By employing the perturbiner method, we study the tree- and one-loop-level amplitudes in (anti)self-dual Yang-Mills, focusing on color-kinematics duality and double copy features; they arise naturally even in the fully off-shell case. In particular, we calculate the respective the Kawai-Lewellen-Tye relations for tree-level Berends-Giele currents and color-kinematics master numerators at one loop, both cases for any number of external particles.

The conformally invariant scalar equation permits the Robin boundary condition at infinity of asymptotically AdS spacetimes. We show how the dynamics of a conformal cubic scalar field on the Reissner-Nordström-anti-de Sitter background depend on the black hole size, charge, and choice of the boundary condition. We study the whole range of admissible charges, including the extremal case. In particular, we observe the transition in stability of the field for large black holes at the specific critical value of the charge. Similarities between Reissner-Nordström and Kerr black hole let us suspect that a similar effect may also occur in rotating black holes.

We investigate spontaneous vectorization in the Einstein-Maxwell-vector (EMV) model, introducing a novel mechanism driven by the interplay between electromagnetic and vector fields. A key innovation in our work is the resolution of an apparent divergence in the vector field near the event horizon, achieved by employing a generalized coordinate transformation. This not only extends the domain of existence for vectorized Reissner-Nordström black holes (VRNBHs), but also refines the theoretical understanding of such solutions. We introduce a new concept of combined charge $\sqrt{{\tilde{Q}}^2+{\tilde{P}}^2}$, which better captures the underlying physics of these black holes and provides a unified framework for analyzing thermodynamics and observable phenomena such as light ring structures. Our findings suggest that VRNBHs exhibit enhanced thermodynamic preference and distinctive light ring properties compared to Reissner-Nordström solutions. Moreover, we demonstrate how this combined charge approach reveals connections to two-charge black hole solutions, offering promising avenues for observational verification within the context of effective field theories.

It has been argued in existing literature that the zero mass limit of Kerr spacetime corresponds to either flat Minkowski spacetime or a wormhole exhibiting a locally flat geometry. In this study, we examine that the zero mass limit of the Kerr–modified gravity (MOG) black hole is equivalent to a wormhole. Moreover, we derive the Kerr-Schild form of the Kerr-MOG black hole through specific coordinate transformations. We further investigate the physical and topological characteristics of the Kerr-MOG black hole within the framework of modified gravity. Our analysis also includes a discussion of the wormhole using cylindrical coordinates, which comprises two distinct coordinate patches. Furthermore, we extend our analysis to the Kerr-Newman black hole and show that the zero mass limit of the Kerr-Newman black hole does not yield a wormhole. However, if we impose an additional criterion such that both the mass parameter and the charge parameters are equal to zero, then the Kerr-Newman black hole will be a wormhole.

In any scalar-tensor theory of gravity exhibiting a screening mechanism, the fifth force mediated by the scalar field is dynamically suppressed at sub-Solar system scales, allowing it to pass existing tests of gravity. As a result, a major research effort has been carried out over the past decades to “outsmart” screened scalars in this game of hide-and-seek. While most of such tests rely on fifth force effects, one should keep in mind that the latter are by no means the only physical feature of scalar-tensor gravity. In particular, this article investigates the possibility of testing screened scalar-tensor models by means of gravitational redshift measurements performed with atomic clocks. Upon deriving the expression for the redshift in this framework, we propose a thought experiment for testing the chameleon model by clock comparisons, which guides us toward more realistic experimental setups, in the laboratory and in space. We find that currently unconstrained regions of the chameleon parameter space could be ruled out by future redshift experiments.

Cotton theory (CT) introduces a higher derivative extension of general relativity (GR) characterized by third-rank field equations. Recently, key distinctions between CT and GR concerning wave and vacuum solutions have been highlighted in Gürses et al. [Wave metrics in the Cotton and conformal Killing gravity theories, Phys. Rev. D 110, 084082 (2024).] and Altas and Tekin [Vanishing of conserved charges in Cotton gravity, Phys. Rev. D 111, L021503 (2025).]. In this study, two particular nonvacuum solutions of CT are investigated within its Codazzi formulation. The motivation is to reveal how this theory might account for or adapt to the effects of nonvacuum sources, and whether it can provide new insights into the behavior of both singular and regular black holes in astrophysical contexts. It is shown that CT generalizes the Kiselev and Dymnikova solutions in GR. Some aspects of the generalized solutions, in particular concerning singularities, thermodynamics, and geodesics, are addressed in comparison to GR.

