arXiv subjects

Kazem Rezazadeh

Publications and source records attributed to Kazem Rezazadeh.

14 recordsLinked to original sources

arXiv2026

Gravitational Bound State Perturbations Inside Black Holes and Isospectrality

We study the bound state solutions for the polar perturbations in the interior of the Schwarzschild black hole. It is shown that for a given value of the spherical harmonic index $\ell$, there are a total of $\ell-1$ bound states for polar perturbations. We show both analytically and numerically that the spectrum of $\ell-2$ of these perturbations coincides exactly with the spectrum of axial perturbations. Consequently, the isospectrality between the bound states of axial and polar perturbations in the interior of the black hole is preserved. Furthermore, the additional mode found in the spectrum of polar perturbations is the algebraically special mode, which also furnishes the ground state of polar perturbations. It is shown that the spectrum of the highly excited states is equally spaced, which, in the semi-classical approximation, yields the black hole area quantization $ΔA = 16 πl_{\mathrm{Pl}}^2$.

gr-qc↗

arXiv2025

A Dark Matter Model with Quadratic Equation of State: Background Evolution and Structure Formation

We propose that dark matter (DM) possesses a quadratic equation of state, which becomes significant at high densities, altering the Universe's evolution during its early stages. We derive the modified background evolution equations for the Hubble parameter $H(z)$ and the DM density parameter $Ω_{\text{dm}}(z)$. We then perturb the governing equations to study the linear growth of matter fluctuations, computing the observable growth factor $fσ_8(z)$. Finally, we compare the model with the latest cosmological data, including Hubble parameter $H(z)$ measurements, and growth factor $fσ_8(z)$ data, up to $z=3$. Our results indicate that the quadratic model, while remaining consistent with background observations, offers a distinct imprint on the growth of structure, providing not only a new phenomenological avenue to address cosmological tensions but also shedding light on the nature of DM.

astro-ph.CO↗

arXiv2025

Cosmological perturbation theory with trinity of scalar fields

We present an explicit formulation of cosmological perturbation theory for three-field models with a flat field space. By performing rotations to align one field with the direction of curvature perturbations and applying the same rotations to the other two field directions, we introduce the semikinematic basis, which is applicable to models with more than two fields. We derive the governing equations in this basis. We also stress a characteristic property of more-than-two-field models: the freedom in choosing the isocurvature perturbations. This framework enables the computation of the curvature and two isocurvature power spectra for any given potential. We numerically solve the background and perturbation equations for three distinct scenarios. First, to validate the consistency of our three-field formalism, we examine an effective two-field model inspired by the two-block case of the multigiant vacua matrix inflation scenario. Next, we analyze a purely three-field system without direct interfield interactions. Finally, we study a three-field case that incorporates direct interactions. For all scenarios, we numerically compute the curvature perturbation power spectra and highlight the effects of rapid turns on the spectra. Finally, we investigate the relationship between these quantities and the observables in the early radiation-dominated era. Through both general arguments and a simple example, we show that three-field inflation can yield a much richer phenomenology. This is particularly true when we assume the initial perturbations in the radiation era include two isocurvature modes.

hep-th↗

arXiv2025

Cosmological Implications of Modified Entropic Gravity

Taking into account the temperature corrections of the energy equipartition law for the bits of information that are coarse-grained on the holographic screen leads to a modification of Einstein's gravitational field equations. In the very high-temperature limit, which corresponds to strong gravitational fields, the modified gravitational equations reduce to the standard Einstein equations of general relativity, but in the low-temperature limit, which corresponds to the weak gravity regime, the modified equations show significant deviations from the standard Einstein equations. We solve the modified Einstein equations for the FRW metric and obtain the modified Friedmann equations. We see that the Friedmann equations obtained with this approach agree with the Friedmann equations previously obtained from the thermodynamic corrections of classical Newtonian mechanics. Using the modified Friedmann equations for a flat universe, we investigate the implications of our modified entropic cosmology (MEC) model. We show that our model can explain the dynamics of the universe without requiring any kind of dark energy. Using the Pantheon supernovae dataset, BAO data, Planck 2018 CMB data, and SH0ES measurements for $H_0$, we test the MEC model against observations. We will see that MEC fits the observational data better than the standard cosmological model of $Λ$CDM. We also see that our model can successfully solve the $H_0$ tension that challenges the standard cosmological model.

