Publications

2024

Gravitational Lensing in More Realistic Dark Matter Halo Models

Ali Tizfahm, Saeed Fakhry, Javad T. Firouzjaee



The measurements of the cosmic microwave background (CMB) have played a significant role in understanding the nature of dark energy. In this article, we investigate the dynamics of the dark energy equation of state, utilizing high-precision CMB data from multiple experiments. We begin by examining the Chevallier-Polarski-Linder (CPL) parameterization, a commonly used and recognized framework for describing the dark energy equation of state. We then explore the general Exponential parameterization, which incorporates CPL as its first-order approximation, and extensions of this parameterization that incorporate nonlinear terms. We constrain these scenarios using CMB data from various missions, including the Planck 2018 legacy release, the Wilkinson Microwave Anisotropy Probe (WMAP), the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT), as well as combinations with lowIn this study, we investigate gravitational lensing within the framework of more realistic dark matter halo models, transcending the limitations of spherical-collapse approximations. Through analytical computations utilizing diverse mass functions, we address critical factors typically overlooked in the standard Press-Schechter formalism, including ellipsoidal-collapse conditions, angular momentum dynamics, dynamical friction, and the cosmological constant. Our analysis incorporates two widely recognized halo density profiles, the Navarro-Frenk-White and Einasto profiles, considering both spherical and ellipsoidal-collapse scenarios. We present relevant calculations of pivotal gravitational lensing observables, such as Einstein radii, lensing optical depths, and time delays, spanning a wide range of redshifts and masses across two distinct lensing models: the point mass and singular isothermal sphere (SIS) lens models. Our findings illuminate that adopting more realistic dark matter halo models leads to heightened lensing effects compared to their spherical-collapse counterparts. Furthermore, our analyses of lensing optical depths and time delays reveal distinct characteristics between point mass and SIS lens models. These outcomes highlight the need for more realistic halo descriptions instead of simple approximations when modeling gravitational lensing, as this approach can potentially better reveal the complex structures of dark matter.

Dynamical dark energy confronted with multiple CMB missions

Mahdi Najafi, Supriya Pan, Eleonora Di Valentino, Javad T. Firouzjaee



The measurements of the cosmic microwave background (CMB) have played a significant role in understanding the nature of dark energy. In this article, we investigate the dynamics of the dark energy equation of state, utilizing high-precision CMB data from multiple experiments. We begin by examining the Chevallier-Polarski-Linder (CPL) parameterization, a commonly used and recognized framework for describing the dark energy equation of state. We then explore the general Exponential parameterization, which incorporates CPL as its first-order approximation, and extensions of this parameterization that incorporate nonlinear terms. We constrain these scenarios using CMB data from various missions, including the Planck 2018 legacy release, the Wilkinson Microwave Anisotropy Probe (WMAP), the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT), as well as combinations with low

Accretion efficiency evolution of Central Supermassive Blackholes in Quasars

Arta Khosravi, Alireza Karamzadeh, Seyed Sajad Tabasi, Javad T. Firouzjaee




The ongoing debate regarding the most accurate accretion model for supermassive black holes at the center of quasars has remained a contentious issue in astrophysics. One significant challenge is the variation in calculated accretion efficiency, with values exceeding the standard range of . This discrepancy is especially pronounced in high redshift supermassive black holes, necessitating the development of a comprehensive model that can address the accretion efficiency for supermassive black holes in both the low and high redshift ranges. In this study, we have focused on low redshift () PG quasars (79 quasars) and high redshift () quasars with standard disks from the flux- and volume-limited QUOTAS+QuasarNET dataset (76 quasars) to establish a model for accretion efficiency. An interesting trend is revealed where in redshift larger than 3, accretion efficiency increases as redshift decreases, while in redshift lower than 0.5, accretion efficiency decreases with reducing redshift. This suggests a peak in accretion efficiency between the low and high redshift quasars. This peak is recognized for the flux- and volume-limited QUOTAS+QuasarNET+DL11 dataset, which is , and it seems to be related to the peak of the star formation rate. (). This result can potentially lead to a more accurate correlation between the star formation rate in quasars and their relationship with the mass of the central supermassive black holes with a more comprehensive disk model in future studies.

