Saeed Fakhry | Shahid Beheshti University (original) (raw)

Papers by Saeed Fakhry

Research paper thumbnail of Compact Binary Merger Rate in Dark-Matter Spikes

arXiv (Cornell University), Jan 5, 2023

Nowadays, the existence of supermassive black holes (SMBHs) in the center of galactic halos is al... more Nowadays, the existence of supermassive black holes (SMBHs) in the center of galactic halos is almost confirmed. An extremely dense region referred to as dark-matter spike is expected to form around central SMBHs as they grow and evolve adiabatically. In this work, we calculate the merger rate of compact binaries in dark-matter spikes while considering halo models with spherical and ellipsoidal collapses. Our findings exhibit that ellipsoidal-collapse dark matter halo models can potentially yield the enhancement of the merger rate of compact binaries. Finally, our results confirm that the merger rate of primordial black hole binaries is consistent with the results estimated by the LIGO-Virgo detectors, while such results can not be realized for primordial black hole-neutron star binaries.

Research paper thumbnail of Effect of a High-Precision Semi-Analytical Mass Function on the Merger Rate of Primordial Black Holes in Dark Matter Halos

arXiv (Cornell University), Dec 16, 2022

In this work, we study the effect of a high-precision semianalytical mass function on the merger ... more In this work, we study the effect of a high-precision semianalytical mass function on the merger rate of primordial black holes (PBHs) in dark matter halos. For this purpose, we first explain a theoretical framework for dark matter halo models and introduce relevant quantities such as halo density profile, concentration parameter, and a high-precision semianalytical function namely Del Popolo (DP) mass function. In the following, we calculate the merger rate of PBHs in the framework of ellipsoidal-collapse dark matter halo models while considering the DP mass function, and compare it with our previous study for the Sheth-Tormen (ST) mass function. The results show that by taking the mass of PBHs as MPBH = 30M , the DP mass function can potentially amplify the merger rate of PBHs. Moreover, we calculate the merger rate of PBHs for the DP mass function as a function of their mass and fraction and compare it with the black hole mergers recorded by the LIGO-Virgo detectors during the latest observing run. Our findings show that the merger rate of PBHs will fall within the LIGO-Virgo band if fPBH O(10 −1). This implies that the DP mass function can be used to strengthen constraints on the fraction of PBHs.

Research paper thumbnail of Effect of a high-precision semianalytical mass function on the merger rate of primordial black holes in dark matter halos

Physical Review D

In this work, we study the effect of a high-precision semianalytical mass function on the merger ... more In this work, we study the effect of a high-precision semianalytical mass function on the merger rate of primordial black holes (PBHs) in dark matter halos. For this purpose, we first explain a theoretical framework for dark matter halo models and introduce relevant quantities such as halo density profile, concentration parameter, and a high-precision semianalytical function namely Del Popolo (DP) mass function. In the following, we calculate the merger rate of PBHs in the framework of ellipsoidal-collapse dark matter halo models while considering the DP mass function, and compare it with our previous study for the Sheth-Tormen (ST) mass function. The results show that by taking the mass of PBHs as MPBH = 30M , the DP mass function can potentially amplify the merger rate of PBHs. Moreover, we calculate the merger rate of PBHs for the DP mass function as a function of their mass and fraction and compare it with the black hole mergers recorded by the LIGO-Virgo detectors during the latest observing run. Our findings show that the merger rate of PBHs will fall within the LIGO-Virgo band if fPBH O(10 −1). This implies that the DP mass function can be used to strengthen constraints on the fraction of PBHs.

Research paper thumbnail of The Merger Rate of Primordial Black Hole–Neutron Star Binaries in Ellipsoidal-collapse Dark Matter Halo Models

The Astrophysical Journal

In this work, we calculate the merger rate of primordial black hole–neutron star (PBH–NS) binarie... more In this work, we calculate the merger rate of primordial black hole–neutron star (PBH–NS) binaries within the framework of ellipsoidal-collapse dark matter models and compare it with that obtained from spherical-collapse dark matter halo models. Our results exhibit that ellipsoidal-collapse dark matter halo models can potentially amplify the merger rate of PBH–NS binaries in such a way that it is very close to the range estimated by the LIGO–Virgo observations. In contrast, spherical-collapse dark matter halo models cannot justify PBH–NS merger events as consistent results with the latest gravitational wave data reported by the LIGO–Virgo collaborations. In addition, we calculate the merger rate of PBH–NS binaries as a function of PBH mass and fraction within the context of ellipsoidal-collapse dark matter halo models. The results indicate that PBH–NS merger events with masses of M PBH ≤ 5M ⊙, M NS ≃ 1.4M ⊙ will be consistent with the LIGO–Virgo observations if f PBH ≃ 1.

