We construct the Kerr-Schild classical double copy of the black string in the Randall-Sundrum II model, deriving the single and zeroth copies and verifying the associated field equations. The single copy gauge field is independent of the holographic coordinate and satisfies a sourceless Maxwell equation on the curved background, in direct analogy with the Coulomb field of the Schwarzschild double copy. The zeroth copy scalar obeys a modified Klein-Gordon equation with a first-order derivative term along the extra dimension; a field redefinition yields a standard Klein-Gordon equation with effective mass $m^2=12/l^2$, induced by the warp factor. We further show that an alternative Kerr-Schild splitting, gravitationally equivalent to the canonical one, produces a physically inequivalent double copy: the gauge field is supported by a conserved but delocalized bulk current, and the zeroth copy satisfies a massless equation that carries no imprint of the warped extra dimension.

We study entanglement harvested by Unruh-DeWitt detectors following Rindler trajectories in compactified and superposed Minkowski spacetime. We consider configuration in which the direction of acceleration (both parallel and antiparallel), separation between detectors and direction of spatial compactification mutually perpendicular to each other. Using the standard entanglement harvesting protocol, we analyze how these features influence the extracted correlations. When detector separation is perpendicular to the direction of acceleration, the harvested entanglement is uniformly suppressed due to increased spacelike separation. Compactification enhances field correlations leading to an increased concurrence and an extended harvesting range at higher accelerations. Additionally, we show that spacetime superposition introduces interference effects that further enlarge the entanglement harvesting region in parameter space, particularly in the high acceleration regime. We also find that the effect of antiparallel acceleration yielding significantly higher entanglement than parallel acceleration prevails in compactified and superposed spacetime.

Tidal Love numbers and other response coefficients of black holes sometimes exhibit a logarithmic dependence on scale, or “running.” We clarify that this coefficient is directly calculable from the structure of the equation obeyed by the field perturbation, and requires no knowledge of the full solution. The derived formula allows us to establish some general results on the existence of logarithmic running. In particular, we show that any static and spherically symmetric spacetime that modifies the Schwarzschild or Reissner-Nordström solutions in a perturbative way must have nonzero logarithmic Love numbers. This applies for instance to all regular black hole metrics. On the other hand, our analysis highlights the importance of the perturbativity assumption; without it, we find explicit black hole solutions beyond general relativity with exactly zero running. We also illustrate the advantage of our method by recovering and extending the known results for the Hayward metric.

Pulsar timing arrays (PTAs) observations will detect gravitational waves (GWs) from the early inspiral phase of supermassive black hole binaries (SMBHBs) with orbital periods of weeks to years. Current PTA analyses generally assume circular binaries; however, dynamical interactions with the surrounding environment can prevent complete circularization, allowing SMBHBs to retain appreciable eccentricities. In this work, we present a gravitational waveform model for eccentric binaries based on the effective-one-body (EOB) formalism, designed for continuous GW searches in PTA data. The model is accurate up to the second post-Newtonian (2PN) order for the conservative dynamics and up to postleading order for the radiation-reaction terms. We provide both a numerically precise and a computationally efficient approximate implementation and evaluate the latter’s accuracy against the full model over a broad range of eccentricities and initial orbital frequencies. Our results show that a substantial region of the parameter space exhibits pronounced orbital evolution, much stronger than in the circular case. We demonstrate the rich harmonic structure of timing residuals induced by eccentric GWs. Properly characterizing eccentric binaries is an essential step toward detecting GWs in PTA data and interpreting the results, ultimately improving our understanding of the supermassive black hole population in the local Universe.

īThe search for dark matter candidates includes primordial black holes (PBHs) as possible constituents. Recent studies show that some PBHs can survive to the present epoch by gaining angular momentum through Hawking radiation of photons and becoming extremal before complete evaporation. While this provides a plausible model in a two-derivative theory of gravity, additional issues arise in effective field theory (EFT)-corrected theories of gravity. In such theories, a rapidly spinning black hole can lead to extremely high tidal forces on a near-horizon observer, with possible observational consequences. In this work, by modeling Hawking radiation as a biased random walk within an EFT of gravity, we show that nearly the same fraction of PBHs survives as in Einstein’s two-derivative theory of gravity. We argue that the resultant near-horizon tidal effects should be detectable in future gravitational-wave observables.

