Papers

This is the fourth paper of a series in which we consider the possibility to use cosmological extra dimensions to explain the accelerated expansion of the Universe and the rotation curve of spiral galaxies without introducing dark matter and dark energy. Here we study gravitational lensing in this setting; we derive an expression for time delay and present a modified likelihood for the mass reconstruction procedure which takes into account the effect of the expansion or contraction of the extra dimensions.

In the last decade, kilometre-scale interferometric gravitational-wave detectors have observed hundreds of compact binary mergers, the majority of which are binary black holes. However, the data are noise-dominated, and multiple independent search algorithms (pipelines) are used to enhance sensitivity and improve robustness. Rather than the standard approach of selecting the most significant pipeline output, we combine the outputs from all pipelines using a conformal prediction-based framework to provide statistically rigorous confidence estimates for candidate events. While combining pipelines improves sensitivity and ranking robustness, it requires a principled statistical framework that remains valid as data properties evolve across observing runs. A key challenge is distribution shifts between simulated datasets used for training and calibration and the real, unlabelled, observations used for testing, which can invalidate coverage guarantees and bias confidence estimates. In this work, we address this challenge by incorporating likelihood-ratio reweighting into our conformal prediction framework to account for covariate shift. Using mock datasets containing simulated signals, we demonstrate that weighted conformal prediction restores well-calibrated coverage under covariate shift and increases the confidence of events near the detection threshold, recovering true signals that would otherwise be missed.

We realize a nearly pure higgsino dark matter candidate with mass of about 1 TeV in the Minimal Supersymmetric Standard Model coupled to the Starobinsky supergravity. The recent LUX-ZEPLIN 248 keV nuclear-recoil event has renewed interest in higgsino dark matter. In this framework, the particle spectrum, including the Higgs boson mass, the higgsino mass and its splitting, is connected to cosmic inflation observables via gravitational mediation of supersymmetry breaking and subsequent renormalization group evolution. We show that the Higgs boson mass predicted by the model agrees with the measured value within the quoted uncertainties for a broad range of bino-induced mass splittings. Wino-induced splitting leads to a Higgs mass several GeV higher and is therefore excluded.

We investigate slow-roll inflation in a modified cosmological framework inspired by a generalized mass-to-horizon relation (MHR), $M=\gamma {c^2 L^n}/{G}$, where $n$ is a real parameter and $\gamma$ a dimensional constant. Using Padmanabhan's emergence paradigm, we derive the modified Friedmann equations for a flat FRW universe and analyze the dynamics of a canonical scalar field (inflaton) under the slow-roll approximation. We study the resulting inflationary phenomenology for power-law and Starobinsky potentials. For power-law potentials, the MHR modification fails to reconcile these models with current CMB constraints on $r$ and $n_s$. In contrast, Starobinsky inflation exhibits significant sensitivity to deviations from $n=1$. A perturbative analysis ($n=1+\Delta$) yields corrections to inflationary observables. We observe that the scalar power-spectrum normalization, under a fixed-Starobinsky prescription, imposes the stringent constraint $0.960 \lesssim n \lesssim 1.040$ for $N=60$ efolds. This is considerably tighter than spectral-index bounds. Our results establish inflation, particularly Starobinsky-like models, as a sensitive probe of generalized horizon thermodynamics and departures from standard MHR scaling.

We generalize the quasi-isotropic solution of the Einstein equations near a cosmological initial singularity to models with a varying speed of light and/or a varying gravitational 'constant'. We construct a formal asymptotic series expansion in a synchronous reference system, taking into account the additional degrees of freedom introduced by the two varying 'constants'. By applying the Landau-Lifshitz quasi-isotropic method, we go beyond the standard results of previous studies that based on the Friedmann-Lemaître-Robertson-Walker (FLRW) model, providing a more general asymptotic analysis of varying speed of light cosmologies. We show that the resulting solutions do not contain the required number of arbitrary functions to qualify as general solutions of these theories.

We prove the black hole uniqueness conjecture in the axially symmetric, stationary, electrovacuum setting, subject to the refined asymptotic analysis of the associated singular harmonic maps, which includes an analyticity hypothesis at the axes. More precisely, it is shown that any asymptotically flat solution of the Einstein–Maxwell equations in this class, with more than one black hole horizon component is either: Majumdar–Papapetrou, up to a duality rotation, in which case all logarithmic angle defects vanish, or every finite axis rod logarithmic angle defect is strictly negative and hence every interaction force is strictly attractive. The proof extends the singular harmonic map method used for vacuum Kerr uniqueness in [18].

