model with $Λ$CDM as well as with some other well known non-interacting cosmological models.

gr-qc↗

arXiv2017

A new interacting two fluid model and its consequences

In the background of a homogeneous and isotropic spacetime with zero spatial curvature, we consider interacting scenarios between two barotropic fluids, one is the pressureless dark matter (DM) and the other one is dark energy (DE), in which the equation of state (EoS) in DE is either constant or time dependent. In particular, for constant EoS in DE, we show that the evolution equations for both fluids can be analytically solved. For all these scenarios, the model parameters have been constrained using the current astronomical observations from Type Ia Supernovae, Hubble parameter measurements, and baryon acoustic oscillations distance measurements. Our analysis shows that both for constant and variable EoS in DE, a very small but nonzero interaction in the dark sector is favored while the EoS in DE can predict a slight phantom nature, i.e. the EoS in DE can cross the phantom divide line `$-1$'. On the other hand, although the models with variable EoS describe the observations better, but the Akaike Information Criterion supports models with minimal number of parameters. However, it is found that all the models are very close to the $Λ$CDM cosmology.

gr-qc↗