We study how environmentally driven spectral instabilities of quasinormal modes respond to parity violating gravitoscalar coupling in black holes. Focusing on dynamical Chern-Simons gravity as a paradigm for parity violation, we perturb the Schwarzschild background with a localized potential bump. Our analysis reveals three distinctive phenomena absent in general relativity: (1) branch reconnections in the complex frequency plane, (2) a counterintuitive mode stabilization that delays overtaking transitions, and (3) scalar mode dominance emerging at intermediate coupling strengths. These frequency domain features show how comparatively weak static sector differences manifest as distinct dynamical signatures, thereby linking parity violating black hole perturbations with non-Hermitian spectral physics. Our results provide a frequency domain characterization of parity violating coupling and motivate future targeted ringdown studies of modified gravity.

For electromagnetism in Minkowski spacetime, the Bessel-Hagen method gives a particularly direct Noetherian derivation of the standard gauge-invariant energy-momentum tensor. The key step is to supplement the form variation generated by an infinitesimal coordinate transformation with a compensating electromagnetic gauge transformation. In this paper we ask whether the same idea can be applied to the massless spin-2 field described by the Fierz-Pauli action. We first prove that no nonzero local tensor quadratic in first derivatives of the symmetric field $h_{\mu \nu }$ can be strictly invariant under the spin-2 gauge transformation $h_{\mu \nu }\mapsto h_{\mu \nu }+{\partial }_{\mu }{\xi }_{\nu }+{\partial }_{\nu }{\xi }_{\mu }$; the direct electromagnetic analogue of the Bessel-Hagen construction therefore cannot exist. Once the inexact nature of the Fierz-Pauli gauge symmetry is treated correctly, however, the Bessel-Hagen construction does produce a gauge-invariant equivalence class of Noether currents. Changing the compensating spin-2 gauge parameter changes the current only by terms proportional to the Fierz-Pauli field equations; performing an independent spin-2 gauge transformation on $h_{\mu \nu }$ changes the current only by a trivial current given by the divergence of an antisymmetric superpotential plus field-equation terms. This provides the natural spin-2 analogue of Bessel-Hagen’s electromagnetic construction, but only in the quotient space of conserved currents, and not as a preferred local gauge-invariant energy-momentum tensor.

Bouncing cosmologies offer an alternative to inflation by resolving the initial singularity through a contracting phase followed by a bounce into expansion. In many such models, the contracting phase is dominated by a single matter component, typically pressureless dust, which leads to an almost scale-invariant spectrum of scalar cosmological perturbations with a slight blue tilt, so that generating the observed red-tilted spectrum within this framework was challenging. In this work, we consider a more realistic scenario in which the contracting phase includes both matter and radiation, as required on physical grounds. We show that the presence of radiation can naturally induce a red tilt in the spectrum of curvature perturbations seeded by quantum vacuum fluctuations in the remote past of the contraction. Since the perturbations of the two fluids are coupled via gravity, vacuum initial conditions must be carefully defined. We demonstrate that, without fine-tuning, the resulting entropy perturbations are subdominant with respect to curvature perturbations. This suggests that a minimal two-component bounce model, involving only ordinary matter and radiation, can connect to the standard expanding cosmology with observationally viable initial conditions.

We investigate a quintessence model involving two scalar fields with double-exponential potentials. This configuration allows the system as a whole to emulate the dynamics of a single field with a shallower potential, enabling scalar fields that individually cannot drive cosmic acceleration to collectively achieve and sustain it. We assess the viability of this model by performing a fully Bayesian analysis and confronting its predictions with observational data, including the Planck 2018 cosmic microwave background (CMB) shift parameters, the newly released Dark Energy Spectroscopic Instrument (DESI) DR2 baryon acoustic oscillation (BAO) measurements, and the Dark Energy Survey Year 5 (DESY5) type Ia supernova (SnIa) sample. Our analysis shows that the two-field quintessence model yields a log Bayes factor relative to the flat $\mathrm{\Lambda }$ cold dark matter of $\mathrm{\Delta }\ln B\sim 4$, indicating moderate evidence against the latter. We also find that the central values of the two slopes of the exponential potentials are both close to 1, whereas the slope of an effective single-field system is constrained to be less than order unity. This property is theoretically desirable from the perspective of higher-dimensional theories. Thus, the two-field quintessence model with exponential potentials provides a physically motivated and compelling mechanism that is consistent with both observational and theoretical requirements.