Research paper thumbnail of On the Merger Rate of Primordial Black Holes in Cosmic Voids

Cornell University - arXiv, Oct 24, 2022

Cosmic voids are known as underdense substructures of the cosmic web that cover a large volume of... more Cosmic voids are known as underdense substructures of the cosmic web that cover a large volume of the Universe. It is known that cosmic voids contain a small number of dark matter halos, so the existence of primordial black holes (PBHs) in these secluded regions of the Universe is not unlikely. In this work, we calculate the merger rate of PBHs in dark matter halos structured in cosmic voids and determine their contribution to gravitational wave events resulting from black hole mergers recorded by the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO)-Advanced Virgo (aVirgo) detectors. Relying on the PBH scenario, the results of our analysis indicate that about 2 ∼ 3 annual events of binary black hole mergers out of all those recorded by the aLIGO-aVirgo detectors should belong to cosmic voids. We also calculate the redshift evolution of the merger rate of PBHs in cosmic voids. The results show that the evolution of the merger rate of PBHs has minimum sensitivity to the redshift changes, which seems reasonable while considering the evolution of cosmic voids. Finally, we specify the behavior of the merger rate of PBHs as a function of their mass and fraction in cosmic voids and we estimate R(MPBH, fPBH) relation, which is well compatible with our findings.

Research paper thumbnail of The Merger Rate of Primordial Black Hole-Neutron Star Binaries in Ellipsoidal-Collapse Dark Matter Halo Models

Cornell University - arXiv, Sep 19, 2022

In this work, we calculate the merger rate of primordial black hole-neutron star (PBH-NS) binarie... more In this work, we calculate the merger rate of primordial black hole-neutron star (PBH-NS) binaries within the framework of ellipsoidal-collapse dark matter models and compare it with that obtained from spherical-collapse dark matter halo models. Our results exhibit that ellipsoidal-collapse dark matter halo models can potentially amplify the merger rate of PBH-NS binaries in such a way that it is very close to the range estimated by the LIGO-Virgo observations. In contrast, spherical-collapse dark matter halo models cannot justify PBH-NS merger events as consistent results with the latest gravitational wave data reported by the LIGO-Virgo collaboration. In addition, we calculate the merger rate of PBH-NS binaries as a function of PBH mass and fraction within the context of ellipsoidalcollapse dark matter halo models. The results indicate that PBH-NS merger events with masses of (M PBH ≤ 5M ⊙ , M NS ≃ 1.4M ⊙) will be consistent with the LIGO-Virgo observations if f PBH ≃ 1.

Research paper thumbnail of The Integrated Sachs-Wolfe Effect in Interacting Dark Matter-Dark Energy Models

Research paper thumbnail of Analytically approximate solutions to higher derivative gravity

Physical Review D

We obtain analytical approximate black hole solutions for higher derivative gravity in the presen... more We obtain analytical approximate black hole solutions for higher derivative gravity in the presence of Maxwell electromagnetic source. We construct near horizon and asymptotic solutions and then use these to obtain an approximate analytic solution using a continued fraction method to get a complete solution. We compute the thermodynamic quantities and check the first law and Smarr formula. Finally, we investigate the null and time-like geodesics of this black hole.

Research paper thumbnail of Cosmological Inflation in F(Q, T) Gravity

Research paper thumbnail of Primordial black hole merger rate in self-interacting dark matter halo models

