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We investigate the structure of strange quark stars (SQSs) in the color─flavor─locked (CFL) phase in the presence of scalar bosonic dark matter within a two─fluid formalism. The quark matter equation of state (EOS) is obtained from perturbative QCD, incorporating the latest Particle Data Group values for the running coupling and strange quark mass, while the dark matter component is modeled as a self-interacting Bose─Einstein condensate. By considering different dark matter masses and varying the pairing gap ∆ and the central dark matter pressure fraction f<SUB>r</SUB>, we analyze the impact of dark matter on the structural properties of SQSs, including the maximum gravitational mass M<SUB>TOV</SUB>, the ratio of dark matter to strange-quark-matter radii R<SUB>DM</SUB>/R<SUB>SQM</SUB>, and the dimensionless tidal deformability Λ. We further examine the compatibility of the resulting mass─radius relations with the recent NICER measurements of compact stars. Within the parameter space considered in this study, we find that M<SUB>TOV</SUB> exhibits a non-monotonic dependence on the dark matter mass, with a critical value beyond which M<SUB>TOV</SUB> decreases. We also show that some pure CFL strange quark star configurations, particularly those associated with very stiff EOSs and larger maximum masses, may not simultaneously remain compatible with the Λ range inferred from GW170817 while occupying the lower mass─gap region. In contrast, the inclusion of dark matter allows two-fluid CFL strange quark star configurations to reproduce the observed properties of massive compact objects in the lower mass─gap region, such as the secondary component of GW190814, while remaining qualitatively compatible with the Λ range inferred from GW170817. We note, however, that the GW170817 constraints were originally inferred within single-fluid compact-star frameworks and therefore provide only qualitative guidance for the present two-fluid halo configurations. Our results suggest that exotic compact-star configurations may populate part of the conventionally defined lower mass─gap region.

Neutrinos can experience fast flavor conversions (FFCs) in highly dense astrophysical environments, such as core-collapse supernovae and neutron star mergers, potentially affecting energy transport and other processes. The simulation of fast flavor conversions under realistic astrophysical conditions requires substantial computational resources and involves significant analytical challenges. While machine learning methods like Multilayer Perceptrons have been used to accurately predict the asymptotic outcomes of FFCs, their 'black-box' nature limits the extraction of direct physical insight. To mitigate this limitation, we employ two distinct interpretable machine learning frameworks-Kolmogorov-Arnold Networks (KANs) and Sparse Identification of Nonlinear Dynamics (SINDy)-to learn interpretable surrogates for the asymptotic input─output mapping from a FFC simulation dataset. Our analysis reveals a fundamental trade-off between predictive accuracy and model simplicity. The KANs demonstrates high fidelity in reconstructing post-conversion neutrino energy spectra, achieving accuracies of up to 90%. In contrast, SINDy yields a low rank, compact closed-form approximation of the input-output mapping, at the expense of some predictive accuracy. Critically, using these structured and sparse surrogates as diagnostic tools, we identify that the system's evolution is most sensitive to the initial number density of heavy-lepton neutrinos when FCCs are triggered, compared to other physical quantities. Ultimately, this work provides a methodological framework for interpretable machine learning that supports genuine data-driven scientific discovery in astronomy and astrophysics, going beyond prediction alone.

The James Webb Space Telescope (JWST) revealed a large population of active galactic nuclei (AGN) with redshifts greater than five. We show that if they emit ultrahigh-energy protons with energies up to <inline-formula><mml:math><mml:mo>≲</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>19</mml:mn></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>eV</mml:mi></mml:math></inline-formula>, the cosmogenic neutrino production in the high-redshift CMB field yields a neutrino flux with a bump at around 50 PeV. This flux is potentially consistent with the current neutrino intensity estimate by the IceCube Neutrino Observatory within the parameter space allowed from the AGN luminosity and number density estimates by JWST. Future neutrino observations that confirm the 50-PeV bump and constrain the small-scale anisotropy will infer ultrahigh-energy cosmic-ray emissions in the early Universe.

Core-collapse supernovae (CCSNe) are powerful sources of gravitational waves (GWs). These signals propagate essentially unobstructed, providing a unique probe of the supernova central engine. In this work, we investigate parameter estimation from the bounce and early ring-down GW signal of rotating CCSNe using machine learning. We infer the peak frequency and peak amplitude of the signal as well as the rotation of the core. We extend previous studies in several directions. We consider a range of progenitor models and nuclear equations of state, and we assess the impact of key physical uncertainties, including bounce time uncertainty and source inclination. We incorporate both current detector noise and the projected sensitivities of next-generation observatories. We find that uncertainty in the bounce time does not significantly affect parameter estimation when the analysis is performed in the Fourier domain. In contrast, orientations when the rotation axis is near the line of sight substantially degrade performance. For optimal orientations, next-generation detectors can constrain rotation out to distances exceeding 100 kpc.

Several theoretical proposals describe horizonless compact objects that can mimic black holes in their gravitational wave signatures; however, their spin-induced quadrupole moments (SIQMs) may reveal their distinct nature. Using the tight bounds on the SIQM of GW241011, we place constraints on the nature of its primary. Across exotic compact object models considered in this Letter, we find that rotating boson stars with quartic self-interactions cannot explain the nature of the primary, whereas models of sufficiently large compactness, <inline-formula><mml:math><mml:mi>C</mml:mi><mml:mo>≳</mml:mo><mml:mn>0.24</mml:mn></mml:math></inline-formula>, may still be viable contenders.

