arXiv subjects

Javad T. Firouzjaee

Publications and source records attributed to Javad T. Firouzjaee.

At least 19 recordsLinked to original sources

arXiv2026

Primordial black hole in Lorentz-violating theories: Insights from Bumblebee gravity

The Bumblebee gravity (BG) model, featuring spontaneous Lorentz symmetry breaking via a vector field non-minimally coupled to curvature, has been widely used to explore Lorentz-violating effects in cosmology. We investigate primordial black hole (PBH) formation within this framework, deriving the complete set of modified perturbation equations. We demonstrate that BG, sourced by a timelike vector field, introduces three distinct enhancements to PBH abundance--modified expansion history, suppressed collapse threshold, and amplified power spectrum--which together render PBHs viable dark matter candidates across the asteroid-mass window for modest Lorentz-violating couplings. However, a systematic analysis of the quadratic action reveals that these phenomenological consequences emerge from a theoretically pathological foundation. The vector sector exhibits an intrinsic ghost instability, while the requirement of a stable symmetry-breaking minimum simultaneously induces a tachyonic instability on timescales far below cosmological scales. The model thus suffers from a fundamental inconsistency: the conditions for cosmological viability and spontaneous symmetry breaking are mutually exclusive within the minimal Bumblebee framework. Our results illustrate both the notable power of Lorentz violation to influence early Universe observables and the necessity of a consistent theoretical foundation for such predictions.

gr-qc↗

arXiv2026

Fragility of stealth solutions in mimetic gravity

We study a broad class of constrained mimetic-type extensions of general relativity with action $S=\int{\rm d}^4x\sqrt{-g}\,\bigl(R/2+λ\,C[g,Ψ]+{\cal L}_{\rm m}\bigr)$, where $R$ is the Ricci scalar, $λ$ is a Lagrange multiplier, $C[g,Ψ]$ is a scalar functional of the metric and generic field content $Ψ$ (possibly involving $Ψ$ and its covariant derivatives) and ${\cal L}_{\rm m}$ is the matter Lagrangian. The branch $\barλ\to 0$, with the bar denoting a background value, provides a simple screening-like limit in which the constrained sector decouples, as in cosmological realizations where $\barλ$ is typically nonzero on large scales while locally one expects $\barλ\simeq 0$. On the exactly stealth branch $\barλ=0$, the constrained sector drops out of the background dynamics, so, on domains where a background profile $\barΨ$ satisfying $\bar C=0$ exists, the theory admits the corresponding general relativity geometries as stealth solutions. As an explicit realization of this mechanism, we consider the scalar field case, where $C=g^{μν}\partial_μϕ\partial_νϕ\pm1=0$ becomes a Hamilton-Jacobi equation selecting geodesic congruences; in this setting, we study spherically symmetric solutions and construct a stealth Kerr profile using Carter separability. We then show, at the general level, that the $\barλ=0$ branch is perturbatively degenerate with general relativity: the constrained sector contributes to the dynamics only through terms weighted by $\barλ$, which vanish on the stealth branch, while still imposing an infinite hierarchy of constraints on the fluctuations. Consequently, the $\barλ\to0$ limit is generically non-uniform, making the would-be screening perturbatively pathological.

gr-qc↗

arXiv2026

When Dark Energy Turns On: Constraints on a Critical Emergence Model

We investigate a specific emergent dark energy scenario, known as critically emergent dark energy (CEDE), in which dark energy is effectively absent in the early Universe and becomes dynamically relevant only after a critical cosmic epoch through a phase transition. We constrain this model using recent cosmological observations, including cosmic microwave background (CMB) data from \emph{Planck} 2018, baryon acoustic oscillation (BAO) measurements from SDSS and DESI DR2, and two independent Type Ia supernova compilations, PantheonPlus and Union3. Our results show that within the CEDE framework a dark energy phase transition is not ruled out. In particular, CMB-only, CMB+SDSS, and CMB+DESI datasets provide evidence for a nonzero transition scale factor and, according to standard statistical indicators such as $Δχ^2$ and Bayesian evidence, can favor CEDE over the $Λ$CDM model. At the same time, we find that CEDE does not fully resolve the Hubble constant tension. Overall, our analysis indicates that dark energy models featuring a phase transition remain a viable and phenomenologically interesting extension of the standard cosmological framework. Upcoming high-precision cosmological surveys will be essential to further assess whether such emergent dark energy scenarios represent a genuine departure from $Λ$CDM or an effective description of current data.