The axion is a promising candidate for solving the strong CP problem. To solve this problem, the global U(1) symmetry must be preserved to a high degree of accuracy. However, it is well known that global symmetries are explicitly violated by quantum gravity effects, giving rise to what is referred to as the axion quality problem. In this paper, we investigate axionic wormholes as a source of explicit U(1) violation in Metric-Affine Gravity. This framework allows for spacetime torsion and non-metricity, which accommodate additional curvature-like and topological terms, such as the Holst and Nieh–Yan terms, that are absent from the metric and Palatini formalisms. We show that non-minimal couplings to these terms modify the wormhole dynamics and enhance the Euclidean wormhole action, thereby alleviating the axion quality problem. We also find that the viable parameter space is enlarged when two of these couplings are simultaneously present. We further identify representative parameter regions where the alleviation of the axion quality problem is compatible with inflationary constraints.

Primordial black holes (PBHs) are a well-motivated dark matter candidate, and their cosmic abundance is constrained by a variety of observational probes. PBHs in the mass range $10^{15}\,\text{g}\,{-}\,10^{17}\,\text{g}$ are evaporating today via Hawking radiation, a process that can heat interstellar dust and modify its thermal emission. Recent studies have used this effect to place constraints on the abundance of non-spinning PBHs. We extend this approach by investigating the influence of PBH spin on dust-heating constraints. Furthermore, we account for secondary photons that originate not only from the decay of gauge bosons but also from the decay of hadrons produced via the fragmentation of primary quarks and gluons emitted through Hawking radiation. By comparing the dust heating rate induced by spinning PBHs with the maximum cooling rate of dust, considering both silicate and graphite grains, we derive new upper limits on the fraction of dark matter in the form of PBHs, $f_{\rm PBH}$. Our results show that the constraints depend on both PBH mass and spin. Smaller PBHs with higher spin yield stronger limits. For example, in the cases we investigated, the strongest constraint is $f_{\rm PBH} \sim 1.5 \times 10^{-4}$ for $M_{\rm PBH} = 10^{15}{\rm g}$ and spin parameter $a_{*} = 0.9999$. Although these limits are less stringent than existing constraints in the same mass range, they provide a distinct and complementary approach to constraining the abundance of PBHs.

We construct a symmetric thin-shell wormhole from two copies of the positive-lapse exterior of a rationally regularized Letelier–Alencar string-cloud black hole with a finite-curvature anti–de Sitter core and string-cloud asymptotics. Using the Darmois–Israel formalism, we derive the surface stresses, energy-condition combinations, conservation law, and effective potential. Every admissible static throat has negative surface energy density, so the weak and dominant energy conditions are violated, whereas the null condition restricted to directions tangent to the shell, and consequently the intrinsic strong condition, holds only in the inner sampled exterior. The radial-vacuum relation of the bulk source removes the external momentum flux. We derive a general local-stability criterion for density- and radius-dependent equations of state and apply it to five surface-fluid models, calibrating one parameter at each static throat. Across the sampled families, barotropic shells remain unstable, whereas the other closures can admit parameter-dependent stable branches. An exact unregularized control shows that the energy-condition structure and the radial-stability signs of the first three closures persist without the rational regulator, whereas the cosmic-Chaplygin stability sign can change while the selected calibration branch remains real and simple. Backward ray tracing shows that throat location primarily reorganizes higher-order and through-throat images, while string-cloud strength and inclination control annular structure and Doppler asymmetry. Comparisons of one- and two-sided source configurations isolate genuine far-side images and show that the central dark region depends on the source distribution.

We develop a largely automatic semi-analytic method for calculating weakly damped quasi-bound states and trapped modes supported by a local minimum of an effective potential. The real part of the frequency is obtained from an arbitrarily high-order local WKB expansion and Padé resummation, while the exponentially small imaginary part is estimated using the Gamow approximation for tunnelling through one or two potential barriers. Because the local quantization requires only derivatives of the potential at its minimum, the method applies readily to non-rational effective potentials and to different compact-object geometries. We provide a Mathematica notebook implementing the procedure. Comparisons with continued-fraction and direct numerical results for massive scalar fields around Schwarzschild and Kerr black holes and for axial trapped modes of a uniform-density star show that the method yields accurate real frequencies and useful estimates of decay rates outside the superradiant regime.