Quantum backreaction effects may quench Hawking evaporation through a "memory burden," allowing primordial black holes (PBHs) with formation masses well below <inline-formula><mml:math><mml:msup><mml:mn>10</mml:mn><mml:mn>15</mml:mn></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>g</mml:mi></mml:math></inline-formula> to survive to the present and contribute to the dark matter. We show that ultrahigh-energy cosmic rays (UHECRs) provide a powerful and previously unexplored probe of this scenario. We compute the proton and neutron emission from memory-burdened PBHs, including the Galactic-halo contribution and the extragalactic proton component, and confront it with the Pierre Auger Observatory proton spectrum and its EeV neutron limits from the Galactic plane. This yields new constraints on the PBH dark-matter fraction as a function of the PBH formation mass and the evaporation-suppression parameter <inline-formula><mml:math><mml:mi>k</mml:mi></mml:math></inline-formula>. For <inline-formula><mml:math><mml:mi>k</mml:mi><mml:mo>≳</mml:mo><mml:mn>3</mml:mn></mml:math></inline-formula> the nonobservation of ultrahigh-energy protons leads to bounds competitive with those from UHE gamma rays, while neutron limits remain comparable to high-energy neutrino constraints. Our results highlights the key role of multimessenger astronomy in constraining beyond-the-standard-model scenarios.

High-frequency gravitational waves, particularly in the range <inline-formula><mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>≳</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:mrow></mml:math></inline-formula>, represent a compelling probe of physics beyond the Standard Model. Due to the absence of direct detection methods in this frequency regime, alternative strategies may be pursued. One promising approach involves the conversion of gravitons into photons in the presence of magnetic fields, a process known as the inverse Gertsenshtein effect. In this study, we explore such graviton-to-photon conversions occurring within the magnetic field environment of the M87 galaxy, utilizing realistic models for the galactic magnetic field and plasma density structure. We use the broadband electromagnetic spectrum of M87, ranging from millimeter to TeV gamma rays, to search for hidden contributions from graviton-photon conversions. In the well-constrained frequency range <inline-formula><mml:math><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msup><mml:mi>─</mml:mi><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>27</mml:mn></mml:mrow></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:mrow></mml:math></inline-formula>, the lack of excess emission allows us to place improved bounds on the gravitational wave strain amplitude <inline-formula><mml:math><mml:msub><mml:mi>h</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math></inline-formula> or on spectral energy density <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>gw</mml:mi></mml:msub><mml:msup><mml:mi>h</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:math></inline-formula>. We find that our results from M87 yield substantially stronger constraints compared to existing bounds derived from Milky Way magnetic field considerations, with improvements ranging from one to five orders of magnitude depending on the frequency band, thereby enhancing the prospects for probing high-frequency gravitational wave backgrounds through indirect electromagnetic signatures.

The remnant of a black hole binary merger settles into a stationary configuration by "ringing down" through the emission of gravitational waves that consist of a superposition of damped exponentials with discrete complex frequencies—the remnant black hole's quasinormal modes. While the frequencies themselves depend solely on the mass and spin of the remnant, the mode amplitudes depend on the merger dynamics. We investigate quasinormal mode excitation by a point particle plunging from the innermost stable circular orbit of a Kerr black hole. Our formalism is general, but we focus on computing the quasinormal mode excitation coefficients in the frequency domain for equatorial orbits, and we analyze their dependence on the remnant black hole spin. We find that higher overtones and subdominant multipoles of the radiation become increasingly significant for rapidly rotating black holes. This suggests that the prospects for detecting overtones and higher-order modes are considerably enhanced for highly spinning merger remnants.

Recent advances in quantum sensing—optical clocks at <inline-formula> <mml:math><mml:mrow><mml:mn>5.5</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>19</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula> systematic uncertainty, frequency-dependent squeezing below the standard quantum limit, quantum magnetometers approaching fundamental sensitivity limits—raise a natural question: can these technologies detect gravitational waves directly, or enhance existing detectors beyond current capabilities? We show that the answer is primarily determined by the coupling mechanism between the gravitational wave and the sensor. Starting from the tidal Hamiltonian in Fermi normal coordinates, we identify three physically distinct mechanisms by which a gravitational wave couples directly to a quantum system, and derive their transducer gains within linearized general relativity and non-relativistic quantum mechanics. Internal atomic coupling (tidal distortion of electronic wavefunctions) yields a transducer gain <inline-formula> <mml:math><mml:mrow><mml:msub><mml:mi>G</mml:mi><mml:mrow><mml:mi>A</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>2.4</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>20</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula>, with vanishing first-order energy shifts for all <inline-formula> <mml:math><mml:mrow><mml:mi>J</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math></inline-formula> clock states—a <inline-formula> <mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mstyle></mml:mstyle><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mn>35</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula> deficit relative to laser interferometry that exceeds any projected quantum enhancement. Center-of-mass coupling (Doppler shifts from geodesic motion) reaches strain sensitivities of <inline-formula> <mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mstyle></mml:mstyle><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>18</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula>, still <inline-formula> <mml:math><mml:mrow><mml:msup><mml:mn>10</mml:mn><mml:mn>4</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula> above LISA requirements. Light propagation coupling (phase accumulation over macroscopic baselines) provides the enormous transducer gain that makes laser interferometry—and atom interferometry—viable. For detectors exploiting this third mechanism, we quantify how much improvement quantum sensors can provide through the detector's noise architecture: LISA's noise budget is predominantly classical, limiting combined quantum enhancement to <inline-formula> <mml:math><mml:mrow><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mo>≍</mml:mo><mml:mn>1.04</mml:mn></mml:mrow></mml:math></inline-formula>, while ground-based detectors in the shot-noise-dominated regime achieve <inline-formula> <mml:math><mml:mrow><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mn>1.6</mml:mn></mml:mrow></mml:math></inline-formula>─<inline-formula> <mml:math><mml:mrow><mml:mn>2.1</mml:mn></mml:mrow></mml:math></inline-formula>. Atom interferometers exploit the same light-propagation mechanism to target the 0.01─10 Hz band between the LISA and LIGO ranges.