Physical Review D, 2022

We study the merger rate of primordial black holes (PBHs) in the self-interacting dark matter (SI... more We study the merger rate of primordial black holes (PBHs) in the self-interacting dark matter (SIDM) halo models. To explore a numerical description for the density profile of the SIDM halo models, we use the result of a previously performed simulation for the SIDM halo models with σ/m = 10 cm 2 g −1. We also propose a concentration-mass-time relation that can explain the evolution of the halo density profile related to the SIDM models. Furthermore, we investigate the encounter condition of PBHs that may have been distributed in the medium of dark matter halos randomly. Under these assumptions, we calculate the merger rate of PBHs within each halo considering the SIDM halo models and compare the results with the one obtained for the cold dark matter (CDM) halo models. We indicate that the merger rate of PBHs for the SIDM halo models during the first epoch (i.e., ∆t ≤ 1 Gyr after the halo virialization) should be lower than the corresponding result for the CDM halo models, while by the time entering the second epoch (i.e., ∆t > 1 Gyr after the halo virialization) sufficient PBH mergers in the SIDM halo models can be generated and even exceed the one resulted from the 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) detectors during the third observing run. The results demonstrate that within the context of the SIDM halo models during the second epoch, the merger rate of 10 M⊙ − 10 M⊙ events falls within the aLIGO window. We also estimate a relation between the fraction of PBHs and their masses, which is well consistent with our findings.

Research paper thumbnail of The integrated Sachs–Wolfe effect in 4D Einstein–Gauss–Bonnet gravity

Physics of the Dark Universe, 2021

A novel 4-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity has been proposed that asserts to by... more A novel 4-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity has been proposed that asserts to bypass the Lovelock's theorem and to result in a non-trivial contribution to the gravitational dynamics in four-dimensional spacetime. In this work, we study the integrated Sachs-Wolfe (ISW) effect in the 4D EGB model. For this purpose, we calculate the evolution of the gravitational potential and the linear growth factor as a function of redshift for the 4D EGB model and compare it with the corresponding result obtained from the Λ-cold dark matter (ΛCDM) model. We also calculate the ISW-auto power spectrum and the ISW-cross power spectrum as functions of cosmic microwave background (CMB) multipoles for the 4D EGB model and compare those with the one obtained from the ΛCDM model. To do this, we use the strongest constraint on the coupling parameter proposed for the 4D EGB model. Additionally, to calculate the ISW effect for the 4D EGB model, we employ three large-scale structure surveys from different wavelengths. The results exhibit that the ISW effect in the 4D EGB model is higher than the one obtained from the ΛCDM model. Hence, we show that the 4D EGB model can amplify the amplitude of the ISW power spectrum, which can be considered as a relative advantage of the 4D EGB model comparing the ΛCDM one. Also, we indicate that the deviation from the ΛCDM model is directly proportional to the value of the dimensionless coupling parameter β.

Research paper thumbnail of Primordial black hole merger rate in ellipsoidal-collapse dark matter halo models

Physical Review D, 2021

We have studied the merger rate of primordial black holes (PBHs) in the ellipsoidal-collapse mode... more 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.

Research paper thumbnail of A stabilization mechanism for excited fermion–boson stars

Classical and Quantum Gravity, 2021

We study numerically the nonlinear stability of excited fermion-boson stars in spherical symmetry... more We study numerically the nonlinear stability of excited fermion-boson stars in spherical symmetry. Such compound hypothetical stars, composed by fermions and bosons, are gravitationally bound, regular, and static configurations described within the coupled Einstein-Klein-Gordon-Euler theoretical framework. The excited configurations are characterized by the presence in the radial profile of the (complex, massive) scalar field-the bosonic piece-of at least one node across the star. The dynamical emergence of one such configuration from the accretion of a cloud of scalar field onto an already-formed neutron star, was numerically revealed in our previous investigation. Prompted by that finding we construct here equilibrium configurations of excited fermion-boson stars and study their stability properties using numerical-relativity simulations. In addition, we also analyze their dynamical formation from generic, constraint-satisfying initial data. Contrary to purely boson stars in the excited state, which are known to be generically unstable, our study reveals the appearance of a cooperative stabilization mechanism between the fermionic and bosonic constituents of those excited-state mixed stars. While similar examples of stabilization mechanisms have been recently discussed in the context of −boson stars and multi-field, multi-frequency boson stars, our results seem to indicate that the stabilization mechanism is a purely gravitational effect and does not depend on the type of matter of the companion star.

