arXiv subjects

Mehrdad Farhoudi

Publications and source records attributed to Mehrdad Farhoudi.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Inflation in f(R,T) Gravity with Chern-Simons Correction

We investigate cosmological inflation within the framework of a linear form of f (R, T ) gravity that incorporates an inflaton scalar field augmented by a Chern-Simons correction induced by aspects of quantum gravity. Utilizing the FLRW metric, we derive the modified Friedmann equations under the slow-roll approximations. We consider two specific forms of the Chern-Simons coupling function, trigonometric and exponential, each paired with the choice of an inflaton potential. Then, we define the essential slow-roll parameters and acquire their required expressions in the proposed model. Subsequently, we compute the scalar spectral index, the tensor spectral index, and the tensor-to- scalar ratio. By adequately constraining the free parameters, the proposed model provides accurate predictions for these inflationary observables that are in good agreement with the Planck 2018 data. Furthermore, the model predictions for the Chern-Simons exponential coupling function impose a stronger limit on the value of the tensor-to-scalar ratio and also provide good agreement with the joint Planck, BK15 and BAO data. Meanwhile, for comparative analysis and better comparison of the model motivations, we also examine the model without the Chern-Simons correction, without the linear form of f (R, T ) gravity, and with a non-linear form of f (R, T ) gravity. As a general conclusion, the obtained findings indicate that the inclusion of the Chern-Simons correction approximately refines the values of the tensor spectral index and tensor-to-scalar ratio in the context of the linear form of f (R, T ) gravity.

gr-qc↗

arXiv2026

Inflation with Gauss-Bonnet Correction and Higgs Potential

We investigate the cosmological inflation for the Einstein-Hilbert action plus the Higgs potential function and the Gauss-Bonnet term coupled with the Higgs scalar field through a dilaton-like coupling. Then, using the Friedmann-Lemaıtre-Robertson-Walker metric and considering the appropriate slow-roll parameters, we derive the necessary equations of motion. In the proposed model, since the e-folding integral cannot be easily solved analytically, we first utilize a well-known Taylor expansion. Then, with a certain range of values derived for the model parameters, utilizing several plots and numerical analysis methods, we obtain results for the tensor-to-scalar ratio and the scalar spectral index that are in good agreement with the latest observational data, particularly from ACT DR6, within the acceptable range of the e-folding values. Meanwhile, a key aspect of this work, crucial for achieving reliable values of the inflationary observables, lies in the adopted functional forms for the potential and coupling functions. Also, in the absence of the Gauss-Bonnet term, we find that the inflationary observables are roughly the same as the predictions of the chaotic inflation model.

gr-qc↗

arXiv2026

Gravitomagnetism from Temporal Dimensional Reduction

We reduce the Taub-NUT metric dimensionally to three spatial dimensions by treating time as an extra curled dimension, and end up with the 3-dimensional Einstein field equations plus a corresponding Maxwell type equations for a gravitomagnetic field, associated with the NUT charge, which also acts as a source for the Einstein field equations. In this approach, the Taub-NUT metric can be envisaged as a (1 + 3)-dimensional analogue of the (1 + 4)-dimensional metric of the Kaluza- Klein theory. Hence, in four dimensions, it unifies gravitation and gravitomagnetism, associated with the NUT charge, in the same footing that the Kaluza-Klein theory unifies gravitation and electromagnetism in five dimensions. In fact, in this way, gravity and gravitomagnetism, associated with the NUT charge, appear as two distinct fields that emerge from the temporal dimensional reduction. We also introduce a relation between the 4-dimensional gravitational constant and the NUT charge.

