The late-time power law tail predicted by Price's law is a generic feature of black hole perturbation theory, yet it is largely absent in numerical relativity waveforms of binary black hole mergers. We show that this suppression arises from the spectral structure of oscillatory sources. For a generic perturbation with carrier frequency $ν$ and characteristic width $σ$, the branch-cut excitation coefficient governing the tail is suppressed by $α=σν$. For a Gaussian pulse, the suppression $\sim e^{-α^2/2}$. This suppression is exact and confirmed by the time domain Regge Wheeler evolutions. The same parameter that controls the transition from broadband to frequency selective black hole response is also responsible for the tail suppression. Moreover, we analytically derive the leading- and next-to-leading-order tail coefficients, finding agreement with numerical fits below the $\sim10\%$ level. Our results provide a first principle explanation for the absence of tails in quasi-circular mergers and their enhancement in head-on and eccentric ones. less

Black holes (BHs) can be studied via fundamentally different observational channels that probe complementary aspects of their physics. While BH imaging provides access to the quasi-static space-time geometry via the strong bending of light rays, gravitational wave (GW) observations probe the dynamical response of the space-time to time-dependent processes in the inspiral, merger and ringdown phases. Both messengers -- electromagnetic imaging probes and ringdown GW spectroscopy --, provide access to essentially the same region -- the one between the BH event horizon and the photon region --, but they do it via conceptually different methods, encoding different physical information. However, it has been shown in the literature that physical quantities supposedly exclusive of each such messenger are actually tightly related to each another via a correspondence that occurs in the eikonal limit (i.e. large values of the multipole number $\ell$) of the geometric-optics approximation. In this paper we clarify the actual identification of observables within such a correspondence and test its accuracy for a bunch of modified spherically symmetric BH geometries proposed in the literature. We find that even for low values of $\ell$ the correspondence is surprisingly accurate in relating the real and imaginary parts of quasi-normal modes in the GW ringdown phase with the critical impact parameter and Lyapunov exponent of nearly-bound light trajectories for every such model analyzed. We discuss the applicability of such a result both for each messenger individually, and also for foreseeable tests of BHs combining both messengers. less

Although cosmic acceleration is well established, its physical origin remains contentious. Most explanations invoke either a non-zero vacuum energy, i.e. a cosmological constant, or new fields. We propose instead a mechanism arising purely from quantum mechanics, without additional constants or local sources. The key point is that quantum theory permits both initial and final boundary conditions on a state, here the wave function of the universe. Pre- and post-selected systems are familiar in laboratory quantum mechanics; we extend the idea to minisuperspace quantum cosmology. As a warm-up, we show that a free non-relativistic particle, initially in a semiclassical wave packet and conditioned on a final quantum state, can have an intermediate peak which accelerates even though the Hamiltonian is free. A semiclassical observer would infer a contrived classical force. We then implement the analogous construction in quantum cosmology using connection variables and unimodular time. A forward semiclassical packet, taken for simplicity to describe pure radiation, is post-selected by a normalizable Chern-Simons soliton. The resulting two-boundary amplitude has a peak which leaves the radiation trajectory and enters an accelerating regime, while the forward Hamiltonian has $Λ=0$. A classical model can mimic this trajectory only by introducing a contrived effective component: near the transition it resembles $w\simeq -1$, but when extrapolated it evolves towards strongly phantom behaviour, $w<-1$. The acceleration is therefore more naturally interpreted as a quantum boundary-condition effect than as a local classical source. We also discuss why the Chern-Simons soliton is a clean final state, the small overlap between initial and final states, and possible tell-tale signatures. less

Much like the aerodynamic deflection of a spinning curveball, a rotating secondary in an extreme-mass-ratio inspiral (EMRI) experiences Magnus and lift forces, in addition to the standard drag force, when traversing a gaseous environment. We present the first framework that incorporates these specific spin-coupled environmental effects (EEs) into the evolution of EMRI. Over the multi-year observation windows of space-based gravitational-wave (GW) detectors, these interactions imprint a unique, distinguishable dephasing signature on the signal. Crucially, a Fisher matrix analysis reveals that gas drag breaks the fundamental vacuum-projection degeneracy between the secondary's spin magnitude and inclination, thereby tightening parameter constraints. Thus, accounting for EEs is not merely a modeling necessity, but a powerful tool for enhancing the detectability of the secondary's intrinsic spin, and could serve as a novel probe of accretion flows harboring massive black holes. less

In this letter, we establish a complete correspondence between the Newman--Penrose and 1+1+2 semitetrad covariant formalisms by expressing all Newman--Penrose spin coefficients, Ricci scalars, and Weyl scalars in terms of the scalar, vector, and tensor variables of the 1+1+2 decomposition. This provides a direct dictionary between two widely used approaches to general relativity and gives a geometrical interpretation of Newman--Penrose quantities in terms of covariantly defined 1+1+2 variables. As an application, we derive necessary conditions for the existence of future outer trapping horizons in locally rotationally symmetric class II spacetimes, expressed in terms of the Ricci and Weyl Newman--Penrose scalars and the cosmological constant. less

We study the primary constraint structure of Newer General Relativity, a gravity theory based on a torsionless teleparallel geometry. The gravitational action is built from a scalar formed by quadratic combinations of the nonmetricity tensor, with arbitrary coefficients $c_i$ in the Lagrangian. We decompose the Lagrangian and compute the canonical momenta conjugate to the metric. We characterize the primary constraints arising from these momenta by identifying when the map between velocities and momenta becomes non-invertible, and organize the outcome through a fully nonlinear decomposition into scalar, vector and tensor sectors. Comparing with previous results in the literature, we recover five and three primary constraints associated with the tensor and vector sectors, respectively. We also identify a previously unreported degeneracy in the scalar sector, which yields either one or two scalar primary constraints depending on the conditions imposed on the parameters $c_i$. Finally, we obtain the primary constraints associated with the covariant formulation of symmetric teleparallel gravity. less