Research paper thumbnail of Perihelion Advance and Trajectory of Charged Test Particles in Reissner-Nordstrom Field via the Higher-Order Geodesic Deviations

Advances in High Energy Physics, 2019

By using the higher-order geodesic deviation equations for charged particles, we apply the method... more By using the higher-order geodesic deviation equations for charged particles, we apply the method described by Kerner et.al. to calculate the perihelion advance and trajectory of charged test particles in the Reissner-Nordstrom space-time. The effect of charge on the perihelion advance is studied and we compared the results with those obtained earlier via the perturbation method. The advantage of this approximation method is to provide a way to calculate the perihelion advance and orbit of planets in the vicinity of massive and compact objects without considering Newtonian and post-Newtonian approximations.

Research paper thumbnail of Glow-spark switching by a dielectric wall in a pin-to-electrolyte discharge

Journal of Plasma Physics, 2015

In this paper, the shape, sound, and current of an electrical discharge in the air between a meta... more In this paper, the shape, sound, and current of an electrical discharge in the air between a metal pin and an electrolyte solution are studied. Two different situations are considered: (A) without, and, (B) with inclusion of a dielectric wall in the discharge circuit. It is found that: (1) the discharge A has a cylindrical shape rather than a branched shape in discharge B, (2) the sound and current of discharge in case A are coherent and deterministic but those of case B are incoherent and stochastic. These differences along with the simulation results of a simple model demonstrate that the discharge in case A is glow, but, that in case B is spark.

Research paper thumbnail of Dynamical formation and stability of fermion-boson stars

Physical Review D, 2020

Gravitationally bound structures composed by fermions and scalar particles known as fermionboson ... more Gravitationally bound structures composed by fermions and scalar particles known as fermionboson stars are regular and static configurations obtained by solving the coupled Einstein-Klein-Gordon-Euler (EKGE) system. In this work, we discuss one possible scenario through which these fermion-boson stars may form by solving numerically the EKGE system under the simplifying assumption of spherical symmetry. Our initial configurations assume an already existing neutron star surrounded by an accreting cloud of a massive and complex scalar field. The results of our simulations show that once part of the initial scalar field is expelled via gravitational cooling the system gradually oscillates around an equilibrium configuration that is asymptotically consistent with a static solution of the system. The formation of fermion-boson stars for large positive values of the coupling constant in the self-interaction term of the scalar-field potential reveal the presence of a node in the scalar field. This suggests that a fermionic core may help stabilize configurations with nodes in the bosonic sector, as happens for purely boson stars in which the ground state and the first excited state coexist.

Research paper thumbnail of Compact Binary Merger Rate in Dark-Matter Spikes

arXiv (Cornell University), Jan 5, 2023

Nowadays, the existence of supermassive black holes (SMBHs) in the center of galactic halos is al... more Nowadays, the existence of supermassive black holes (SMBHs) in the center of galactic halos is almost confirmed. An extremely dense region referred to as dark-matter spike is expected to form around central SMBHs as they grow and evolve adiabatically. In this work, we calculate the merger rate of compact binaries in dark-matter spikes while considering halo models with spherical and ellipsoidal collapses. Our findings exhibit that ellipsoidal-collapse dark matter halo models can potentially yield the enhancement of the merger rate of compact binaries. Finally, our results confirm that the merger rate of primordial black hole binaries is consistent with the results estimated by the LIGO-Virgo detectors, while such results can not be realized for primordial black hole-neutron star binaries.

Research paper thumbnail of Effect of a High-Precision Semi-Analytical Mass Function on the Merger Rate of Primordial Black Holes in Dark Matter Halos

arXiv (Cornell University), Dec 16, 2022

In this work, we study the effect of a high-precision semianalytical mass function on the merger ... more In this work, we study the effect of a high-precision semianalytical mass function on the merger rate of primordial black holes (PBHs) in dark matter halos. For this purpose, we first explain a theoretical framework for dark matter halo models and introduce relevant quantities such as halo density profile, concentration parameter, and a high-precision semianalytical function namely Del Popolo (DP) mass function. In the following, we calculate the merger rate of PBHs in the framework of ellipsoidal-collapse dark matter halo models while considering the DP mass function, and compare it with our previous study for the Sheth-Tormen (ST) mass function. The results show that by taking the mass of PBHs as MPBH = 30M , the DP mass function can potentially amplify the merger rate of PBHs. Moreover, we calculate the merger rate of PBHs for the DP mass function as a function of their mass and fraction and compare it with the black hole mergers recorded by the LIGO-Virgo detectors during the latest observing run. Our findings show that the merger rate of PBHs will fall within the LIGO-Virgo band if fPBH O(10 −1). This implies that the DP mass function can be used to strengthen constraints on the fraction of PBHs.