gr-qc↗

arXiv2024

Spherical Collapse Approach for Non-standard Dark Matter Models and Enhanced Early Galaxy Formation in JWST

Using the spherical collapse approach, we investigate the impact of two alternative dark matter models, each characterized by distinct non-zero equations of state, one constant and the other time dependent on the nonlinear regime. Specifically, we compare these models to standard cold dark matter (CDM) by analyzing their influence on the linear density threshold for nonrelativistic component collapse and virial overdensity. Additionally, we explore the number count of collapsed objects, or dark matter halos, analogous to the number count of galaxy clusters. Finally, in light of recent discoveries by the James Webb Space Telescope (JWST), indicating the potential for more efficient early galaxy formation at higher redshifts, we investigate how alternative dark matter assumptions can enhance structure formation efficiency during the early universe.

astro-ph.CO↗

arXiv2024

Inflation Based on the Tsallis Entropy

We study the inflationary scenario in the Tsallis entropy-based cosmology. The Friedmann equations in this setup can be derived by using the first law of thermodynamics. To derive the relations of the power spectra of the scalar and tensor perturbations in this setup, we reconstruct an $f(R)$ gravity model which is thermodynamically equivalent to our model in the slow-roll approximation. In this way, we find the inflationary observables, including the scalar spectral index and the tensor-to-scalar ratio in our scenario. Then, we investigate two different potentials in our scenario, including the quadratic potential and the potential associated with the natural inflation in which the inflaton is an axion or a pseudo-Nambu-Goldstone boson. We examine their observational viability in light of the Planck 2018 CMB data. We show that although the results of these potentials are in tension with the observations in the standard inflationary setting, their consistency with the observations can be significantly improved within the setup of the Tsallis entropy-based inflation. Moreover, we place constraints on the parameters of the considered inflationary models by using the current observational data.

gr-qc↗

arXiv2023

Transient Acceleration after Non-minimal M-flation Preheating

Light massive preheat fields acquire a non-vanishing dispersion during parametric resonance from their quantum particle production. This in turn will modify the inflaton potential, which in some cases can induce a transient period of acceleration. We illustrate this phenomenon in the setup of non-supersymmetric non-minimal M-flation (non-$\mathbb{M}$-flation) which has some motivations from the brane compactifications in string theory. Implementing a lattice simulation by the LATTICEEASY code, we compute the potential correction term in our scenario and show that the modified term indeed causes the universe to make a transition from the decelerated expansion to a temporary phase of acceleration. The correction term reduces to some extent the number density of the particles generated during preheating, but the efficiency of preheating remains still enough to have successful particle production after inflation. We also compute the spectrum of the gravitational waves (GWs) generated during preheating in our setup by using the LATTICEEASY code. Although the peak frequency remains almost the same, the inclusion of the correction term reduces the amplitude of the gravitational spectrum by almost one order of magnitude.

hep-th↗

arXiv2022

NANOGrav Signal from the End of Inflation and the LIGO Mass and Heavier Primordial Black Holes

Releasing the 12.5-year pulsar timing array data, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported the evidence for a stochastic common-spectrum which would herald the detection of a stochastic gravitational wave background (SGWB) for the first time. We investigate if the signal could be generated from the end of a $\sim 10$ MeV but still phenomenologically viable double-field inflation when the field configuration settles to its true vacuum. During the double-field inflation at such scales, bubbles of true vacuum that can collapse to LIGO mass and heavier primordial black holes form. We show that only when this process happens with a first-order phase transition, the produced gravitational wave spectrum can match with the NANOGrav acclaimed SGWB signal. We show that the produced gravitational wave spectrum matches the NANOGrav SGWB signal only when this process happens through a first-order phase transition. Using LATTICEEASY, we also examine the previous observation in the literature that by lowering the scale of preheating, despite the shift of the peak frequency of the gravitational wave profile to smaller values, the amplitude of the SGWB could be kept almost constant. We notice that this observation breaks down at the preheating scale, $M\lesssim 10^{-14}~m_{{}_{\rm Pl}}$.