Structure formation in various dynamical dark energy scenarios

Masoume Reyhani, Mahdi Najafi, Javad T. Firouzjaee, Elenora Di Valentino

DOI: 10.1016/j.dark.2024.101477



This research investigates the impact of the nature of Dark Energy (DE) on structure formation, focusing on the matter power spectrum and the Integrated Sachs–Wolfe effect (ISW). By analyzing the matter power spectrum at redshifts z= 0 and z= 5, as well as the ISW effect on the scale of ℓ= 10− 100, the study provides valuable insights into the influence of DE equations of state (EoS) on structure formation. The findings reveal that dynamical DE models exhibit a stronger matter power spectrum compared to constant DE models, with the JBP model demonstrating the highest amplitude and the CPL model the weakest. Additionally, the study delves into the ISW effect, highlighting the time evolution of the ISW source term F (a) and its derivative d F (a)/d a, and demonstrating that models with constant DE EoS exhibit a stronger amplitude of F (a) overall, while dynamical models such as CPL exhibit the highest amplitude

The Interpretability of LSTM Models for Predicting Oil Company Stocks: Impact of Correlated Features

Javad T. Firouzjaee, Pouriya Khalilian



Oil companies are among the largest companies in the world whose economic indicators in the global stock market have a great impact on the world economy (Stevens, 2018) and market due to their relation to gold (Aijaz et al., 2016), crude oil (Henriques & Sadorsky, 2008), and the dollar (Huang et al., 1996). This study investigates the impact of correlated features on the interpretability of long short‐term memory (LSTM) (Peters, 2001) models for predicting oil company stocks. To achieve this, we designed a standard long short‐term memory (LSTM) network and trained it using various correlated data sets. Our approach is aimed at improving the accuracy of stock price prediction by considering the multiple factors affecting the market, such as crude oil prices, gold prices, and the US dollar. The results demonstrate that adding a feature correlated with oil stocks does not improve the interpretability of LSTM models 

2023

SVR Algorithm as a Tool for More Optimal Intergalactic Medium Simulation in the Epoch of Reionization

S. Mobina Hosseini, Mahsa Berahman, Seyed Sajad Tabasi, Javad T. Firouzjaee




All kinds of simulations of the intergalactic medium, such as hydrodynamic simulation, N-body simulation, numerical and semi-numerical simulation, etc., have been used to realize the history of this medium. One of these simulations is 21SSD, which is specifically focused on the epoch of reionization. This simulation deepens our understanding of the physics behind the intergalactic medium by considering the free parameters related to the Wouthuysen-Field coupling fluctuations and X-ray and Lyman line transfers in the intergalactic medium, and by presenting the plots of the power spectrum, brightness temperature, etc. in different redshifts. However, due to many physical phenomena that play significant roles in this epoch, simulations of the intergalactic medium are usually extremely complex, time-consuming, and require very powerful hardware. In this work, by using the Support Vector Regression algorithm and based on the 21SSD simulation datasets, we have tried to make the machine fully understand the brightness temperature changes in terms of redshift for different astrophysical free parameters values. At first, we trained the machine with the results of the 21SSD simulation. Then, the machine was able to predict the brightness temperature in terms of redshift with very high accuracy for other interval coefficients. Although we have used this algorithm to estimate the brightness temperature, it seems that this algorithm can be easily used for other parts of cosmology and astrophysics. With its help, it is possible to save time and obtain results with extraordinary accuracy similar to complex simulations, even with normal hardware.