Research paper thumbnail of Effect of a high-precision semianalytical mass function on the merger rate of primordial black holes in dark matter halos

Physical Review D

In this work, we study the effect of a high-precision semianalytical mass function on the merger ... more In this work, we study the effect of a high-precision semianalytical mass function on the merger rate of primordial black holes (PBHs) in dark matter halos. For this purpose, we first explain a theoretical framework for dark matter halo models and introduce relevant quantities such as halo density profile, concentration parameter, and a high-precision semianalytical function namely Del Popolo (DP) mass function. In the following, we calculate the merger rate of PBHs in the framework of ellipsoidal-collapse dark matter halo models while considering the DP mass function, and compare it with our previous study for the Sheth-Tormen (ST) mass function. The results show that by taking the mass of PBHs as MPBH = 30M , the DP mass function can potentially amplify the merger rate of PBHs. Moreover, we calculate the merger rate of PBHs for the DP mass function as a function of their mass and fraction and compare it with the black hole mergers recorded by the LIGO-Virgo detectors during the latest observing run. Our findings show that the merger rate of PBHs will fall within the LIGO-Virgo band if fPBH O(10 −1). This implies that the DP mass function can be used to strengthen constraints on the fraction of PBHs.

Research paper thumbnail of The Merger Rate of Primordial Black Hole–Neutron Star Binaries in Ellipsoidal-collapse Dark Matter Halo Models

The Astrophysical Journal

In this work, we calculate the merger rate of primordial black hole–neutron star (PBH–NS) binarie... more In this work, we calculate the merger rate of primordial black hole–neutron star (PBH–NS) binaries within the framework of ellipsoidal-collapse dark matter models and compare it with that obtained from spherical-collapse dark matter halo models. Our results exhibit that ellipsoidal-collapse dark matter halo models can potentially amplify the merger rate of PBH–NS binaries in such a way that it is very close to the range estimated by the LIGO–Virgo observations. In contrast, spherical-collapse dark matter halo models cannot justify PBH–NS merger events as consistent results with the latest gravitational wave data reported by the LIGO–Virgo collaborations. In addition, we calculate the merger rate of PBH–NS binaries as a function of PBH mass and fraction within the context of ellipsoidal-collapse dark matter halo models. The results indicate that PBH–NS merger events with masses of M PBH ≤ 5M ⊙, M NS ≃ 1.4M ⊙ will be consistent with the LIGO–Virgo observations if f PBH ≃ 1.

Research paper thumbnail of On the Merger Rate of Primordial Black Holes in Cosmic Voids

Cornell University - arXiv, Oct 24, 2022

Cosmic voids are known as underdense substructures of the cosmic web that cover a large volume of... more Cosmic voids are known as underdense substructures of the cosmic web that cover a large volume of the Universe. It is known that cosmic voids contain a small number of dark matter halos, so the existence of primordial black holes (PBHs) in these secluded regions of the Universe is not unlikely. In this work, we calculate the merger rate of PBHs in dark matter halos structured in cosmic voids and determine their contribution to gravitational wave events resulting from black hole mergers recorded by the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO)-Advanced Virgo (aVirgo) detectors. Relying on the PBH scenario, the results of our analysis indicate that about 2 ∼ 3 annual events of binary black hole mergers out of all those recorded by the aLIGO-aVirgo detectors should belong to cosmic voids. We also calculate the redshift evolution of the merger rate of PBHs in cosmic voids. The results show that the evolution of the merger rate of PBHs has minimum sensitivity to the redshift changes, which seems reasonable while considering the evolution of cosmic voids. Finally, we specify the behavior of the merger rate of PBHs as a function of their mass and fraction in cosmic voids and we estimate R(MPBH, fPBH) relation, which is well compatible with our findings.

