Whether studying gravitational waves from extreme mass ratio inspirals or exploring the analogy between massless spin-particle waves, black hole perturbation theory proves indispensable. At the heart of developing a universal perturbation framework for such problems lies the challenge of formulating a coordinate-independent, unified wave equation that is universally applicable to any black hole spacetime.This paper resolves this central issue in type-D spacetimes by introducing a generating function H and establishing new definitions for the key spin coefficients. Specifically, the spin coefficients ρ, μ, τ, and π are redefined, respectively, as the directional derivatives of the logarithm of the generating function along the null tetrad (lμ , nμ , mμ , m̅μ ), and the field quantities are rescaled using H. It is thereby found that the field equations governing massless particles of spins 0, 1/2, 1, 3/2, and 2 in arbitrary type-D black hole spacetimes can all be described by a single, unified equation. This finding is particularly remarkable, as unifying these field equations is already a significant challenge in flat spacetime, let alone in the intricate spacetime around black holes.Consequently, this work will inevitably prompt a re-examination of the shared characteristics among various types of particles in black hole spacetimes. Meanwhile, we verify the correctness of the new definition for the spin coefficients, and provide the explicit form of the unified equation for nearly all known type-D black hole backgrounds. This lays a solid foundation not only for studying gravitational waves from extreme mass ratio inspirals but also for exploring the analogy between massless spin-particle waves in any type-D black hole background.

We present an exact static and spherically symmetric black hole configuration in the framework of symmetric teleparallel gravity, where non-metricity is coupled to an axion field. Starting from the modified field equations, we obtain analytical solutions characterized by the mass parameter M and an axion–geometry coupling constant α . The resulting metric exhibits a regular horizon structure with well-behaved curvature invariants. We analyze the thermodynamic quantities including the Hawking temperature, entropy, and heat capacity, and show that the system maintains thermodynamic consistency across the allowed parameter range. As for the analysis of the heat capacity we show that it remains negative for all admissible horizon radii, indicating that the black hole is thermodynamically unstable and does not undergo any phase transition. Furthermore, we study the motion of massive and massless test particles, identifying the conditions for circular orbits and the photon sphere. The axion–non-metricity coupling modifies both the effective potential and the shadow radius, producing observable deviations from the Schwarzschild limit. Using the photon-sphere/eikonal correspondence, the quasinormal mode (QNM) frequencies are computed, revealing that an increase in α enhances both the oscillation frequency and damping rate, ensuring linear stability. Overall, the axion-non-metricity interaction imprints coherent modifications on the black hole s thermodynamics, shadow, and ringdown spectrum, offering potential signatures for testing deviations from general relativity.

Low-frequency and broadband sound absorption by nested Helmholtz Resonator based on Sonic Black Holes

Matter accretion and greybody factor of charged bumblebee gravity black hole

In light of increasing observational evidence supporting the existence of ultra-compact objects, we adopt the term astrophysical black hole to refer to any object having a huge mass confined within a sufficiently small region of spacetime. This terminology encompasses both the classical black hole solutions predicted by general relativity, as well as alternative compact objects that may not possess an event horizon. We propose models of Astrophysical Black holes (ABHs) without event horizons (EHs), as a more viable explanation for the long-term quenching phenomenon in galaxies. At the same time, the short-term quenching is explained here in terms of an efficient feedback expected in the models of stellar-mass astrophysical black holes (StMABHs). We have calculated the radiative flux from the disk in a general spherically symmetric metric background and used it to contrast the distinctive features of the BHs and ABHs scenarios. We demonstrate the relative ease of wind generation from the accretion disk surrounding an ABH without an event horizon, compared to a BH, and highlight the significant strength of these winds. The nature of the feedbacks arising from accretion onto a BH and an ABH in the 'quasar' and 'radio' modes are compared and some possible observational signatures of the StMABHs are pointed out.

