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The speedup of parameter estimation is an active field of research in gravitational-wave data analysis. In this paper, we present GP15, a deep-learning method that merges residual networks and normalizing flows into a general-purpose, image-based estimator of binary black hole (BBH) parameters. Building on our early work, we map BBH spectrograms from the Advanced LIGO and Advanced Virgo detectors to color channels in an RGB image amenable to be processed with residual networks. GP15 is trained on simulated data for BBH mergers obtained with the IMRPhenomXPHM waveform approximant and tested for all three-detector events from the gravitational-wave transient catalogs GWTC-3 and GWTC-2.1 reported by the LIGO-Virgo-KAGRA (LVK) Collaboration. Overall, our model yields good agreement with the LVK results over most parameters. Our simple model can produce large amounts of posterior samples on the order of a second, complementing existing approaches with normalizing flows based on time or frequency representation of gravitational-wave data. We also discuss current shortcomings of our model and possible improvements for future extensions (e.g., including noise conditioning from the detectors' power spectral density or splitting the parameter space into intrinsic and extrinsic subspaces).

Gravitational-wave memory is a low-frequency, nonoscillatory component of the radiation field that provides a potentially powerful but as yet undetected probe of strong-field gravity. We present the first calculation of gravitational memory from full inspiral-merger-ringdown waveforms in a theory beyond general relativity, focusing on scalar-Gauss-Bonnet gravity as a theoretically well-motivated and numerically accessible extension of GR. Starting from the general memory formulas in Horndeski gravity, we derive explicit spin-weighted spherical-harmonic expressions for the tensor null memory in scalar-Gauss-Bonnet theory and evaluate them on existing numerical-relativity waveforms for both shift-symmetric and dynamically scalarizing binary black hole mergers. We find that the dominant effect is an indirect modification of the tensor memory through changes in the nonlinear merger dynamics, while the direct scalar contribution to the tensor memory remains suppressed by orders of magnitude for the systems considered in this work. For the largest deviations in our dataset, the final memory amplitude differs from the corresponding GR prediction by a few percent and by up to <inline-formula><mml:math><mml:mrow><mml:mo>~</mml:mo><mml:mn>4</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math></inline-formula> when compared to the GR template that minimizes the waveform mismatch in a detector-oriented analysis. We further show that including memory increases the mismatch between GR and scalar-Gauss-Bonnet waveforms by more than an order of magnitude, indicating that memory can provide complementary information for testing gravity with third-generation detectors, especially for low-mass binaries.

A fundamental challenge in low-frequency gravitational-wave detectors is the limited signal-to-noise ratio (SNR) of calibration lines, particularly in torsion-bar systems where the response is governed by rotational dynamics. In this work, we resolve this issue by optimizing the geometrical configuration of a torque-coupled gravity field calibrator (GCal), achieving an improvement in calibration-line SNR by more than an order of magnitude compared with conventional layouts. For the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed-meter (CHRONOS), the calibration signal appears as a monochromatic line within the 0.1--<inline-formula><tex-math>$10~\mathrm{Hz}$</tex-math></inline-formula> band. At <inline-formula><tex-math>$1~\mathrm{Hz}$</tex-math></inline-formula>, the strain-equivalent calibration amplitude reaches <inline-formula><tex-math>$|h_{\rm GCal}| = 1.16 \times 10^{-14}$</tex-math></inline-formula>, corresponding to an SNR density of <inline-formula><tex-math>$|h_{\rm GCal}|/S_h = 4.16 \times 10^{3}$</tex-math></inline-formula>. This demonstrates for the first time that a high-SNR calibration line can be directly injected into the sub-Hz band of a torsion-bar detector. A first-order perturbative error propagation analysis yields a total fractional systematic uncertainty of <inline-formula><tex-math>$\delta h_{\rm GCal}/h_{\rm GCal} = 0.24\%$</tex-math></inline-formula>, dominated by geometric alignment uncertainties, while contributions from mass uncertainties and the gravitational constant remain subdominant. The corresponding absolute systematic uncertainty is <inline-formula><tex-math>$\delta h_{\rm GCal} \sim 10^{-17}$</tex-math></inline-formula> at <inline-formula><tex-math>$1~\mathrm{Hz}$</tex-math></inline-formula>. These results establish torque-coupled gravitational calibration as a practical solution to the longstanding low-SNR problem in sub-Hz torsion-bar detectors and provide a robust pathway toward precision absolute calibration in the low-frequency regime.