We study binary mergers of dark matter cores in the Bose-Einstein condensate (BECDM) model. We include two scenarios: scalar self-interaction and the presence of a gravitationally coupled ideal gas. Using 3D simulations of the Gross-Pitaevskii-Poisson and Schrödinger-Poisson-Euler systems, we analyze the properties of the resulting remnants. We find that the final core-mass ratio reaches a stable average value after the merger. Repulsive self-interaction increases the mass of the final solitonic core, while attractive interaction enhances mass loss. In mergers involving an ideal gas, namely of fermion-boson stars, a stable solitonic core always forms in the bosonic component, even when the gas dominates, whereas the gas itself does not form a compact core. We explain these results using energy scalings and find that without self-interaction, equilibrium cores follow $E\propto -M^3$, which leads to an almost universal merger fraction. Self-interaction changes this scaling, because repulsive $g$ moves the system toward a milder $E\propto -M^2$ scaling and increases mass retention, while attractive $g$ strengthens binding and favors mass ejection. In the case of interaction with an ideal gas, this component only modifies the gravitational background and does not change the intrinsic scaling of the bosonic part. These results show that the merger outcome is not universal, but controlled by the interaction strength, while solitonic BECDM cores remain robust across diverse environments, including gas.

Recent studies of Galactic surveys, such as Gaia, have revealed that the Milky Way’s gravitational potential comes from a matter distribution that is triaxial and rotated with respect to the Galactic center-Sun axis. This, in turn, could mean that the dark matter halo also shares these properties. In this work, by fitting to the Fermi-LAT gamma-ray observations, we test the compatibility of the morphology of the Galactic Center excess (GCE) from dark matter annihilation with a triaxial dark matter halo. In particular, we consider both untilted triaxial halos and halos whose principal axes are tilted with respect to the Galactic disk. In our fits of the Fermi-LAT data, by testing over a large library of galactic diffuse emission models, we quantify how the halo triaxiality and tilt affect the line-of-sight-integrated annihilation signal and, consequently, the preferred GCE spatial templates. We find that the GCE spectrum and inner cuspiness are robust against variations in the triaxiality and tilt of the dark matter halo. However, in terms of its overall morphology, the GCE in the gamma-ray data can discriminate between choices for the dark matter halo’s triaxiality and tilt. Finally, we find that the GCE is more compatible with originating from a triaxial and tilted halo of dark matter than originating from a triaxial and tilted halo of stars, a result important for understanding the GCE’s origin.

To address the computational bottleneck of analyzing type II supernova samples from surveys like Legacy Survey of Space and Time, we present two stella-based neural network surrogates: the interaction model for low-energy explosions with potential circumstellar material (CSM) interaction and the photospheric model for standard interaction-free SNe IIP. Each surrogate follows a two-stage design in which an autoencoder first compresses stella spectral energy distributions (SEDs) into a latent representation and a separate neural-network emulator then maps physical parameters to that latent space. Both models incorporate latent mixup regularization to improve latent-space continuity, while using different network backbones tailored to their respective regimes: ResNet blocks for the interaction model and 2D CNNs for the photospheric model. On test sets, the interaction model and photospheric model achieve normalized SED reconstruction MSEs of $\approx 9.1\times {10}^{-5}$ and $1.0\times {10}^{-4}$, respectively. For the low-luminosity SN 2005cs, the interaction model favors a low-mass progenitor ($M_{\mathrm{ZAMS}}=10.40_{-0.05}^{+0.04}{M}_{\odot }$) with evidence for confined dense CSM, highlighting the likely presence of CSM interaction and offering a physical scenario consistent with direct imaging that helps resolve the historical mass discrepancy. For SN 2012aw, serving as a validation benchmark, the interaction model demonstrates consistency with previous studies by recovering a progenitor mass ($M_{\mathrm{ZAMS}}=11.05_{-0.06}^{+0.06}{M}_{\odot }$). For the archetypal SN 1999em, the photospheric model derives a progenitor mass ($M_{\mathrm{ZAMS}}=10.05_{-0.04}^{+0.07}{M}_{\odot }$) broadly consistent with direct preexplosion imaging limits without explicit CSM modeling, demonstrating that the photospheric model captures the essential physics of standard type IIP explosions. This model-level implementation can reduce full Bayesian parameter-inference runtime from days to minutes, providing a practical foundation for near-real-time physical characterization in large survey streams.

