arXiv subjects

K. Rezazadeh

Publications and source records attributed to K. Rezazadeh.

11 recordsLinked to original sources

arXiv2026

Static Spherically Symmetric Solutions in Modified Entropic Gravity

Within the entropic gravity paradigm introduced by Verlinde, one assumes that the microscopic degrees of freedom residing on the holographic screen obey the equipartition law of energy. Nevertheless, implications of statistical mechanics suggest that this energy sharing can acquire corrections that depend on temperature. Taking such modifications into account leads to altered gravitational field equations when derived from thermodynamic considerations. We solve the resulting modified Einstein equations in the case of a static, spherically symmetric spacetime and determine the general structure of the metric components. Our findings indicate that if the temperature correction function behaves as $f(T)\propto T^{2}$, the corresponding spacetime geometry reduces to the flat Minkowski metric. Therefore, to obtain small deviations from flatness, it is necessary to consider slight departures from this purely quadratic temperature dependence. Such deviations can be interpreted as encoding additional gravitational effects, potentially associated with matter contributions that are not entirely described by the holographic screen. As a result of these effects, the effective gravitational potential acquires a logarithmic correction, which can give rise to deviations from Newtonian gravity. This aspect is especially significant in the regime of very weak gravitational fields, where these corrections may affect particle motion and could carry implications for both astrophysical and cosmological contexts.

gr-qc↗

arXiv2025

Relativistic MOND Theory from Modified Entropic Gravity

We derive a relativistic extension of Modified Newtonian Dynamics (MOND) within the framework of entropic gravity by introducing temperature-dependent corrections to the equipartition law on a holographic screen. Starting from a general modification of the surface degrees of freedom and employing the Unruh relation between acceleration and temperature, we obtain modified Einstein equations in which the geometric sector acquires explicit thermal corrections. Solving these equations for a static, spherically symmetric spacetime in the weak-field, low-temperature regime yields a corrected metric that smoothly approaches Minkowski space at large radii and naturally contains a characteristic acceleration scale. In the very-low-acceleration regime, the model reproduces MOND-like deviations from Newtonian dynamics while providing a relativistic underpinning for that phenomenology. We confront the theory with rotation-curve data for NGC~3198 and perform a Bayesian parameter inference, comparing our relativistic MOND (RMOND) model with both a baryons-only Newtonian dynamics (ND) model and a halo of dark matter (DM) model. We find that RMOND and DM models both fit the data significantly better than the baryons-only ND prediction, and that RMOND provides particularly improved agreement at $r\gtrsim 20\,\mathrm{kpc}$. These results suggest that temperature-corrected entropic gravity provides a viable relativistic framework for MOND phenomenology, motivating further observational tests, including gravitational lensing and extended galaxy samples.

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arXiv2022

Structure Formation in Dark Matter Particle Production Cosmology

We investigate a cosmological scenario in which the dark matter particles can be created during the evolution of the Universe. By regarding the Universe as an open thermodynamic system and using non-equilibrium thermodynamics, we examine the mechanism of gravitational particle production. In this setup, we study the large-scale structure (LSS) formation of the Universe in the Newtonian regime of perturbations and derive the equations governing the evolution of the dark matter overdensities. Then, we implement the cosmological data from Planck 2018 CMB measurements, SNe Ia and BAO observations, as well as the Riess et al. (2019) local measurement for $H_0$ to provide some cosmological constraints for the parameters of our model. We see that the best case of our scenario ($χ_{\rm tot}^{2}=3834.40$) fits the observational data better than the baseline $Λ$CDM model ($χ_{\rm tot}^{2} = 3838.00$) at the background level. We moreover estimate the growth factor of linear perturbations and show that the best case of our model ($χ_{fσ_{8}}^{2}=39.85$) fits the LSS data significantly better than the $Λ$CDM model ($χ_{fσ_{8}}^{2}=45.29$). Consequently, our model also makes a better performance at the level of the linear perturbations compared to the standard cosmological model.

astro-ph.CO↗

arXiv2022

Cascading Dark Energy

The standard cosmological model is in the midst of a stress test, thanks to the tension between supernovae-based measurements of the Hubble constant $H_{0}$ and inferences of its values from Cosmic Microwave Background (CMB) anisotropies. Numerous explanations for the present-day cosmic acceleration require the presence of a new fundamental scalar field, as do Early Dark Energy (EDE) solutions to the Hubble tension. This raises the possibility that multiple fields cooperatively contribute to the dark energy component in bursts throughout cosmic time due to distinct initial conditions and couplings. Here, this Cascading Dark Energy (CDE) scenario is illustrated through a realization that effectively reduces to a two-field model, with two epochs in which dark energy is cosmologically significant. The model is compared to measurements of the CMB, baryon acoustic oscillations, as well as both PANTHEON and SH0ES observations of Type-Ia supernovae. Neglecting the linear perturbations, it is found that this scenario ameliorates the Hubble tension, improving over purely late-time models of dark energy, and the agreement between the galaxy survey measurements of baryon acoustic oscillations.