We study rotating hybrid stars, with particular emphasis on the effect of spin on the deconfinement phase transition and star properties. Our analysis is based on a hybrid equation of state with a phase transition from hadronic matter containing hyperons to color-superconducting quark matter, where the quark phase is modeled within a relativistic density functional approach. By varying the strength of the vector repulsion and diquark pairing couplings in the microscopic quark Lagrangian, we construct a set of hybrid star sequences with different quark-matter onset densities. This framework ensures consistency with astrophysical and gravitational wave constraints on mass, radius, and tidal deformability. We show how increasing the rotational frequency affects the maximum gravitational mass, oblateness, central energy density of compact stars, as well as the onset of quasiradial oscillations and nonaxisymmetric instabilities. Our results indicate that, for the most favorable parameter sets with strong vector coupling, hybrid stars containing a color-superconducting quark-matter core can account for the fastest-spinning and most massive known neutron star, PSR J0952-0607. In contrast, this pulsar cannot be explained by the purely hadronic equation of state considered in this study. These findings are highly relevant for future observations with the Square Kilometre Array (SKA), which will significantly expand the known pulsar population and enable precise measurements of masses and spin frequencies.

We have incorporated a hadron interaction tuning based on accelerator data into our atmospheric neutrino flux calculation, which has been used to analyze atmospheric neutrino oscillations at Super-Kamiokande. This new approach enables a more direct evaluation of the flux uncertainty than a conventional tuning using atmospheric muons. The neutrino flux calculated with this new tuning is 5%--10% smaller but still consistent with our previously published prediction within its uncertainty. The flavor ratio <inline-formula><tex-math>$(\nu _{\mu }+\bar{\nu }_{\mu })/(\nu _e+\bar{\nu }_e)$</tex-math></inline-formula> and <inline-formula><tex-math>$\bar{\nu }/\nu$</tex-math></inline-formula> ratio were consistent with the previous prediction. Based on the measurement errors of the accelerator data, we evaluated the flux uncertainty associated with the new tuning to be 7%--9% in the <inline-formula><tex-math>$E_{\nu } \lt $</tex-math></inline-formula> 1 GeV region, which was difficult to assess with the conventional tuning. The flux uncertainty in the <inline-formula><tex-math>$1\lt E_{\nu }\lt 10$</tex-math></inline-formula> GeV region was evaluated to be 5%--7%, which is an improvement over the conventional tuning.

The minimal left-right symmetric model (mLRSM) provides an elegant and testable framework for addressing the origin of neutrino masses. We examine the constraints on the sub-GeV right-handed (RH) neutrino in the type-II seesaw scenario of the mLRSM without left-right mixing, taking limits from collider searches, meson decays, supernovae, neutrinoless double beta (0νββ) decay and cosmology. Specifically, we derive the 0νββ decay constraints using the advanced effective field theory approach and up-to-date nuclear matrix element calculations. Besides, we update the SN1987A cooling bound with the state-of-the-art simulations, provide new constraints from the energy deposition in the supernova ejecta, and incorporate the stringent RH neutrino lifetime upper limit τ ≲ 0.023 s from the big bang nucleosynthesis. Our results identify the parameter region compatible with all current experimental and observational constraints, where the RH neutrino mass lies between 700 MeV and 1 GeV and the RH W boson mass is slightly below 20 TeV. This region is exclusively probed by the future tonne-scale 0νββ decay experiments, providing a unique window to test the mLRSM and the possible origin of neutrino masses.

