arXiv subjects

Ahmad Sheykhi

Publications and source records attributed to Ahmad Sheykhi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Two-Field Quintessential Higgs model and the Swampland

We study a two-field model where a quintessence field with an exponential potential $e^{-βϕ/M_P}$ is coupled to the Higgs field. It is claimed that this model is consistent with the proposed Swampland conjecture. We check this claim by calculating its inflationary observables. Although, these observables are in good agreement with the latest CMB data, but we find an upper bound $β\lesssim 8\times 10^{-3}$ that strongly disfavors the Swampland conjecture.

astro-ph.CO↗

arXiv2025

Horizon Entropy Refined: Quantum Contributions and Cosmological Insights

We study the effects of quantum fluctuations on the event horizon area and their implications for corrections to the Bekenstein-Hawking entropy. These quantum corrections are incorporated into the framework of large-scale gravitational systems, utilizing the holographic principle to derive modified Friedmann equations. By redefining the Bekenstein-Hawking entropy, our model predicts significant alterations to the Friedmann equations within specific parameter ranges, offering novel perspectives on cosmological scales. Using distance modulus data from the Pantheon supernova sample, we demonstrate the model's potential to constrain the parameters governing quantum corrections and address unresolved cosmological issues. Crucially, our analysis reveals that quantum fluctuations can increase the area of the event horizon by up to 47\%. Beyond this threshold, theoretical predictions encounter substantial challenges when compared with observational data. This approach bridges quantum gravity and observational cosmology, opening new avenues for testing and refining theoretical models.

hep-th↗

arXiv2024

Growth of cosmic perturbations in the modified $f(R,T)$ gravity

We explore the generalized $f(R,T)$ modified theory of gravity, where the gravitational Lagrangian is a function of Ricci scalar $R$ and the trace of the energy-momentum tensor $T$. We derive modified field equations to the linear order of perturbations in the context of $f(R,T)$ model. We then investigate the growth of perturbations in the context of $f(R,T)$ modified gravity. Primary numerical investigations based on matter power spectra diagrams indicate a structure growth suppression in $f(R,T)$ gravity, which exhibits consistency with local measurements. Also, we notice that matter-geometry interaction in $f(R,T)$ model would results in the specific feature named as "matter acoustic oscillations" appeared in matter power spectra diagrams. Moreover, we put constraints on the cosmological parameters of $f(R,T)$ model, utilizing current observations, chiefly cosmic microwave background (CMB), weak lensing, supernovae, baryon acoustic oscillations, and redshift-space distortions data. Numerical results based on MCMC calculations imply that $f(R,T)$ is a qualified theory of modified gravity in reconciling Planck CMB data with local probes of large scale structures, by reporting lower values for the structure growth parameter $σ_8$ compared to the standard model of cosmology.

gr-qc↗

arXiv2024

Ghost Dark Energy in Tsallis and Barrow Cosmology

According to the thermodynamics-gravity conjecture, any modification to the entropy expression leads to the modified cosmological field equations. Based on this, we investigate the cosmological consequences of the modified Friedmann equations when the entropy associated with the horizon is in the form of Tsallis/Barrow entropy and the dark energy (DE) component is in the form of ghost dark energy (GDE). We perform a dynamical system analysis and see that the Tsallis GDE(TGDE) and Barrow GDE(BGDE) can exhibit a correct phase space evolution for suitable range of the free parameters($0<Δ<1$ for BGDE and $β<3/2$ for TGDE). We find that in BGDE and TGDE (with $Q=3b^2 H(ρ_D+ρ_m)$), there exist an early radiation dominated phase of expansion which is absent for GDE model in standard cosmology. It is worth mentioning that seeking an unstable phase of matter dominated in TGDE leads to a new constraint on $β$($<3/2$). Using the resulting range of free parameters from latter step we find that both models are capable to explain the cosmic evolution from deceleration to an accelerated phase. We observe that increasing $Δ$ ($β$) parameters leads to a delay in the cosmic phase transition. We conclude that BGDE and TGDE are viable cosmological models which predict a consistent phase transition of the cosmic expansion for suitable ranges of the parameters. We also calculate the squared sound speed for both models and find out that they are unstable against perturbations...