astro-ph.CO↗

arXiv2022

Non-Gaussianity and Secondary Gravitational Waves from Primordial Black Holes Production in $α$-attractor Inflation

We study the non-Gaussianity and secondary Gravitational Waves (GWs) in the process of the Primordial Black Holes (PBHs) production from inflation. In our work, we focus on the $α$-attractor inflation model in which a tiny bump in the inflaton potential enhances the amplitude of the curvature perturbations at some scales and consequently leads to the PBHs production with different mass scales. We implement the computational code BINGO which calculates the non-Gaussianity parameter in different triangle configurations. Our examination implies that in this setup, the non-Gaussianity gets amplified significantly in the equilateral shape around the scales in which the power spectrum of the scalar perturbations undergoes a sharp declination. The imprints of these non-Gaussianities can be probed in the scales corresponding to the BBN and $μ$-distortion events, or in smaller scales, and detection of such signatures in the future observations may confirm the idea of our model for the generation of PBHs or rule it out. Moreover, we investigate the secondary GWs in this framework and show that in our model, the peak of the present fractional energy density is obtained as $Ω_{\rm GW0} \sim 10^{-8}$ at different frequencies which depends on the model parameters. These results lie well within the sensitivity region of some GWs detectors at some frequencies, and therefore the observational compatibility of our model can be evaluated by the forthcoming data from these detectors. We further provide some estimations for the tilts of the induced GWs spectrum in the different intervals of frequency, and demonstrate that the spectrum obeys the power-law relation $Ω_{\rm GW0}\sim f^{n}$ in those frequency bands.

gr-qc↗

arXiv2021

Mechanism of primordial black holes production and secondary gravitational waves in $α$-attractor Galileon inflationary scenario

We study the process of the Primordial Black Holes (PBHs) production in the novel framework, namely $α$-attractor Galileon inflation (G-inflation) model. In our framework, we take the Galileon function as $G(ϕ)=G_{I}(ϕ)\left(1+G_{II}(ϕ)\right)$, where the part $G_{I}(ϕ)$ is motivated from the $α$-attractor inflationary scenario in its original non-canonical frame, and it ensures for the model to be consistent with the Planck 2018 observations at the CMB scales. The part $G_{II}(ϕ)$ is invoked to enhance the curvature perturbations at some smaller scales which in turn gives rise to PBHs formation. By fine-tuning of the model parameters, we find three parameter sets which successfully produce a sufficiently large peak in the curvature power spectrum. We show that these parameter sets produce PBHs with masses ${\cal O}(10)M_\odot$, ${\cal O}(10^{-5})M_\odot$, and ${\cal O}(10^{-13})M_\odot$ which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and around $0.98\%$ of the current Dark Matter (DM) content of the universe, respectively. Additionally, we study the secondary Gravitational Waves (GWs) in our setup and show that our model anticipates the peak of their present fractional energy density as $Ω_{GW0} \sim 10^{-8}$ for all the three parameter sets, but at different frequencies. These predictions can be located well inside the sensitivity region of some GWs detectors, and therefore the compatibility of our model can be assessed in light of the future data. We further estimate the tilts of the included GWs spectrum in the different ranges of frequency, and confirm that spectrum follows the power-law relation $Ω_{GW0}\sim f^{n}$ in those frequency bands.

astro-ph.CO↗

arXiv2021

Primordial black holes formation and secondary gravitational waves in nonminimal derivative coupling inflation