We investigate the fate of asymptotic particle states in supersymmetric quantum field theories on de Sitter backgrounds, distinguishing algebraic field content from localized LSZ-like excitations. We study how long-wavelength gravitational fluctuations may suppress pole residues and redistribute spectral weight from isolated contributions toward continuum sectors. The analysis is formulated within a Kallen-Lehmann spectral framework and is intended as a structural infrared mechanism rather than a non-perturbative proof of de Sitter quantum gravity. The paper develops three connected levels. First, we analyze residue suppression in a quasi-de Sitter setting and its possible transmission through Yukawa and gravitational interactions. Second, we formulate the spectral decomposition as a map between the full field-theoretic spectrum and the observable particle sector. Third, we examine a conditional cosmological realization of redistributed spectral weight, including possible effects on expansion, structure growth, and lensing. This cosmological layer is a downstream phenomenological test and is not used as evidence that pole loss has occurred. In the current MCMC analysis we find S8 approximately 0.803 and, when the smooth fraction is varied, fS8 = 0.0025 +/- 0.0017, consistent with the benchmark value 0.00262. These results test the internal consistency of the proposed realization but do not establish the dark-sector assignment or the underlying infrared mechanism.

Anti-de Sitter (AdS) spacetimes are not globally hyperbolic, so boundary conditions are generally needed for well-defined dynamics. Ishibashi and Wald (IW) determined boundary conditions yielding well-posed dynamics with conserved positive energy for scalar, electromagnetic, and linearized gravitational fields, using nonlocal potentials and individual spherical harmonic modes. In this paper, we determine which IW conditions are local in the primary fields and which are AdS-invariant. In four spacetime dimensions, the standard AdS-invariant boundary conditions set the rescaled magnetic field to zero for electromagnetism and the rescaled magnetic Weyl tensor to zero for Einstein gravity on the conformal boundary. We generalize these standard boundary conditions by setting arbitrary linear combinations of the electric and magnetic fields, or the corresponding Weyl tensors, to zero, and we conjecture that these exhaust the local, AdS-invariant, conservative boundary conditions in AdS$_{1,3}$. Boundary conditions with non-AdS-invariance and in different spacetime dimensions are also studied. The AdS-invariant conditions in AdS$_{1,3}$ extend to Yang-Mills theory and nonlinear gravity in a general Fefferman-Graham setting. We construct phase spaces for these general boundary conditions. The covariant phase spaces for general conditions are naturally associated with Lagrangians containing Pontryagin terms and, in gravity, a Gauss-Bonnet term. In gravity, except for the standard boundary condition, no nontrivial asymptotic symmetries exist and all conserved charges such as ADM mass vanish identically, as in a closed universe. Spacetimes with Killing fields including AdS then exhibit linearization instability in the sense of Fischer and Marsden and Moncrief. For nonstandard boundary conditions, the modified symplectic structure yields new expressions for the entropy formula of black holes.

Bimetric theory is an extension of general relativity that perturbatively describes one massless and one massive graviton. It admits observationally viable cosmologies, passes local tests of gravity, and provides candidates for dynamical dark energy and spin-2 dark matter. Scattering amplitudes provide a complementary probe of its consistency, e.g. through analyticity, unitarity, and causality. While the tree-level $2\to2$ amplitudes of the closely related theory of massive gravity have previously been computed and analysed, the richer amplitude structure of bimetric theory has mostly been studied for selected processes and helicity sectors. We present the complete set of tree-level $2\to2$ scattering amplitudes of ghost-free bimetric theory around proportional Minkowski backgrounds. We expand the action through quartic order in the mass eigenstates and use a computer-algebra workflow to compute all of the $199$ symmetry-inequivalent helicity amplitudes. We find a clear hierarchy: amplitudes with a single massive external state vanish, those with exactly two are independent of the nonlinear bimetric parameters, and nonlinear ghost-free parameter dependence first appears with three massive external states. Taking the massive-gravity limit yields the complete set of tree-level $2\to2$ massive gravity amplitudes, which agree exactly with previous results. Finally, all amplitudes grow with energy at most as $E^6$, corresponding to the characteristic $\Lambda_3$ strong-coupling scale, and this maximal growth cannot be eliminated by any nontrivial choice of the theory parameters.