hep-th↗

arXiv2025

Chameleon Gravity as an Alternative to Dark Matter

By studying the chameleon gravity on galaxy scales, we investigate the effects of the chameleon dark matter. To perform this task, we consider the dynamics of the chameleon scalar field in the region of galactic halos in static spherically symmetric spacetimes that differ only slightly from classical general relativity, but have similar symmetry behind this region. Hence, we demonstrate its ability to act as dark matter. In fact, by obtaining the expression for the tangential speed in this region, we apply this approach to explain the issue of flat galactic rotation curves. We obtain that the mass associated with the chameleon scalar field varies linearly with the radius of galaxy, and hence, the tangential speed in that region is constant, which is consistent with observational data without any need to introduce a mysterious dark matter. Accordingly, we show that the presented chameleon gravity has a well-defined Newtonian limit and can describe the geometry of spacetime in the region of flat galactic rotation curves, and is consistent with the corresponding results of the $ ΛCDM $ model with the NFW profile. We also consider a test particle moving in a timelike geodesic and obtain the fifth-force, which varies proportionally to the inverse of the radius of galaxy. Moreover, we obtain the effects on the angle representing the deflection of light and on the time representing the radar echo delay of photons passing through the region of galactic halos. We show that as the radius of galaxy increases, the effect on the angle of light passing through the region around galactic halos decreases. Also, the obtained result indicates that the relevant time delay decreases with increasing radius towards the end of the galactic halo.

gr-qc↗

arXiv2025

Symmetries of Weyl Superconductors with Different Pairings

We examine the Bogoliubov-de Gennes Hamiltonian and its symmetries for a time-reversal symmetry broken three dimensional Weyl superconductor. In the limit of vanishing pairing potential, we specify that this Hamiltonian is invariant under two sets of continues symmetries, i.e. the $U(1)$ gauge symmetry and the $U(1)_A$ axial symmetry. Although a pairing of the Bardeen-Cooper-Schrieffer type spontaneously breaks both of these symmetries, we show that a Fulde-Ferrell-Larkin-Ovchinnikov type pairing spontaneously breaks only the $U(1)$ gauge symmetry (that is then restored via the well-known scalar phase mode of superconductivity). Consequently, in the former case, two Nambu-Goldstone modes are required in the system to restore the broken symmetries. We indicate that one of these two modes is an emergent pseudo-scalar phase mode. We also demonstrate that such a phase mode leads to a pseudo-Meissner effect.

hep-th↗

arXiv2025

Metric Field as Emergence of Hilbert Space

First, we explain some ambiguities of spacetime and metric field as fundamental concepts. Then, from the Unruh effect point of view and using the Gelfand-Naimark-Segal construction, we construct an operator as a quanta of acceleration that we call quantum acceleration operator (QAO). Thereupon, we investigate the relation between the vacuum of two different frames in the Minkowski space. Also, we show that the vacuum of each accelerated frame in the Minkowski space can be obtained by applying such a QAO to the Minkowski vacuum. Furthermore, utilizing these QAOs, we augment the Hilbert space and then extract the metric field of a general frame of the Minkowski spacetime. In this approach, these concepts emerge from the Hilbert space through the constructed QAOs. Accordingly, such an augmented Hilbert space includes quantum field theory in a general frame and can be considered as a fundamental concept instead of the classical metric field and the standard Hilbert space.

gr-qc↗

arXiv2024

Perturbation Spectra of Warm Inflation in $f(Q, T)$ Gravity

We investigate the warm inflationary scenario within the context of the linear version of f (Q, T ) gravity, coupled with both the inflaton scalar field and the radiation field, under the conditions of the strong dissipation regime. First, we calculate the modified Friedmann equations and the modified slow-roll parameters. Subsequently, we apply the slow-roll approximations to derive the scalar power spectrum and the tensor power spectrum. Also, we develop formulations of the scalar and tensor perturbations for the f (Q, T ) gravity with the warm inflation scenario. Furthermore, we scrutinize two different forms of the dissipation coefficient, a constant and a function of the inflaton field, to determine the scalar spectral index, the tensor-to-scalar ratio and the temperature for the power-law potential case. By imposing some constraints on the free parameters of the model, we attain results in good agreement with both the Planck 2018 data and the joint Planck, BK15 and BAO data for the tensor-to-scalar ratio, and consistent results aligned with the Planck 2018 data for the scalar spectral index. In addition, the obtained results are within the range of observational data for the amplitude of the scalar power spectrum. Consequently, we are able to revive the power-law potential that was previously ruled out by observational data. Moreover, for both dissipation coefficients, the model leads to a scalar spectral index with the blue and red tilts in agreement with the WMAP three years data.