astro-ph.CO↗

arXiv2026

Primordial observables of explicit diffeomorphism violation in gravity

We investigate the potential for current and future gravitational-wave detectors to observe imprints of explicit diffeomorphism violation in primordial signals. Starting from a simple model with known effects, we derive the strain amplitude and power spectrum for primordial gravitational waves, both of which are affected by the symmetry breaking. Through this, we directly find predictions for the tensor spectral index and tensor-to-scalar which are different from general relativity. By considering the known sensitivity curves for NANOGrav, SKA, THEIA, $μ$-ARES, ASTROD-GW, LISA, BBO, DECIGO, CE, AION-km, AEDGE, ET, and aLIGO, we place observability limits on the parameters controlling the diffeomorphism violation. For instance, we find that aLIGO could observe signals for $s_{00} \lesssim -0.1$, while more sensitive future detectors like LISA and DECIGO could probe violations as small as $s_{00} \approx -5 \times 10^{-4}$ and $-3 \times 10^{-3}$, respectively. Finally, we consider the existing constraints on the number of relativistic degrees of freedom $ΔN_{\rm eff}$ which is tightly constrained by Big-Bang Nucleosynthesis (BBN) and we find that $ΔN_{\rm eff}$ only weakly depends on the symmetry breaking but places a lower bound on the coefficients which is consistent with available bounds from the speed of gravitational waves.

gr-qc↗

arXiv2025

Reduced-order autoregressive dynamics of a complex financial system: a PCA-based approach

This study analyzes the dynamic interactions among the NASDAQ index, crude oil, gold, and the US dollar using a reduced-order modeling approach. Time-delay embedding and principal component analysis are employed to encode high-dimensional financial dynamics, followed by linear regression in the reduced space. Correlation and lagged regression analyses reveal heterogeneous cross-asset dependencies. Model performance, evaluated using the coefficient of determination ($R^2$), demonstrates that a limited number of principal components is sufficient to capture the dominant dynamics of each asset, with varying complexity across markets.

q-fin.ST↗

arXiv2025

A Multi-Probe ISW Study of Dark Energy Models with Negative Energy Density: Galaxy Correlations, Lensing Bispectrum, and Planck ISW-Lensing Likelihood

We investigate the late-time imprints of three dark energy (DE) models, namely, the Chevallier-Polarski-Linder (CPL) parametrization, $Λ_{\rm s}$CDM, and an Omnipotent DE model, on cosmological observables sensitive to the time evolution of gravitational potentials. While CPL serves as a reference parameterization, the Omnipotent and $Λ_{\rm s}$CDM scenarios were originally proposed as possible solutions to the $H_0$ tension and are selected here because they can yield negative dark energy. These models are examined within a multi-probe framework based on the Integrated Sachs-Wolfe (ISW) effect and the lensing-ISW bispectrum. By analyzing both two- and three-point Cosmic Microwave Background (CMB) correlations, we assess how their late-time dynamics modify the growth and decay of large-scale gravitational potentials compared to the standard $Λ$CDM cosmology. Despite producing nearly indistinguishable CMB angular spectra at high multipoles, these models yield distinctive signatures in the low-$\ell$ ISW plateau as well as in higher-order statistics related to ISW, highlighting the power of both large-scale CMB anisotropies and higher-order CMB statistics in testing dark energy physics. Our results demonstrate that combining complementary ISW probes provides an effective way to discriminate between dark energy scenarios and will be crucial to determine whether negative or sign-switching dark energy is ultimately favored or disfavored by forthcoming data.

