We investigate the quasinormal modes (QNMs) and greybody factors of dilaton-Euler-Heisenberg (dEH) de Sitter (dS) black holes in string-inspired Euler-Heisenberg gravity. Since the axial gravitational and electromagnetic perturbations decouple, we treat them independently. By applying the asymptotic iteration method (AIM) alongside a sixth-order WKB approximation, we compute the quasinormal frequencies and find excellent agreement between the two approaches. We also find that the QNM spectra depend sensitively on the magnetic charge Q_m, cosmological constant Λ, and nonlinear coupling ε, with a notable topological anomaly appearing in the electromagnetic frequency trajectories. Additionally, larger values of Q_m or the multipole number l generally suppress wave transmission, while the electromagnetic sector with ε=1 exhibits an anomalous response.
Abstract For the static and spherically symmetric case, we study the global consistency of the inverse reconstruction of generalized Rastall theory, G αβ +λH(R)g αβ = κ T αβ , when the geometry is fixed before the generalized Rastall function H(R). The analysis addresses the existence of a global inverse r=r(R) and the compatibility among the different radial branches when several radii produce the same value of the Ricci scalar curvature R. To illustrate the relevance of the analysis, we consider the regular Fan--Wang family with vanishing independent Schwarzschild mass. It is shown that R(r) is globally invertible if and only if μ=3 and 0<ν ≦ 1. The case ν=1 admits the elementary inverse r(R)=(24M F q 2 /R) 1/5 -q, whereas for 0<ν<1 the inverse is unique even though, in general, it cannot be expressed explicitly. It is also shown that the Hayward geometry employed by Lin and Qian and identified by (μ,ν)=(3,3) possesses two physical inverses over R min < R<0; the functions reconstructed on the two branches differ for ω ≦ 1/3, so the reconstruction is multivalued. For ω ≠ -1,1/3, a global single-valued function H(R) exists throughout the regular Fan--Wang family if and only if μ=3 and 0<ν ≦ 1. In this sector, H ∈ C 1 \ C 2 for 0<ν<1 and H ∈ C 0 \ C 1 for ν=1.
Abstract Precise measurements of high-energy cosmic-ray electrons and positrons have revealed spectral features that are difficult to reproduce with a single smooth power-law background. The rising positron fraction measured by PAMELA and AMS-02, together with the all-electron excess reported by ATIC and the high-precision all-electron spectrum measured by DAMPE, has motivated interpretations ranging from nearby astrophysical accelerators to dark-matter annihilation or decay. In this work, we revisit the conventional diffuse electron background and the possible contribution from nearby pulsars within a common propagation framework. The diffuse component is modeled using GALPROP configurations calibrated against cosmic-ray nuclei and diffuse gamma-ray observations. We then use the Green-function solution for nearby discrete sources with radiative losses to study pulsar contributions with both burst-like and continuous injection histories, including the effect of stochastic inverse-Compton cooling on the propagated spectra. We also use the highest-energy DAMPE data points as an illustrative case to compare possible local-source contributions from pulsars and supernova-remnant-like burst sources. The spectral shape of such features provides a useful diagnostic for distinguishing physically plausible nearby-source features from more exotic interpretations.
Abstract We investigate the dynamics of test particles in a Hayward–AdS black hole with string fluid, with particular emphasis on how the relevant parameters influence particle motion. The stability of circular orbits is studied using the effective potential method, which yields analytic expressions for the particle energy and angular momentum in terms of the black hole parameters. The effects of these parameters on the innermost stable circular orbit and the effective force are also examined. In addition, we calculate the pericenter precession frequency and investigate the small oscillatory motion of a particle around a stable orbit, examining how the corresponding frequency is modified by the black hole parameters. We further compare particle dynamics in the Hayward–AdS spacetime with string fluid to those in the Hayward–AdS, the Hayward Reissner Nordstrom, and the conventional Hayward black holes. Lastly, we examine the influence of the string fluid and the Hayward parameter on the quasinormal spectrum and the time-domain dynamics of electromagnetic perturbations, and find that these two terms have reciprocal effects on the quasinormal scale.
Abstract The main objective of this manuscript is to explore the viable cosmological bounce solutions in the background of gravity, where Q denotes the non-metricity scalar describing the curvature of spacetime, represents the matter-Lagrangian density, and T is the trace of the energy-momentum tensor. For this purpose, we consider an isotropic and homogeneous universe with bulk viscous fluid to examine the implications of this framework on the dynamics of the early universe. The central focus of this work is to uncover the role of bulk viscosity on the cosmic evolution, especially in non-singular bounces. Furthermore, we investigate the behavior of various cosmological parameters to identify the viable bounce solutions. We find that the positive energy density, negative pressure profile and violation of energy conditions (ECs) support the realization of a cosmological bounce. These results emphasize the importance of bulk viscosity in shaping the cosmic dynamics. We demonstrate that system's entropy evolution satisfies the second law of thermodynamics, confirming its thermodynamic consistency over time. Moreover, the examination of cosmographic parameters and adiabatic sound speed demonstrates the model's kinematic feasibility and stability. Our findings imply that gravity stands as a consistent and alternative theory for standard cosmology, providing a significant theoretical framework to study the gravitational interactions. This modified gravitational theory (MGT) successfully accommodates important aspect of cosmic evolutions and offers a new perspective on geometric and coupling effects that may govern the dynamics of the universe beyond the predictions of general relativity.