Research paper thumbnail of The Merger Rate of Primordial Black Hole-Neutron Star Binaries in Ellipsoidal-Collapse Dark Matter Halo Models

Cornell University - arXiv, Sep 19, 2022

In this work, we calculate the merger rate of primordial black hole-neutron star (PBH-NS) binarie... more In this work, we calculate the merger rate of primordial black hole-neutron star (PBH-NS) binaries within the framework of ellipsoidal-collapse dark matter models and compare it with that obtained from spherical-collapse dark matter halo models. Our results exhibit that ellipsoidal-collapse dark matter halo models can potentially amplify the merger rate of PBH-NS binaries in such a way that it is very close to the range estimated by the LIGO-Virgo observations. In contrast, spherical-collapse dark matter halo models cannot justify PBH-NS merger events as consistent results with the latest gravitational wave data reported by the LIGO-Virgo collaboration. In addition, we calculate the merger rate of PBH-NS binaries as a function of PBH mass and fraction within the context of ellipsoidalcollapse dark matter halo models. The results indicate that PBH-NS merger events with masses of (M PBH ≤ 5M ⊙ , M NS ≃ 1.4M ⊙) will be consistent with the LIGO-Virgo observations if f PBH ≃ 1.

Research paper thumbnail of The Integrated Sachs-Wolfe Effect in Interacting Dark Matter-Dark Energy Models

Research paper thumbnail of Analytically approximate solutions to higher derivative gravity

Physical Review D

We obtain analytical approximate black hole solutions for higher derivative gravity in the presen... more We obtain analytical approximate black hole solutions for higher derivative gravity in the presence of Maxwell electromagnetic source. We construct near horizon and asymptotic solutions and then use these to obtain an approximate analytic solution using a continued fraction method to get a complete solution. We compute the thermodynamic quantities and check the first law and Smarr formula. Finally, we investigate the null and time-like geodesics of this black hole.

Research paper thumbnail of Cosmological Inflation in F(Q, T) Gravity

Research paper thumbnail of Primordial black hole merger rate in self-interacting dark matter halo models

Physical Review D, 2022

We study the merger rate of primordial black holes (PBHs) in the self-interacting dark matter (SI... more We study the merger rate of primordial black holes (PBHs) in the self-interacting dark matter (SIDM) halo models. To explore a numerical description for the density profile of the SIDM halo models, we use the result of a previously performed simulation for the SIDM halo models with σ/m = 10 cm 2 g −1. We also propose a concentration-mass-time relation that can explain the evolution of the halo density profile related to the SIDM models. Furthermore, we investigate the encounter condition of PBHs that may have been distributed in the medium of dark matter halos randomly. Under these assumptions, we calculate the merger rate of PBHs within each halo considering the SIDM halo models and compare the results with the one obtained for the cold dark matter (CDM) halo models. We indicate that the merger rate of PBHs for the SIDM halo models during the first epoch (i.e., ∆t ≤ 1 Gyr after the halo virialization) should be lower than the corresponding result for the CDM halo models, while by the time entering the second epoch (i.e., ∆t > 1 Gyr after the halo virialization) sufficient PBH mergers in the SIDM halo models can be generated and even exceed the one resulted from the 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) detectors during the third observing run. The results demonstrate that within the context of the SIDM halo models during the second epoch, the merger rate of 10 M⊙ − 10 M⊙ events falls within the aLIGO window. We also estimate a relation between the fraction of PBHs and their masses, which is well consistent with our findings.

Research paper thumbnail of The integrated Sachs–Wolfe effect in 4D Einstein–Gauss–Bonnet gravity

Physics of the Dark Universe, 2021

A novel 4-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity has been proposed that asserts to by... more A novel 4-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity has been proposed that asserts to bypass the Lovelock's theorem and to result in a non-trivial contribution to the gravitational dynamics in four-dimensional spacetime. In this work, we study the integrated Sachs-Wolfe (ISW) effect in the 4D EGB model. For this purpose, we calculate the evolution of the gravitational potential and the linear growth factor as a function of redshift for the 4D EGB model and compare it with the corresponding result obtained from the Λ-cold dark matter (ΛCDM) model. We also calculate the ISW-auto power spectrum and the ISW-cross power spectrum as functions of cosmic microwave background (CMB) multipoles for the 4D EGB model and compare those with the one obtained from the ΛCDM model. To do this, we use the strongest constraint on the coupling parameter proposed for the 4D EGB model. Additionally, to calculate the ISW effect for the 4D EGB model, we employ three large-scale structure surveys from different wavelengths. The results exhibit that the ISW effect in the 4D EGB model is higher than the one obtained from the ΛCDM model. Hence, we show that the 4D EGB model can amplify the amplitude of the ISW power spectrum, which can be considered as a relative advantage of the 4D EGB model comparing the ΛCDM one. Also, we indicate that the deviation from the ΛCDM model is directly proportional to the value of the dimensionless coupling parameter β.

