arXiv subjects

Abolhassan Mohammadi

Publications and source records attributed to Abolhassan Mohammadi.

At least 19 recordsLinked to original sources

arXiv2026

Probing Gravitational Wave Signatures from Periodic Orbits of Regular Black Holes in Asymptotically Safe Gravity

We investigate bound and periodic timelike geodesics and their associated gravitational-wave (GW) signatures in the spacetime of a regular black hole arising in asymptotically safe gravity (ASG). The geometry incorporates quantum corrections via a running gravitational coupling, encoded in a dimensional scaling parameter $ξ$, that modifies the near-horizon structure while preserving asymptotic flatness. We derive the effective potential for massive test particles and determine the conditions for stable circular and bound motion as functions of $ξ$, including the shift in the innermost stable circular orbit (ISCO). The three topological integers $(z,w,v)$, which represent the number of zooms, whirls, and vertices per radial cycle, are used to categorize the test particles' periodic orbits using Levin's zoom -- whirl taxonomy. Moreover, we employ the rational frequency ratio $q = \frac{ω_ϕ}{ω_r} - 1$ to find closed orbits, where $ω_ϕ$ and $ω_r$ stand for the azimuthal and radial frequencies, respectively. We examine how the orbital frequency spectrum is altered, whirl behaviour is enhanced, and deviations from the Schwarzschild limit are produced by the quantum parameter $ξ$. The GW forms for extreme mass-ratio inspirals (EMRIs) are calculated within the quadrupole approximation. We find that as $ξ$ increases, the signals that are released exhibit detectable amplitude modulations and phase shifts. The corresponding typical strain spectra fall within the anticipated sensitivity limits of space-based detectors such as LISA, Taiji, and TianQin, as they peak in the millihertz frequency band. Peak strain increases monotonically with $ξ$, indicating that observational restrictions on quantum-gravity-induced deviations from classical general relativity in the strong-field domain can be obtained from precise measurements of zoom -- whirl dynamics in EMRIs.

gr-qc↗

arXiv2026

Primordial Black Hole Abundances and Scalar Induced Gravitational Waves from Finite-Width Power Spectra in a Stiff Thermal History

We study the formation of primordial black holes (PBHs) from large primordial perturbations that re-enter the horizon during an epoch with equation of state ${\rm w}\geq1/3$. We consider a log-normal curvature power spectrum of finite width $Δ$ and determine the collapse amplitude by numerical-relativity simulations of a self-gravitating perfect fluid. Threshold scans are performed for five values of $\rm w$ and five spectral widths, and the resulting numerical thresholds are used in the PBH abundance and scalar-induced gravitational-wave (SIGW) calculations. For comparison, we also evaluate the semi-analytical $q$-function prescription. It reproduces the numerical trend close to radiation domination and for nearly monochromatic profiles, but it is not a reliable threshold estimator for generic finite-width profiles in stiffer backgrounds. We show how collapse thresholds increase with both $Δ$ and $\rm w$, changing the curvature amplitude required for PBHs to constitute all of the dark matter and, consequently, the normalization of the accompanying SIGW signal.

astro-ph.CO↗

arXiv2026

Effects of the inflaton mass on the pre-inflationary dynamics and primordial power spectra in loop quantum cosmology

Considering the mass parameters as free phenomenological parameters, we investigate inflation driven by a canonical scalar field with different potentials in LQC. We find that a smaller mass value makes it easier to obtain a sufficient number of e-folds for the quadratic potential, while for the Starobinsky potential, more e-folds are obtained for larger mass values. By estimating the probability of slow-roll inflation, we find that it remains very close to unity for all considered cases. We compute the primordial power spectrum using three approaches: the dressed metric, the hybrid, and the alternative mass function approaches for the case that the kinetic energy dominated at bounce. The resulting power spectrum for both potentials and different mass values shows the same qualitative pattern in all three approaches. It is suppressed at small $k$, amplified and oscillating over an intermediate range, and nearly scale-invariant at large $k$. The approaches mainly differ in how fast the power spectrum converges to this regime, with the hybrid approach converging the fastest, followed by the alternative mass function and dressed metric approaches. We find that the pivot scale $k_\star$ is highly sensitive to the inflaton mass, changing by more than an order of magnitude for a mass variation of only a few percent. Using the tensor power spectrum at $k_\star$, we calculate the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$, finding good agreement with current data. Finally, the scalar power spectra are fed into the CAMB code to obtain the angular power spectrum and compare with the Planck 2018 data and the best-fit $Λ$CDM model. All three approaches are consistent with data at high multipoles, while at low multipoles the hybrid approach gives the closest agreement and the dressed metric approach shows the largest deviation.