gr-qc↗

arXiv2023

Thermodynamics of Deformed AdS-Schwarzschild Black Hole

By implementing the gravitational decoupling method, we find the deformed AdS-Schwarzschild black hole solution when there is also an additional gravitational source, which obeys the weak energy condition. We also deliberately choose its energy density to be a certain monotonic function consistent with the constraints. In the method, there is a positive parameter that can adjust the strength of the effects of the geometric deformations on the background geometry, which we refer to as a deformation parameter. The condition of having an event horizon limits the value of the deformation parameter to an upper bound. After deriving various thermodynamic quantities as a function of the event horizon radius, we mostly focus on the effects of the deformation parameter on the horizon structure, the thermodynamics of the solution and the temperature of the Hawking- Page phase transition. The results show that with the increase of the deformation parameter: the minimum horizon radius required for a black hole to have local thermodynamic equilibrium and the minimum temperature below which there is no black hole decrease, and the horizon radius of the phase transition and the temperature of the first-order Hawking-Page phase transition increase. Furthermore, when the deformation parameter vanishes, the obtained thermodynamic behavior of the black hole is consistent with that stated in the literature.

gr-qc↗

arXiv2023

Cosmological Solutions of Chameleon Scalar Field Model

We investigate cosmological solutions of the chameleon model with a non-minimal coupling between the matter and the scalar field through a conformal factor with gravitational strength. By considering the spatially flat FLRW metric and the matter density as a non-relativistic perfect fluid, we focus on the matter-dominated phase and the late-time accelerated phase of the universe. In this regard, we manipulate and scrutinize the related field equations for the density parameters of the matter and the scalar fields with respect to the e-folding. Since the scalar field fluctuations depend on the background and the field equations become highly non-linear, we probe and derive the governing equations in the context of various cases of the relation between the kinetic and potential energies of the chameleon scalar field, or indeed, for some specific cases of the scalar field equation of state parameter. Thereupon, we schematically plot those density parameters for two different values of the chameleon non-minimal coupling parameter, and discuss the results. In both considered phases, we specify that, when the kinetic energy of the chameleon scalar field is much less than its potential energy (i.e., when the scalar field equation of state parameter is $\simeq - 1 $), the behavior of the chameleon model is similar to the $ΛCDM$ model. Such compatibility suggests that the chameleon model is phenomenologically viable and can be tested with the observational data.

gr-qc↗

arXiv2022

Current Density of Majorana Bound States

It is known that a non-local complex fermion can be written in terms of two Majorana fermions. We exploit this fact to explain the system of two Majorana zero modes bound to a vortex and an anti-vortex, on the surface of a topological insulator in contact with an s-wave superconductor, as a non-local complex fermion. Although the current density of a single zero mode vanishes, by starting with a wave packet consisted of the positive and negative energy complex fermions, we specify that a time-dependent oscillatory motion emerges in the system. We also show that the amplitude and frequency of the oscillations depend on the relative distance of those two zero modes. Therefore, the observation of this oscillatory motion can be considered as a signature of the Majorana zero modes. Also, as the frequency of such an oscillatory motion depends on the distance between the two zero modes, it can be adjusted to bring this frequency within the resolution of observations. Furthermore, we indicate that the predicted oscillatory current is the reminiscent of the zitterbewegung effect.

hep-th↗

arXiv2022

Cosmological Inflation in f(Q, T) Gravity

We study the cosmological inflation within the context of f(Q, T) gravity, wherein Q is the nonmetricity scalar and T is the trace of the matter energy-momentum tensor. By choosing a linear combination of Q and T, we first analyze the realization of an inflationary scenario driven via the geometrical effects of the linear f(Q, T) gravity and then, we obtain the modified slow-roll parameters, the scalar and the tensor spectral indices, and the tensor-to-scalar ratio for the proposed model. In addition, by choosing three inflationary potentials, i.e. the power-law, hyperbolic and natural potentials, and by applying the slow-roll approximations, we calculate these inflationary observables in the presence of an inflaton scalar field. The results indicate that by properly restricting the free parameters, the proposed model provides appropriate predictions that are consistent with the observational data obtained from the Planck 2018. Also, we specify that the contribution of linear model of f(Q, T) gravity with the hyperbolic and natural potentials can impose different restrictions on the parameters of these potentials. Furthermore, the predictions of natural inflation in this model are in good agreement with the joint Planck, BK15 and BAO data, justifying the use of the f(Q, T) gravity.

