Dark matter near a black hole and effective extra-dimensional corrections can change the same horizon-scale observables. This creates a simple but important question: if an image differs from Kerr, what caused the difference? We study this problem with DARK-HIDE. The dark-matter branch is described by rotating metrics with radial mass functions, while the hidden-dimensional branch is a rotating braneworld metric with a non-electromagnetic tidal charge. We compare photon regions, critical curves, controlled image morphology, a shadow-size likelihood calibrated to EHT results, and local ZAMO escape cones. A strong negative tidal charge is easy to separate from Kerr and from the two benchmark dark-matter profiles. The difficult case appears after the tidal charge is continuously adjusted to mimic the dark-matter critical curve and image proxy. At $\varepsilon/M=0.025$, the best $P+I$ mimics occur at $q/M^2=-0.01917$ for Einasto and $-0.01117$ for cored cNFW, with small standardized separations of $0.084$ and $0.051$. A ray-bundle caustic test does not pass the required convergence and topology checks, so it is excluded from inference. After marginalizing over spin and isotropic inclination, current EHT shadow-size constraints leave both dark-matter amplitudes prior dominated. They mildly suppress large negative tidal charge, but remain fully compatible with $q=0$. Local escape cones retain a small, smooth, and well-resolved difference between the matched branches. Thus, present shadow size alone cannot break the DARK-HIDE degeneracy, while local photon transport keeps additional strong-field information. less

Reheating after inflation is one of the strongest sources of gravitational waves (GW), producing a stochastic background (SGWB) with a non-thermal spectrum peaked at frequencies of order a few GHz. Detecting it is difficult: experiments based on the inverse Gertsenshtein effect in intense magnetic fields reach the MHz but not the GHz band, where the typical strain is around $10^{-30}$. The same window contains the coalescence of light primordial black hole (PBH) binaries, whose merger frequency $f\simeq4.4\ \mathrm{kHz}\,(M_\odot/M)$ falls in the MHz--GHz range for planetary to sub-planetary masses; since such objects are necessarily sub-solar, their detection would be strong evidence for PBHs as a component of the dark matter. We propose a solid-state detector at GHz frequencies that could integrate over months to years the GW continuously arriving from the Big Bang and search for light PBH binary coalescence. As a concrete realization we consider a modular array of $\sim10^3$ ultra-pure sapphire $(10\ \mathrm{cm})^3$ monocrystals forming a cubic-metre detector read out by cryogenic single-phonon sensors, whose segmentation provides thermal isolation, favourable counting statistics and coincidence-based background rejection. We also compare candidate materials, finding diamond superior per unit volume but limited by the unavailability of large single crystals. Finally, we contrast the two targets. The stationary background is a shot-noise-limited counting problem, best served by a narrow, resonance-enhanced, long-integration search; the loud transient chirp of a nearby merger is better caught by a fast, broad-band search with coincidence tagging. Because the phonon spectrum is continuous, a modular solid-state array can serve both, by staggering resonant cells across the band while running a broad-band subset for chirp tracking. less

With current advances in gravitational wave (GW) detection made by the worldwide LIGO-Virgo-KAGRA (LVK) network of detectors, ever-more sensitive tests of gravity in the strong-field regime are now possible. This enables one to test gauge theories beyond Einstein-Hilbert action, such as Weyl gauge theories of gravity. The only anomaly-free (quantum) gauge theory of a space-time symmetry beyond Poincaré is based on Weyl gauge group (of dilatations and Poincaré symmetry) with Weyl conformal geometry as its natural underlying geometry. This gauge theory has spontaneous breaking of Weyl gauge symmetry to Einstein-Hilbert and Proca actions, plus a positive cosmological constant. We investigate the GW polarisation modes of Weyl (quadratic) gauge theory of gravity in Weyl geometry and compare our findings to the most recent experimental data. We show how the geodesic deviation equation from Riemannian geometry translates to Weyl geometry, and explain why it is crucial to perform the analysis around de Sitter background, which is the correct low-energy limit of Weyl quadratic gravity, to not alter the GW content, and then compute the polarisation modes. In addition to the two transverse-traceless tensor modes predicted by Einstein-Hilbert action, we find two additional vector modes induced by the transverse fluctuations of the Weyl gauge field. If detected, these vectors modes would be important evidence for Weyl gauge symmetry. less