gr-qc↗

arXiv2024

Cosmology from String T-duality and zero-point length

Inspired by String T-duality and taking into account the zero-point length correction, $l_0$, to the gravitational potential, we construct modified Friedmann equations by applying the first law of thermodynamics on the apparent horizon of the Friedmann-Robertson-Walker (FRW) Universe. The cosmological viability of this extended scenario is investigated by studying influences on the evolution of the early Universe and performing the cosmographic analysis. Furthermore, we explore the inflationary paradigm under the slow-roll condition. By testing the model against observational data, the zero-point length is constrained around the Planck scale, in compliance with the original assumption from String T-duality. We also study the growth of density perturbations in the linear regime. It is shown that the zero-point length stands out as an alternative characterization for the broken-power-law spectrum, which provides a better fitting for the experimental measurements comparing to the simple power-law potential. Finally, we focus on implications for the primordial gravitational waves (PGWs) spectrum. Should the zero-point length be running over energy scales, as is the case for all parameters and coupling constants in quantum gravity under renormalization group considerations, deviations from General Relativity (GR) might be potentially tested through upcoming PGW observatories.

gr-qc↗

arXiv2024

Apparent Dark Matter Inspired by Einstein Equation of State

The purpose of this article is twofold. First, by means of Padmanabhan's proposal on the emergence nature of gravity, we recover the $Λ$CDM model and the effect of the dark matter in the context of cosmology. Toward this goal, we use the key idea of Padmanabhan that states cosmic space emerges as the cosmic time progress and link the emergence of space to the difference between the number of degrees of freedom on the boundary and in the bulk. Interestingly enough, we show that the effect of the cold dark matter in the cosmological setup can be understood by assuming an interaction between the numbers of degrees of freedom in the bulk. In the second part, we follow the Jacobson's argument and obtain the modified Einstein field equations with additional dark matter component emerging due to the interaction term between dark energy and baryonic matter related by $Ω_{DM,0}=\sqrt{2 αΩ_{M,0} Ω_{DE,0}}$, where $α$ is a coupling constant. Finally, a correspondence with Yukawa cosmology is pointed out, and the role of massive gravitons as a possibility in explaining the nature of the dark sector as well as the theoretical origin of the Modified Newtonian Dynamics (MOND) are addressed. We speculate that the interaction coupling $α$ fundamentally measures the entanglement between the gravitons and matter fields and there exists a fundamental limitation in measuring the gravitons wavelength.

gr-qc↗

arXiv2024

Corrections to Friedmann equations inspired by Kaniadakis entropy

Adopting the thermodynamics-gravity conjecture, and assuming the entropy associated with the apparent horizon of the Friedmann-Robertson-Walker (FRW) universe has the form of the generalized Kaniadakis entropy, we extract the modified Friedmann equations describing the evolution of the universe using the first law of thermodynamics on the apparent horizon. We then investigate the validity of the generalized second law of thermodynamics for the universe enclosed by the apparent horizon.

gr-qc↗

arXiv2023

Slow-roll inflation and growth of perturbations in Kaniadakis modification of Friedmann cosmology

Kaniadakis entropy is a one-parameter deformation of the classical Boltzmann-Gibbs-Shannon entropy, arising from a self-consistent relativistic statistical theory. Assuming a Kaniadakis-type generalization of the entropy associated with the apparent horizon of Friedmann-Robertson-Walker (FRW) Universe and using the gravity-thermodynamics conjecture, a new cosmological scenario is obtained based on the modified Friedmann equations. By employing such modified equations, we analyze the slow-roll inflation, driven by a scalar field with power-law potential, at the early stages of the Universe. We explore the phenomenological consistency of this model by computation of the scalar spectral index and tensor-to-scalar ratio. Comparison with the latest Planck data allows us to constrain Kaniadakis parameter to $κ\lesssim\mathcal{O}(10^{-12}\div10^{-11})$, which is discussed in relation to other observational bounds in the past literature. We also disclose the effects of Kaniadakis correction term on the growth of perturbations at the early stages of the Universe by employing the spherically symmetric collapse formalism in the linear regime of density perturbations. We find out that the profile of density contrast is non-trivially affected in this scenario. Interestingly enough, we observe that increasing Kaniadakis parameter $κ$ corresponds to a faster growth of perturbations in a Universe governed by the corrected Friedmann equations. Finally, we comment on the consistency of the primordial power spectrum for scalar perturbations with the best data-fit provided by Planck.