gr-qc↗

arXiv2022

Cosmic Acceleration and Geodesic Deviation in Chameleon Scalar Field Model

While considering the chameleon scalar field model with the spatially flat FLRW background, we investigate the late-time acceleration phase of the universe, wherein we apply the typical potential usually used in this model. Through setting some constraints on the free parameters of the model, we indicate that the non-minimal coupling between the matter and the scalar field in such a model should be strongly coupled in order to have an accelerated expansion of the universe at the late-time. We also investigate the relative acceleration of the parallel geodesics by obtaining the geodesic deviation equation in the context of chameleon model. Then, through the null deviation vector fields, we obtain the observer area-distance as a measurable quantity to compare the model with other relevant models.

gr-qc↗

arXiv2022

Primordial Black Hole Merger Rate in Self-Interacting Dark Matter Halo Models

We study the merger rate of primordial black holes (PBHs) in self-interacting dark matter (SIDM) halo models. To explore a numerical description for the density profile of SIDM halo models, we use the result of a previously performed simulation for SIDM halo models with $σ/m=10~{\rm cm^{2}g^{-1}}$. We also propose a concentration-mass-time relation that can explain the evolution of the halo density profile related to SIDM models. Furthermore, we investigate the encounter condition of PBHs that may have been randomly distributed in the medium of dark matter halos. Under these assumptions, we calculate the merger rate of PBHs within each halo considering SIDM halo models and compare the results with that obtained for cold dark matter (CDM) halo models. To do this, we employ the definition of the time after halo virialization as a function of halo mass. We indicate that SIDM halo models for $f_{\rm PBH}>0.32$ can generate sufficient PBH mergers in such a way that those exceed the one resulted from CDM halo models. By considering the spherical-collapse halo mass function, we obtain similar results for the cumulative merger rate of PBHs. Moreover, we calculate the redshift evolution of the PBH total merger rate. To determine a constraint on the PBH abundance, we study the merger rate of PBHs in terms of their fraction and masses and compare those with the black hole merger rate estimated by the Advanced LIGO (aLIGO)-Advanced Virgo (aVirgo) detectors during the third observing run. The results demonstrate that within the context of SIDM halo models, the merger rate of $10~M_{\odot}-10~M_{\odot}$ events can potentially fall within the aLIGO-aVirgo window. We also estimate a relation between the fraction of PBHs and their masses, which is well consistent with our findings.

astro-ph.CO↗

arXiv2021

A `Third' Quantization Constructed for Gauge Theory of Gravity

In general, a global and unique vacuum state cannot be constructed for a curved space. As a remedy, we introduce a curved space background geometry with a Minkowski metric tensor and locally non-zero curvature and torsion. Based on this geometry, we propose a `third'/vacuum quantization model as a consequence of Unruh effect. Accordingly, we introduce a `third' quantization scalar field as a general coordinate transformation of spacetime for the second quantization fields. Then we show that in the classical limit, the `third' quantization fields appear as Riemannian manifolds with an emergent metric on which the second quantization fields are located. This way, the standard model of field theory turns out as an effective theory. Moreover, using the proposed `third' quantization fields, we build a $U(1)\times SU(4)$ Yang-Mills gauge theory for gravity. According to this gravitational model, we indicate that an analytical solution of the presented gravitational model, for the `third' quantum field particle trajectory (such as a star), corresponds to the trajectory of a test particle in the Mannheim-Kazanas space. Furthermore, by using non-perturbative methods and lattice gauge theory results, we render a solution for the potential of the constructed model that can explain the galaxy rotation curves and gravitational lensing without any need to dark matter. We also address the cosmic microwave background phenomenon and the expansion of the universe.

hep-th↗

arXiv2021

Electron dynamics in noncommutative geometry with magnetic field and Zitterbewegung phenomenon

Starting from a gauge invariant Dirac Hamiltonian with noncommutativity of space sector in the presence of an external uniform magnetic field, the resulting Dirac equation has been solved for electrons and its corresponding zitterbewegung (ZBW) phenomenon has been studied. The corresponding energy spectrum is shown to be different from previous studies wherein the non-gauge invariant Dirac Hamiltonian has been used. The effects of noncommutativity alter the amplitude as well as the cyclotron and the ZBW frequencies of the average velocity of charge carriers. This result is contrary to previous studies wherein there was no magnetic field and hence, neither the amplitude nor the frequency of the motion was affected. Moreover, all of the ZBW frequencies of the Landau energy-levels appear in the results. Also, in weak magnetic fields, we have calculated the average velocity of charge carriers for two initially localized spin-up and spin-down cases. The plotted trajectories reveal difference between these two cases. In addition, the ZBW phenomenon has been shown that manifests itself as a circular motion whose direction is spin dependent while accompanied by the cyclotron motion.

