arXiv subjects

A. Sheykhi

Publications and source records attributed to A. Sheykhi.

At least 19 recordsLinked to original sources

arXiv2026

Constraining Modified Mass-to-Horizon Cosmology Through Primordial Inflationary Observables

We investigate slow-roll inflation in a modified cosmological framework inspired by a generalized mass-to-horizon relation (MHR), $M=γ{c^2 L^n}/{G}$, where $n$ is a real parameter and $γ$ a dimensional constant. Using Padmanabhan's emergence paradigm, we derive the modified Friedmann equations for a flat FRW universe and analyze the dynamics of a canonical scalar field (inflaton) under the slow-roll approximation. We study the resulting inflationary phenomenology for power-law and Starobinsky potentials. For power-law potentials, the MHR modification fails to reconcile these models with current CMB constraints on $r$ and $n_s$. In contrast, Starobinsky inflation exhibits significant sensitivity to deviations from $n=1$. A perturbative analysis ($n=1+Δ$) yields corrections to inflationary observables. We observe that the scalar power-spectrum normalization, under a fixed-Starobinsky prescription, imposes the stringent constraint $0.960 \lesssim n \lesssim 1.040$ for $N=60$ efolds. This is considerably tighter than spectral-index bounds. Our results establish inflation, particularly Starobinsky-like models, as a sensitive probe of generalized horizon thermodynamics and departures from standard MHR scaling.

gr-qc↗

arXiv2026

Kaniadakis Holographic Dark Energy with Particle Horizon as IR Cutoff

We construct a holographic dark-energy model using Kaniadakis entropy with the particle horizon as the infrared cutoff, consistently modifying both the holographic density and the Friedmann background. In standard Einstein gravity, noninteracting particle-horizon holographic dark energy (HDE) does not produce the sufficiently negative pressure required for late-time accelerated expansion. We show that the Kaniadakis deformation changes this behavior while the particle horizon is retained. For the representative parameter choice $K=1.90\times10^{-36}$, with $Ω_{\rm DE0}=0.7$ and $c^2=0.64$, the deceleration parameter changes sign at $z\simeq0.585$ in the noninteracting case. Including the interaction strengths $b^2=0.03$ and $0.06$ changes the transition only slightly, giving $z\simeq0.586$ and $0.589$, respectively. Thus, for the parameter set considered here, the interaction does not generate the acceleration; rather, the transition is already present in the noninteracting Kaniadakis model and the interaction produces only a small shift in its timing. We derive the autonomous evolution equations, the effective dark-energy equation of state, and the deceleration parameter. The adiabatic squared sound speed is also examined as a diagnostic of the effective-fluid stability, while statefinder variables are used to characterize deviations from $Λ$CDM. Finally, we verify that the $K\to0$ limit continuously recovers standard Einstein-gravity particle-horizon HDE, for which the noninteracting model remains decelerating.

gr-qc↗

arXiv2026

`It from Bit': is there a second law of quantum complexity?

At a deeper level the principle of least action is interpreted as the law of least entropy increase consistent with Prigogine's principle of minimum entropy production, and the implications of quasistatic information quantization rule (Proc. R. Soc. A 480: 20240024), are explored for the conjectured second law of quantum complexity. It is thus shown that the conjectured second law of complexity is derivable from the information quantization rule such that long after heat-death the quantum state complexity evolves as $C(t)=C_{\rm max}\exp(-1/t)$, increasing with time to a saturation value $C_{\rm max}$ that is exponential in the equilibrium entropy $S_{\rm max}$. For the out-of-equilibrium circumstances, however, the quantum complexity can decrease with the time, asymptotically tending to a minimum determined by the distance from equilibrium.

quant-ph↗

arXiv2026

Ghost Dark Energy in the Modified Kaniadakis Cosmology

We investigate ghost dark energy (GDE) in a cosmological framework derived from Kaniadakis entropy. By applying the first law of thermodynamics to the FRW apparent horizon, we obtain modified Friedmann equations that include a correction term characterized by the Kaniadakis parameter $λ$. We then study the evolution of a flat universe containing pressureless matter and interacting GDE within this modified gravity setup. Our numerical analysis reveals that the Kaniadakis correction mildly affects the dark energy equation of state and shifts the transition to cosmic acceleration. Stability analysis via the squared sound speed shows the model remains generally unstable, though the instability is moderated for larger $λ$. Statefinder diagnostics indicate that the model approaches the $Λ$CDM fixed point in the future, with deviations decreasing as $λ$ increases.