astro-ph.CO↗

arXiv2020

The growth of DM and DE perturbations in DBI non-canonical scalar field scenario

We study the effect of varying sound speed on clustering dark energy in the Dirac-Born-Infeld (DBI) scenario. The DBI action is included in the class of $k$-essence models, and it has an important role in describing the effective degrees of freedom of D-branes in the string theory. In the DBI setup, we take the anti-de Sitter (AdS) warp factor $f(ϕ)=f_0\, ϕ^{-4}$, and investigate the self-interacting quartic potential $V(ϕ)=λϕ^{4}/4$. We calculate the full expression of the effective sound speed for our model, and show that it can evolve with time during the cosmological evolution. Besides, the adiabatic sound speed evolves with time here, and this influences the background dynamics to some extent. We show that the effective sound speed is very close to the adiabatic sound speed. We examine the effect of the variable sound speed on growth of the perturbations in both the linear and non-linear regimes. In the linear regime, we apply the Pseudo-Newtonian formalism, and show that dark energy suppresses the growth of perturbations at low redshifts. From study the Integrated Sachs-Wolf (ISW) effect in our setup, we see that the model manifests some deviation from the concordance $Λ$CDM model. In the non-linear regime, we follow the approach of spherical collapse model, and calculate the linear overdensity, the virial overdensity, overdensity at the turn around and the rate of expansion of collapsed region. We further compute relative number density of halo objects above a given mass in our setting, and show that the number of structures with respect to the $Λ$CDM model is reduced more in the high mass tail at high redshifts.

gr-qc↗

arXiv2018

Warm DBI inflation with constant sound speed

We study inflation with the Dirac-Born-Infeld (DBI) noncanonical scalar field in both the cold and warm scenarios. We consider the Anti-de Sitter warp factor $f(ϕ)=f_{0}/ϕ^{4}$ for the DBI inflation and check viability of the quartic potential $V(ϕ)=λϕ^{4}/4$ in light of the Planck 2015 observational results. In the cold DBI setting, we find that the prediction of this potential in the $r-n_s$ plane is in conflict with Planck 2015 TT,TE,EE+lowP data. This motivates us to focus on the warm DBI inflation with constant sound speed. We conclude that in contrary to the case of cold scenario, the $r-n_s$ result of warm DBI model can be compatible with the 68\% CL constraints of Planck 2015 TT,TE,EE+lowP data in the intermediate and high dissipation regimes, whereas it fails to be observationally viable in the weak dissipation regime. Also, the prediction of this model for the running of the scalar spectral index $dn_s/d\ln k$ is in good agreement with the constraint of Planck 2015 TT,TE,EE+lowP data. Finally, we show that the warm DBI inflation can provide a reasonable solution to the swampland conjecture that challenges the de Sitter limit in the standard inflation.

gr-qc↗

arXiv2018

Structure formation in clustering DBI dark energy model with constant sound speed

Within the framework of DBI non-canonical scalar field model of dark energy, we study the growth of dark matter perturbations in the both linear and non-linear regimes. In our DBI model, we consider the anti-de Sitter warp factor $f(ϕ)=f_0\, ϕ^{-4}$ with constant $f_0>0$ and assume the DBI dark energy to be clustered and its sound speed $c_s$ to be constant. For a spatially flat FRW universe filled with pressureless dark matter and DBI dark energy, we first obtain the evolutionary behaviors of the background quantities. Our results show that in our DBI model, the universe starts from a matter dominated epoch and approaches to the de Sitter universe at late times, as expected. Also the DBI potential behaves like the power law one $V(ϕ)\propto ϕ^n$. In addition, we use the Pseudo-Newtonian formalism to obtain the growth factor of dark matter perturbations in the linear regime. We conclude that for smaller $c_s$ (or $f_0$), the growth factor of dark matter is smaller for clustering DBI model compared to the homogeneous one. In the following, in the non-linear regime based on the spherical collapse model, we obtain the linear overdensity $δ_c(z_c)$, the virial overdensity $Δ_{\rm vir}(z_c)$, overdensity at the turn around $ζ(z_c)$ and the rate of expansion of collapsed region $h_{\rm ta}(z)$. We point out that for the smaller $c_s$ (or $\tilde{f}_0$), the values of $δ_c(z_c)$, $Δ_{\rm vir}(z_c)$, $ζ(z_c)$ and $h_{\rm ta}(z)$ in non-clustering DBI models deviate more than the $Λ$CDM compared to the clustering DBI. Finally, with the help of spherical collapse parameters we calculated the relative number density of halo objects above a given mass and conclude that the differences between clustering and homogeneous DBI models are more pronounced for higher-mass halos at high redshift.