A rotating version of a known static, spherically symmetric, zero Ricci scalar Lorentzian wormhole is constructed. It turns out that for this given nonrotating geometry, the standard Newman-Janis algorithm does not produce a rotating wormhole and, therefore, the method pioneered by Azreg-Aïnou has to be used. The rotating spacetime thus obtained is shown to be regular with wormhole features, though it is no longer a $R=0$ spacetime. The required matter is found to violate the energy conditions, as expected. A few other characteristic properties of this new rotating spacetime are mentioned. Finally, we calculate the shadow for this geometry and discuss its features vis-a-vis the Kerr geometry and available event horizon telescope observations.

Precession in black-hole binaries is caused by a misalignment between the total spin and the orbital angular momentum. The gravitational-wave emission of such systems is anisotropic, which leads to an asymmetry in the $\pm m$ multipoles when decomposed into a spherical harmonic basis. This asymmetric emission can impart a kick to the merger remnant black hole as a consequence of linear momentum conservation. Despite the astrophysical importance of kicks, multipole asymmetries contribute very little to the overall signal strength and, therefore, the majority of current gravitational-wave models do not include them. Recent efforts have been made to include asymmetries in waveform models. However, those efforts focus on capturing finer features of precessing waveforms without making explicit considerations of remnant kick velocities. Here we close that gap and present a comprehensive analysis of the linear momentum flux expressed in terms of multipole asymmetries. As expected, large asymmetries are needed to achieve the largest kick velocities. Interestingly, the same large asymmetries may lead to negligible kick velocities if the antisymmetric and symmetric waveform parts are perpendicular to each other around merger. We also present a phenomenological tool for testing the performance of waveform models with multipole asymmetries. This tool helped us to fix an inconsistency in the phase definition of the imrphenomxo4a waveform model.

Black hole solutions are explored in the Lorentz gauge theory of gravity. The fields of the theory are the gauge potential in the adjoint and a scalar in the fundamental representation of the Lorentz group, a metric tensor then emerging as a composite field in a symmetry-broken phase. Three distinct such phases of the theory are considered. In the SO(3) phase, the fundamental field is identified with a generalized Painlevé-Gullstrand-Lemaître coordinate time. In the static spherically symmetric case it is a stealth scalar, and the general vacuum solution is then parametrized by two constants, one related to the black hole mass and the other to an observer. Also, formulations of pregeometric first order electromagnetism are considered in order to construct a consistent realization of a charged black hole. In the SO(1, 2) phase of the theory, the Schwarzschild solution is a configuration wherein the fundamental field is real outside and imaginary inside the horizon. In this phase the field can be associated with an effective radial pressure resulting in additional singularities and asymptotic nonflatness. Finally, a symmetry-broken phase which would correspond to solutions in an alternative attempt at a Lorentz gauge theory is shown to be incompatible with black holes.

Recently, metrics that describe regular black holes, extreme black holes, or traversable wormholes have been widely discussed. These spacetimes, appearing in scenarios such as the brane world, are contingent on the values of the parameters, with each metric encompassing all three objects. We are considering various known models for these black hole/wormhole interpolating spacetimes and showing that, starting from the macroscopic black holes, all of them must evaporate into macroscopic wormholes, thus avoiding existential problems related to the final stages of black hole evaporation and issues of quantum gravity and black hole remnants. For this purpose, we are calculating the energy emission rates of black holes and the appropriate lifetimes. We argue that some of our conclusions should hold regardless of the specific model, as long as it permits an extremal black hole state with zero temperature at a particular value of the coupling constant.

We investigate the thermodynamics of Reissner-Nordström Gauss-Bonnet (RN-GB) black holes in anti–de Sitter (AdS) space with three horizon geometries ($k=+1,0,-1$) within the grand canonical ensemble. Using the recently developed topological approach to black hole thermodynamics, inspired by Duan’s $\varphi$-mapping theory, we analyze the black holes by treating both critical points in the phase diagram and black hole solutions as defects in the thermodynamic parameter space. Our results show that the Gauss-Bonnet coupling significantly alters the topological classification of RN-GB AdS black holes, distinguishing them from their RN AdS counterparts in the grand canonical ensemble while aligning with their canonical ensemble counterparts. Complementary analyses of local stability using specific heat validate the implication of topological analysis. Furthermore, an evaluation of global stability via Gibbs free energy provides a comprehensive understanding on system phase structure. Notably, for $k=+1$, topological analysis suggests liquid-gas-type phase transitions, whereas global analysis favors Hawking-Page transitions. For $k=-1$, topology indicates a single stable black hole branch, yet the global analysis reveals the presence of Hawking-Page transitions.