In this paper, we explore the feasibility of detecting gravitomagnetic effects generated by gravitational waves, by monitoring the relative orientation of the angular momentum vectors of test particles. We analyze the response of the relative angular momentum direction to all six polarization modes of gravitational waves and estimate the magnitude of its variation during gravitational wave events. Our findings indicate that when test particles possess magnetic moments, applying an external magnetic field of appropriate strength can induce resonant precession of the angular momentum direction under the influence of gravitational waves. This resonance may significantly amplify the gravitational wave signal, potentially enabling its detection with future gyroscope-based detectors. Such detectors would complement existing gravitational wave observatories that rely on gravitoelectric effects.

In this work, we investigate the generation and propagation of gravitational waves within a minimal gravitational SME (Standard Model Extension). Starting from the modified graviton dispersion relation derived in the linearized gravity sector, we analyze the polarization properties of gravitational waves in the transverse─traceless tensor sector. We then construct the retarded Green function associated with the Lorentz─violating wave operator, explicitly verifying the causal structure of the theory and identifying the modified propagation speeds of the tensorial modes. In addition, we study the source─induced emission of gravitational waves from a binary black─hole system. We show that the gravitational waveform preserves the standard quadrupolar amplitude and polarization structure, while Lorentz─violating effects enter exclusively through a modification of the retarded time. As a result, the spatial components of the metric perturbation h<SUB>ij</SUB>(t, r) acquire a phase shift determined by the SME coefficients. Finally, we estimate phenomenological bounds to the model under consideration.

Gravitational memory provides a distinctive low-frequency probe of gravity, but explicit merger studies beyond general relativity remain limited. In this work, we investigate memory from binary black hole mergers in Ricci-coupled scalar-Gauss-Bonnet gravity, a natural extension of scalar-Gauss-Bonnet theory that admits an additional scalar breathing polarization. Based on numerical-relativity waveforms of binary black hole coalescences, we show that the change in the scalar charge of the system across merger generates a significant scalar-memory contribution. For a GW150914-like system, this effect modifies the memory signal in a gravitational-wave detector on the same observable timescale and by an amount comparable to the pure scalar-Gauss-Bonnet correction to tensor memory. Thus, it can substantially enhance the total deviation from the general-relativity prediction over a broad range of source and detector configurations. We argue that this identifies a general mechanism: whenever a compact-binary merger changes the asymptotic charge of an additional gravitational field, and that field sources an observable extra polarization, the resulting memory can provide a leading low-frequency signature of new gravitational physics.

Ultra-high-energy neutrinos and cosmic rays are excellent probes of astroparticle physics phenomena. For astroparticle physics analyses, robust and accurate reconstruction of signal parameters such as arrival direction and energy is essential. Radio detection is an established detector concept explored by many observatories; however, current reconstruction methods ignore bin-to-bin noise correlations, which limits reconstruction resolution and, so far, has prevented calculations of event-by-event uncertainties. In this work, we present a likelihood description of neutrino or cosmic-ray signals in radio detectors with correlated noise, as present in all neutrino and cosmic-ray radio detectors. We demonstrate, with simulation studies of both neutrinos and cosmic-ray radio signals, that signal parameters such as energy and direction, including event-by-event uncertainties with correct coverage, can be obtained. This method reduces reconstruction uncertainties and biases compared to previous approaches. Additionally, the Likelihood can be used for event selection and enables differentiable end-to-end detector optimization. The reconstruction code is available through the open-source software NuRadioReco.

The interior crust and much of the liquid core of neutron stars is believed to be a quantum liquid mixture of neutron and proton superfluids and a relativistic electron liquid. Quantized vortices in the neutron superfluid and quantized flux lines in the proton superconductor are topological defects of these hadronic condensates. I consider the formation of the superfluid state in young neutron stars under nonequilibrium conditions imposed by the neutrino cooling rate. The nonequilibrium phase transition implies that the onset of superfluidity is accompanied by the generation of quantized vortices based on the mechanism envisioned by Kibble in the context cosmic string formation in evolutionary models of an expanding universe, and further developed by Zurek for nonequilibrium phase transitions in quantum liquids such as <inline-formula><mml:math><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math></inline-formula>. I discuss the Kibble-Zurek mechanism (KZM) and scaling relations for topological defect formation starting from the Cooper pair fluctuation propagator for temperatures approaching <inline-formula><mml:math><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math></inline-formula>. I then calculate the predicted vortex densities based on Urca and modified Urca cooling mechanisms in the cores of neutron stars for several models of the superfluid gap and transition temperature of the interior neutron superfluid. In all cases studied the KZM leads to a large density of topological defects in the condensate phase, which in 3D form a random network of vortex lines and loops, i.e. the generation of quantum turbulence.