gr-qc↗

arXiv2017

Tachyon inflation with steep potentials

Within the framework of tachyon inflation, we consider different steep potentials and check their viability in light of the Planck 2015 data. We see that in this scenario, the inverse power-law potential $V(ϕ)=V_{0}(ϕ/ϕ_{0})^{-n}$ with $n=2$ leads to the power-law inflation with the scale factor $a(t)\propto t^{q}$ where $q>1$, while with $n<2$, it gives rise to the intermediate inflation with the scale factor $a(t)\propto\exp\left(At^{f}\right)$ where $A>0$ and $0 2$ can be compatible with the 95\% CL region of Planck 2015 TT, TE, EE+lowP data. We further conclude that the exponential potential $V(ϕ)=V_{0}e^{-ϕ/ϕ_{0}}$, the inverse $\cosh$ potential $V(ϕ)=V_{0}/\cosh(ϕ/ϕ_{0})$, and the mutated exponential potential $V(ϕ)=V_{0}\left[1+(n-1)^{-(n-1)}(ϕ/ϕ_{0})^{n}\right]e^{-ϕ/ϕ_{0}}$ with $n=4$, can be consistent with the 95\% CL region of Planck 2015 TT, TE, EE+lowP data. Moreover, using the $r-n_s$ constraints on the model parameters, we also estimate the running of the scalar spectral index $dn_{s}/d\ln k$ and the local non-Gaussianity parameter $f_{\rm NL}^{\rm local}$. We find that the lower and upper bounds evaluated for these observables are compatible with the Planck 2015 results.

gr-qc↗

arXiv2016

Brans-Dicke inflation in light of the Planck 2015 data

We study inflation in the Brans-Dicke gravity as a special model of the scalar-tensor gravity. We obtain the inflationary observables containing the scalar spectral index, the tensor-to-scalar ratio, the running of the scalar spectral index and the equilateral non-Gaussianity parameter in terms of the general form of the potential in the Jordan frame. Then, we compare the results for various inflationary potentials in light of the Planck 2015 data. Our study shows that in the Brans-Dicke gravity, the power-law, inverse power-law and exponential potentials are ruled out by the Planck 2015 data. But, the hilltop, Higgs, Coleman-Weinberg and natural potentials can be compatible with Planck 2015 TT,TE,EE+lowP data at 95\% CL. Moreover, the D-brane, SB SUSY and displaced quadratic potentials can be in well agreement with the observational data since their results can lie inside the 68\% CL region of Planck 2015 TT,TE,EE+lowP data.

astro-ph.CO↗

arXiv2016

Power-law and intermediate inflationary models in f(T)-gravity

We study inflation in the framework of $f(T)$-gravity in the presence of a canonical scalar field. After reviewing the basic equations governing the background cosmology in $f(T)$-gravity, we turn to study the cosmological perturbations and obtain the evolutionary equations for the scalar and tensor perturbations. Solving those equations, we find the power spectra for the scalar and tensor perturbations. Then, we consider a power-law form for the $f(T)$ function in the action and examine the inflationary models with the power-law and intermediate scale factors. We see that in contrast with the standard inflationary scenario based on the Einstein gravity, in the considered $f(T)$-gravity scenario, the power-law and intermediate inflationary models can be compatible with the observational results of Planck 2015 at 68\% CL. In our $f(T)$-gravity setting, the potentials responsible for both the power-law and intermediate inflationary models have the power-law form $V(ϕ) \propto {ϕ^m}$ but the power $m$ is different for them. Therefore, we can refine some of power-law inflationary potentials in the framework of $f(T)$-gravity while they are disfavored by the observational data in the standard inflationary scenario. Interestingly enough, the self-interacting quartic potential $V(ϕ) \propto {ϕ^4}$ which has special reheating properties, can be consistent with the Planck 2015 data in our $f(T)$-gravity scenario while it is ruled out in the standard inflationary scenario.

gr-qc↗

arXiv2015

Intermediate inflation from a non-canonical scalar field

We study the intermediate inflation in a non-canonical scalar field framework with a power-like Lagrangian. We show that in contrast with the standard canonical intermediate inflation, our non-canonical model is compatible with the observational results of Planck 2015. Also, we estimate the equilateral non-Gaussianity parameter which is in well agreement with the prediction of Planck 2015. Then, we obtain an approximation for the energy scale at the initial time of inflation and show that it can be of order of the Planck energy scale, i.e. ${M_P} \sim {10^{18}}\,{\rm{GeV}}$. We will see that after a short period of time, inflation enters in the slow-roll regime that its energy scale is of order ${M_P}/100 \sim \;{10^{16}}{\rm{GeV}}$ and the horizon exit takes place in this energy scale. We also examine an idea in our non-canonical model to overcome the central drawback of intermediate inflation which is the fact that inflation never ends. We solve this problem without disturbing significantly the nature of the intermediate inflation until the time of horizon exit.

gr-qc↗