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arXiv2026

Starobinsky Inflation in k-Essence Framework: Attractor Dynamics, Reheating, and Consistency with ACT DR6

The recent ACT DR6 has shifted the preferred value of the scalar spectral index upward so that many well-established inflationary models have been disfavoured, including the Starobinsky potential. Despite this, the Starobinsky potential remains exceptionally well-motivated, with origins in $R^2$ gravity, no-scale supergravity, and the $α$-attractor framework. In this work, we show that the Starobinsky potential can be fully revived within a k-essence framework, described by the Lagrangian $\mathcal{L} = F(ϕ)X - V(ϕ)$, with a power-law kinetic coupling $F(ϕ) = 1+Aϕ^n$ and no modification to the gravitational sector. Solving the background equations numerically, we find that the predictions for $n_s$, $α_s$, and $r$ fall within the $1σ$ region of ACT DR6 for a well-defined range of the coupling parameters. The attractor behavior of the inflationary solution is confirmed both analytically through the Hamilton-Jacobi formalism and numerically via a phase-space analysis. For the reheating phase, it is discussed that due to the nature of the Starobinsky potential, the effective equation of state parameter is fixed as $w_{\rm re} = 0$, resulting in a reheating temperature $T_{\rm re} \sim 10^{14}~{\rm GeV}$, well above the BBN bound. The relic gravitational wave spectrum is also computed and it is found that they can lie within the sensitivity bound of the BBO. These results demonstrate that the Starobinsky potential remains a theoretically viable candidate for inflation and that its incompatibility with ACT DR6 in the canonical setting can be resolved by introducing a simple non-canonical kinetic coupling without any modification to the underlying gravitational theory.

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arXiv2026

Periodic orbits as probes of charged loop quantum gravity black holes through gravitational waves

Gravitational waves from extreme-mass-ratio inspirals (EMRI) provide a direct probe of the strong-field geometry of black holes. Motivated by this, we study the motion of test particles and the resulting gravitational wave emission in the spacetime of a charged black hole inspired by loop quantum gravity (LQG), where the classical singularity is replaced by a smooth transition surface arising from the LQG polymerization, in which its radius is set by the LQG area gap condition. As a result, the polymerization parameter $δ_b$ is uniquely determined by the mass $M$ and charge parameter $Q$, so that all cases examined in this work contain LQG correction. By constructing the effective potential, the innermost stable circular orbit (ISCO) and the marginally bound orbit (MBO) are determined. Periodic orbits are classified using the Levin-Perez-Giz zoom-whirl taxonomy, showing how the orbit topology shapes the waveform, so that each closed trajectory is labeled by the triple integer $(z, w, v)$ and located through the rational frequency ratio $q = ω_ϕ/ω_r - 1$. Within the quadrupole approximation, the gravitational waveforms for an EMRIs are estimated, and the resulting polarizations are obtained in the time-domain and frequency-domain. The resulting polarizations in the time-domain exhibit a zoom-whirl morphology, with the waveform amplitude and phase dependent on the LQG parameter. The characteristic strain peaks in the millihertz band for all values of the charge parameter $Q$, and they exceed the projected sensitivities of LISA, Taiji, and TianQin, suggesting that future observations could place meaningful constraints on the LQG polymerization parameter in the strong-field regime.

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arXiv2025

Spinning Primordial Black Holes and Scalar Induced Gravitational Waves from Single Field Inflation

We investigate the formation of primordial black holes (PBHs), their spin and abundance, in a single-field inflationary model based on a mutated hilltop potential inserted with a small step-like feature. This step induces a brief phase of ultra-slow-roll inflation, producing the large enhancement of the scalar power spectrum required for an appreciable amount of PBH abundance. Instead of the commonly used analytical power spectra, we compute the primordial power spectrum accurately by numerically solving the Mukhanov-Sasaki equation. Using the obtained power spectrum, we apply peak theory with $\nabla^2 ζ$ treated as a Gaussian random field and parametrize the curvature profile by its amplitude $μ$ and characteristic width $K$. Confining the study to Type-I PBH, the threshold value is calculated using two robust methods: the average of the compaction function and the q-function method. Using the result, the dimensionless spin parameter of the resulting PBHs is calculated at linear order and found to be $\sqrt{\langle a_\star^2 \rangle} \sim 10^{-3}$; however, it can be higher for smaller masses. We present detailed predictions for two representative parameter sets, calculate the present-day PBH mass function $f_{\rm PBH}(M)$ and the associated scalar-induced gravitational waves (SIGW). The first produces PBHs of mass $M \simeq 10^{-13}\,M_\odot$ that can account for $100\%$ of dark matter, while the second yields $M \simeq 10^{-2}\,M_\odot$ PBHs contributing approximately $2.4\%$ of the dark matter density. The predicted signals of SIGWs lie within the sensitivity bands of future experiments such as LISA, DECIGO, BBO, and SKA. In particular, the second parameter set produces a SIGWs compatible with the recent NANOGrav evidence for a low-frequency gravitational-wave signal.

