arXiv subjects

Ahmad Sheykhi

Publications and source records attributed to Ahmad Sheykhi.

At least 19 recordsLinked to original sources

arXiv2026

Greybody Factors and Hawking Spectra of Quantum-Corrected Black Holes

We investigate Hawking radiation of a quantum-corrected Schwarzschild black hole with vacuum polarization and conformal anomaly effects. The corrected geometry has two horizons, a non-monotonic Hawking temperature that vanishes at a critical mass, and a logarithmic entropy correction. We compute greybody factors, spectra, and emission rates for massless bosonic fields of various spins and angular momenta. Quantum corrections suppress low-energy greybody factors and enhance high-energy ones. During late evaporation, where quantum influences become significant, the decreasing temperature strongly suppresses emission. In this case, the higher-spin modes become negligible, and the scalar $s=l=0$ mode dominates. The black hole lifetime is greatly extended as it asymptotically approaches a stable extremal remnant.

hep-th↗

arXiv2026

Hawking radiation from a semi-classical Schwarzschild black hole

This study investigates the evaporation process of a Schwarzschild black hole, incorporating quantum corrections arising from conformal anomaly and vacuum polarization. We show that these quantum corrections significantly modify the Hawking radiation and thermodynamic behavior of the black hole. In particular, the Hawking temperature reaches a maximum value and subsequently decreases as the black hole loses mass, eventually vanishing in the extremal limit. The evaporation process therefore approaches an extremal remnant configuration with zero Hawking temperature. Furthermore, we demonstrate that the black hole entropy receives logarithmic corrections due to the modified thermodynamic structure. Hawking radiation is also analyzed using the Parikh-Wilczek tunneling method, which confirms the consistency of the obtained thermodynamic behavior within the quantum-corrected framework.

hep-th↗

arXiv2026

Entanglement islands and information recovery from near-extremal regular black holes

We investigate the Page curve and information recovery in a near-extremal regular black hole inspired by T-duality, in which the central singularity is resolved by a minimal length scale. By integrating the first law of thermodynamics at fixed minimal length, we obtain a black-hole entropy containing an intrinsically quantum logarithmic correction, while the area-law contribution vanishes in the extremal limit. Consequently, the extremal remnant carries a finite entropy of purely quantum origin. Using the island prescription in the near-horizon regime, we evaluate the generalized entropy of radiation and compare the standard area functional with an alternative functional constructed from the corrected thermodynamic entropy of the black hole. In the near-extremal limit, the latter reduces analytically to the classical extremization problem with an effective coupling, and it places the physical island closer to the outer horizon. Although both prescriptions produce a Page transition, they predict different saturation values and Page times. While the standard functional yields a plateau controlled by the area term, the corrected prescription saturates at the full thermodynamic entropy of the outer horizon and, in the extremal limit, at the logarithmic remnant entropy. Including evaporation and backreaction, the Page curve develops the expected descending branch and asymptotes to the entropy of the cold extremal remnant rather than to zero. Our results indicate that a thermodynamically consistent description of information recovery from regular near-extremal black holes requires incorporating the intrinsic quantum correction to the gravitational entropy.

hep-th↗

arXiv2026

An Effective Framework for Entropy Modifications via Stochastic Metric Fluctuations

Deviations from the area law of horizon entropy are often modeled phenomenologically through modified Friedmann equations in cosmology. In this work, we explore an effective stochastic interpretation of such modifications by considering a Friedmann--Robertson--Walker (FRW) universe perturbed by a conformal, time-dependent noise factor with vanishing mean. Averaging the Einstein equations to second order in the fluctuation amplitude yields an effective correction to the Friedmann equation. We show that, in the spatially flat case with vanishing cosmological constant, this correction can be effectively matched to the general form induced by a deformed entropy-area relation. Within this phenomenological framework, the noise variance can be suitably chosen to match the functional forms associated with several generalized entropy models, including Renyi, (dual) Kaniadakis, Barrow, logarithmic, and MOND-inspired hypergeometric entropies. The present analysis therefore provides an effective correspondence between stochastic metric fluctuations and entropy deformations.