Stellar toroidal magnetic fields are known to be unstable to the Tayler instability. Here we demonstrate the existence of a complementary current-driven instability of essentially arbitrary toroidal-field configurations in stably stratified nonrotating stars with the following properties: (i) in ideal magnetohydrodynamics, it grows on the Alfvén timescale ${\tau }_{\mathrm{A}}$; (ii) under certain conditions, it may reveal itself by driving shellular differential rotation about an arbitrary axis perpendicular to the magnetic-field symmetry axis; (iii) it is large scale in the angular directions $\theta$ and $\phi$, and develops at radial wave numbers $k\lesssim \mathcal{N}{\tau }_{\mathrm{A}}/R$, where $\mathcal{N}$ is the Brunt-Väisälä frequency and $R$ is the stellar radius. Thus, unlike the Tayler instability, the proposed instability is intrinsically global. Consequently, it may be less susceptible to dissipative suppression than the Tayler instability and can prevail over it in some regimes. This instability may have broad implications for magnetic field generation in stars and could modify scenarios of magnetic field amplification within the Tayler-Spruit dynamo, contributing to models of efficient angular-momentum transport and chemical mixing in stellar interiors.

Pair production in spatially inhomogeneous chirped electric fields with super-Gaussian pulse shape is investigated using the Dirac-Heisenberg-Wigner formalism, and the effect of super-Gaussian pulse shapes on the reduced momentum spectrum and the reduced total yield of created particles is mainly concerned. It is found that with the variation of the super-Gaussian envelope exponent, the momentum spectrum exhibits more pronounced oscillations, shifting, and broadening. The total yield of created particles increases monotonically with the increase of the super-Gaussian envelope exponent in the high-frequency fields with small chirp and low-frequency fields with any chirp. Meanwhile, the total yield of created particles under the super-Gaussian pulse electric fields is approximately twice that produced with the conventional Gaussian pulse envelope. These results can provide theoretical guidance for optimizing the form of external field to enhance the vacuum pair-production rate.

The ordering of neutrino masses remains a key unknown in particle physics and cosmology. While upcoming oscillation experiments are expected to determine the mass ordering at low energies, it is important to explore complementary probes that access the underlying mechanism of neutrino mass generation. In this work, we show that future high-energy electron-positron colliders can provide sensitivity to the neutrino mass ordering through the lepton-flavor structure of heavy neutral lepton (HNL) interactions. In the minimal type-I seesaw scenario with two nearly degenerate HNLs, the flavor composition of the heavy–light neutrino mixings is strongly correlated with the light-neutrino mass spectrum, leading to distinct collider signatures for normal and inverted mass orderings. We demonstrate that future $Z$ factories such as CEPC and FCC-ee can probe these flavor patterns over a wide region of parameter space, establishing collider searches for HNLs as a complementary approach to neutrino mass ordering studies.