astro-ph.CO↗

arXiv2025

Power Law Plateau Inflation and Primary Gravitational Waves in the light of ACT

We investigate Power-Law Plateau (PLP) inflation in standard gravity and its consistency with ACT DR6 data. While many inflationary models, including the Starobinsky inflation, are disfavored by ACT observations, the PLP potential remains viable across a broad range of its parameters. Then, the dynamics of the reheating phase are investigated, where we mainly focus on the reheating temperature and its relationship with the inflationary phase and primordial gravitational waves. Incorporating the overproduction of the primordial gravitational waves can affect the effective number of relativistic species during the bounce. The constraint data on $ΔN_{\rm eff}$ can impose a lower bound on the reheating temperature. This constraint will be more efficient for a stiff equation of state. It is determined that for $ω_{re} > 0.58$, this constraint would be efficient. Combining the result of the reheating temperature and the inflationary phase, it is concluded that to have both a viable result standing in $1σ$ of ACT DR6 and also to satisfy the reheating lower bound, the total number of e-folds during the inflationary phase should be $N_k \lesssim 62$. Higher e-folds of expansion result in a reheating temperature below the bound, which is disfavored. Finally, for the constraint values of the reheating temperature, the energy spectrum of the gravitational waves has been explored. The results indicate that there is a higher chance of detection for lower reheating temperatures and higher reheating equation of state.

astro-ph.CO↗

arXiv2025

The primordial angular power spectrum from the alternative mass function in loop quantum cosmology

We investigate the cosmological impacts of the alternative effective mass function of the modified Mukhanov-Sasaki equation in loop quantum cosmology, which is obtained from the polymerization of the classical mass function derived in the comoving gauge. This alternative effective mass function is distinct from those in the dressed metric and the hybrid approaches and is able to generate a new structure in the primordial power spectrum. After taking the Starobinsky potential and employing a particular polymerization ansatz for the inverse Hubble rate, the effective mass function is characterized by a free parameter $ξ$. When $ξ\ge 0.1$, there appears a wave-packet structure in the region preceding the almost scale invariant regime of the power spectrum and both the location and the height of the wave packet are affected by the choice of $ξ$. For the angular power spectrum, we find $ξ=0.2$ provides the best-fit curve to the result from the $Λ$CDM model. Our study presents a concrete example in which the fine structure of the primordial power spectrum sensitively relies on the parameters in the polymerization ansatz.

gr-qc↗

arXiv2025

Starobinsky like inflation and EGB Gravity in the light of ACT

The ACT data shows an enhancement in the value of the scalar spectral index, as $n_s = 0.9743 \pm 0.0034$, leading to disfavoring many inflationary models, including the Starobinsky model. To satisfy the constraint made by ACT, we will investigate the Starobinsky potential within the Einstein-Gauss-Bonnet(EGB) gravity theory. EGB gravity is motivated by the higher-dimensional theory, which includes quadratic curvature correction terms and a coupling between the scalar field and the GB term that modifies the dynamical equations. The model is considered by using the slow-roll approximation method and the exact numerical approach for two different coupling functions. The results indicate that the model is in good agreement with the data and the results stand in $1σ$ of the ACT $r-n_s$ plane. Considering the running of the scalar spectral index also implies the consistency of the model with data. In addition, the parametric space of the free parameters of the EGB coupling is explored, where we find the acceptable region of the parameters in which the resulting $n_s$ and $r$ stay in $1σ$ of the ACT data. Next, the reheating phase is considered. It is determined that the model can simultaneously satisfy the constraint of ACT data and the reheating temperature constraints.

astro-ph.CO↗

arXiv2025

Non-Standard Thermal History and Formation of Primordial Black Holes in Einstein-Gauss-Bonnet Gravity