gr-qc↗

arXiv2026

Slow-roll inflation in (dual) Kaniadakis cosmology

We investigate slow-roll inflation within the framework of Kaniadakis and dual Kaniadakis cosmology, where the usual entropy formalism is generalized through a deformation parameter $κ$. By deriving the modified Friedmann equations and the corresponding inflationary dynamics induced by Kaniadakis entropy, we analyze the deviations from standard inflation arising from $κ$-corrections.To test the viability of these models, we evaluate two representative inflationary potentials: the power-law and the Mexican-hat (symmetry breaking) potentials. We compute the scalar and tensor spectral indices, the tensor-to-scalar ratio, and the primordial power spectrum, comparing them against the latest Planck and BICEP observational constraints. Our results show that in the standard Kaniadakis formulation, viable slow-roll inflationary scenarios compatible with observations can be obtained, although the allowed values of the deformation parameter are suppressed, yielding bounds of $κ\sim \mathcal{O}(10^{-9})$ and $κ\sim \mathcal{O}(10^{-37})$ for the power-law and Mexican-hat potentials, respectively. Conversely, for the dual Kaniadakis formulation, we find no physical parameter space that simultaneously satisfies the observational bounds for these potentials, rendering this formulation phenomenologically disfavored. These findings suggest that while standard Kaniadakis cosmology can leave potentially observable imprints on primordial perturbations, the constraints on $κ$ during inflation are significantly weaker than those from late-time cosmological bounds, hinting at a possible energy-scale dependence of the non-extensive deformation parameter.

gr-qc↗

arXiv2026

Big-Bang Nucleosynthesis and WIMP Dark Matter Freeze-Out as Probes of Yukawa Cosmology

We investigate Big-Bang Nucleosynthesis (BBN) in the context of Yukawa cosmology. We first derive the modified Friedmann equations by starting from the first law of thermodynamics on the apparent horizon. Using observational data on $^4\mathrm{He}$, deuterium, and $^7\mathrm{Li}$ abundances, we place stringent bounds on the Yukawa coupling $α$. The $^4\mathrm{He}$ and deuterium constraints are mutually consistent ($-0.24 \lesssim α\lesssim 0.12$), while $^7\mathrm{Li}$ requires $α\in [-0.76,\,-0.72]$. This indicate that Yukawa cosmology cannot resolve the Lithium Problem. We then extend our analysis to WIMP freeze-out, and show that the modified Hubble parameter alters the relic abundance, yielding an independent constraint $-0.017 \lesssim α\lesssim 0.018$ from $Ω_{\mathrm{CDM}}h^2 = 0.120 \pm 0.001$. We also derive the modified time-temperature relation, and show that the positive $α$ raises the early Universe temperature. Our analysis demonstrates that BBN and dark matter relic abundance serve as complementary probes of modified gravity. Our studies confirm that Yukawa cosmology is a testable framework for early-Universe physics.

astro-ph.CO↗

arXiv2026

Unified dark sector and Hubble-tension alleviation in scalar-vector-tensor gravity

We investigate a scalar-vector-tensor theory in which matter is minimally coupled to a Jordan-frame metric $\tilde g_{μν}=(1+Ξ)g_{μν}$, while a massive vector sector interacts with the baryonic current. We show that the conformal scalar coupling modifies the physical expansion rate measured by matter observers, leading to an enhancement of the Hubble constant inferred at low redshift. We stress, however, that the Hubble rate is not a conformal invariant, whereas the acoustic angular scale $θ_s$ is, and we derive the exact integral condition that the scalar field evolution must satisfy. We show that a single-signed scalar velocity cannot satisfy it, and we construct instead a two-epoch phenomenological evolution which matches $θ_s$ exactly while retaining the late-time enhancement, at the cost of a small pre-recombination shift of the effective gravitational coupling. Importantly, the recombination temperature is unmodified, since particle masses are constant in the Jordan frame, only the expansion rate at that epoch being altered. The scalar potential naturally acts as a dynamical dark-energy sector, while the vector sector provides two distinct contributions. The temporal component, determined algebraically by the baryon current, yields an apparent matter-like term in the background expansion that is not a true fluid but rather a manifestation of the interaction energy. The propagating spatial modes, on the other hand, form a vector condensate that behaves as a collisionless pressureless component and can play the cosmological role of cold dark matter. Hence, the framework connects scalar dynamics, effective dark-energy evolution, and the $H_0$ tension within a single setup.

gr-qc↗

arXiv2026

Landauer's Principle as a Criterion for Thermodynamic Consistency in Generalized Black Hole Entropies

Under the assumption that black hole horizon area is quantized, each Hawking evaporation step, during which the black hole loses mass and transitions to a lower area level, is interpreted as the erasure of one bit of information. In this paper, by employing Landauer's principle, we test the consistency of various black hole entropy relations with this information-theoretic framework. For the Bekenstein-Hawking entropy, the energy emitted per step saturates the Landauer bound. We extend this analysis to a broad class of generalized entropy models, yielding three distinct outcomes. In the first category, Landauer's principle imposes constraints on the Hawking temperature and, consequently, on the black hole mass. In the second, it restricts the free parameters of the entropy model. The third category, exemplified by Kaniadakis entropy, proves incompatible with Landauer's principle. For all compatible models, we derive the area quantization parameter and the corresponding area spectrum. While this parameter is constant for Bekenstein-Hawking entropy, it becomes level-number-dependent for many generalized models. Nonetheless, the relative spacing between successive area levels vanishes in the classical limit. Our findings point to a deep link between information theory and black hole thermodynamics.