gr-qc↗

arXiv2026

Resolving Galactic and Cluster Dynamics Without Dark Matter: Tsallis Entropy as the Unique Foundation of Emergent Gravity

While modified entropy models-such as Barrow, Tsallis, Kaniadakis,Power-law, Logarithmic, and Rényi entropies-have been widely explored in cosmological contexts, their implications on galactic scales remain largely untested. These generalizations of the Bekenstein-Hawking entropy encode quantum gravitational, nonextensive, or fractal spacetime effects and can alter the gravitational entropy-area relation. In this paper, we demonstrate that the entropic force framework, when applied to galactic rotation curves and the baryonic mass of galaxy clusters, uniquely selects Tsallis entropy as the specific generalized entropy formulation. We then extend this Tsallis modified gravity to globular clusters to complete the structural hierarchy from galaxies to galaxy clusters to globular clusters and to investigate its behavior as a function of system scale. We will show that the nonextensive parameter exhibits no correlation with any of the macroscopic quantities characterizing gravitational systems, such as mass, radius, temperature, or density. Furthermore, it has previously been shown that entropy is not well-defined within the standard thermodynamic approach to gravity. The adoption of nonextensive statistics provides a foundation for entanglement, thereby enabling a consistent definition of entanglement entropy. We predict the existence of galaxy clusters with $δ= 1$ (i.e., clusters whose dynamics require no dark matter) analogous to $δ= 1$ systems already observed at galactic and globular cluster scales. This prediction provides a unique observational test to discriminate Tsallis gravity from $Λ$CDM and MOND. Therefore, for the entropic gravity paradigm to be consistent with observational data across all scales-from globular clusters to galaxies to galaxy clusters-it is inevitably required to be built upon \textit{Tsallis} entropy.

astro-ph.GA↗

arXiv2026

Structure of Anisotropic Magnetized Neutron Stars in f(R,T) Gravity with Realistic Equation of State

In this study, within the framework of f(R,T) modified gravity, we investigate the influence of coupling parameter, magnetic field and anisotropy parameter on the neutron star structure. This work employs an accurate equation of state (EoS), derived from realistic microscopic calculations based on the AV18 nucleon-nucleon potential, to compute the structure of this compact object. Here, determination of Schwarzschild radius, compactness, gravitational surface redshift and Kretschmann scalar within the f(R, T) gravity, confirms that our theoretical results are consistent with the observational constraints. While established physical EoSs within the framework of Einstein gravity have successfully characterized a broad range of compact objects, they remain inadequate in explaining certain massive objects residing within the mass gap (2.5 to 5 Msun). We show that some compact objects residing in the mass gap interpreted as candidates of neutron stars within the framework of f(R, T) gravity. Finally, we compare our results with the observational data from LIGO/Virgo/KAGRA and NICER, setting the parameters of the f(R, T) theory and anisotropy to successfully reproduce the masses and radii of the GW170817, PSR J0952-0607 and PSR J0740+6620 and the masses of the secondary components of GW190814 and GW200210-092254.

gr-qc↗

arXiv2026

Modified Entropy from Action Principle

We propose a modified gravity theory by extending the Einstein-Hilbert action with an arbitrary function of the Ricci scalar and the Kretschmann scalar invariants. The resulting modified Friedmann equations for a spatially flat FRW universe are derived, which remain free of higher-order derivatives and reduce to the standard Friedmann equations in the limiting case. Employing the gravity-thermodynamics conjecture, we investigate the thermodynamic behavior at the apparent horizon and derive the corresponding modified entropy. Using the first law of thermodynamics together with the modified Friedmann equations, we obtain a general expression for the apparent horizon entropy. This formalism allows us to compute the modified entropy for various well-known entropy models. Our approach establishes a consistent thermodynamic framework linking modified gravity theories constructed from curvature invariants to generalized entropy functions on the cosmological apparent horizon.