Dark matter (DM) environments around black holes (BHs) can influence their mergers through dynamical friction, causing gravitational wave (GW) dephasing during the inspiral phase. While this effect is well studied for collisionless dark matter (CDM), it remains unexplored for self-interacting dark matter (SIDM) due to the typically low DM density in SIDM halo cores. In this work, by considering BH mergers within SIDM spikes, which can arise from models with a massive force mediator, we show that the GWs emitted are dephased in a distinct manner. To incorporate the feedback of the BH orbital motion that can significantly modify the DM profiles, we use <inline-formula><mml:math><mml:mi>N</mml:mi></mml:math></inline-formula>-body simulations to analyze GW dephasing in binary BH inspirals within CDM and SIDM spikes. By tracking the binary's motion in different DM environments, we show that the Laser Interferometer Space Antenna (LISA) can observe GW dephasing arising from SIDM spikes in particular scenarios. Our results indicate that these observations offer a possibility of distinguishing between binary-BH inspirals in different DM environments.

We study stochastic gravitational waves (GWs) generated in a postinflationary magnetogenesis scenario with time-dependent gauge couplings during inflation and reheating. In this setup, magnetic anisotropic stress directly sources gravitational waves, while the induced curvature perturbations generate an additional scalar-induced GW component. We compare the spectral behavior of the two contributions and find that the magnetic-originated GW dominates the peak amplitude, whereas the scalar-induced contribution becomes important on larger scales. For <inline-formula><mml:math><mml:mrow><mml:msub><mml:mrow><mml:mi>n</mml:mi></mml:mrow><mml:mrow><mml:mi>b</mml:mi></mml:mrow></mml:msub><mml:mo>≥</mml:mo><mml:mn>3</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:math></inline-formula>, both contributions exhibit the universal infrared scaling <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>GW</mml:mi></mml:msub><mml:mo>(</mml:mo><mml:mi>f</mml:mi><mml:mo>&lt;</mml:mo><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mi>peak</mml:mi></mml:mrow></mml:msub><mml:mo>)</mml:mo><mml:mo>∝</mml:mo><mml:msup><mml:mi>f</mml:mi><mml:mn>3</mml:mn></mml:msup></mml:math></inline-formula>, while their ultraviolet slopes differ, leading to distinct spectral signatures. For suitable reheating and magnetogenesis parameters, the resulting GW signal naturally extends into the nano-Hz range relevant for pulsar timing array observations, and it remains consistent with other bounds. The distinct spectral features of the two components may provide a useful probe of reheating dynamics and primordial magnetogenesis.

We propose the jiggled interferometer (JIGI), a novel ground-based gravitational wave detector employing low-frequency noise mitigation similar to that of space-based detectors. Using rapidly repeated free-fall test masses, JIGI eliminates seismic and suspension thermal noise during free fall. Compared to the juggled interferometer, it offers improved angular stability and avoids tracking lasers. We analyze detrending—a required step to remove actuation-induced noise—and show sensitivity gains of about four orders of magnitude in the 0.1─0.3 Hz band, relative to seismic and suspension noise extrapolated from the Cosmic Explorer.

We present a fast Bayesian inference framework to address the growing computational cost of gravitational-wave parameter estimation. The increased cost is driven by improved broadband detector sensitivity, particularly at low frequencies, due to advances in detector commissioning, resulting in longer in-band signals and a higher detection rate. Waveform models now incorporate features like higher-order modes, further increasing the complexity of standard inference methods. Our framework employs meshfree likelihood interpolation with radial basis functions to accelerate Bayesian inference using the IMRPhenomXHM waveform model that incorporates higher modes of the gravitational wave signal. In the initial start-up stage, interpolation nodes are placed within a constant-match metric ellipsoid in the intrinsic parameter space. During sampling, likelihood is evaluated directly using the precomputed interpolants, bypassing the costly steps of on-the-fly waveform generation and overlap-integral computation. We speed-up sampler convergence by sampling in a rotated parameter space aligned with the eigenbasis of the metric ellipsoid, where the parameters are uncorrelated by construction. This method yields unbiased parameter recovery when applied to 104 simulated neutron-star─black-hole signals (NSBH) in LIGO─Virgo data, while reducing computational cost by up to an order of magnitude for the longest-duration signal. The meshfree framework equally applies to symmetric compact binary systems dominated by the quadrupole mode, supporting parameter estimation across a broad range of sources. Applied to a simulated NSBH signal in Einstein Telescope data where the effects of Earth's rotation are neglected for simplicity, our method achieves an <inline-formula><mml:math><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>4</mml:mn></mml:msup><mml:mo>)</mml:mo></mml:math></inline-formula> speed-up, demonstrating its potential use in the 3G era.