gr-qc↗

arXiv2026

Constraints on Rényi Entropy through Primordial Big-Bang Nucleosynthesis and Baryogenesis

The Rényi entropy, a one-parameter generalization of Boltzmann-Gibbs entropy, offers a promising framework for probing quantum gravitational effects in cosmology. By modifying the entropy-area relation of the apparent horizon, Rényi entropy can alter the expansion dynamics of the early universe, with potential implications for Big-Bang Nucleosynthesis. In this work, we derive the modified Friedmann equations within the Rényi entropy paradigm and investigate their impact on the primordial abundances of light elements Deuterium ($D$), Helium-4 ($_{}^{4}\textit{He}$), Lithium-7 ($_{}^{7}\textit{Li}$) and baryogenesis. Using observational constraints from Planck, primordial abundance data and observational data on baryogenesis, we put stringent bounds on the Rényi parameter. Furthermore, we explore whether Rényi entropy corrections could mitigate the long-standing Lithium discrepancy. This study provides the first systematic constraints on Rényi cosmology from BBN and highlights the role of nonextensive thermodynamics in early-universe physics. Our analysis shows that the obtained ranges for the Rényi parameter $λ$ exhibit a overlap for the Helium-4 and Deuterium, but this overlapping region-despite a small discrepancy-is inconsistent with the range obtained from Lithium-7. This small mismatch between the ranges raises the possibility of alleviating the \textit{Lithium Problem} in the modified cosmology scenarios. Furthermore, we present the relationship between the cosmic time and temperature within the framework of Rényi cosmology. We observe that an increase in the Rényi parameter increase the temperature of the early universe.

physics.gen-ph↗

arXiv2026

From information bounds to infrared gravity: implications of Sharma-Mittal entropy

The Sharma--Mittal (SM) entropy provides a two-parameter generalization encompassing both the Rényi and Tsallis statistical frameworks. In this work, we investigate its thermodynamic and gravitational implications in the context of black hole physics and emergent gravity. Specifically, we examine the compatibility of the gravitational realization of the SM entropy with the Bekenstein bound and show that the corresponding framework consistently interpolates between the Rényi and Bekenstein--Hawking entropies in the appropriate limits. By incorporating Landauer's principle into black hole thermodynamics, we obtain a modified mass-loss relation associated with one-bit information erasure, exhibiting nontrivial parameter-dependent asymptotic behavior in both the small- and large-mass regimes. Furthermore, within Verlinde's entropic gravity framework, we derive modified gravitational force and acceleration laws induced by the SM entropy. We show that the resulting acceleration deviates from the Newtonian prediction at large distances and naturally reproduces a MOND-like regime for the specific parameter relation $R/δ= 3/2$. This condition establishes a direct connection between the SM entropy parameters and the MOND acceleration scale $a_0$. Our findings highlight the potential of the SM framework to provide a unified link between black hole thermodynamics, information theory, and infrared modifications of gravity, while offering new insights into phenomena traditionally attributed to the dark matter paradigm.

gr-qc↗

arXiv2026

Constraining Zero-Point Length from Gravitational Baryogenesis

The existence of a fundamental zero-point length, $l_0$, a minimal spacetime scale predicted by T-duality in string theory or quantum gravity theories, modifies the entropy associated with the horizon of spacetime. In the cosmological setup, this leads to correction to the Friedmann equations governing the evolution of the Universe. In this paper, we investigate the implications of zero-point length $l_0$-corrected gravity for gravitational baryogenesis and early universe thermodynamics, deriving constraints on $l_0$ from observational baryon asymmetry data. We observe that under the condition of non-equilibrium thermodynamics, $l_0$ generates $\dot{\mathcal{R}}\neq 0$ during radiation epoch, where $\mathcal{R}$ is the Ricci scalar. This yields a baryon asymmetry parameter $η\propto l_0^2 T_D^9/M_{\rm Pl}^7$. The observed baryon asymmetry $η\sim 9.9 \times 10^{-11}$ constrains $l_0 \lesssim 7.1 \times 10^{-33} m$, approximately $440$ times the Planck length. Furthermore, our analysis reveals that the zero-point length correction in the Friedmann equation, effectively slows the expansion rate at high energies, resulting in a modified time-temperature relationship where the Universe maintains higher temperatures for longer time during early epochs compared to standard cosmology. Our results establish zero-point length cosmology as a testable framework connecting quantum gravity to cosmological observables, with implications for early universe thermal history and fundamental length scales.