gr-qc↗

arXiv2023

Entropic corrections to Friedmann equations and bouncing universe due to the zero-point length

We employ Verlinde's entropic force scenario to extract the modified Friedmann equations by taking into account the zero-point length correction to the gravitational potential. Starting form the modified gravitational potential due to the zero-point length, we first find the logarithmic corrections to the entropy expression and then we derive the modified Friedman equations. Interestingly enough, we observe that the corrected Friedmann equations are similar to the Friedmann equations in braneworld scenario. In addition, from the corrected Friedmann equations, we pointed out a possible connection to the GUP principle which might have implications on the Hubble tension. To this end, we discuss the evolution of the scale factor under the effect of zero-point length. Finally, having in mind that the minimal length is of the Planck order, we obtain the critical density and the bouncing behavior of the universe with a critical density and a minimal scale factor of the order of Planck length.

gr-qc↗

arXiv2023

Modified cosmology through Kaniadakis entropy

We explore a cosmological model inspired by the modified Kaniadakis entropy and disclose the influences of the modified Friedmann equations on the evolution of the Universe. We find that in modified Kaniadakis cosmology with only pressure-less matter, one can reproduce the accelerated Universe without invoking any kind of dark energy. We delved into the evolution of the Universe during the radiation-dominated era as well. We also investigate the behavior of the scale factor and the deceleration parameter for a multiple-component Universe consisting of pressure-less matter and a cosmological constant/dark energy. Interestingly enough, the predicted age of the Universe in the modified Kaniadakis cosmology becomes larger compared to the standard cosmology which may alleviate the age problem. Furthermore, The findings reveal that in the modified Kaniadakis cosmology, the transition from a decelerated phase to an accelerated Universe occurs at higher redshifts compared to the standard cosmology. These results shed light on the potential implications of incorporating Kaniadakis entropy into cosmological models and provide valuable insights into the behavior of the Universe in different cosmological scenarios. Moreover, they emphasize the crucial role of modifications to the geometry component and the significance of such modifications in understanding the dynamics of the Universe.

gr-qc↗

arXiv2023

Harvesting energy driven by Comisso-Asenjo process from Kerr-MOG black holes

Magnetic reconnection is a process that plays a critical role in plasma astrophysics by converting magnetic energy into plasma particle energy. Recently, Comisso and Asenjo demonstrated that rapid magnetic reconnection within a black hole's ergosphere can efficiently extract energy from a rotating black hole. In this paper, by considering a Kerr black hole in the MOdified gravity (MOG) framework, we investigate the impact of the MOG parameter $α$ on the rotational energy extraction via the Comisso-Asenjo process (CAP). To model energy extraction from supermassive black holes located in the center of galaxies, we set the value of $α$ within the range inferred from the recent observation of Sgr A* by the Event Horizon Telescope (EHT). Our results indicate that the Kerr-MOG black hole is a more efficient host for CAP-based rotational energy extraction compared to the Kerr black hole, since it amplifies the power of energy extraction and efficiency of the plasma energization process. We show that, from the energy extraction viewpoint, the CAP is more efficient than the Blandford-Znajek process (BZP). The latter is another magnetic field-based energy extraction model which is widely believed to be an engine for powering the high-energy astrophysics jets emerging from the supermassive black holes at active galactic nuclei. In particular, we show that the ratio of the energy extraction power of CAP to BZP in the presence of the MOG parameter is greater than that of the Kerr black hole. Our results promise this phenomenological message that the MOG-induced correction on the Kerr black hole background plays an important role in favor of energy extraction via the CAP.