A long-standing problem in electroweak phase transition studies is the spurious gauge dependence of bubble nucleation rate calculations, which introduces large systematic uncertainties in predictions of gravitational waves and related cosmological phenomena. We address this issue by presenting a systematic study of gauge-invariant bubble nucleation within a phenomenologically realistic effective field theory framework. Using the three-dimensional thermal effective field theory formalism with a consistent power-counting scheme, we rigorously establish the gauge invariance of the physical nucleation rate up to two-loop order. This proof eliminates the gauge-parameter dependence of key phase transition parameters, providing a robust theoretical foundation for precise calculations of gravitational waves, electroweak baryogenesis, and primordial magnetogenesis.

Neutrino flavor evolution in dense astrophysical environments such as core-collapse supernova (CCSN) is influenced by collective effects. While the Fast Flavor Instability (FFI) and the Collisional Flavor Instability (CFI) are recognized as key drivers of rapid flavor conversion, their nonlinear competition with collisional damping in spatially inhomogeneous systems remains poorly understood. Motivated by recent findings that FFI and resonancelike CFI cooccur in the postbounce phase in CCSN, we scrutinize their dynamic competitions and asymptotic states. To this end, we perform numerical simulations of the quantum kinetic neutrino transport, incorporating both spatial advection and the collision terms. We demonstrate that the interplay between these coexisting neutrino flavor instabilities and collisions leads to rich dynamics. Rather than merely inducing simple decoherence, collisional damping can substantially alter the overall dynamics of collective flavor oscillations, driving the system through complex evolutionary pathways. In all cases where flavor instability develops, we find that the system converges to the same flavor-equilibrated asymptotic state, despite the diversity of intermediate dynamics. Our results suggest that realistic collisional effects drive the system to an asymptotic state distinct from the one predicted by the collisionless FFI picture. This highlights the importance of incorporating collisional effects when modeling the asymptotic outcome of flavor conversion in CCSN models.

In a gravitational lensing system, the relative transverse velocities of the lens, source, and observer induce a frequency shift in the observed radiation. While this shift is typically negligible in most astrophysical contexts, strategies for its detection have been proposed for both electromagnetic and gravitational waves. This paper provides a rigorous theoretical treatment of the effect, deriving general expressions for the frequency shift within a lensing system embedded in a cosmological spacetime. Our formulation remains valid for arbitrary distances and velocities---including highly relativistic regimes---under any Friedmann-Lemaître-Robertson-Walker metric. Expanding upon previous papers on moving lenses, we provide a detailed analysis of frequency effects induced by lenses moving at relativistic speeds. Furthermore, we extend standard lensing theory by deriving an exact formula for the variation in the source's viewing direction. This result is of interest for strongly anisotropic emitters, such as compact binary systems emitting gravitational waves. Finally, we quantify the apparent misalignment between the lens and the source's two images produced by time-delay effects in lens systems moving with high velocity.

We study the non-Gaussian features of the gravitational wave (GW) background generated by a population of inspiraling supermassive black hole (SMBH) binaries. We show that the SMBH GW amplitude distribution (GWAD) features a universal heavy power-law tail <inline-formula><mml:math><mml:mo>∝</mml:mo><mml:msup><mml:mi>A</mml:mi><mml:mrow><mml:mo>-</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>, while the low-amplitude tail depends on the SMBH merger rate and the energy-loss mechanisms of the binaries. The distribution of the induced timing residuals inherits this heavy tail. As a result, the ensemble averaged statistical moments of order three and higher diverge, limiting their usefulness as measures of non-Gaussianity, and the GW background from SMBH binaries exhibits the single loud source principle, according to which the strongest signals are more likely to be caused by a small number of loud sources. We confirm that the variance-averaged Gaussian approximation accurately describes the timing residual statistics. This approximation justifies a factored likelihood structure that combines standard Gaussian-process PTA posteriors with the non-Gaussian population prior, enabling consistent incorporation of non-Gaussian effects into SMBH model inference. We provide a fast and flexible Python implementation to compute the distribution of timing residuals from a given SMBH merger rate or GWAD.