gr-qc↗

arXiv2025

Emergence of the cosmic space inspired by mass-to-horizon entropy

The conception of gravity as an emergent phenomenon, rooted in the thermodynamics of spacetime, offers a radical departure from its geometric description. This paper investigates the emergence of cosmic space by synthesizing two key thermodynamic approaches: the equilibrium perspective, where the first law of thermodynamics is applied to the apparent horizon, and the dynamic perspective of Padmanabhan, where the cosmic space emerges as cosmic time progresses. The central element of our study is the incorporation of a mass-to-horizon entropy relation, $M=γ{c^2 L^n}/{G}$, where $M$ denotes the effective mass associated with the system, $L$ corresponds to the cosmological horizon, and $γ$ is a constant with dimensions $[L]^{1-n}$. We first use this relation within the Clausius relation and apply the first law of thermodynamics, $dE=T_h dS_h+WdV$, on the apparent horizon to derive the modified Friedmann equations. Subsequently, we embed the mass-to-horizon entropy relation into Padmanabhan's cosmic emergence proposal, the dependence of the volume change on the degrees of freedom in the bulk and on the boundary, and show its consistency with the thermodynamically derived equations. The successful reconstruction of the modified Friedmann equations through these independent yet convergent thermodynamic routes strongly suggests that the mass-to-horizon entropy is a fundamental bridge between the information-theoretic microstructure of spacetime and its effective cosmological description. Finally, we show that the generalized second law of thermodynamics is fulfilled for the universe enveloped by the apparent horizon.

gr-qc↗

arXiv2025

Barrow Cosmology and Big-Bang Nucleosynthesis

Using thermodynamics-gravity conjecture, we present the formal derivation of the modified Friedmann equations inspired by the Barrow entropy, $S\sim A ^{1+δ/2}$, where $0\leqδ\leq 1$ is the Barrow exponent and $A$ is the horizon area. We then constrain the exponent $δ$ by using Big-Bang Nucleosynthesis (BBN) observational data. In order to impose the upper bound on the Barrow exponent $δ$, we set the observational bound on $\left| \frac{δT_f} {T_f }\right|$. We find out that the Barrow parameter $δ$ should be around $ δ\simeq 0.01$ in order not to spoil the BBN era. Next we derive the bound on the Barrow exponent $δ$ in a different approach in which we analyze the effects of Barrow cosmology on the primordial abundances of light elements i.e. Helium $_{}^{4}\textit{He}$, Deuterium $D$ and Lithium $_{}^{7}\textit{Li}$. We observe that the deviation from standard Bekenstein-Hawking expression is small as expected. Additionally we present the relation between cosmic time $t$ and temperature $T$ in the context of modified Barrow cosmology. We confirm that the temperature of the early universe increases as the Barrow exponent $δ$ (fractal structure of the horizon) increases, too.

gr-qc↗

arXiv2025

Three dimensional charged black holes in Gauss-Bonnet gravity

By using the zero-point length effect, we construct a new class of charged black hole solutions in the framework of three dimensional Gauss-Bonnet (GB) gravity with Maxwell electrodynamics. The gravitational and electromagnetic potentials are finite and regular everywhere, however, the computation of scalar curvature invariants suggest the presence of a singularity at the origin. We also explore thermodynamics of the obtained solutions and reveal that the entropy of the black hole decreases due to the stringy effects. The thermodynamic and conserved quantities are computed and also the validity of the first law of thermodynamics on the black hole horizon is verified. Finally, the spinning black hole solution is also reported.