Relativistic redshift of light and its implication for astrophysical objects

Javad T. Firouzjaee

DOI: 10.1016/j.newast.2023.102011


We study cosmological Lemaitre–Tolman–Bondi (LTB) black hole thermodynamics immersed in a quintom universe. We investigate some thermodynamic aspects of such a black hole in detail. We apply two methods of treating particles’ tunneling from the apparent horizons and calculate the black hole’s temperature in each method; the results of which are the same. In addition, by considering specific time slices in the cosmic history, we study the thermodynamic features of this black hole in these specific cosmic epochs. Also, we discuss the information loss problem and the remnant content of the cosmological black hole in different cosmic epochs in this context. We show that approximately in all the cosmic history, the temperature of the black hole’s apparent horizon is more than the temperature of the cosmological apparent horizon.

A New Constraint on the Simulation of the Intergalactic Medium through the Evolution of the Neutral Hydrogen Fraction in the Epoch of Reionization

S. Mobina Hosseini, Bahareh Soleimanpour Salmasi, Seyed Sajad Tabasi, Javad T. Firouzjaee

DOI: 10.3847/1538-4357/ad0459


The thermal history of the intergalactic medium is full of extremely useful data in the field of astrophysics and cosmology. In other words, by examining this environment in different redshifts, the effects of cosmology and astrophysics can be observed side by side. Therefore, simulation is our very powerful tool to reach a suitable model for the intergalactic medium, both in terms of cosmology and astrophysics. In this work, we have simulated the intergalactic medium with the help of the 21cmFAST code and compared the evolution of the neutral hydrogen fraction in different initial conditions. Considerable works arbitrarily determine many important effective parameters in the thermal history of the intergalactic medium without any constraints, and usually, there is a lot of flexibility for modeling. Nonetheless, in this work, by focusing on the evolution of the neutral hydrogen fraction in different models and comparing it with observational data, we have eliminated many models and introduced only limited simulation models that could confirm the observations with sufficient accuracy. This issue becomes thoroughly vital from the point that, in addition to restricting the models through the neutral hydrogen fraction, it can also impose restrictions on the parameters affecting its changes. However, we hope that in future works, by enhancing the observational data and increasing their accuracy, more compatible models with the history of the intergalactic medium can be achieved.

Compact Binary Merger Rate in Dark-matter Spikes

Saeed Fakhry , Zahra Salehnia, Azin Shirmohammadi , Mina Ghodsi Yengejeh, Javad T. Firouzjaee

DOI: 10.3847/1538-4357/acc1dd

Today, 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 cannot be realized for binary neutron stars and primordial black hole-neutron star binaries.

Modeling the Central Supermassive Black Hole Mass of Quasars via the LSTM Approach

Seyed Sajad Tabasi, Reyhaneh Vojoudi Salmani, Pouriya Khaliliyan, Javad T. Firouzjaee

DOI: 10.3847/1538-4357/ace03f


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 3 approximately 4 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(M_PBH, f_PBH) relation, which is well compatible with our findings.

2022

On the merger rate of primordial black holes in cosmic voids

Saeed Fakhry, Seyed Sajad Tabasi, Javad T. Firouzjaee

DOI: 10.1016/j.dark.2023.101244


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 3 ∼ 4 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 ( M_ PBH , f_PBH ) relation, which is well compatible with our findings.

On the General Entangled State and Quantum Decoherence

Abasalt Rostami, Javad T. Firouzjaee

DOI: 10.3390/universe8100508


We study the primary entanglement effect on the decoherence of reduced-density matrices of scalar fields, which interact with other fields or independent mode functions. We study the (leading) tree-level evolution of the scalar bispectrum due to a coupling between two scalar fields. We show that the primary entanglement has a significant role in the decoherence of the given quantum state. We find that the existence of such an entanglement could couple dynamical equations coming from a Schrödinger equation. We show that if one wants to see no effect of the entanglement parameter in the decohering of the quantum system, then the ground state eigenvalues of the interaction terms in the Hamiltonian cannot be independent of each other Generally, including the primary entanglement destroys the independence of the interaction terms in the ground state. We show that the imaginary part of the entanglement parameter plays an important role in the decoherence process without posing any specific restriction to the interaction terms. Our results could be generalized to every scalar quantum field theory with a well-defined quantization of its fluctuations in a given curved space-time.