Quasistationary state gravitationally bound to Lense-Thirring Bumblebee-ModMax black hole

Analysis of Kiselev black hole in f(τ, T) gravity through quasinormal modes, greybody factors and thermodynamic quantities

A rationally designed intramolecular aptamer switch for rapidly sensing oxytetracycline.

Metafluid models for higher-dimensional sonic black holes

We study massless scalar, electromagnetic, and Dirac perturbations of the exact asymptotically flat regular black hole supported by a phantom Dirac-Born-Infeld scalar. Using the Padé-improved WKB method, with a time-domain Prony check for the scalar fundamental mode, we compute representative quasinormal frequencies and find that larger regularity shifts the spectrum toward smaller oscillation frequencies and damping rates, whereas the quality factor changes only weakly. The spectral shifts remain well above the estimated numerical uncertainty, demonstrating that the DBI regularity scale leaves a robust spin-dependent imprint on ringdown.

Thermodynamics and shadows of Kerr black holes endowed with a global monopole charge

Dirac quasinormal modes of charged black holes in f(R, T) gravity with nonlinear electrodynamics

The generation of broadband elastic Airy beam based on partitioned acoustic black hole metasurface

Thermodynamics and phase transitions of charged-AdS black holes in dRGT massive gravity with nonlinear electrodynamics

Noether charges and the first law of thermodynamics for multifractional Schwarzschild black hole in the q-derivative theory

Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanohertz GW Universe. We explore the physics of GW signals that may explain the data, with a focus on GW backgrounds (GWBs) considering both astrophysical and cosmological origins. We describe how: ▪ An astrophysical nanohertz GWB emerges as the superposition of individual signals from inspiraling massive black hole binaries. ▪ Environment coupling, eccentricity, and sparse sampling cause great uncertainty in the theoretical prediction of the supermassive black hole signal but also offer a way to determine the origin of the signal. ▪ PTA data offer unprecedented opportunities to constrain high-energy physics beyond the Standard Model by probing early Universe GWBs that originated during or after inflation. ▪ Different early Universe GWBs, typically created by nonlinear and out-of-equilibrium dynamics, can explain the PTA data (e.g., those from inflation scenarios, first-order phase transitions, or topological defects). ▪ The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics (probing, e.g., the equation of state of the early Universe, the origin of cosmological perturbations, the nature of dark matter, or whether exotic objects like primordial black holes or cosmic strings exist).

A Black Hole Star at Cosmic Noon: Extreme Balmer Break, Photospheric Continuum, and Broad Absorption by Thick Winds in a Little Red Dot at z = 1.7

The Complex Kinematics of the Young Stars Orbiting the Supermassive Black Hole in the Galactic Center Can Be Explained by the Presence of an Intermediate-mass Companion of Sgr A*

Dynamical Evidence for a Billion Solar-mass Black Hole in Galaxy NGC 4061 from ALMA 12CO(2–1) Kinematics

The horizon of a black hole, the 'surface of no return', is characterized by its rotation frequency ΩH and surface gravity κ. A striking signature is that any infalling object appears to orbit at ΩH owing to frame dragging, while its emitted signals decay exponentially at a rate set by κ as a consequence of gravitational redshift. Recent theoretical work1 predicts that gravitational waves from binary black-hole mergers carry direct imprints of the properties of the merger remnant in the form of a 'direct wave'. This gravitational-wave component oscillates near 2ΩH, reflecting the horizon's frame dragging, and decays at an increasing rate characterized by κ, with additional screening from the black hole's spacetime. Here we report observational evidence of a direct wave in GW2501142, with a 90% credible matched-filter signal-to-noise ratio of ( ) in the LIGO Hanford (Livingston) detector. The measured properties are in full agreement with theoretical predictions for a Kerr black hole. These findings establish an observational channel to directly measure frame-dragging effects in black-hole ergospheres and explore (near-)horizon physics in dynamical, strong-gravity regimes.