gr-qc↗

arXiv2026

New Black hole Solutions in $f(\mathbb{Q})$ Gravity

We investigate static and spherically symmetric vacuum solutions in the symmetric teleparallel $f(\mathbb{Q})$ modified theory of gravity. Starting from a recently proposed classification of affine connections compatible with both the symmetries of spacetime and the constraints of symmetric teleparallel geometry, we develop a systematic approach to solve the full field equations. We first identify two distinct classes of connections that satisfy the off-diagonal metric field equations and the connection constraints. For an arbitrary $f(\mathbb{Q})$ function when the non-metricity scalar $\mathbb{Q}$ vanishes, we recover exact analytical solutions equivalent to those of general relativity, including the Schwarzschild and Schwarzschild (anti)de-Sitter metrics. We then extend our analysis beyond general relativity by considering the quadratic model $f(\mathbb{Q})=\mathbb{Q}+α~\mathbb{Q}^2$ with a small parameter $α$. Using a perturbative approach, we derive asymptotically flat, analytical solutions up to second order in $α$. These solutions exhibit corrections to the standard Schwarzschild metric, characterized by new integration constants that can be interpreted as connection hair. We explore the asymptotic behavior of these solutions and disclose that the horizon radius receives corrections that can be expressed compactly using the Lambert $\mathcal{W}$ function. Our results provide new, non-trivial vacuum solutions within $f(\mathbb{Q})$ gravity and highlight the rich structure introduced by the non-metricity connection.

gr-qc↗

arXiv2026

Hints Beyond $Λ$CDM from Barrow and Tsallis Holographic Dark Energy with GO cutoff

Barrow and Tsallis Holographic Dark Energy (HDE) are two recent extensions of the standard HDE framework, obtained by introducing generalized entropy corrections through the Barrow and Tsallis formalisms. In this work, we examine the cosmological consequences of Barrow and Tsallis HDE implemented with the Granda-Oliveros (GO) infrared (IR) cutoff. After deriving the modified Friedmann equations within the thermodynamic-gravity conjecture, we study the background evolution in both non-interacting and interacting dark sector scenarios, emphasizing the role of the entropic parameter in shaping late-time dynamics. We then confront the model with state-of-the-art observations, including PantheonPlus and Union3 Type Ia supernovae, Cosmic Chronometers and DESI DR2 BAO measurements. Using Bayesian MCMC methods, we constrain the model parameters and compare the performance of BHDE with that of $Λ$CDM. Our results show that BHDE is compatible with current data and can exhibit a mild statistical preference over the concordance model for certain dataset combinations. Overall, the analysis underscores the relevance of generalized entropy frameworks in late-time cosmology and identifies Barrow-Tsallis holography with the GO cutoff as a competitive alternative to $Λ$CDM.

gr-qc↗

arXiv2026

Holographic dark energy in modified Kaniadakis cosmology

It is well-known that any modification to the entropy expression not only change the energy density of the holographic dark energy, but also modifies the cosmological field equations through thermodynamics-gravity correspondence. Here we propose a Kaniadakis holographic dark energy (KHDE) in the background of the modified Kaniadakis cosmology by incorporating the effects of Kaniadakis entropy into the Friedmann equations. We choose the Hubble radius, $L=H^{-1}$, as system's IR cutoff and determine the cosmological implications of this model. We first consider a dark energy (DE) dominated universe and reveal that this model mimics the cosmological constant with $w_{DE}=-1$. This implies that the theoretical origin of the cosmological constant, $Λ$, may be understood through KHDE in the context of Kaniadakis cosmology. Remarkably, we observe that in the absence of interaction between DE and dark matter (DM), and in contrast to HDE in standard cosmology, our model can explain the current acceleration of the cosmic expansion for the Hubble radius as IR cutoff. When the interaction between DE and DM is taken into account, we see that the total equation of state parameter (EoS), $w_{tot}=p_{tot}/ρ_{tot}$ can cross the phantom line at the present time. We also analyze the squared speed of sound, $v_s^2$, for this model and find out that $(v_s^2<0)$ for interacting KHDE. Investigating the statefinder, confirms the distinction between KHDE and $Λ$CDM model. It is seen that the statefinder diagram move away from the point of $\left\lbrace r,s\right\rbrace= \left\lbrace 1,0\right\rbrace$ with increasing the interaction parameter.

gr-qc↗

arXiv2025

Observational constraints on the modified cosmology inspired by string T-duality

We explore the cosmological consequences of a modified cosmology inspired by string T-duality. We incorporate the zero-point length correction, $l_0$, into the gravitational potential and derive the modified Friedmann equations via thermodynamic approach at the apparent horizon of a Friedmann-Robertson-Walker (FRW) universe. The resulting framework introduces a dimensionless coupling parameter $β\sim l_0^2H_0^2$ quantifying deviations from the standard $Λ$CDM model. Using Bayesian inference with \textsc{Cobaya} and MCMC sampling, we constrain the model parameter against late-time observations, including PantheonPlus and Union3 Type~Ia supernovae, cosmic chronometers, DESI~DR2 BAO measurements, and Amati-calibrated GRBs. The joint analysis yields an upper bound $β\lesssim \mathcal{O}(10^{-3})$ (68\% C.L.), implying that departures from $Λ$CDM are extremely small within current precision. Model comparison through the Akaike Information Criterion shows that the $Λ$CDM and T-duality models provide statistically equivalent fits to the data, exhibiting only a marginal preference for $Λ$CDM. These results provide the first quantitative observational constraints on string T-duality inspired modified cosmology and underscore the potential of future high-precision surveys to test quantum-gravity induced corrections in a late-time universe.

gr-qc↗

arXiv2024

"IT FROM BIT": How does information shape the structures in the universe?