We investigate a quantum massive scalar field in the interior of a charged and spherically-symmetric (Reissner-Nordström) black hole. We examine the behaviour for varying values of the black hole charge, field mass and coupling constant when the field is in two quantum states: Hartle-Hawking (representing a black hole in thermal equilibrium) and Unruh (representing a black hole evaporating via the emission of Hawking radiation). We show that the vacuum polarization as well as the angular components of the quantum stress-energy tensor diverge on the Cauchy horizon, in stark contrast to what happens for massless fields. We also calculate the energy fluxes in Eddington-Finkelstein coordinates $\{u,v\}$. We show that these fluxes on the Cauchy horizon do not generically vanish. This implies, in particular, that in regular, Kruskal coordinates $\{U,V\}$ the ingoing flux diverges like $V^{-2}$ on the Cauchy horizon (where $V=0$). This divergence suggests that its backreaction via the semiclassical Einstein equations would yield a strong singularity, as opposed to its weaker, classical counterpart. Interestingly, there are exceptions, in which the energy fluxes in Eddington-Finkelstein coordinates vanish: (i) in the extremal limit (where the black hole is maximally charged); (ii) certain fine-tuned regions of parameter space, where the fluxes change sign.

Gravitational waves carry information on the spins of merging binary-black holes. The orientations of their spins relative to their orbits ("tilts")—while challenging to measure—differentiate the astrophysical formation mechanisms by which merging black hole binaries may form. Multiple analyses have reported tentative evidence that black hole spins are preferentially oriented in the plane of the binary orbit. We show that these results are likely extrapolated from more easily measurable "effective spin" parameters which characterize the gravitational-wave inspiral of black hole binaries but may not uniquely constrain their astrophysical formation. In particular, we reproduce a peak in the spin tilt distribution with information about the marginal effective spin distributions alone. We propose a geometric picture to compare constraints on effective spins to standard population modeling assumptions, suggesting that current inferences of a preferred spin orientation are spurious.

We present GW Explorer: A Beginner's Guide, an outreach curriculum designed to introduce high school students to gravitational-wave (GW) astrophysics through interactive Python Jupyter notebooks. Most existing GW resources target beginning audiences and advanced students, leaving a gap at the pre-college level that we directly address. The curriculum integrates foundational physics with hands-on computation implemented through both self-directed and workshop-based instructional formats. In the self-directed format, students completed the curriculum independently on cloud-based platforms such as Google Colab. During the workshop format, students worked through the same activities under the guidance of University of Nevada, Las Vegas graduate student mentors. Topics span gravity, spacetime, GW sources, interferometric detection, and data analysis. An implementation in local high school classrooms informed the content and pacing, and survey results demonstrate gains in conceptual understanding and coding confidence. GW Explorer offers a scalable, open-access framework for authentic astrophysics research in the high school classroom.

We study the electromagnetic radiation and wave-optical imaging of an oscillating electric dipole orbiting a Kerr black hole. We derive the effective 4-current associated with a pointlike oscillating electric dipole in curved spacetime, and use black hole perturbation theory to compute the resulting radiation field at future null infinity, from first principles. We then develop a wave-optical imaging framework for a moving electromagnetic source in curved spacetime. We obtain images of an orbiting electric dipole, displaying relativistic beaming, gravitational lensing, and Einstein rings. We study polarization-dependent scattering by comparing the images produced by spinning dipoles with opposite helicities, finding a displacement that roughly decreases with the inverse of the radiation frequency, as expected for a beyond-geometric-optics effect. Our results provide a first-principles benchmark for beyond-geometric-optics descriptions of electromagnetic radiation in Kerr spacetime.