astro-ph.HE↗

arXiv2023

On observational signatures of multi-fractional theory

We study the multi-fractional theory with $q$-derivatives, where the multi-fractional measure is considered to be in the time direction. The evolution of power spectra and also the expansion history of the universe are investigated in the $q$-derivatives theory. According to the matter power spectra diagrams, the structure growth would increase in the multi-fractional model, expressing incompatibility with low redshift measurements of large scale structures. Furthermore, concerning the diagrams of Hubble parameter evolution, there is a reduction in the value of Hubble constant which conflicts with local cosmological constraints. Thus, primary numerical investigations imply that $q$-derivatives theory has no potential to relieve observational tensions. We also explore the multi-fractional model with current observational data, principally Planck 2018, weak lensing, supernovae, baryon acoustic oscillations (BAO), and redshift-space distortions (RSD) measurements. Numerical analysis reveals that the degeneracy between multi-fractional parameters makes them remain unconstrained under observations. Furthermore, observational constraints on $H_0$ and $σ_8$, detect no significant departure from standard model of cosmology.

gr-qc↗

arXiv2023

Growth of perturbations in higher dimensional Gauss-Bonnet FRW cosmology

We explore the influences of the higher order Gauss Bonnet (GB) correction terms on the growth of perturbations at the early stage of a (n + 1)-dimensional Friedmann-Robertson-Walker (FRW) universe. Considering a cosmological constant in the FRW background, we study the linear perturbations by adopting the spherically symmetric collapse (SC) formalism. In light of the modifications that appear in the field equations, we disclose the role of the GB coupling constant α, as well as the extra dimensions n > 3 on the growth of perturbations. It, in essence, is done by defining a dimensionless parameter \{beta}=(n-2)(n-3) \{alpha} H_0^2 in which H_0 is the Hubble constant. We find that the matter density contrast starts growing at the early stages of the universe and, as the universe expands, it grows faster compared to the standard cosmology. Besides, in the framework of GB gravity, the growth of matter perturbations in higher dimensions is faster than its standard counterpart (n = 3). Further, in the presence of α, the growth of perturbations increases as it increases. This is an expected result, since the higher order GB correction terms increase the strength of the gravity and thus support the growth of perturbations. For the existing cosmological model, we also investigate the behavior of quantities such as density abundance, deceleration, and the jerk parameter. Finally, we study the imprint of the GB parameter and the higher dimensions in the evolution of the mass function of the dark matter halos.

gr-qc↗

arXiv2023

Evolution of Spherical Overdensities in Energy-Momentum-Squared Gravity

Employing the spherical collapse (SC) formalism, we investigate the linear evolution of the matter over-density for energy-momentum-squared gravity (EMSG), which in practical phenomenological terms, one may imagine as an extension of the ΛCDM model of cosmology. The underlying model, while still having a cosmological constant, is a non-linear material extension of the general theory of relativity (GTR) and includes correction terms that are dominant in the high-energy regime, the early universe. Considering the Friedman{Robertson{Walker (FRW) background in the presence of a cosmological constant, we find the effects of the modifications arising from EMSG on the growth of perturbations at the early stages of the universe. Considering both possible negative and positive values of the model parameter of EMSG, we discuss its role in the evolution of the matter density contrast and growth function in the level of linear perturbations. While EMSG leaves imprints distinguishable from ΛCDM, we find that the negative range of the ESMG model parameter is not well-behaved, indicating an anomaly in the parameter space of the model. In this regard, for the evaluation of the galaxy cluster number count in the framework of EMSG, we equivalently provide an analysis of the number count of the gravitationally collapsed objects (or the dark matter halos). We show that the galaxy cluster number count decreases compared to the ΛCDM model. In agreement with the hierarchical model of structure formation, in EMSG cosmology the more massive structures are less abundant, meaning that form at later times.

astro-ph.CO↗

arXiv2023

Apparent dark matter as a non-local manifestation of emergent gravity

We disclose a close correspondence between Verlinde's Emergent Gravity (VEG) theory and the non-local gravity theories. Such non-local effects can play crucial role at small distances as well as in large scale structures. In particular, we argue that the emergent gravity effectively is a manifestation of the entanglement entropy and can modify Newton's law of gravity as well as address the flat rotation curves of spiral galaxies. In the cosmological setup, we have considered three different models for the apparent dark matter density. In the first model, we have found that Friedmann equations get modified due to the presence of the apparent dark matter (DM) in such a way that Newton's constant of gravity shifts as $G\rightarrow G\left(1+ζ\right)$, where $ζ$ is a dimensionless small parameter. Using the flat rotating curves we estimate $ζ\sim 10^{-7}$. For such a model, we find out that by rescaling the radial coordinate, $r$, the curvature space constant, $k$, and the scale factor of the universe, $a$, the effect of apparent DM can change the geometry of the universe and can shift the curvature space constant as $k_{\star}=k (1-ζ)$. To this end, we study a more realistic model with evolving densities where we obtain the $Λ$CDM model in Verlinde's gravity and comment on the Hubble tension problem with evolving densities.