In this work, the strong lensing effect of the memory signal was considered. In the geometric optics limit, the lensed memory signal becomes oscillatory, while the unlensed is basically monotonic. This is because only the high frequency Fourier modes contribute strongly to the lensed signal. Because of the step-function-like behavior of the unlensed memory waveform, the lensed waveform possesses characteristic morphology that is dependent on the type of the image, but independent of the lens model and the binary system. That is, for each type of the lensed image, the lensed memory waveform has an approximate reflection symmetry about a symmetrical axis in the time domain. More specifically, for the type I and type III images, the lensed memory signals are nearly odd under the reflection, while the type II signal is roughly even. In addition, at the symmetrical axis, the sign of the slope for the type I image is different from that for the type III image. These universal characteristic features would help determine the type of the lensed image. This is particularly because the memory waveform can be well approximated by a suitable step function, which involves just two parameters, the overall amplitude and the time of arrival. It is fast and cheap to simulate this approximated waveform. Once the type of the lensed image is determined with the approximated memory waveform, one can use the appropriate waveform template for the oscillatory component of the gravitational wave to perform the parameter estimation.

We present an improved subdominant-mode amplitude (SMA) test of general relativity (GR), which probes amplitude-level deviations in the higher-order modes of gravitational-wave (GW) signals from binary black hole mergers while keeping the dominant quadrupole mode fixed. Using a comprehensive parameter-estimation campaign, we benchmark the test against Gaussian noise fluctuations, waveform modeling systematics, and physical effects such as spin precession and orbital eccentricity. When applied to numerical-relativity simulations, the SMA test performs reliably for aligned-spin and mildly precessing systems but exhibits measurable biases for strongly precessing or eccentric binaries. Although designed to detect amplitude deviations, the test also responds coherently to phase perturbations, yielding apparent GR violations when applied to phase-modified waveforms. Applied to recent GW detections, we report the strongest constraint on the hexadecapolar (4, 4) mode amplitude deviation, <inline-formula><mml:math><mml:mi>δ</mml:mi><mml:msub><mml:mi>A</mml:mi><mml:mn>44</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mo>-</mml:mo><mml:mn>0.3</mml:mn><mml:msubsup><mml:mn>0</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>3.45</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.16</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>, consistent with GR. With these results, this work establishes the SMA test as a robust and broadly sensitive null test of general relativity and demonstrates a systematic approach for assessing the robustness of GW tests of GR.

With evidence for a nanohertz gravitational-wave background now established by pulsar timing arrays, the search focuses on identifying individual supermassive black hole binaries. We show that these binaries produce a distinct spatial correlation pattern across the array, acting as a deterministic analog to the stochastic Hellings and Downs curve. We derive a closed analytic expression for this single-source overlap reduction function, <inline-formula><mml:math><mml:msub><mml:mi>ϒ</mml:mi><mml:mrow><mml:mi>a</mml:mi><mml:mi>b</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula>, factorizing the signal into a source-dependent amplitude and a purely geometric fingerprint. Using simulated datasets, we demonstrate that this fingerprint breaks the degeneracy between an individual binary and a stochastic background. Including these cross-correlations yields Bayes factors of 1611 favoring the continuous-wave model over a Hellings and Downs correlated background model and 159 favoring the continuous-wave model over an uncorrelated red-noise model. Furthermore, these new cross-correlations improve sky localization by a factor of <inline-formula><mml:math><mml:mn>11</mml:mn><mml:mi>x</mml:mi></mml:math></inline-formula> over an uncorrelated search. Finally, while coherent matched filtering offers higher theoretical sensitivity, we argue that a cross-correlation-based search for individual binaries provides a robust alternative that hedges against the possibility of overfitting to noise fluctuations by focusing on the evidence for the correlations. Indeed, the geometric fingerprints we present here show that spatial correlations can also be used to identify the first nanohertz gravitational-wave sources.

Long-lived particles (LLPs) provide an interesting window into physics beyond the Standard Model, offering characteristic signatures at colliders and in cosmology. In this work, we investigate LLPs decays into dark matter. If the lifetime of LLPs are longer than <mml:math><mml:mrow><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> s, the decay products can disrupt the synthesis of light nuclei in the early universe and alter Big Bang Nucleosynthesis (BBN) predictions. If the LLP is much heavier than the dark matter particle, the decay contributes to the number of effective neutrino species, <mml:math><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mi>e</mml:mi><mml:mi>f</mml:mi><mml:mi>f</mml:mi></mml:mrow></mml:msub></mml:math>. We describe these decays via dimension-six effective operators and outline the parameter space in which such decays obey cosmological bounds stemming from BBN, structure formation, Cosmic Microwave Background, and Baryon Acoustic Oscillation data.

We study the quasinormal spectrum of a massive test scalar field in the exact asymptotically flat regular black-hole geometry supported by a phantom Dirac─Born─Infeld scalar. Using high-order WKB approximation improved by Padé resummation, together with characteristic time-domain integration and Prony extraction, we compute the fundamental mode and the first two overtones for representative values of the regularity parameter and the field mass. We show that increasing the field mass raises the oscillation frequency and reduces the damping rate, while increasing the regularity scale generally makes the ringing softer and longer lived. The time-domain profiles are in very good agreement with the WKB─Padé results and confirm the robustness of the spectrum. For sufficiently large field mass, the damping tends to zero, indicating the onset of quasiresonant behavior, although in the time domain these modes are eventually masked by oscillatory late-time tails. Our results show that massive scalar ringing provides a sensitive probe of this DBI-supported regular geometry.