We associate a gravitational Hilbert space $\mathbf{H}_\sigma$ to any closed compact $(d-1)$-manifold $\sigma$ with real metric. A quantum state $\mathcal{J}(\sigma)$ is a $d$-manifold bounded by $\sigma$ and equipped with elliptic data. An inner product is defined by gluing states pairwise across $\sigma$ and evaluated by viewing the resulting closed $d$-manifold as a boundary condition on the gravitational path integral (GPI) over $(d+1)$-manifolds. If $\sigma$ is nonempty and the GPI is dominated by a single $(d+1)$-manifold $M$ in the $G_N\to 0$ limit, then $M$ contains a Lorentzian CRT fixed-point set, providing $\mathcal{J}(\sigma)$ with a classical spacetime interpretation. Conversely, given a finite Lorentzian domain with edge $\sigma$, a state $\mathcal{J}(\sigma)$ may be associated to it by deforming its initial data off the real Lorentzian section and retaining only elliptic data. This establishes a broad correspondence between non-asymptotic spacetime regions and quantum states. Assuming that $\mathbf{H}_\sigma$ factorizes over connected components of $\sigma$, our framework admits operators and partial traces. This allows us to explore the information-theoretic structure of the states we define. As an example, we construct a family of states by deforming partial Cauchy slices $\Sigma$ that straddle a two-sided black hole; $\sigma$ consists of two spheres. We construct the reduced state on one sphere and find that its Rényi entropies are positive, monotonic, and sensitive to all aspects of $\Sigma$ and its complex deformation. The von Neumann entropy, however, is controlled only by the maximin surface in the causal domain of $\Sigma$, independently of other parameters, so long as the complex deformation does not vanish. Our proposal may thus explain the efficacy of tensor network toy models of holography while transcending their limitations.

As parameters are varied, the set of saddle points contributing to a (path) integral may change discontinuously, leading to a corresponding change in the asymptotic expansion of the integral. This behavior is known as the Stokes phenomenon. We explore this phenomenon in the context of the analytic continuation problem relating the AdS/CFT and dS/CFT correspondences. In this paper, we study these correspondences for three-dimensional pure gravity and two-dimensional Liouville theory, using independent calculations in bulk minisuperspace and in the boundary Liouville zero-mode. In the bulk, the dS contour selects a single saddle and yields the tunneling wave function. Upon continuation to AdS, the contour instead selects an infinite family of saddles. The boundary calculation independently reproduces the same Stokes structure at leading semiclassical order, providing a nontrivial holographic consistency check. Our construction also offers a contour prescription for the conformal factor problem in Euclidean AdS$_3$ quantum gravity within minisuperspace.

Ultralight dark matter could be an axial vector field, a simple possibility which motivates new experiments. Couplings to fermions, through an axial vector current or a dark electric dipole moment operator, lead to enhanced spin torques compared to axion dark matter. An axial vector also has an analogue of the axion-photon coupling, though its consistent realization requires a photon mass. Under this "axial-photon" coupling, the effect of a background magnetic field is suppressed, and the strongest experimental probes involve polarimetry, electric fields in superconducting cavities, and the cosmic microwave background. Since a massive axial vector is dual to a massive two-form, our results also apply to "Kalb-Ramond" dark matter motivated by string theory.

We derive an entropy formula for recently proposed tensor network models which prepare diffeomorphism invariant states of topological quantum field theories with non-compact and/or continuous gauge groups. We show that our entropy formula generalizes the notion of “topological entanglement entropy” to incorporate the infinite number of particle-like excitations in such theories. When our networks are endowed with gauge group $\mathrm{SL}(2,\mathbb{R})$, we can interpret them as models of three-dimensional gravity with small Newton's constant and possibly non-invertible metrics.

We extend the Dark Matter-Baryon Separability Condition to show that the same framework developed for dark matter deficient galaxies naturally admits a conjugate branch describing preferential baryonic depletion. Using the recently discovered almost-dark galaxy TTT J1237327+143535 as a worked example, we derive a family of dynamical consistency relations parameterized by the unknown progenitor ratio $\mu_i$, including thresholds for $\sigma_{\rm los}$, $M_{\rm dyn}$, the enclosed baryonic fraction and the dynamical mass to light ratio. We further connect the separability framework to the expected globular cluster population, providing an independent consistency test of the inferred halo mass. We also provide some bounds on the baryon ratio using the tidal properties of Virgo Cluster. Future measurements of stellar kinematics, gas content and globular clusters can therefore determine whether this galaxy occupies the positive separability branch and also test whether dark matter deficient and baryon depleted systems can be described within a common framework.