gr-qc↗

arXiv2023

Towards high temperature holographic superconductors

We explore a holographic superconductor model in which a real scalar field is non-minimally coupled to a gauge field. We consider several types of the non-minimal coupling function h($ψ$) including exponential, hyperbolic (cosh), power-law and fractional forms. We investigate the influences of the non-minimal coupling parameter $α$ on condensation, critical temperature and conductivity. We can categorize our results in two groups. In the first group, conductor/superconductor phase transition is easier to occur for larger values of $α$, while in the second group stronger effects of the non-minimal coupling makes the formation of scalar hair harder. Although the real and imaginary parts of conductivity are impressed by different forms of h($ψ$), they follow some universal behaviors such as connecting with each other through Kramers-Kronig relation in low frequency regime or the appearance of gap frequency at low temperatures. We find the best form of forms of non-minimal coupling function that gives us better information in wide range of non-minimal coupling constant and temperature. Choosing the best form of h($ψ$), we construct a family of solutions for holographic conductor/superconductor phase transitions to discover the effect of the hyperscaling violation when the gauge and scalar fields are non-minimally coupled. we find that the critical temperature increases for higher effects of hyperscaling violation $θ$ and non-minimal coupling constant $α$. By increasing these two parameters, we obtain lower values of condensation which means that conductor/superconductor phase transition will acquire easier. Furthermore, we understand that the hyperscaling violation affects the conductivity $σ$ of the holographic superconductors and changes the expected relation in the gap frequency. Some universal behaviors like infinite DC conductivity are observed.

hep-th↗

arXiv2023

Constraining the Lorentz-Violating Bumblebee Vector Field with Big Bang Nucleosynthesis and Gravitational Baryogenesis

By keeping the cosmological principle i.e., an isotropic and homogeneous universe, we consider the cosmology of a vector-tensor theory of gravitation known as the \textit{bumblebee} model. In this model a single Lorentz-violating timelike vector field with a nonzero vacuum expectation value (VEV) couples to the Ricci tensor and scalar, as well. Taking the ansatz $B(t)\sim t^β$ for the time evolution of the vector field we derive the relevant dynamic equations of the Universe, where $β$ is a free parameter. In particular, by employing observational data coming from the Big Bang Nucleosynthesis (BBN) and the matter-antimatter asymmetry in the Baryogenesis era, we impose some constraints on the VEV of the bumblebee timelike vector field i.e., $ξb^2$, and the exponent parameter $β$. The former and the latter limit the size of Lorentz violation, and the rate of the time evolution of the background Lorentz-violating bumblebee field, respectively.

gr-qc↗

arXiv2023

Black hole geometrothermodynamics and critical phenomena: a look from Tsallis entropy-based perspective

We analyze geometrothermodynamics of charged anti-de Sitter (AdS) black holes with a global monopole in the framework of Tsallis statistics. The latter arises from a non-additive generalization of Boltzmann-Gibbs entropy, which is still recovered as a particular sub-case. We examine the effects of Tsallis entropy on small-large black holes phase-transitions and critical exponents, comparing the results with the liquid-gas change of phase occurring in van der Waals fluids. We also discuss some physical constraints on Tsallis parameter and the impact on sparsity of BH radiation. Thermodynamic curvature behavior is then explored from a geometric perspective within Ruppeiner formalism on the $S-P$ coordinate space. Similar considerations are extended to charged $(2+1)$-dimensional Banados-Teitelboim-Zanelli (BTZ) black holes by computing the entropy-pressure and entropy volume corrected curvatures. Our analysis shows that Tsallis prescription has non-trivial implications on black hole geometrothermodynamics, emphasizing the role of non-additive entropy on the correspondence between black holes and van der Waals systems.

hep-th↗