astro-ph.CO↗

arXiv2025

Cosmological Inflation in $f(Q,\mathcal{L}_{m})$ Gravity

Cosmological inflation remains a key paradigm for explaining the earliest stages of the Universe, yet the theoretical limitations of General Relativity (GR) motivate the development of alternative formulations capable of addressing both early and late cosmic acceleration. In this work, we investigate cosmological inflation within the $f(Q,\mathcal{L}_{m})$ gravity framework based on symmetric teleparallel geometry, where the non-metricity scalar $Q$ couples directly to the matter Lagrangian. We formulate the slow-roll dynamics and derive analytical predictions for the scalar spectral index $n_{s}$ and tensor-to-scalar ratio $r$ in both linear and nonlinear non-minimal coupling models, assuming a power-law inflaton potential. Our findings show that the linear case, $f(Q,\mathcal{L}_{m})=-αQ + 2\mathcal{L}_{m}+β$, becomes compatible with Planck+BK15+BAO constraints for positive $α$ and $β$, producing narrow viable contours in parameter space. In contrast, the nonlinear model, $f(Q,\mathcal{L}_{m})=-αQ+(2\mathcal{L}_{m})^{2}+β$, achieves observational viability only for negative $α$ and $β$, and its predictions predominantly fall inside the $68\%$ confidence region of joint data. These results demonstrate that $f(Q,\mathcal{L}_{m})$ gravity produces distinct inflationary regimes, providing a highly competitive alternative to GR.

gr-qc↗

arXiv2025

Quantum Dynamics and Information Measures in PT and Anti-PT-Symmetric Systems

In this study, we investigate qubit dynamics under PT and Anti-PT-symmetric non-Hermitian Hamiltonians, focusing on phase evolution, decoherence, quantum speed limits (QSL), and Rényi entanglement entropies. Using similarity transformations and Dyson maps, we analyze the reduced density matrix evolution in bosonic environments. Anti-PT-symmetric systems show enhanced robustness against decoherence, with slower entropy growth and longer coherence times compared to PT-symmetric counterparts. QSL behavior is non-monotonic, reflecting rapid initial evolution followed by a gradual decrease. Higher-order Rényi entropies reveal that Anti-PT-symmetric qubits preserve quantum information more effectively, offering advantages for memory and cryptographic applications.

quant-ph↗

arXiv2025

Ultradense Dark Matter Halos with Poisson Noise from Stellar-Mass Primordial Black Holes

In this work, we investigate the impact of Poisson noise from stellar-mass primordial black holes (PBHs) on the formation of ultradense dark matter halos (UDMHs). Our findings reveal that the discrete spatial distribution of PBHs significantly enhances small-scale density fluctuations, particularly for massive stellar-mass PBHs. Our results indicate that the modified power spectrum, incorporating both adiabatic and isocurvature contributions from PBH-induced Poisson noise, strongly depends on PBH mass and fraction. Specifically, increasing PBH mass shifts the differential mass function of UDMHs toward higher masses, while variations in the suppression parameter $n$ modulate the efficiency of UDMH formation at small scales. For lower values of $n$, our findings show a significant boost in UDMH abundance, favoring multi-component dark matter scenarios. Conversely, at higher values of $n$, the predicted UDMH distributions align more closely with single-component models dominated by stellar-mass PBHs. Furthermore, our analysis demonstrates that more realistic halo mass functions, which account for angular momentum and dynamical friction, consistently predict higher UDMH abundances compared to traditional Press-Schechter formalism.

astro-ph.CO↗

arXiv2025

High-redshift Galaxies from JWST Observations in More Realistic Dark Matter Halo Models

The James Webb Space Telescope (JWST) has unveiled unexpectedly massive galaxy candidates at high redshifts, challenging standard $Λ$CDM cosmological predictions. In this work, we study the predictions of more realistic dark matter halo models combined with modified matter power spectra for interpreting JWST observations of high-redshift galaxies. We employ three halo mass functions: the conventional Sheth-Tormen (ST) model and two physically motivated alternatives introduced by Del Popolo (DP1 and DP2). Our analysis of cumulative stellar mass densities at $z \simeq 8$--$10$ reveals that the standard ST mass function systematically underpredicts JWST observations, achieving marginal consistency only with high star formation efficiencies. In contrast, the DP1 and DP2 models demonstrate significantly improved agreement with observations even within standard $Λ$CDM, with statistical consistency within $1$--$2σ$ for moderate star formation efficiencies. When combined with modified power spectra, these refined halo models achieve suitable agreement with JWST data across broad parameter ranges, particularly for steeper spectral indices that amplify high-mass halo formation. Crucially, we find that moderate star formation efficiencies coupled with small-scale power enhancements provide robust reconciliation between theory and observations, eliminating the need for extreme astrophysical assumptions. Our results demonstrate that incorporating realistic halo collapse physics, often neglected in standard analyses, can substantially alleviate apparent tensions between JWST observations and $Λ$CDM predictions, highlighting the critical importance of small-scale structure formation physics in early cosmic epochs.

astro-ph.GA↗

arXiv2025

Mapping Crisis-Driven Market Dynamics: A Transfer Entropy and Kramers-Moyal Approach to Financial Networks