Recent proposals suggested quantum clock interferometry for tests of the Einstein equivalence principle. However, atom interferometric models often include relativistic effects only in an ad hoc fashion. Here, instead, we start from the multiparticle nature of quantum-delocalizable atoms in curved spacetime and generalize the special-relativistic center of mass (c.m.) and relative coordinates that have previously been studied for Minkowski spacetime to obtain the light-matter dynamics in curved spacetime. In particular, for a local Schwarzschild observer located at the surface of the Earth using Fermi-Walker coordinates, we find gravitational correction terms for the Poincaré symmetry generators and use them to derive general relativistic c.m. and relative coordinates. In these coordinates we obtain the Hamiltonian of a fully first-quantized two-particle atom interacting with the electromagnetic field in curved spacetime that naturally incorporates special and general relativistic effects.

Matter-wave interferometry is susceptible to noninertial noise sources, which can induce dephasing and a resulting loss of interferometric visibility. Here, we focus on inertial torsion noise (ITN), which arises from the rotational motion of the experimental apparatus suspended by a thin wire and subject to random external torques. We provide analytical expressions for the ITN noise starting from generalized Langevin equations describing the experimental box, which can then be used together with the transfer function to obtain the dephasing factor. We verify the theoretical modeling and the validity of the approximations using Monte Carlo simulations, obtaining good agreement between theory and numerics. As an application, we estimate the size of the effects for the next generation of interferometry experiments with femtogram particles, which could be used as the building block for entanglement-based tests of the quantum nature of gravity. We find that the ambient gas is a weak source of ITN, posing mild restrictions on the ambient pressure and temperature. We discuss the general ITN constraints by assuming a Langevin equation parametrized by three phenomenological parameters.

Black hole binaries with small mass ratios will be critical targets for the forthcoming Laser Interferometer Space Antenna (LISA) mission. They also serve as useful tools for understanding the properties of binaries at general mass ratios. In its early stages, such a binary’s gravitational-wave-driven inspiral can be modeled as the smaller body flowing through a sequence of geodesic orbits of the larger black hole’s spacetime. Its motion through this sequence is determined by the rate at which backreaction changes an orbit’s integrals of motion $E$, $L_z$, and $Q$. Key to the motion being close to a geodesic at any moment is the idea that the effect of backreaction is small compared to a “restoring force” arising from the potential which governs geodesic motion. This restoring force holds the small body on a geodesic trajectory as the backreaction causes that geodesic to slowly evolve. As the inspiraling body approaches the last stable orbit (LSO), the restoring force becomes weaker and the backreaction becomes stronger. Once the small body evolves past the LSO, its trajectory converges to a plunging geodesic. This work aims to smoothly connect these two disparate regimes: the slowly evolving adiabatic inspiral and the final plunge. Past work has focused on this transition to plunge for circular systems. Here, we study the transition for binaries with eccentricity. A well-defined eccentric transition will make it possible to develop small-mass-ratio binary waveform models that terminate in a physically reasonable way, rather than abruptly terminating as an inspiral-only model ends. A model that can explore the parameter space of eccentricity may also be useful for understanding the final cycles of eccentric binaries at less extreme mass ratios, such as those likely to be observed by ground-based detectors.

Models of gravitational collapse provide important means to test whether nonclassical space-time effects, motivated, for instance, by quantum gravity, can be realized in generic ways in physically relevant situations. Here, a detailed analysis of marginally bound Lemaître-Tolman-Bondi space-times is given in emergent modified gravity, which in particular includes a covariant formulation of holonomy modifications usually considered in models of loop quantum gravity. As a result, generic collapse in this setting is shown to imply a physical singularity that removes the bouncing behavior seen in vacuum space-times with the same type of modifications.