We investigate the influence of the Garfinkle-Horowitz-Strominger (GHS) dilaton black hole on different quantum resources of Dirac fields beyond the single-mode approximation. By employing the negativity to characterize quantum entanglement and the $l_1$-norm and the relative entropy of coherence to characterize quantum coherence, we demonstrate that these resources exhibit remarkably different responses to the gravitational field. Specifically, increasing the dilaton parameter continuously suppresses quantum entanglement, leaving only a finite residual amount in the strong-gravity regime, whereas quantum coherence is enhanced, indicating that the dilaton-induced spacetime affects nonlocal quantum correlations and local quantum superposition in fundamentally different ways. Furthermore, we show that an initially maximally entangled state does not always possess the largest negativity after propagating in the GHS dilaton spacetime; instead, under appropriate conditions, certain non-maximally entangled states can retain stronger entanglement than the maximally entangled one. These findings reveal the resource-dependent nature of gravitational effects in dilaton black hole backgrounds and provide new insights into the manipulation and protection of quantum resources for relativistic quantum information processing in curved spacetime.

Point particles are generically ill defined in fully nonlinear general relativity, although they are naturally well defined in linear perturbation theory. In a previous paper, two of us showed how they can be rigorously defined from matched asymptotic expansions at second order in perturbation theory on a generic background spacetime. Here we show how the resulting field equations can be recast in a more practical, more fully “skeletonized” form. We also show that this form of the field equations enables simple derivations of equations of motion. Finally, we show how calculations in second-order perturbation theory can bypass the use of singular “punctures” and make rigorous use of off-the-shelf regularization methods such as Blanchet and Damour's Hadamard regularization, opening new avenues to second-order self-force calculations in black hole binaries.

We provide a unified analysis of scalar-field quantization and detector response in relativistic rotating frames, comparing rigid and Trocheris-Takeno (TT) descriptions in unbounded Minkowski spacetime. We distinguish three questions that are often conflated: (i) how a fixed quantum state is represented in rotating coordinates, (ii) whether rotating time evolution selects a global ground state, and (iii) how a rotating detector responds to the vacuum. Coordinate-transformed rigid and TT modes span the same positive-frequency subspace as inertial modes: their Bogoliubov beta coefficients vanish and their mode sums reproduce the Minkowski Wightman function. Neither rotating time flow selects a global scalar ground state, although for different reasons: the rigid generator is not globally timelike and is unbounded below, whereas TT time translation is not a Killing symmetry. Nevertheless, a circular Unruh-DeWitt detector in the Minkowski vacuum has a stationary, nonzero response. We relate this response to negative corotating frequencies, evaluate it after an explicit Hadamard subtraction, and show that it is nonthermal. Thus vanishing Bogoliubov mixing, the absence of a symmetry-selected rotating vacuum, and detector excitation are mutually consistent.

This work concerns a procedure for removing gauge dependence from graviton corrections to the effective field equations for a massless, minimally coupled scalar on de Sitter background. The procedure involves combining diagrams in the same way as the scattering amplitude for the $t$-channel exchange of the massless scalar between two massive particles, but without taking the asymptotic limits which are problematic in cosmology. Implementing this at 1-loop on flat space background requires five classes of diagrams, in addition to the naive exchange, and the combination of those diagrams does eliminate dependence on the graviton gauge fixing functional. When those same diagrams are generalized to de Sitter background, their combination cancels the most important (“nonlocal”) secular gauge dependence, but it leaves (“local”) secular gauge dependence that is associated with external legs. Local secular gauge dependence can be removed by adding two additional classes of diagrams, and making secular corrections to the external wavefunctions. The purpose of this paper is to include these additional corrections on de Sitter background in order to derive fully gauge independent effective field equations at 1-loop order.

Adopting pseudo-random noise (PRN) codes for inter-satellite ranging, the resulting data-encoded PRN signal induces excessive laser phase measurement noise under the currently adopted integrated scheme in space-based gravitational wave (GW) detection systems. To suppress the noise, a scheme of inter-satellite integrated laser link is presented in this Letter, in which the data-encoded PRN signal for inter-satellite absolute distance measurement and communication is phase-modulated onto the ultra-stable oscillator (USO) clock signal before phase modulation onto the laser carrier. Theoretical analysis and simulation results show that in the proposed scheme the noise is significantly suppressed compared with the currently-adopted one, while the noise stemming from the data-encoded PRN signal on clock noise transfer is limited within the requirement.