Based on a synthesis of three main ingredients: (i) the Shannon information in nonequilibrium systems, (ii) the semiclassical energy-time quantization rule, and (iii) the quasistatic information-energy correspondence, a new general rule for the quantization of quasistatic information states supported by an environment away from equilibrium is introduced if the history of the environment is known as a function of time in terms of its thermodynamic potential for information $T(t)ΔS(t)$ that is a free energy measuring the distance from equilibrium $ΔS(t)$, and $T(t)$ is the mean temperature of the environment at time $t$. This all new quasistatic information-time quantization rule is applied to the expanding universe using a phenomenological thermodynamic potential for information in the matter dominated era in order to find the eigen-informations of the persistent structures that are supported by the universe (or the local environments therein) at any given epoch, thus providing an information-theoretic foundation for formation of structures and rise of complexity with time that embodies the cosmic evolution as epitomized by the late Wheeler's famous conjecture ``{\it it from bit}". This theoretical procedure must also open new avenues for further research into the quantum theory of information and complexity in nonequilibrium thermodynamics.

cond-mat.stat-mech↗

arXiv2024

Holographic dark energy in Barrow cosmology with Granda-Oliveros IR cutoff

Applying the modified Barrow entropy, inspired by the quantum fluctuation effects, to the cosmological background, and using thermodynamics-gravity conjuncture, the Friedmann equations get modified as well. In this paper, we explore the holographic dark energy with Granda-Oliveros (GO) IR cutoff, in the context of the modified Barrow cosmology. First, we assume two dark components of the universe evolves independently and obtain the cosmological parameters and explore the cosmic evolution. Second, we consider an interaction term between dark energy (DE) and dark matter (DM). We observe that the Barrow parameter $δ$ crucially affects the cosmic dynamics, causes the transition from the decelerating phase to the accelerating phase occurs later. We find out that the equation of state parameter is in the quintessence region in the past and crosses the phantom divide at the present time. Finally, we examine the squared speed of sound analysis for this model. According to the squared sound speed diagrams, the results indicate that the presence of interaction between DM and DE as well as increasing in the value of $δ$ leads to the manifestation of signs of instability in the past $(v_s^2<0)$. Furthermore, by examining the statefinder, we find that presence of $δ$ also makes a distinction between holographic dark energy in Barrow cosmology with GO-IR cutoff and the $Λ$CDM model. In fact, increasing $δ$ causes the statefinder diagram move away from the point of $\left\lbrace r,s\right\rbrace= \left\lbrace 1,0\right\rbrace$ at $z=0$.

gr-qc↗

arXiv2023

Nonextensive entropies impact onto thermodynamics and phase structure of Kerr-Newman black holes

Taking the nonextensive Tsallis and Rényi entropies into account, we explore thermodynamic properties and phase transitions of the Kerr-Newman black holes (KNBH) in the microcanonical and canonical ensembles. We also compare our results with those obtained by attributing the Bekenstein-Hawking entropy bound to the mentioned black holes. Our analysis indicates that, similarly to the standard Boltzmann picture, isolated KNBH in the microcanonical approach are stable against axisymmetric perturbations in both Tsallis and Rényi models. On the other hand, in considering the case when the black holes are enveloped by a bath of thermal radiation in the canonical treatment, the KNBH based on the Tsallis and Rényi entropies can be stable for some values of the entropy parameters, in contrast to the traditional Boltzmann framework. For the case of Rényi entropy, we find that a Hawking-Page transition and a first order small black hole/large black hole transition can occur in a similar fashion as in rotating black holes in an anti-de Sitter space. Finally, we employ the Ruppeneir geometrothermodynamic technique to provide a new perspective on studying the nature of interactions between black hole microstructures, revealing a non-trivial impact of nonextensive entropies.