physics.gen-ph↗

arXiv2025

Big-Bang nucleosynthesis constraints on (dual) Kaniadakis cosmology

We investigate the concept of Kaniadakis entropy and its dual formulation, examining their implications for gravitational dynamics within the framework where gravity emerges as an entropic force resulting from changes in the informational content of a physical system. In this context, we derive a modified form of Newton's law of gravitation that reflects the corrections introduced by both Kaniadakis entropy and its dual state. Furthermore, we apply the emergent gravity scenario at large scales and derive the modified Friedmann equations incorporating corrections from (dual) Kaniadakis entropy. Our results provide deeper insights into the interplay between thermodynamics and gravitational dynamics. In order to constrain the model parameter, we study the Big-Bang Nucleosynthesis in the context of (dual) Kaniadakis cosmology. We explore an alternative method to establish limits on the Kaniadakis parameter, denoted as $K$, by examining how (dual) Kaniadakis cosmology influences the primordial abundances of light elements i.e. Helium $_{}^{4}\textit{He}$, Deuterium $D$ and Lithium $_{}^{7}\textit{Li}$. Our analysis indicates that the obtained ranges for the dual Kaniadakis parameter (unlike the Kaniadakis parameter) exhibit overlap for the aforementioned light elements, and the allowed values fall within the range $ -0.8\times 10^{-78}\lesssim \tilde {K^*}\lesssim 0.8\times 10^{-78}$, which shows that the deviations from the conventional Bekenstein-Hawking formula are minimal, as expected. This consistency between the ranges suggests a potential solution to the well-known \textit{Lithium problem}. Furthermore, we discuss the relationship between cosmic time $t$ and temperature $T$ within the framework of (dual) Kaniadakis cosmology.

gr-qc↗

arXiv2025

Thermodynamical properties of nonsingular universe

We disclose the thermodynamical properties of the apparent horizon in a nonsingular universe. We take into account the zero-point length correction to the gravitational potential and derive the modified entropy expression that includes zero-point length correction terms. We apply the first law of thermodynamics on the apparent horizon as well as the emergent gravity scenario to derive the modified Friedmann equations. Further, we examine the time evolution of the total entropy, including the entropy of the apparent horizon and the matter field entropy inside the horizon and find out that the generalized second law of thermodynamics is satisfied. We also investigate the cosmological implications of the modified cosmology through zero-point length. We observe that the zero-point length correction does not change the general profile of the universe evolution, however, it shifts the time of the phase transition in a universe filled with matter and cosmological constant. We explore the age of the universe for our model and observe that the predicted age of the universe becomes larger compared to the standard cosmology. By calculating the explicit form of Ricci and Kretchmann invariants, we confirm that in our model, the initial singularity of the universe is removed. This is an expected result, because the main motivation for considering zero-point length correction in the gravitational potential is to remove singularity at the origin.

gr-qc↗

arXiv2025

Modified $f(R,T)$ theory in light of gravitational wave standard sirens

In this paper, we ponder observational constraints on the modified $f(R,T)$ gravity, where the gravitational action is a function of Ricci scalar $R$ plus the trace of the energy-momentum tensor $T$, regarding the functional form $f(R,T)=R+2f(T)$ with $f(T)=8πGλT$. For this purpose, we utilize recently available data, including cosmic microwave background, weak lensing, supernovae, baryon acoustic oscillations, and redshift-space distortions measurements, together with forcasted gravitational wave (GW) data from Laser Interferometer Space Antenna (LISA). Notably, we examine the potentiality of simulated GW data from LISA standard sirens (SS) sources to enhance cosmological constraints on the $f(R,T)$ model parameters. In this regard, we create three LISA mock catalogs, namely Pop III, Delay, and No Delay, to improve the obtained constraints on cosmological parameters of $f(R,T)$ gravity from current observations. Numerical analysis reveals that mock GW data from LISA SS sources make marginal improvements on constraining the $f(R,T)$ model cosmological parameters.

gr-qc↗

arXiv2025

Dark Universe inspired by the Kaluza-Klein gravity and impact on Primordial Gravitational Waves

We explore the potential implications of Kaluza-Klein (KK) gravity in unifying the dark sector of the Universe. Through dimensional reduction in KK gravity, the 5D spacetime framework can be reformulated in terms of a 4D spacetime metric, along with additional scalar and vector fields. From the 4D perspective, this suggests the existence of a tower of particle states, including KK gravitons with spin-0 and spin-1 states, in addition to the massless spin-2 gravitons of general relativity (GR). The key idea in the present paper is the analogy with superconductivity theory. By assuming a minimal coupling between an additional complex scalar field and the gauge field, a "mass" term emerges for the spin-1 gravitons. This, in turn, leads to long-range gravitational effects that could modify Newton's law of gravity through Yukawa-type corrections. Assuming an environment-dependent mass for the spin-1 graviton, near the galactic center the repulsive force from this spin-1 graviton is suppressed by an additional attractive component from Newton's constant corrections, resulting in a Newtonian-like, attraction-dominated effect. In the galaxy's outer regions, the repulsive force fades due to its short range, making dark matter appear only as an effective outcome of the dominant attractive corrections. This approach also explains dark matter's emergence as an apparent effects on cosmological scales while our model is equivalent to the scalar-vector-tensor gravity theory. Finally, we examine the impact of dark matter on the primordial gravitational wave (PGW) spectrum and show that it is sensitive to dark matter effects, providing an opportunity to test this theory through future GW observatories.

gr-qc↗