Financial markets are dynamic, interconnected systems where local shocks can trigger widespread instability, challenging portfolio managers and policymakers. Traditional correlation analysis often miss the directionality and temporal dynamics of information flow. To address this, we present a unified framework integrating Transfer Entropy (TE) and the N-dimensional Kramers-Moyal (KM) expansion to map static and time-resolved coupling among four major indices: Nasdaq Composite (^IXIC), WTI crude oil (WTI), gold (GC=F), and the US Dollar Index (DX-Y.NYB). TE captures directional information flow. KM models non-linear stochastic dynamics, revealing interactions often overlooked by linear methods. Using daily data from August 11, 2014, to September 8, 2024, we compute returns, confirm non-stationary using a conduct sliding-window TE and KM analyses. We find that during the COVID-19 pandemic (March-June 2020) and the Russia-Ukraine crisis (Feb-Apr 2022), average TE increases by 35% and 28%, respectively, indicating heightened directional flow. Drift coefficients highlight gold-dollar interactions as a persistent safe-haven channel, while oil-equity linkages show regime shifts, weakening under stress and rebounding quickly. Our results expose the shortcomings of linear measures and underscore the value of combining information-theoretic and stochastic drift methods. This approach offers actionable insights for adaptive hedging and informs macro-prudential policy by revealing the evolving architecture of systemic risk.

q-fin.ST↗

arXiv2024

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

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. In addition, emulators also have become widely used in recent years. Especially because they are very fast, they are used in many works instead of actual complete simulations. Emulators are trained with the help of simulator results and can work with very high accuracy similar to real simulators. Usually, reliable machine learning algorithms, such as neural networks, are used to program these emulators. In this work, we show that one of the best machine learning algorithms that can be used in general and have extremely high accuracy is the Support Vector Regression algorithm. This algorithm works better than the Multilayer Perceptron algorithm not only from the practical point of view but also from the theoretical point of view. To prove this issue, we have considered the 21SSD hydrodynamical simulation in addition to the theoretical arguments, and we have used both algorithms to be trained by this simulation for comparison. Finally, we have shown that the Support Vector Regression algorithm can be a much better and more accurate algorithm than neural network algorithms in future emulations, and by mastering it, one can neglect other algorithms with full confidence. Although this work is focused only on an intergalactic medium simulation in the Epoch of Reionization, it can be used for all other simulations regardless of their type. In other words, it can be employed for all hydrodynamic simulations, N-body simulations, etc. in all cosmic epochs and rely on its results.

astro-ph.CO↗

arXiv2024

Toward Gravitational Lensing in Modified Theories of Gravity

In this study, we investigate gravitational lensing within modified gravity frameworks, focusing on the Hu-Sawicki $f(R)$ and normal branch Dvali-Gabadadze-Porrati (nDGP) models, and we compare these results with those obtained from general relativity (GR). Our results reveal that both modified gravity models consistently enhance key lensing parameters relative to GR, including the Einstein radius, lensing optical depth, and time delays. Notably, we find that the Hu-Sawicki $f(R)$ and nDGP models yield significantly larger Einstein radii and higher lensing probabilities, especially at greater redshifts, indicating an increased likelihood of lensing events under modified gravity. Our analysis of time delays further shows that the broader mass distributions in these frameworks lead to pronounced differences in high-mass lens systems, providing potential observational markers of modified gravity. Additionally, we observe amplified magnification factors in wave optics regimes, highlighting the potential for gravitational wave (GW) lensing to differentiate modified gravity effects from GR predictions. Through these findings, we propose modified gravity theories as compelling alternatives to GR in explaining cosmic phenomena, with promising implications for future high-precision gravitational lensing surveys.

gr-qc↗

arXiv2024

Accretion Efficiency Evolution of Central Supermassive Black Holes in Quasars

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 $0.038 < ε< 0.42$. 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. The selection effect was removed from model construction by creating a flux- and volume-limited sample, as the range of values for estimating the accretion efficiency factor varied through different redshifts. In this study, we have focused on low redshift ($z < 0.5$) Palomar-Green quasars (79 quasars) and high redshift ($z \geq 3$) quasars with standard disks from the flux- and volume-limited QUOTAS+QuasarNET dataset (75 quasars) to establish a model for accretion efficiency. By considering the QUOTAS+QuasarNET+DL11 dataset, a peak can be seen around $z \sim 2.708$, and it seems to be related to the peak of the star formation rate ($1 < z_{SFR} < 3$). Consequently, the observed maximum and minimum values of accretion efficiency in standard disks, through the considered bond (3$σ$), display a significantly wider range than previously noted and differentiate over time. In redshifts higher than 2.708, the accretion efficiency shows patterns of increase as redshift decreases, while in redshifts lower than 2.708, accretion efficiency is seen to decrease with reducing redshift. 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.