In this paper, we explore hidden symmetries in electrically charged, four-dimensional rotating Kerr-Newman black hole within f(R) gravity. By deriving the Killing and Killing-Yano tensors, we establish their role in the spacetime structure. The gyromagnetic ratio is calculated and shown to retain its universal value of g = 2, consistent with all four-dimensional black holes. These findings show that the gyromagnetic ratio remains consistent in this modified gravity setting. Moreover, they highlight the connection between hidden symmetries and the ability to separate the Hamilton-Jacobi equation in f(R) gravity. This work advances the study of black holes in modified gravity, offering implications for both theoretical frameworks and observational cosmology such as gravitational wave analyses.

A recent analysis of the false-vacuum decay in non-perturbative regimes is here extended in the presence of Einstein gravity, computing the corresponding effective potential and decay rate. We consider a λϕ4 scalar field theory and we observe that, in comparison to the usual perturbative decay rate, the higher the coupling λ, the greater the decay probability. We evaluate the running of the self-interaction coupling and obtain a weakly coupled theory at lower energies, proving that Einstein gravity grants an even more reliable weak coupling approximation with the universe cooling down. We also provide an extended study of a non-minimal coupling ξ of the scalar field with gravity showing how this term makes the false-vacuum decay more difficult. Minima can also disappear at large coupling ξ. We discuss possible applications of these results to cosmological phase transitions, gravitational-wave astronomy, and condensed matter systems.

The large-scale limits on the relic signals of gravitational radiation complement the bounds coming from the interferometric detectors (in the audio band) and from the pulsar timing arrays (in the nHz range). Within this inclusive perspective, the spectral energy density of the gravitons is sharply suppressed in the aHz region even though the high-frequency signal can be comparatively much larger both in the kHz and GHz domains. For there are no direct tests on the expansion rate prior to the formation of the light nuclei, a modified postinflationary timeline affects the total number of e-folds and additionally suppresses the tensor-to-scalar ratio by making the relic signals effectively invisible in the aHz range. The expansion rate prior to nucleosynthesis is further bounded by the evolution of the hypercharge field and the large-scale magnetism also constrains the decelerated expansion rate. The magnetogenesis requirements are compatible with a potentially detectable spectral energy density of the relic gravitons between the MHz and the THz while the tensor-to-scalar ratio remains suppressed in the aHz region. A maximum of the spectral energy density of the gravitons in the audio domain leads instead to a larger magnetic field when the scale of the gravitational collapse of the protogalaxy (of the order of the Mpc) gets comparable to the Hubble radius before equality. Along a converse viewpoint, the results obtained here imply that a long decelerated stage expanding faster than radiation does not affect the high-frequency range but reduces the effective number of e-folds by so enhancing the tensor-to-scalar ratio, possibly beyond its observational limit.

Gravitational observables for binary systems exhibit a simple polynomial dependence on the masses m<SUB>1</SUB>, m<SUB>2</SUB> of the two scattering objects when they are written in terms of the appropriate kinematic variables in the post-Minkowskian (PM) regime. We point out that this property, combined with particle interchange symmetry, allows one to reconstruct the leading and subleading PM waveforms from their probe limit, m<SUB>1</SUB> ≫ m<SUB>2</SUB>. As an application, focusing on their re-expansion in the small-velocity or post-Newtonian (PN) regime, we calculate the probe-limit waveforms up to 10PN, and then exploit this observation to deduce from them the waveforms for generic masses in the center-of-mass frame up to 5PN. To this end, we employ both the amplitude-based waveform integrands and the tail formula. This combined approach simplifies substantially the PN expansion of the full subleading PM waveform.

We propose a quantum-enhanced sensing scheme for the detection of wavelike dark matter and high-frequency gravitational waves using two-dimensional ion crystals in a Penning trap. The protocol employs spin-motion squeezed states to improve the signal-to-noise ratio and enable a super-Heisenberg scaling with respect to the number of ions over a broad parameter range. We analyze the sensitivity of the protocol to representative wavelike dark matter candidates, including the axionlike particle and the dark photon, as well as to high-frequency gravitational waves, taking into account the decoherence and dephasing of the ion spins. Our results indicate that two-dimensional ion crystals and this new protocol provide a promising platform for probing previously unexplored parameter space in searches for light dark matter and high-frequency gravitational waves.