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

Saeed Fakhry , Zahra Salehnia , Azin Shirmohammadi , Javad T. Firouzjaee

DOI: 10.3847/1538-4357/aca523


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.

New accretion constraint on the evaporation of primordial black holes

Seyed Sajad Tabasi, Mahsa Berahman, Javad T. Firouzjaee

DOI: 10.1103/PhysRevD.107.083523


In this paper, we investigate the processes of evaporation and accretion of primordial black holes during the radiation-dominated era and the matter-dominated era. This subject is very important since usually these two processes are considered independent of each other. In other words, previous works consider them in such a way that they do not have a direct effect on each other, and as a result, their effects on the mass of primordial black holes are calculated separately. The calculations of this paper indicate that assuming these two processes independent of each other will lead to wrong results that only give correct answers within certain limits. In fact, in general, it is a mistake to consider the static state for the event horizon of primordial black holes and perform calculations related to their evaporation, while the radius of primordial black holes is constantly changing due to accretion. In addition, we show that considering the dynamic event horizon in some masses and in some times can lead to the shutdown of the Hawking evaporation process. This study is much more accurate and detailed than our previous study. These calculations show well the mass evolution of primordial black holes from the time of formation to the end of the matter-dominated era, taking into account both the main processes governing black holes: evaporation and accretion.

The integrated Sachs–Wolfe effect in interacting dark matter–dark energy models

Mina Ghodsi Yengejeh, Saeed Fakhry , Javad T. Firouzjaee , Hojatollah Fathi

DOI: 10.1016/j.dark.2022.101144


Inspired by string theory, Heisenberg’s uncertainty principle can be generalized to include the photon-electron gravitational interaction, which leads to the Generalized Uncertainty Principle (GUP). Although GUP considers gravitational uncertainty at the minimum fundamental length scale in physics, it does not consider the effects of spacetime curvature on quantum mechanical uncertainty relations. The Extended Uncertainty Principle (EUP) is a generalization of Heisenberg’s Uncertainty Principle that, unlike the GUP, applies to large length scales. GEUP is also a linear combination of EUP and GUP that creates minimal uncertainty on large length scales. The Einstein-Gauss-Bonnet theory (EGB) can be considered as one of the most promising candidates for modified gravity. In this paper, by using GUP, EUP, and GEUP, we intend to obtain the Hawking temperature of a four-dimensional EGB black hole in the asymptotically flat and (Anti)-de Sitter spacetime. We show that coupling constant, cosmological constant, mass, and radius significantly affect Hawking temperature and decrease or increase Hawking temperature depending on the chosen horizons.

2021

A new constraint on the Hawking evaporation of primordial black holes in the radiation-dominated era