We investigate the total scattering cross section ($\sigma$), the differential cross section $\left(\frac{d\sigma }{d{Q}^2}\right)$, the longitudinal [$A_L(E_e,Q^2)$] and perpendicular [$A_P(E_e,Q^2)$] spin asymmetries of the polarized target proton, as well as the longitudinal [$P_L(E_e,Q^2)$], perpendicular [$P_P(E_e,Q^2)$], and transverse [$P_T(E_e,Q^2)$] polarization components of the final neutron, in the weak charged current induced electron-proton scattering relevant to the future experiments at the Thomas Jefferson National Accelerator Facility (JLab) and Mainz Microtron (MAMI). The analysis is performed assuming time-reversal (T) invariance as well as without assuming T invariance, allowing for a nonvanishing transverse polarization component of the final nucleon, perpendicular to the production plane. Numerical results are presented for the above mentioned observables, and their sensitivities to the various parametrizations of the axial vector form factor $g_1(Q^2)$ and a nonzero weak electric form factor $g_2(Q^2)$ are examined. We find that the cross section depends strongly on the parametrizations used for the axial vector form factor. Moreover, the dipole parametrization of $g_1(Q^2)$ with a higher value of the axial dipole mass $M_A$ simulates the apparent enhancement in $\sigma$ obtained using the nondipole parametrizations like the $z$-expansion and Faddeev equation form. The cross sections are found to depend only weakly on the weak electric form factor $g_2(Q^2)$, which is associated with the violation of G-invariance. On the contrary, the spin observables both $A_{L,P}(E_e,Q^2)$ and $P_{L,P}(E_e,Q^2)$ are found to be strongly dependent on $g_2(Q^2)$. This study may be useful in the analysis of the neutrino oscillation experiments to provide an alternative constrain on the parametrization of axial vector form factor, which currently has large uncertainties.

Color-kinematics duality states that the kinematic numerators of the cubic tree-level Yang-Mills scattering amplitudes obey the same symmetry properties that the color factors obey due to the Jacobi identity. We present a novel strategy for deriving this duality, based on the differential forms on a superspace. This space of superforms carries a generalization of a Batalin-Vilkovisky (BV) algebra (${\mathrm{BV}}^{\square }$ algebra). We show that the homotopy algebra of color-stripped Yang-Mills theory is obtained as a quotient of this space in which a subspace, which is an ideal “up to homotopy,” is modded out. This algebra is a subsector of a ${\mathrm{BV}}_{\infty }^{\square }$ algebra. Deriving the latter would provide a first-principle proof of color-kinematics duality from field theory.

Gravitational waves (GWs) are unique messengers as they travel through the Universe without alteration except for gravitational lensing. Their long wavelengths make them susceptible to diffraction by cosmic structures, providing an unprecedented opportunity to map dark matter substructures. Identifying lensed events requires the analysis of thousands to millions of simulated events to reach high statistical significances. This is computationally prohibitive with standard GW parameter estimation methods. We exploit dingo-lensing, a deep-learning algorithm that accelerates the inference from CPU days to minutes to thoroughly reanalyze GW231123, the most promising lensing candidate to date. By performing more than 200,000 simulations with three different waveform models, we find that its statistical significance is below $4\sigma$ and the event cannot be claimed as lensed. We observe that 8% of GW231123-like nonlensed simulations favor lensing, which could be explained by the self-similarity of short-duration signals. Still, 58% of GW231123-like lensed simulations have larger support for lensing, showing that higher detection statistics are possible. We show that analyzing simulations with different waveform models only lowers the significance, highlighting the relevance of waveform systematics. Although GW231123 exposes the challenges of claiming the first GW lensing detection, our deep-learning methods have demonstrated to be powerful enough to enable the upcoming discovery of lensed GWs.

We propose hydrogenated carbon structures as targets with a remarkable sensitivity to dark matter–nucleon interactions, in the mass range between 1 MeV and 100 MeV. The ejection of a proton following the interaction with a dark matter particle is a quasielastic process, with an extremely small energy threshold and a clear experimental signature. The proposed detectors are simple, technologically ready, and inexpensive. Yet, they can be considerably more sensitive than current experiments. They also allow strong directionality to be used toward efficient background rejection.

We present a novel framework for deriving on-shell recursion relations, with a specific focus on biadjoint and pure Yang-Mills theories. Starting from the double-cover Cachazo-He-Yuan factorization formulas, we identify a suitable set of independent kinematic variables that enables the reconstruction of amputated currents from amplitudes. As a by-product, this new recursive structure recasts the Bern-Carrasco- Johansson numerators into an explicitly on-shell factorized form.