We fully classify the phenomenology of gravitational wave emission from scaling cosmic string networks with varying tension and compute the spectral indices of the resulting stochastic backgrounds. In string compactifications, periods of varying tension occur when moduli acquire a time dependence. We present concrete examples in type IIB string theory as D3- and NS5-branes wrapping internal cycles, which become dynamical due to the effect of moduli potentials. Moreover, we use swampland constraints to derive general bounds on the allowed time variation of the effective string tension in Friedmann-Lemaitre-Robertson-Walker backgrounds and on the resulting spectral indices.

Gravitational-wave detectors operate in inherently nonstationary environments, requiring robust detector characterization (DetChar) to distinguish instrumental transients from astrophysical signals. Traditional DetChar frameworks typically rely on morphological classifiers or energy-based projections, such as band-limited root-mean-square (BLRMS) metrics, which can conflate global amplitude scaling with physical reconfigurations of the spectrum. In this work, we introduce Fisher information velocity, a novel geometric channel that models the detector's power spectral density as a point on a Riemannian manifold. By tracking the kinematic drift of the noise floor and utilizing exterior algebra to calculate tangent divergence (<inline-formula><mml:math><mml:mi>sin</mml:mi><mml:mi>θ</mml:mi></mml:math></inline-formula>), we mathematically decouple simple energy surges from spectral warps, or differential redistributions of power across frequency bands. Applying this framework via the SGN-DRIFT streaming pipeline to <inline-formula><mml:math><mml:mo>~</mml:mo><mml:mn>40</mml:mn></mml:math></inline-formula> hours of high-cadence Advanced LIGO O4a data, we evaluate <inline-formula><mml:math><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>282</mml:mn></mml:math></inline-formula>, 080 independent manifold velocity samples. High-resolution phase space mapping reveals a bimodal taxonomy of severe instrumental nonstationarity, classifying events into structural pivots (87.2%) and isotropic surges (12.8%). Among codetected events, the geometric channel achieves higher significance than standard BLRMS monitors in 74% of cases with a median sensitivity ratio of <inline-formula><mml:math><mml:mi>Γ</mml:mi><mml:mo>=</mml:mo><mml:mn>1.65</mml:mn></mml:math></inline-formula>. The two channels detect largely nonoverlapping populations, increasing the total anomaly catalog by 87% over BLRMS alone. Systematic validation on 10 confirmed GWTC-4.0 events and <inline-formula><mml:math><mml:mo>~</mml:mo><mml:mn>5</mml:mn></mml:math></inline-formula>, 000 simulated injections demonstrates robust insensitivity to astrophysical signals, establishing this geometric channel as a sensitive, complementary, and veto-safe diagnostic for current and next-generation detector networks.

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 quasistatic 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 <inline-formula><mml:math><mml:mo>ℓ</mml:mo></mml:math></inline-formula>) of the geometric-optics approximation. In this paper we clarify the actual identification of observables within such a correspondence and test its accuracy for modified spherically symmetric BH geometries proposed in the literature. We find that even for low values of <inline-formula><mml:math><mml:mo>ℓ</mml:mo></mml:math></inline-formula> the correspondence is surprisingly accurate in relating the real and imaginary parts of quasinormal 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.

GW250114 is the loudest gravitational wave event to date observed by LIGO-Virgo-KAGRA Collaboration. Owing to its high signal-to-noise ratio (SNR), previous analyses based on quasinormal mode (QNM) superpositions have suggested evidence of the fundamental and the first overtone of the <inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mi>m</mml:mi><mml:mo>=</mml:mo><mml:mn>2</mml:mn></mml:math></inline-formula> mode in this event. However, QNMs are not orthogonal and the inclusion of multiple QNMs induces correlations among them, which can hinder the robust identification of subdominant QNMs. To address this challenge, we apply an analysis based on orthonormalized QNMs [S. Morisaki , Phys. Rev. D 112, 124083 (2025)PRVDAQ2470-001010.1103/lhkp-w6hm] to GW250114. We find that, in the model including three <inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mi>m</mml:mi><mml:mo>=</mml:mo><mml:mn>2</mml:mn></mml:math></inline-formula> QNMs up to the second overtone, the first overtone of the <inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mi>m</mml:mi><mml:mo>=</mml:mo><mml:mn>2</mml:mn></mml:math></inline-formula> mode is more strongly supported than in previous nonorthogonal analyses, with the inferred significance increasing from 86.2% to 99.9%. Furthermore, we estimate deviations from the Kerr prediction using the orthonormal QNM framework and find no significant deviation, consistent with previous analyses. These results demonstrate that the orthonormal QNM framework provides a more robust way to identify subdominant modes in high-SNR ringdown signals, highlighting its potential for future gravitational wave observations.