Seyed Sajad Tabasi, Javad T. Firouzjaee

DOI: 10.1140/epjc/s10052-023-11379-0


In this paper, we revisit the evaporation and accretion of primordial black holes (PBHs) during cosmic history and compare them to see if both of these processes are constantly active for PBHs or not. Our calculations indicate that during the radiation-dominated era, PBHs absorb ambient radiation due to accretion, and their apparent horizon grows rapidly. This growth causes the Hawking radiation process to practically fail and all the particles that escape as radiation from PBHs to fall back into them. Nevertheless, our emphasis is that the accretion efficiency factor also plays a very important role here and its exact determination is essential. We have shown that the lower mass limit for PBHs that have not yet evaporated should approximately be 10^14 g rather than 10^15 g. Finally, we study the effects of Hawking radiation quiescence in cosmology and reject models based on the evaporation of PBHs in the radiation-dominated era.
All kinds of simulations of the intergalactic medium, such as hydrodynamic simulation, N-body simulation, numerical and semi-numerical simulation, etc., have been used to realize the history of this medium. One of these simulations is 21SSD, which is specifically focused on the epoch of reionization. This simulation deepens our understanding of the physics behind the intergalactic medium by considering the free parameters related to the Wouthuysen-Field coupling fluctuations and X-ray and Lyman line transfers in the intergalactic medium, and by presenting the plots of the power spectrum, brightness temperature, etc. in different redshifts. However, due to many physical phenomena that play significant roles in this epoch, simulations of the intergalactic medium are usually extremely complex, time-consuming, and require very powerful hardware. In this work, by using the Support Vector Regression algorithm and based on the 21SSD simulation datasets, we have tried to make the machine fully understand the brightness temperature changes in terms of redshift for different astrophysical free parameters values. At first, we trained the machine with the results of the 21SSD simulation. Then, the machine was able to predict the brightness temperature in terms of redshift with very high accuracy for other interval coefficients. Although we have used this algorithm to estimate the brightness temperature, it seems that this algorithm can be easily used for other parts of cosmology and astrophysics. With its help, it is possible to save time and obtain results with extraordinary accuracy similar to complex simulations, even with normal hardware.
All kinds of simulations of the intergalactic medium, such as hydrodynamic simulation, N-body simulation, numerical and semi-numerical simulation, etc., have been used to realize the history of this medium. One of these simulations is 21SSD, which is specifically focused on the epoch of reionization. This simulation deepens our understanding of the physics behind the intergalactic medium by considering the free parameters related to the Wouthuysen-Field coupling fluctuations and X-ray and Lyman line transfers in the intergalactic medium, and by presenting the plots of the power spectrum, brightness temperature, etc. in different redshifts. However, due to many physical phenomena that play significant roles in this epoch, simulations of the intergalactic medium are usually extremely complex, time-consuming, and require very powerful hardware. In this work, by using the Support Vector Regression algorithm and based on the 21SSD simulation datasets, we have tried to make the machine fully understand the brightness temperature changes in terms of redshift for different astrophysical free parameters values. At first, we trained the machine with the results of the 21SSD simulation. Then, the machine was able to predict the brightness temperature in terms of redshift with very high accuracy for other interval coefficients. Although we have used this algorithm to estimate the brightness temperature, it seems that this algorithm can be easily used for other parts of cosmology and astrophysics. With its help, it is possible to save time and obtain results with extraordinary accuracy similar to complex simulations, even with normal hardware.
All kinds of simulations of the intergalactic medium, such as hydrodynamic simulation, N-body simulation, numerical and semi-numerical simulation, etc., have been used to realize the history of this medium. One of these simulations is 21SSD, which is specifically focused on the epoch of reionization. This simulation deepens our understanding of the physics behind the intergalactic medium by considering the free parameters related to the Wouthuysen-Field coupling fluctuations and X-ray and Lyman line transfers in the intergalactic medium, and by presenting the plots of the power spectrum, brightness temperature, etc. in different redshifts. However, due to many physical phenomena that play significant roles in this epoch, simulations of the intergalactic medium are usually extremely complex, time-consuming, and require very powerful hardware. In this work, by using the Support Vector Regression algorithm and based on the 21SSD simulation datasets, we have tried to make the machine fully understand the brightness temperature changes in terms of redshift for different astrophysical free parameters values. At first, we trained the machine with the results of the 21SSD simulation. Then, the machine was able to predict the brightness temperature in terms of redshift with very high accuracy for other interval coefficients. Although we have used this algorithm to estimate the brightness temperature, it seems that this algorithm can be easily used for other parts of cosmology and astrophysics. With its help, it is possible to save time and obtain results with extraordinary accuracy similar to complex simulations, even with normal hardware.