Research paper thumbnail of Primordial black hole merger rate in ellipsoidal-collapse dark matter halo models

Physical Review D, 2021

We have studied the merger rate of primordial black holes (PBHs) in the ellipsoidal-collapse mode... more 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.

Research paper thumbnail of A stabilization mechanism for excited fermion–boson stars

Classical and Quantum Gravity, 2021

We study numerically the nonlinear stability of excited fermion-boson stars in spherical symmetry... more We study numerically the nonlinear stability of excited fermion-boson stars in spherical symmetry. Such compound hypothetical stars, composed by fermions and bosons, are gravitationally bound, regular, and static configurations described within the coupled Einstein-Klein-Gordon-Euler theoretical framework. The excited configurations are characterized by the presence in the radial profile of the (complex, massive) scalar field-the bosonic piece-of at least one node across the star. The dynamical emergence of one such configuration from the accretion of a cloud of scalar field onto an already-formed neutron star, was numerically revealed in our previous investigation. Prompted by that finding we construct here equilibrium configurations of excited fermion-boson stars and study their stability properties using numerical-relativity simulations. In addition, we also analyze their dynamical formation from generic, constraint-satisfying initial data. Contrary to purely boson stars in the excited state, which are known to be generically unstable, our study reveals the appearance of a cooperative stabilization mechanism between the fermionic and bosonic constituents of those excited-state mixed stars. While similar examples of stabilization mechanisms have been recently discussed in the context of −boson stars and multi-field, multi-frequency boson stars, our results seem to indicate that the stabilization mechanism is a purely gravitational effect and does not depend on the type of matter of the companion star.

Research paper thumbnail of Perihelion Advance and Trajectory of Charged Test Particles in Reissner-Nordstrom Field via the Higher-Order Geodesic Deviations

Advances in High Energy Physics, 2019

By using the higher-order geodesic deviation equations for charged particles, we apply the method... more By using the higher-order geodesic deviation equations for charged particles, we apply the method described by Kerner et.al. to calculate the perihelion advance and trajectory of charged test particles in the Reissner-Nordstrom space-time. The effect of charge on the perihelion advance is studied and we compared the results with those obtained earlier via the perturbation method. The advantage of this approximation method is to provide a way to calculate the perihelion advance and orbit of planets in the vicinity of massive and compact objects without considering Newtonian and post-Newtonian approximations.

Research paper thumbnail of Glow-spark switching by a dielectric wall in a pin-to-electrolyte discharge

Journal of Plasma Physics, 2015

In this paper, the shape, sound, and current of an electrical discharge in the air between a meta... more In this paper, the shape, sound, and current of an electrical discharge in the air between a metal pin and an electrolyte solution are studied. Two different situations are considered: (A) without, and, (B) with inclusion of a dielectric wall in the discharge circuit. It is found that: (1) the discharge A has a cylindrical shape rather than a branched shape in discharge B, (2) the sound and current of discharge in case A are coherent and deterministic but those of case B are incoherent and stochastic. These differences along with the simulation results of a simple model demonstrate that the discharge in case A is glow, but, that in case B is spark.

Research paper thumbnail of Dynamical formation and stability of fermion-boson stars

Physical Review D, 2020

Gravitationally bound structures composed by fermions and scalar particles known as fermionboson ... more Gravitationally bound structures composed by fermions and scalar particles known as fermionboson stars are regular and static configurations obtained by solving the coupled Einstein-Klein-Gordon-Euler (EKGE) system. In this work, we discuss one possible scenario through which these fermion-boson stars may form by solving numerically the EKGE system under the simplifying assumption of spherical symmetry. Our initial configurations assume an already existing neutron star surrounded by an accreting cloud of a massive and complex scalar field. The results of our simulations show that once part of the initial scalar field is expelled via gravitational cooling the system gradually oscillates around an equilibrium configuration that is asymptotically consistent with a static solution of the system. The formation of fermion-boson stars for large positive values of the coupling constant in the self-interaction term of the scalar-field potential reveal the presence of a node in the scalar field. This suggests that a fermionic core may help stabilize configurations with nodes in the bosonic sector, as happens for purely boson stars in which the ground state and the first excited state coexist.