The ringdown gravitational wave from a binary black hole (BBH) merger is a superposition of quasi-normal modes (QNMs) of the remnant black hole. In general relativity (GR), QNMs are damped harmonic oscillations with frequencies and damping times uniquely determined by the remnant's mass and spin. The measurement of the ringdown modes and performing black hole spectroscopy provides a tool to test the validity of GR. We present a ringdown analysis based on reconstruction of GW signals with coherent WaveBurst (cWB). This method yields tighter constraints on the QNM frequency and damping time than previous measurements. The improved precision results from the noise reduction achieved by the cWB reconstruction and the enhanced ringdown analysis, which probes the remnant properties at earlier times, closer to the merger. We have analyzed publicly available binary black hole (BBH) detections from the third Gravitational-Wave Transient Catalog (GWTC-3). For all events considered, the measured frequency and damping time of the dominant $(l,m)=(2,2)$ mode are found to be consistent with the predictions of GR. A combined analysis further strengthens these constraints, yielding fractional deviations in frequency $δf_{220} = 0.003_{-0.028}^{+0.028}$ and damping time $δτ_{220} = 0.050_{-0.086}^{+0.081}$, consistent with zero within the quoted uncertainties. less

Space-based gravitational-wave (GW) detectors, including LISA and Taiji, offer unprecedented access to regimes where alternative theories of gravity may deviate from General Relativity (GR). Gravitational Quantum Field Theory (GQFT) provides a novel framework in which the Poincaré-type inhomogeneous spin symmetry of Weyl-type fermions in the Standard Model is elevated to a gauge symmetry. Within this construction, the fundamental gravitational field is identified with a gravigauge field which behaves as a Goldstone-type bi-covariant vector field. Unlike GR, GQFT predicts additional polarization states: one transverse scalar (breathing) mode and two vector modes. In this work, we focus on the transverse, isotropic scalar mode and investigate its detectability with Taiji. To isolate this mode, we employ the null-response channel (NRC), a specific interferometric combination designed to suppress contributions from other polarizations. We implement an analytical, dynamic orbital model to realistically simulate a triangular constellation. We compute the response functions and sensitivity curves for various interferometric channels, compare them with the standard Michelson channel, and demonstrate the effectiveness of the NRC approach. Our results show that the NRC provides a reliable, waveform-independent criterion for testing non-GR polarizations, and we anticipate that it will serve as a valuable tool for probing gravitational theories in future space-based GW missions. less

The classical formation of a Schwarzschild black hole from a regular, non-singular configuration has recently been shown to be impossible within general relativity: the geometry inevitably develops a discontinuity at the origin, a phenomenon termed Minkowski breaking by Ovalle, Casadio, and Kamenshchik [PRD 113 (2026), 064042]. This obstruction signals that the transition to the Schwarzschild point mass must be a discrete, quantum event. We uncover the thermodynamic footprint of this transition. Using the explicit family of regular Schwarzschild black holes with a de Sitter core, we show that the inner Killing horizon carries a formal Bekenstein-Hawking entropy $S_{\rm inner} = A_{\rm inner}/4$ that is absent in the singular Schwarzschild state. This entropy is hidden from external observers in equilibrium but, assuming the generalized second law, must be released when the inner horizon disappears. As the collapse parameter $n$ decreases, the inner horizon shrinks and its entropy is gradually released during classical evolution, until the horizon finally vanishes at $n=0$ with the Minkowski breaking. The surface gravity diverges as $n\to0^+$, with the semiclassical description breaking down at $n \sim 1/\ln(h/\ell_P)$; the final disappearance is therefore a deep quantum process. For the $n=3$ regular black hole, the stored entropy is approximately $59\%$ of $A/4$; in the semiclassical limit $n\gg1$, it approaches the full $A/4$. The integer nature of $n$ implies a quantized entropy spectrum, with the Schwarzschild black hole as the ground state within the OCK family. We discuss how the classical mass-inflation instability may be circumvented by the quantum disappearance of the Cauchy horizon, and clarify the continuous vs. discrete nature of the collapse. less

Gravitational waves affect the propagation of electromagnetic waves in laser cavities, modulating the frequency of emitted photons. We use this effect to search for high-frequency gravitational waves between 100 kHz and 100 MHz using optical precision spectroscopy. Our limits constrain much of this frequency range for the first time. We discuss future improvements of the technique, which we expect to enhance the sensitivity by eight orders of magnitude, and to extend the frequency coverage up to at least 1 GHz. less

Gravitational waves affect the propagation of electromagnetic waves in laser cavities, modulating the frequency of emitted photons. We use this effect to search for high-frequency gravitational waves between 100 kHz and 100 MHz using optical precision spectroscopy. Our limits constrain much of this frequency range for the first time. We discuss future improvements of the technique, which we expect to enhance the sensitivity by eight orders of magnitude, and to extend the frequency coverage up to at least 1 GHz. less