Inflation provides a suitable environment for the formation of Primordial Black Holes(PBHs). In this article, we examine the formation of primordial black holes in the Mutated Hilltop inflation model coupled with the Einstein-Gauss-Bonnet term. A suitable choice of the coupling function with adjusted parameters can produce the USR regime during the inflationary phase, which lasts for some number of e-folds. The scalar field in this regime remains almost unchanged, and the first slow-roll parameter $ε_1$ drops dramatically, leading to a significant enhancement to the curvature power spectrum for small scales so that it grows up to order of $\mathcal{O}(0.01)$; a crucial feature for producing PBH and secondary gravitational waves (GWs). We investigate the formation of PBHs for different sets of parameters. By considering the behavior of the scalar power spectrum, it is realized that the peak in the scalar power spectrum occurs in different scales. Then, the presented model is capable of predicting PBH formation in a wide range mass, from $\mathcal{O}(10^{-14})M_{\odot}$ to $\mathcal{O}(10)M_{\odot}$, which are compatible with the LIGO-Virgo data. PBHs with mass $\mathcal{O}(10^{-5})M_{\odot}$ can account for the micro-lensing event in OGLE as well as the asteroid masses $\mathcal{O}(10^{-15})M_{\odot}-\mathcal{O}(10^{-12})M_{\odot}$ PBH can be attributed to $100\% $ dark matter present in the universe. The generated perturbations during inflation re-enter the horizons after the end of inflation, where the universe may be in a non-standard epoch rather than the radiation-dominant phase, with an equation of state $1/3< ω\leq 1$...

gr-qc↗

arXiv2024

Non-commutativity in modified loop cosmology

In this study, we explore the pre-inflationary dynamics of the universe using a noncommutative extension of the mLQC-I framework. By incorporating a scalar field potential, we show that key features of Loop Quantum Cosmology (LQC), such as the quantum bounce and the super-inflationary phase, are preserved. Numerical solutions to the modified Hamiltonian equations, with initial conditions set at the quantum bounce, reveal that the universe's early expansion rate is sensitive to the shape of the potential. For a chaotic potential, the inclusion of noncommutativity results in a faster expansion rate, whereas for the Starobinsky potential, the expansion rate decreases with increasing noncommutative parameter $θ$. Additionally, higher values of $θ$ lead to an increased time derivative of the Hubble parameter, causing a shorter yet more expansive super-inflationary phase. Over time, Hubble parameters for different values of $θ$ converge. For the Starobinsky potential, the Hubble parameter consistently decreases with larger $θ$, resulting in a prolonged super-inflationary stage. The study also addresses the validation of the Hamiltonian constraint during the evolutionary time.

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arXiv2024

Exploring the pre-inflationary dynamics in loop quantum cosmology with a DBI scalar field

Loop quantum cosmology is a symmetry-reduced application of loop quantum gravity. The theory predicts a bounce for the universe at the Planck scale and resolves the singularity of standard cosmology. The dynamics is also governed by an effective Hamiltonian, which predicts a modified Friedmann equation containing the quadratic terms of the energy density. The term plays an essential role in the high energy regime, but the equations return to the standard form in the low energy regime. The evolution of the universe in the pre-inflationary period is studied in the framework of loop quantum cosmology, where the DBI scalar field is assumed to be the dominant component of the universe. Using the numerical method, we provide the evolution of the DBI field. The background evolution shows that there are three phases as: bouncing phase, transition phase and slow-roll inflationary phase. There is also a short period of super-inflation just at the beginning of the bounce phase. The field first climbs the potential and then reaches the turning point where $\dotϕ$ disappears and the potential energy becomes the dominant part of the energy density. This is the time when the slow roll inflation begins and the field slowly rolls down the potential. The results indicate that there are a few e-fold expansions in the bounce phase, about $N = 3.5-4$, and the universe experiences about $N = 59$ e-fold expansions in the slow-roll inflation phase.