Parity invariance of general relativity imposes exact relations between waveforms and remnant properties of binary-black-hole configurations related by reversal of the in-plane spins and observer direction. We summarize these relations and construct consistency tests for spin-precessing waveform and remnant models. Applying a conservative, unmaximized overlap diagnostic to seven state-of-the-art waveform models, we find that models built from aligned-spin calibrated waveforms and parity-covariant precession dynamics satisfy the symmetry to numerical precision. The original IMRPhenomXO4a and IMRPhenomXPNR implementations instead exhibit median parity residuals of approximately $1.6\times10^{-3}$ with broad upper tails, while NRSur7dq4 shows its largest violations at high mass ratio and large spin magnitude. The asymmetric extension of SEOBNRv5PHM exhibits a smaller, localized violation, while NRSur7dq4Remnant shows larger residuals in the recoil than in the final mass and spin. We trace these violations to an observer-dependent phase anchor in the IMRPhenom antisymmetric mode, non-invariant parameterization of the NRSur7dq4 surrogate fits, and one non-invariant NR-calibrated coefficient in the asymmetric SEOBNRv5PHM extension. Local corrections restore waveform parity to numerical precision for the IMRPhenom and SEOBNRv5PHM cases; restoring it in NRSur7dq4 requires refitting the surrogate. Reanalyses of eight GWTC-5.0 events show no qualitative change in astrophysical interpretation, although the IMRPhenom correction yields Jensen-Shannon divergences up to $5\times10^{-2}$ in individual marginalized posteriors. Exact symmetries thus provide inexpensive, model-independent consistency tests and should be imposed by construction in calibrated waveform models.

We develop a Plebański–Demiański-adapted parametrization of the general Ovcharenko–Podolský solution and construct new families of generalized black holes in four-dimensional Einstein–Maxwell theory. These Petrov type~D spacetimes possess non-null, fully non-aligned electromagnetic fields. A restriction imposed in the previous parametrization is not required by the field equations in shifted coordinates. Removing it retains an independent charge parameter $q$, which may be real or purely imaginary. For $q^2\geq0$, both the metric and electromagnetic field admit a smooth aligned limit to the charged Plebański–Demiański solution with $\Lambda=0$ and $e^2+g^2=q^2$. We impose angular conditions in the Griffiths–Podolský form allowing Killing horizon cross sections with spherical topology. The resulting eight-parameter class extends the seven-parameter class of Ovcharenko and Podolský [arXiv:2508.04850]. The remaining normalization condition is generically quartic, defining four algebraic branches. We give explicit metrics and electromagnetic fields for the non-twisting and $l=0$ twisting families and analyze their black hole and acceleration horizons, extremality, conicity, and further limits. We determine the effective-NUT-free branches of the latter family and their degenerate cases. Using the genuine Griffiths–Podolský form, we establish an explicit correspondence between the non-accelerating effective-NUT-free subclass and the Kerr–Newman–Bertotti–Robinson family and express its electromagnetic flux charges in our parameters. The $q^2<0$ sector includes the genuine uncharged Kerr–Bertotti–Robinson solution recently identified by Ovcharenko and Podolský [arXiv:2608.21672].

We develop an on-shell description of low-frequency gravitational waveforms from black-hole mergers beyond leading order. Treating the strongly coupled merger as effective hard $S$-matrix data, we organize its long-wavelength response using soft theorems and the KMOC formalism. At next-to-leading order, the quantum soft theorem contains logarithmic terms absent from the classical soft theorem. We show that these extra terms cancel in the full KMOC in-in observable between the one-loop radiative amplitude and the corresponding graviton cut, leaving precisely the classical logarithmic contributions associated with gravitational drag and early-time acceleration. We also identify the $1/\omega$ corrections from remnant recoil and Christodoulou non-linear memory. These results reveal a hierarchy of merger information accessible at low frequency: logarithmic tails depend only on asymptotic hard data, recoil probes total radiated momentum, while non-linear memory probes the angular distribution of the emitted radiation.