We characterize and phenomenologically model the merger-ringdown of gravitational waves emitted by a small compact object that plunges and merges into a Kerr black hole from equatorial-eccentric inspirals. The waveforms are generated employing a time-domain Teukolsky code sourced with trajectories computed using the effective-one-body framework. We span values of the Kerr spin <inline-formula><mml:math><mml:mi>a</mml:mi><mml:mo>∈</mml:mo><mml:mo>[</mml:mo><mml:mo>-</mml:mo><mml:mn>0.9</mml:mn><mml:mo>,</mml:mo><mml:mn>0.9</mml:mn><mml:mo>]</mml:mo></mml:math></inline-formula>, eccentricity at the last stable orbit (LSO) <inline-formula><mml:math><mml:msub><mml:mi>e</mml:mi><mml:mrow><mml:mi>LSO</mml:mi></mml:mrow></mml:msub><mml:mo>∈</mml:mo><mml:mo>[</mml:mo><mml:mn>0</mml:mn><mml:mo>,</mml:mo><mml:mn>0.9</mml:mn><mml:mo>]</mml:mo></mml:math></inline-formula>, and relativistic anomaly <inline-formula><mml:math><mml:msub><mml:mi>ξ</mml:mi><mml:mrow><mml:mi>LSO</mml:mi></mml:mrow></mml:msub><mml:mo>∈</mml:mo><mml:mo>[</mml:mo><mml:mn>0</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn><mml:mi>π</mml:mi><mml:mo>]</mml:mo></mml:math></inline-formula>. We characterize the last peak of the waveform and ringdown features across the parameter space, finding that the eccentricity mainly affects the last peak features, while it has a smaller impact on the ringdown signal. In contrast, the relativistic anomaly measured at the LSO influences the morphology of the last peak in a restricted portion of the parameter space and has no impact on the ringdown part. We perform the analysis for all the spin-weighted spherical harmonic modes normally included in the SEOBNR family of models, <inline-formula><mml:math><mml:mo>(</mml:mo><mml:mo>ℓ</mml:mo><mml:mo>,</mml:mo><mml:mi>m</mml:mi><mml:mo>)</mml:mo><mml:mo>∈</mml:mo><mml:mo>{</mml:mo><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>3</mml:mn><mml:mo>,</mml:mo><mml:mn>3</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>4</mml:mn><mml:mo>,</mml:mo><mml:mn>4</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>5</mml:mn><mml:mo>,</mml:mo><mml:mn>5</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mo>,</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>3</mml:mn><mml:mo>,</mml:mo><mml:mn>2</mml:mn><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mo>(</mml:mo><mml:mn>4</mml:mn><mml:mo>,</mml:mo><mml:mn>3</mml:mn><mml:mo>)</mml:mo><mml:mo>}</mml:mo></mml:math></inline-formula>. Finally, we introduce a merger-ringdown model for SEOBE-TML, a forthcoming inspiral-merger-ringdown waveform model for eccentric spin-aligned binary black holes in the test-mass limit, whose features can be extended to comparable-mass regimes. The model also accounts for quasinormal mode mixing during the ringdown. It provides a first step toward incorporating the impact of residual eccentricity close to merger into spin-aligned effective-one-body merger-ringdown models for binary black holes.