We develop a theoretical framework to describe the full interaction between vector vortex light fields and gravitational waves (GWs). Using perturbation theory and the canonical quantization of the electromagnetic field, we calculate the quantum transition probabilities of vector Bessel beams propagating through GWs. We demonstrate that GWs induce fourteen different quantum transition channels across orbital angular momentum (OAM) $l$ and spin angular momentum (SAM) $σ$, mapping initial states $\ket{σ,l}$ to $\ket{σ+Δσ,l+j-Δσ}$, where $Δσ\in \{-2, 0, 2\}$ represents the change in SAM and $j \in \{-3, \dots, 3\}$ denotes the change in total angular momentum. Among these channels, SAM-conserving transitions between OAM states, specifically $\ket{σ, l}\rightarrow \ket{σ, l\pm 1}$, provide the most viable mechanism for experimental detection. Conversely, spin-flip transitions are shown to be heavily suppressed relative to OAM transitions. Additionally, the reversal of SAM induces an asymmetric shift in the OAM transition channels, reflecting the underlying coupling between SAM and OAM during the gravitational interaction. Based on these transition channels, we propose a new cavity-based GW detection configuration. By relying on quantum transitions rather than macroscopic arm-length changes, this scheme is inherently insensitive to displacement-based disturbances like seismic noise, offering a new paradigm and frequency bands for GW observation. less

We investigate the geodesic structure and optical appearance of compact objects with primary scalar hair in shift- and parity-symmetric beyond Horndeski gravity. The analytic solution considered here depends on a theory parameter and a dimensionless mass parameter \cite{Bakopoulos:2023sdm}. For a fixed theory parameter, varying the mass traces a family of static spacetimes that can interpolate between timelike naked singularities, regular solitons, regular black holes, Reissner-Nordström-like black holes, multi-horizon black holes, and Schwarzschild-like black holes. We classify these branches by their horizon structure and analyze null and timelike geodesics, focusing on light rings, innermost stable circular orbits, and static spheres. We then compute thin-disk optical images by ray tracing. We find that the number of horizons is not directly encoded in the image: horizonless objects can show shadow-like central depressions, while multi-horizon black holes can closely resemble single-horizon black holes when their exterior light ring and disk structures are similar. Thus, the optical appearance is governed mainly by the photon potential and the disk inner edge, with the deeper horizon structure leaving only an indirect imprint. Quantitative radial-profile diagnostics confirm that the degeneracy is mainly morphological: the profiles differ at fixed impact parameter, but become much closer after rescaling by the critical impact parameter. These results provide a concrete example of how distinct compact object branches in beyond Horndeski gravity can share similar observational signatures. less

We study black hole (BH) formation from the nonlinear growth and collapse of primordial perturbations during the matter-dominated era. Modelling cold dark matter (CDM) as pressureless dust, we describe the collapse in a fully nonlinear relativistic framework using the Lemaître-Tolman-Bondi (LTB) and quasi-spherical Szekeres solutions as exact perturbations of a spatially-flat Friedmann-Lemaître-Robertson-Walker (FLRW) $Λ$CDM background. At first order in relativistic scalar perturbation theory, the growing mode of any relevant quantity can be expressed in terms of the conserved gauge-invariant curvature perturbation $\mathcal{R}_c$, which acts as a potential for the 3-curvature of hypersurfaces orthogonal to the matter 4-velocity. We use this result to express the active gravitational mass and curvature functions of the LTB and Szekeres models in terms of the initial values of $\mathcal{R}_c$ and its spatial derivatives. From these initial curvature data we derive: (i) the turn-around, collapse, and apparent-horizon formation times, and (ii) the regularity conditions required for BH formation. We show that sinusoidal and Gaussian profiles do not provide viable BH-forming channels, whereas broad compensated curvature peaks, naturally predicted by peak theory, do. We then estimate the formation times of $10^{3}-10^{6}~\mathrm{M}_\odot$ massive BH seeds produced by the direct collapse of primordial CDM curvature peaks, finding full BH formation at redshifts $z>5$, with core collapse beginning at $10 \lesssim z \lesssim 16$. Finally, we characterize the local dynamics and singularity type of the collapse (point-like, cigar-like, or pancake-like) directly from the initial comoving curvature data, clarifying the role of the initial shear in selecting the collapse end-state. less

We extend the frequency-domain analysis of quasinormal modes in a dynamical, spherically symmetric black hole spacetime undergoing constant-rate mass evolution. In particular, we report a novel feature of the spectrum: the presence of purely imaginary eigenvalues in addition to the usual light-ring modes. We study the frequencies of these modes both analytically and numerically. The analytical calculation uses a novel formalism based on recent advances in connection coefficients of Heun functions. We then compute the frequencies numerically using a spectral method on hyperboloidal slices and find excellent agreement between the two approaches. Finally, we validate the frequency-domain results against an independent set of time-domain simulations. Our analysis shows that the purely imaginary modes govern the late-time signal through exponentially decaying tails. In the Schwarzschild limit, both frequency- and time-domain studies consistently show that the purely imaginary modes give rise to the familiar Schwarzschild power-law tail. less