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arXiv2023

Exploring new subclass of k-inflation: tachyon inflation in $R+ηT$ gravity model

It is explained that any scalar field in $f(R,T)$ gravity model could present a new subclass of the k-essence model for inflation. While the case of the quintessence has been studied, there is an empty gap required to be filled by investigating more types of field models. Here, we are aiming to partially fill the gap and concentrate on the tachyon field. In this study, we explore the phenomenon of tachyon inflation in the context of the alternative gravity theory $f(R,T) = R + ηT$. By applying slow-roll approximations, we examine the model in detail for three different potentials: power-law, generalized T-mode, and inverse hyperbolic cosh. Using observational data and Python coding, we determine a range of values for the model's free parameters that allow it to fit the data perfectly. In essence, this study provides a comprehensive analysis of tachyon inflation in the $R + ηT$ gravity theory, offering new insights into this alternative framework and its potential to explain tachyon inflation.

gr-qc↗

arXiv2023

Wormhole solution in modified teleparallel-Rastall gravity and energy conditions

The possibility of static and spherically symmetric traversable wormhole solution in modified teleparallel Rastall gravity (MTRG) in a non-diagonal tetrad is investigated. Rastall assumption modifies the field equations and energy-momentum conservation law. These modifications lead to different exact asymptotically flat traversable wormhole solutions. By imposing different constraints on modified energy-momentum conservation law, different exact solutions are found and the obtained results for teleparallel Rastall gravity (TRG), with specific choices of $f(T)$, are studied. It is shown that the Rastall parameters have a key role in these models and in all of those exact traversable wormhole solutions the null energy condition (NEC) and weak energy condition (WEC) of matter or energy which has surrounded the wormhole are valid at the throat of the wormhole and through the space time as well.

gr-qc↗

arXiv2022

Constant-roll inflation driven by holographic dark energy

The scenario of constant-roll inflation is investigated assuming that it is caused by holographic dark energy with energy density $3M_p^2 c^2 / L^2$. The energy density with varying $c$ parameter and Hubble radius as infrared cutoff provides a proper description for the late time acceleration expansion. This motivates us to study the role of the same holographic dark energy in the scenario of constant-roll inflation. One of the main features of constant-roll inflation is that the second slow-roll parameter is constant which is not necessarily small. This feature brings a differential equation for the parameter $c$. After finding an analytical solution for the parameter, the observable parameters are obtained at the time of inflation, and by comparing the model with data, the constants of the model are determined. The energy scale of inflation is estimated which is about $10^{16} \; {\rm GeV}$. Finally, a correspondence between the HDE and canonical scalar field is considered and the behavior of the constructed potential and its magnitude is discussed.

gr-qc↗

arXiv2022

Brane inflation: swampland criteria, TCC, and reheating predictions

We consider inflation in a five -dimensional space time with the inflaton field confined to live on a brane world. In this scenario, we study different types of potentials for the inflaton, discuss their observational consequences, and compare with data. We find that some class of potentials are in good agreement with observation and that the value of the inflaton field can be sub-Planckian. Moreover, we investigate the swampland criteria in this scenario and determine the consistency of the model with the conjectures. Doing so, we could determine models that simultaneously satisfy both observational data and swampland criteria. More constraints are applied by studying the reheating phase where the acceptable range for the reheating temperature imposes some bounds on the models. As the last step, the result of trans-Planckian censorship conjecture for the model is considered where it is shown the constraint of TCC will be very strong and it could be used to applied limit on the brane tension.

gr-qc↗

arXiv2021

Null geodesic of Schwarzschild AdS with Gaussian matter distribution

One of the best ways to understand the gravitation of a massive object is by studying the photon's motion around it. We study the null geodesic of a regular black hole in anti-de Sitter spacetime, including a Gaussian matter distribution. Obtaining the effective potential and possible motions of the photon are discussed for different energy levels. The nature of the effective potential implies that the photon is prevented from reaching the black hole's center. Different types of possible orbits are considered. A photon with negative energy is trapped in a potential hole and has a back and forth motion between two horizons of the metric. However, for specific values of positive energy, the trapped photon still has a back and forth motion; however, it crosses the horizons in every direction. The effective potential has an unstable point outside the horizons, which indicates the possible circular motion of the photon. The closest approach of the photon and the bending angle are also investigated.

gr-qc↗

arXiv2021

Holographic warm inflation

The increasing interest in studying the role of holographic dark energy in the evolution of the very early universe motivates us to study it for the scenario of warm inflation. Due to this scenario, the holographic dark energy, which now drives inflation, has an interaction with the radiation. The case of interacting dark energy also has received increasing interest in studying the late time cosmology. The Infrared cutoff is taken as the Hubble length and all corrections are assumed to be exhibited by the parameter $c$, which appears in the holographic dark energy. By comparing the predictions of the model with observational data, the free constants of the model could be determined. Then, by using these values of the constants, the energy density of inflation is estimated. Next, we consider the validity of the fundamental assumptions of the warm inflation, e.g. $T/H > 1$, which is necessary to be held during inflation, for the obtained values of the constant. Gathering all outcomes, the model could be count as a suitable candidate for warm inflation.

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