astro-ph.GA↗

arXiv2024

Event Horizon Telescope observations exclude compact objects in baseline mimetic gravity

Mimetic gravity has gained significant appeal in cosmological contexts, but static spherically symmetric space-times within the baseline theory are highly non-trivial: the two natural solutions are a naked singularity and a black hole space-time obtained through an appropriate gluing procedure. We study the shadow properties of these two objects, finding both to be pathological. In particular, the naked singularity does not cast a shadow, whereas the black hole casts a shadow which is too small. We argue that the Event Horizon Telescope images of M87$^{\star}$ and Sgr A$^{\star}$ rule out the baseline version of mimetic gravity, preventing the theory from successfully accounting for the dark sector on cosmological scales. Our results highlight an interesting complementarity between black hole imaging observations and modified gravity theories of cosmological interest.

gr-qc↗

arXiv2024

Robust Preference for Dynamical Dark Energy in DESI BAO and SN Measurements

Recent Baryon Acoustic Oscillation (BAO) measurements released by DESI, when combined with Cosmic Microwave Background (CMB) data from Planck and two different samples of Type Ia supernovae (Pantheon-Plus and DESY5) reveal a preference for Dynamical Dark Energy (DDE) characterized by a present-day quintessence-like equation of state that crossed into the phantom regime in the past. A core ansatz for this result is assuming a linear Chevallier-Polarski-Linder (CPL) parameterization $w(a) = w_0 + w_a (1-a)$ to describe the evolution of the DE equation of state (EoS). In this paper, we test if and to what extent this assumption impacts the results. To prevent broadening uncertainties in cosmological parameter inference and facilitate direct comparison with the baseline CPL case, we focus on 4 alternative well-known models that, just like CPL, consist of only two free parameters: the present-day DE EoS ($w_0$) and a parameter quantifying its dynamical evolution ($w_a$). We demonstrate that the preference for DDE remains robust regardless of the parameterization: $w_0$ consistently remains in the quintessence regime, while $w_a$ consistently indicates a preference for a dynamical evolution towards the phantom regime. This tendency is significantly strengthened by DESY5 SN measurements. By comparing the best-fit $χ^2$ obtained within each DDE model, we notice that the linear CPL parameterization is not the best-fitting case. Among the models considered, the EoS proposed by Barboza and Alcaniz consistently leads to the most significant improvement.

astro-ph.CO↗

arXiv2024

Gravitational Lensing in More Realistic Dark Matter Halo Models

In this study, we explore gravitational lensing using more realistic dark matter halo models, moving beyond the limitations of spherical-collapse approximations. Through analytical calculations employing various mass functions, we address critical factors often neglected in the standard Press-Schechter formalism, such as 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 provide detailed calculations of key gravitational lensing observables, including Einstein radii, lensing optical depths, and time delays, across a broad range of redshifts and masses using two different lensing models: the point mass and singular isothermal sphere (SIS) models. Our results show that using more realistic dark matter halo models enhances lensing effects compared to their spherical-collapse counterparts. Additionally, our analyses of lensing optical depths and time delays reveal distinct differences between the point mass and SIS lens models. These findings underscore the importance of using realistic halo descriptions instead of simplified approximations when modeling gravitational lensing, as this approach can more accurately capture the complex structures of dark matter.

astro-ph.CO↗

arXiv2024

A note on the stability of the Cauchy horizon in regular black holes without mass inflation

Unlike generic models of regular black holes (BHs) with nonzero surface gravity on both Cauchy and event horizons, an inner-degenerate counterpart with zero Cauchy horizon surface gravity was recently proposed. For this regular BH solution with spherical symmetry, we examine the stability of the Cauchy horizon from both classic and quantum mechanics viewpoints. We find that the classical perturbations do not lead to mass inflation at the Cauchy horizon, indicating classically stability, while quantum fluctuations cause it to be unstable quantum mechanically.

gr-qc↗