We computethe transmission properties of electromagnetic (EM), gravitational wave, and static gravitational perturbations through geometric throats in spherically symmetric spacetimes. On the ultrastatic Ellis-Bronnikov background, decomposition of the four-dimensional Maxwell equations into vector spherical harmonics yields an effective Schrödinger problem with centrifugal barrier <inline-formula><mml:math><mml:msubsup><mml:mi>V</mml:mi><mml:mo>ℓ</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mi>EM</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mo>ℓ</mml:mo><mml:mo>(</mml:mo><mml:mo>ℓ</mml:mo><mml:mo>+</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo><mml:mo>/</mml:mo><mml:mo>(</mml:mo><mml:msup><mml:mi>σ</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:msubsup><mml:mi>r</mml:mi><mml:mn>0</mml:mn><mml:mn>2</mml:mn></mml:msubsup><mml:mo>)</mml:mo></mml:math></inline-formula> peaked at the throat. For the lowest physical EM mode (<inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:math></inline-formula>), frequencies below the barrier-top frequency <inline-formula><mml:math><mml:msub><mml:mi>ω</mml:mi><mml:mi>max</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:msqrt><mml:mn>2</mml:mn></mml:msqrt><mml:mo>/</mml:mo><mml:msub><mml:mi>r</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula> are strongly suppressed by sub-barrier tunneling. Gravitational wave perturbations (<inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>≥</mml:mo><mml:mn>2</mml:mn></mml:math></inline-formula>) see a qualitatively similar barrier and are likewise strongly suppressed below their respective barrier-top frequencies. By contrast, the static gravitational monopole (<inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:math></inline-formula>), governed by the linearized Einstein equations on the same background, satisfies the source-free conservation law <inline-formula><mml:math><mml:mo>(</mml:mo><mml:msup><mml:mi>a</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:msup><mml:mi>Φ</mml:mi><mml:mo>'</mml:mo></mml:msup><mml:msup><mml:mo>)</mml:mo><mml:mo>'</mml:mo></mml:msup><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:math></inline-formula> with no potential barrier, yielding the exact solution <inline-formula><mml:math><mml:mi>Φ</mml:mi><mml:mo>∝</mml:mo><mml:mi>arctan</mml:mi><mml:mo>(</mml:mo><mml:mi>σ</mml:mi><mml:mo>/</mml:mo><mml:msub><mml:mi>r</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:mo>)</mml:mo></mml:math></inline-formula>. We extend these results to a one-parameter family of throat geometries with varying profile shapes, and to a reflected-Schwarzschild (Damour-Solodukhin-type) wormhole, demonstrating that the qualitative asymmetry—strong sub-barrier suppression for all propagating radiation (<inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>≥</mml:mo><mml:mn>1</mml:mn></mml:math></inline-formula>) versus polynomial attenuation for the static monopole (<inline-formula><mml:math><mml:mo>ℓ</mml:mo><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:math></inline-formula>)—is universal for static, spherically symmetric throats. Numerov integration, WKB estimates, and exact analytical solutions are compared throughout. The results establish a structural constraint-wave asymmetry arising from the multipole decomposition of the field equations, independent of the matter content sourcing the geometry, on a fixed background.

A particular solution to the three-body problem is the circular Lagrange three-body system, where the masses move in circular orbits such that they always constitute an equilateral triangle. Such a system has been found to emit gravitational waves with a waveform similar to that of a binary system. In this work, we study the gravitational waveform degeneracy between quasicircular binary systems and Lagrange three-body systems up to 0.5PN order. Assuming we know the parameters of a given binary system, we determine the parameters of the Lagrange triple that produces the same waveform as that of the binary. We show that there exists a mass quadrupole degeneracy in both the plus and cross modes, characterized by two parameters. We also find that there are binary systems and linearly stable Lagrange three-body systems that can have the same mass quadrupole waveform up to the coalescence time. In such cases, the normalized overlap of the waveforms with respect to the power spectral density of the advanced LIGO design remains above 0.97 as long as the binary has nearly symmetric masses. Beyond the mass quadrupole, there is a unique degeneracy at the 0.5PN. However, the Lagrange triple that satisfies this degeneracy is unstable.

Recent advances in the observation of black-hole candidates have renewed interest in probing their near-horizon structure and in searching for departures from the standard singular solutions of general relativity. In this context, significant effort has been devoted to regular black holes and to horizonless black-hole mimickers, motivated primarily by quantum-gravitational effects. Depending on the value of the regularization parameter relative to the object mass, typical spherically symmetric solutions can describe either of these two scenarios. Regular black-hole configurations generically feature an outer and an inner horizon surrounding a maximally symmetric core; the inner horizon in turn triggers mass inflation and semiclassical instabilities. The horizonless branch of the same solutions, by contrast, supports stable inner light rings when sufficiently compact, yet is itself subject to instabilities associated with long-lived quasinormal modes. Here, we investigate how to emulate these spacetimes in an analog-gravity platform based on surface-gravity waves in a shallow-water basin, with the aim of reproducing these instabilities experimentally. We begin by identifying the flow profiles and boundary conditions required to replicate the relevant effective geometries. In particular, we show that the inner-core metrics can be simulated with a nonrotating central-drainage configuration, and we propose a graded-drainage profile to connect them to an asymptotically flat exterior region. We then assess the experimental feasibility of studying the instabilities mentioned above with current technology. Our conclusion is that, while the required setup is realizable in principle, alternative media, such as Bose-Einstein condensates, may offer a more practical route to faithfully capturing the targeted physical features.

Primordial black holes (PBHs) provide a well-motivated nonparticle candidate for dark matter, requiring an enhancement of curvature perturbations on small inflationary scales consistent with observational constraints. In this work we study PBH production within Horndeski gravity, accounting for compatibility with the GW170817 constraint on the gravitational-wave (GW) speed and imposing a constant coupling to the Ricci scalar. Under these conditions, and assuming an inflaton field characterized by a canonical kinetic term and a smooth potential, the inflationary dynamics is controlled by the cubic Horndeski interaction. By investigating standard functional forms of the latter we identify the specific kinetic structure that allows enhancement of the effective friction on the inflaton, thereby inducing a transient ultraslow-roll phase embedded within a standard slow-roll evolution. For representative parameter choices we find that pronounced amplifications in the scalar power spectrum are generated, leading to the formation of asteroid-mass PBHs with masses of order <inline-formula><mml:math><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>16</mml:mn></mml:mrow></mml:msup><mml:mo>)</mml:mo><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula>, which can account for a substantial fraction of the dark matter abundance, reaching <inline-formula><mml:math><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mi>PBH</mml:mi></mml:mrow></mml:msub><mml:mo>≃</mml:mo><mml:mn>0.9</mml:mn></mml:math></inline-formula>, while satisfying current observational constraints. The resulting characteristic features in the scalar power spectrum also imply potentially observable scalar-induced gravitational-wave signatures.