hep-th↗

arXiv2023

Note on agegraphic dark energy inspired by modified Barrow entropy

We revisit agegraphic dark energy (ADE) model when the entropy associated with the apparent horizon is in the form of Barrow entropy, $S\sim A^{1+δ/2}$, where $0\leqδ\leq1$ indicates the amount of the quantum-gravitational deformation effects of the horizon. The modification to the entropy expression, not only change the energy density of ADE, but also modifies the Friedmann equations due to thermodynamics-gravity conjecture. Based on this, we investigate the cosmological consequences of ADE through modified Barrow cosmology and disclose the effects of Barrow exponent $δ$ on the evolutions of the cosmological parameters. We observe that, depending on the values of $δ$, the transition from early decelerated phase to the late time accelerated phase occurs, and the equation of state (EoS) parameter $ w_{de} $ varies from quintessence $ -1<w_{de}<-1/3 $ to the phantom regime $ (w_{de}<-1)$. When $δ=0$, all results of ADE in standard cosmology are restored.

gr-qc↗

arXiv2021

New black hole solutions in three-dimensional $\mathit{f(R)}$ gravity

We construct two new classes of analytical solutions in three-dimensional spacetime and in the framework of $f(R)$ gravity. The first class represents a non-rotating black hole (BH) while the second class corresponds to a rotating BH solution. The Ricci scalar of these BH solutions have non-trivial values and are described by the gravitational mass $M$, two angular momentums $J$ and $J_1$, and an effective cosmological constant $Λ_{eff}$. Moreover, these solutions do not restore the $3$-dimensional Bañados-Teitelboim-Zanelli (BTZ) solutions of general relativity (GR) which implies the novelty of the obtained BHs in $f(R)$ gravity. Depending on the range of the parameters, these solutions admit rotating/non-rotating asymptotically AdS/dS BH interpretation in spite that the field equation of $f(R)$ has no cosmological constant. Interestingly enough, we observe that in contrast to BTZ solution which has only causal singularity and scalar invariants are constant everywhere, the scalar invariants of these solutions indicate strong singularity for the spacetime. Furthermore, we construct the forms of the $f(R)$ function showing that they behave as polynomial functions. Finally, we show that the obtained solutions are stable from the viewpoint that heat capacity has a positive value, and also from the condition of Ostrogradski which state that the second derivative of $f(R)$ should have a positive value.

physics.gen-ph↗

arXiv2021

Lifshitz scaling effects on the holographic paramagnetic-ferromagnetic phase transition

We disclose the effects of Lifshitz dynamical exponent $z$ on the properties of holographic paramagnetic-ferromagnetic phase transition in the background of Lifshitz spacetime. To preserve the conformal invariance in higher dimensions, we consider the Power-Maxwell (PM) electrodynamics as our gauge field. We introduce a massive $2$-form coupled to the PM field and perform the numerical shooting method in the probe limit by assuming the PM and the $2$-form fields do not back-react on the background geometry. The obtained results indicate that the critical temperature decreases with increasing the strength of the power parameter $q$ and dynamical exponent $z$. Besides, the formation of the magnetic moment in the black hole background is harder in the absence of an external magnetic field. At low temperatures, and in the absence of an external magnetic field, our result show the spontaneous magnetization and the ferromagnetic phase transition. We find that the critical exponent takes the universal value $β= 1/2$ regardless of the parameters $q, z, d$, which is in agreement with the mean field theory. In the presence of an external magnetic field, the magnetic susceptibility satisfies the Curie-Weiss law.

hep-th↗

arXiv2021

Holographic paramagnetic-ferromagnetic phase transition of Power-Maxwell-Gauss-Bonnet black holes

Based on the shooting method, we numerically investigate the properties of holographic paramagnetism-ferromagnetism phase transition in the presence of higher order Gauss-Bonnet (\emph{GB}) correction terms on the gravity side. On the matter field side, however, we consider the effects of the Power-Maxwell (\emph{PM}) nonlinear electrodynamics on the phase transition of this system. For this purpose, we introduce a massive $2-$form coupled to \emph{PM} field, and neglect the effects of $2-$form fields and gauge field on the background geometry. We observe that increasing the strength of both the power parameter $q$ and \emph{GB} coupling constant $α$ decrease the critical temperature of the holographic model, and lead to the harder formation of magnetic moment in the black hole background. Interestingly, we find out that at low temperatures, the spontaneous magnetization and ferromagnetic phase transition happen in the absence of external magnetic field. In this case, the critical exponent for magnetic moment has the mean field value, $1/2$, regardless of the values of $q$ and $α$. In the presence of external magnetic field, however, the magnetic susceptibility satisfies the Curie-Weiss law.

hep-th↗