hep-th↗

arXiv2021

Towards Amplituhedron via One-Dimensional Theory

Inspired by the closed contour of momentum conservation in an interaction, we introduce an integrable one-dimensional theory that underlies some integrable models such as the Kadomtsev-Petviashvili (KP)-hierarchy and the amplituhedron. In this regard, by defining the action and partition function of the presented theory, while introducing a perturbation, we obtain its scattering matrix (S-matrix) with Grassmannian structure. This Grassmannian corresponds to a chord diagram, which specifies the closed contour of the one-dimensional theory. Then, we extract a solution of the KP-hierarchy using the S-matrix of theory. Furthermore, we indicate that the volume of phase-space of the one-dimensional manifold of the theory is equal to a corresponding Grassmannian integral. Actually, without any use of supersymmetry, we obtain a sort of general structure in comparison with the conventional Grassmannian integral and the resulted amplituhedron that is closely related to the Yang-Mills scattering amplitudes in four dimensions. The proposed theory is capable to express both the tree- and loop-levels amplituhedron (without employing hidden particles) and scattering amplitude in four dimensions in the twistor space as particular cases.

hep-th↗

arXiv2021

QED Treatment of Linear Elastic Waves in Asymmetric Environments

Considering the importance of correctly understanding the dynamics of microstructure materials for their applications in related technologies, by eliminating the shortcomings and some overlooked physical concepts in the existing asymmetric elastic theories, we have presented an asymmetric elastodynamic model based on a U(1) gauge theory with quantum electrodynamics (QED) structure. Accordingly, we have shown that there is a correspondence between an elastic theory, which can explain the behavior of elastic waves within an asymmetric elastic medium, and QED. More specific, we have indicated that the corresponding elastic wave equations are somehow analogous to QED ones. In this regard, by adding vibrational degrees of freedom and introducing a gauge property of the waves of displacement for the waves of rotation, we have generalized and modified the related Cosserat theory (CT) for an elastic environment. Thus on macro scales, the elastic waves can possess the QED treatment. This analogy provides a new paradigm of fermions and bosons. Also, from experimental point of view, we have shown that the behavior of elastic waves in a granular medium is equivalent to behavior of light in dispersive media, which can be explained using QED. Hence, contrary to the Cosserat and discrete models, this amended CT has qualitatively been indicated to be consistent with the corresponding empirical observations.

physics.class-ph↗

arXiv2021

Primordial Black Hole Merger Rate in Ellipsoidal-Collapse Dark Matter Halo Models

We have studied the merger rate of primordial black holes (PBHs) in the ellipsoidal-collapse model of halo to explain the dark matter abundance by the PBH merger estimated from the gravitational waves detections via the Advanced LIGO (aLIGO) detectors. We have indicated that the PBH merger rate within each halo for the ellipsoidal models is more significant than for the spherical models. We have specified that the PBH merger rate per unit time and per unit volume for the ellipsoidal-collapse halo models is about one order of magnitude higher than the corresponding spherical models. Moreover, we have calculated the evolution of the PBH total merger rate as a function of redshift. The results indicate that the evolution for the ellipsoidal halo models is more sensitive than spherical halo models, as expected from the models. Finally, we have presented a constraint on the PBH abundance within the context of ellipsoidal and spherical models. By comparing the results with the aLIGO mergers during the third observing run (O3), we have shown that the merger rate in the ellipsoidal-collapse halo models falls within the aLIGO window, while the same result is not valid for the spherical-collapse ones. Furthermore, we have compared the total merger rate of PBHs in terms of their fraction in the ellipsoidal-collapse halo models for several masses of PBHs. The results suggest that the total merger rate of PBHs changes inversely with their masses. We have also estimated the relation between the fraction of PBHs and their masses in the ellipsoidal-collapse halo model and have shown it for a narrow mass distribution of PBHs. The outcome shows that the constraint inferred from the PBH merger rate for the ellipsoidal-collapse halo models can be potentially stronger than the corresponding result obtained for the spherical-collapse ones.

astro-ph.CO↗