Quantum field theory (QFT) on fractal spacetimes is a program aiming at quantizing the gravitational interaction consistently at all energy scales thanks to an intrinsically or dynamically induced multiscale or multifractal-like spacetime geometry that regularizes the infinities of standard QFT. We reach the goal of this program and formulate a field theory of quantum gravity which is shown to be super-renormalizable and unitary at all perturbative orders. Viable and unviable ways to test this proposal through black holes and gravitational waves are discussed.

We investigate the dynamical and thermodynamic evolution of a Schwarzschild black hole in Modified Gravity (MOG) perturbed by a scalar gravitational wave breathing mode. By evaluating the linearized modified Einstein equations at the near-horizon boundary, we reduce the spatial wave operator to a closed-form temporal ordinary differential equation, thereby explicitly deriving the damped-oscillatory kinematics of the scalar strain. Using a quasi-adiabatic approximation, we show that the effective surface gravity and dynamical temperature are linearly modulated by the perturbation amplitude and velocity. These rapid geometric fluctuations break the semiclassical adiabatic regime, triggering explicitly non-thermal particle creation analogous to the dynamical Casimir effect. Furthermore, we resolve a local thermodynamic paradox concerning apparent horizon area fluctuations. We prove that first-order geometric perturbations <mml:math><mml:mrow><mml:mi>O</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mrow><mml:mi>h</mml:mi></mml:mrow><mml:mrow><mml:mi>b</mml:mi></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> are fully reversible kinematic artifacts, whereas irreversible entropy generation is a strictly second-order <mml:math><mml:mrow><mml:mi>O</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:msubsup><mml:mrow><mml:mi>h</mml:mi></mml:mrow><mml:mrow><mml:mi>b</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msubsup><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> effect driven by the Raychaudhuri expansion, thereby preserving the Generalized Second Law. Finally, we apply these mechanisms to the black hole information paradox. We show that treating the MOG deformation parameter as a quantum-scale running coupling, <mml:math><mml:mrow><mml:mi>α</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>M</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>, mathematically decouples the effective gravitational charge from linear mass scaling. This dynamically forces the evaporating black hole toward the extremal limit (<mml:math><mml:mrow><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:msub><mml:mo>→</mml:mo><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>), smoothly quenching the Hawking temperature to zero and yielding a thermodynamically stable, information-preserving remnant.

We numerically study a formation of near extremal horizons from a gravitational collapse of radially symmetric gravitational waves in <inline-formula> <mml:math><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:math></inline-formula> dimensions within the framework of pure Einstein gravity with positive cosmological constant. Evolution of a regular initial data with cosmological horizon leads to a formation of a black hole with mass exceeding <inline-formula> <mml:math><mml:mrow><mml:mn>99</mml:mn><mml:mi>%</mml:mi></mml:mrow></mml:math></inline-formula> of the extremal value corresponding to the black hole and cosmological horizons coinciding. We demonstrate how our results fit within the framework of characteristic gluing, and present some evidence that the third law of black hole thermodynamics may not hold in the cosmological context, where the extremality corresponds to the maximal mass of the Schwarzschild black hole in de-Sitter space.

We present a framework based on the standard type-I seesaw model that relates the baryon asymmetry of the universe to the dark matter (DM) density. The framework, which we name Asymgenesis, relies on the presence of primordial charge asymmetries seeded either in the dark sector or in the visible sector. A higher-dimensional portal operator reshuffles this initial asymmetry into both sectors, eventually resulting in a nonzero <mml:math><mml:mrow><mml:mi>B</mml:mi><mml:mo>−</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math> asymmetry and an asymmetric DM component. Compared to conventional asymmetric-dark-matter (ADM) schemes, our framework imposes far milder requirements on the portal interaction. In particular, the portal interaction need not violate <mml:math><mml:mrow><mml:mi>B</mml:mi><mml:mo>−</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math>, and the temperature scales of efficient <mml:math><mml:mrow><mml:mi>B</mml:mi><mml:mo>−</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math> violation and efficient charge-transfer interaction mediated by the portal operator can be separated. We develop the formalism in detail and argue that the flexibility of our framework enlarges the model-building landscape for ADM.

We consider the impact on cosmological first-order phase transitions (FOPTs) of low-temperature thermal corrections to the effective potential. These are corrections from degrees of freedom whose field-dependent masses in the true vacuum are much larger than the nucleation temperature, though in the false vacuum the field-dependent masses may be much smaller than the nucleation temperature. We focus on the regime in which the thin-wall approximation is valid and in which these corrections are small enough that they do not alter the vacuum structure of the theory. Although the general form of these corrections to the thermal effective potential can be quite complicated, we argue that the net effect of all such corrections can be well modeled in this limit with a single new parameter. We determine the shift in the parameters of the FOPT in terms of this new parameter and the impact on gravitational wave signals and cosmological observables.