We study the possibility of the Primordial Black Holes (PBHs) formation with the aim of finding a considerable fraction of Dark Matter (DM), using the gravitationally enhanced friction mechanism which arises from a nonminimal derivative coupling between the scalar field and the gravity. Assuming the nonminimal coupling parameter as a special function of the scalar field and considering the potential of natural inflation, we find three parameter sets that produce a period of ultra slow-roll inflation. This leads to sufficiently large enhancement in the curvature power spectra to form PBHs. We show that under the gravitationally enhanced friction mechanism, PBHs with a mass around ${\cal O}\big(10^{-12}\big)M_\odot$ can constitute around $96\%$ of the total DM and so this class of PBHs can be taken as a great candidate for DM. We further study the secondary Gravitational Waves (GWs) in our setting and show that our model predicts the peak of the present fractional energy density as $Ω_{GW0} \sim 10^{-8}$ at different frequencies for all the three parameter sets. This value lies well inside the sensitivity region of some GWs detectors at some frequencies, and therefore the observational compatibility of our model can be appraised by the data from these detectors.

gr-qc↗

arXiv2020

Non-Minimal M-flation

We show how in a matrix inflationary model in which there is a non-minimal coupling between the matrix inflatons and gravity --hence dubbed Non-$\MM$-flation-- some of the disadvantages of the minimal model can be avoided. In particular, the number of D3 branes can be reduced substantially to $\lesssim \mathcal{O}(100)$, which can alleviate the ``potential'' backreaction problem of large number of D3 branes on the background geometry. This is achieved by values of non-minimal coupling of order few hundred, which is much smaller than that of Higgs Inflation. The prediction of the model in the symmetry breaking part of the potential, which is a local attractor and can support eternal inflation, is compatible with the latest PLANCK results. In contrast to the minimal model, the spectator fields can partially or completely reheat the universe, depending on the symmetry-breaking vacuum expectation value and the non-minimal coupling parameter. We also comment on how the presence of gauge species keep the UV cutoff at around the Planck scale in the Einstein frame and, in contrast to the Higgs inflation, the problem of field displacements beyond the cutoff does not occur.

hep-th↗

arXiv2018

Resurrecting the Power-law, Intermediate, and Logamediate Inflations in the DBI Scenario with Constant Sound Speed

We investigate the power-law, intermediate, and logamediate inflationary models in the framework of DBI non-canonical scalar field with constant sound speed. In the DBI setting, we first represent the power spectrum of both scalar density and tensor gravitational perturbations. Then, we derive different inflationary observables including the scalar spectral index $n_s$, the running of the scalar spectral index $dn_s/d\ln k$, and the tensor-to-scalar ratio $r$. We show that the 95\% CL constraint of the Planck 2015 T+E data on the non-Gaussianity parameter $f_{\rm NL}^{\rm DBI}$ leads to the sound speed bound $c_{s}\geq0.087$ in the DBI inflation. Moreover, our results imply that, although the predictions of the power-law, intermediate, and logamediate inflations in the standard canonical framework ($c_s=1$) are not consistent with the Planck 2015 data, in the DBI scenario with constant sound speed $c_s<1$, the result of the $r-n_s$ diagram for these models can lie inside the 68\% CL region favored by the Planck 2015 TT,TE,EE+lowP data. We also specify the parameter space of the power-law, intermediate, and logamediate inflations for which our models are compatible with the 68\% or 95\% CL regions of the Planck 2015 TT,TE,EE+lowP data. Using the allowed ranges of the parameter space of the intermediate and logamediate inflationary models, we estimate the running of the scalar spectral index and find that it is compatible with the 95\% CL constraint from the Planck 2015 TT,TE,EE+lowP data.

astro-ph.CO↗

arXiv2017

Logamediate Inflation in f(T) Teleparallel Gravity

We study logamediate inflation in the context of $f(T)$ teleparallel gravity. $f(T)$-gravity is a generalization of the teleparallel gravity which is formulated on the Weitzenböck spacetime, characterized by the vanishing curvature tensor (absolute parallelism) and the non-vanishing torsion tensor. We consider an $f(T)$-gravity model which is sourced by a canonical scalar field. Assuming a power-law $f(T)$ function in the action, we investigate an inflationary universe with a logamediate scale factor. Our results show that, although logamediate inflation is completely ruled out by observational data in the standard inflationary scenario based on Einstein gravity, it can be compatible with the 68\% confidence limit joint region of Planck 2015 TT,TE,EE+lowP data in the framework of $f(T)$-gravity.

gr-qc↗