The generalization of photon spheres by considering the trajectories of massive particles leads to the definition of massive particle surfaces (MPS). These surfaces, built with the trajectories of massive particles, have a partial umbilicity property. Using the geodesic and Gaussian curvature of the Jacobi metric (a Riemannian metric), we derive a general condition for the existence of a massive particle surface defined for an asymptotically flat spacetime metric. Our results can be applied to the worldlines of charged massive particle surfaces. We provide a simple characterization for timelike and null trajectories using a Riemannian geometric approach. We are able to recover the results for the existence of light rings (LR’s) and timelike circular orbits (TCO’s). We show how an event horizon gets characterized using the curvatures of a Riemannian metric. We discuss several examples, where we derive conditions for the existence of photon sphere and a massive particle surface. We calculate the radius of the photon sphere and the radius of the innermost stable circular orbits (ISCO).

We investigate the scenario in which primordial black holes (PBHs) with masses $M_{\mathrm{PBH}}\lesssim {10}^9\mathrm{g}$ undergo Hawking evaporation, around the big-bang nucleosynthesis (BBN) epoch. The evaporation process modifies the Universe’s expansion rate and the baryon-to-photon ratio, leading to an alteration of the primordial abundance of light nuclei. We present numerical solutions for the set of equations describing this physics, considering different values of PBH masses and abundances at their formation, showing how their evaporation impacts the abundances of light nuclei, obtained by incorporating the nonstandard Hubble rate and baryon-to-photon ratio into the BBN code parthenope. The results are then used to place upper bounds for the PBH relative abundance at formation in the range ${10}^8\mathrm{g}\lesssim M_{\mathrm{PBH}}\lesssim {10}^9\mathrm{g}$, providing the strongest constraints existing to date in this mass range.

The primordial power spectrum of curvature perturbations has been well measured on large scales but remains fairly unconstrained at smaller scales, where significant deviations from $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) may occur. Measurements of 21-cm intensity mapping in the dark ages promise to access very small scales that have yet to be probed, extending beyond the reach of cosmic microwave background and galaxy surveys. In this paper, we investigate how small-scale power-law enhancements—or blue tilts—of the primordial power spectrum affect the 21-cm power spectrum. We consider generic enhancements due to curvature modes, isocurvature modes, and runnings of the spectral tilt. We present forecasts for Earth- and lunar-based instruments to detect a blue-tilted primordial spectrum. We find that an Earth-based instrument capable of reaching the dark ages could detect any enhancements of power on nearly all the scales it can observe, which depends on the baseline of the interferometer. The smallest scales observed by such an instrument can only detect a very strong enhancement. However, an instrument on the far side of the Moon of the same size would be able to probe shallower slopes with higher precision. We forecast results for instruments with 100 km (3000 km) baselines and find that they can probe up to scales of order $k_{\max }\sim 8{\mathrm{Mpc}}^{-1}(k_{\max }\sim 250{\mathrm{Mpc}}^{-1})$, thereby providing invaluable information on exotic physics and testing inflationary models on scales not otherwise accessible.

We investigate the effects of Chern-Simons-Gauss-Bonnet gravity on fundamental metrics. This theory involves perturbative corrections to general relativity, as well as two scalar fields, the axion and the dilaton, which arise from Chern-Simons and Gauss-Bonnet gravity modifications respectively. The combined Chern-Simons-Gauss-Bonnet gravity is motivated by a wide range of theoretical and phenomenological perspectives, including particle physics, string theory, and parity violation in the gravitational sector. In this work, we provide the complete set of field equations and equations of motion of the Chern-Simons-Gauss-Bonnet modified gravity theory for a suite of fundamental metrics (Friedmann-Lemaître-Robertson-Walker, Schwarzschild, spherically symmetric, and perturbed Minkowski), under no prior assumptions on the behavior of the fields. The full set of field equations and equations of motion can be numerically solved and applied to specific observables under certain assumptions, and can be used to place constraints on the Chern-Simons-Gauss-Bonnet modified gravity theory.

We use a custom-made calibrator to measure individual detectors’ polarization angles of BICEP3, a small aperture telescope observing the cosmic microwave background (CMB) at 95 GHz from the South Pole. We describe our calibration strategy and the statistical and systematic uncertainties associated with the measurement. We reach an unprecedented precision for such measurement on a CMB experiment, with a repeatability for each detector pair of 0.02°. We show that the relative angles measured using this method are in excellent agreement with those extracted from CMB data. Because the absolute measurement is currently limited by a systematic uncertainty, we do not derive cosmic birefringence constraints from BICEP3 data in this work. Rather, we forecast the sensitivity of BICEP3 sky maps for such analysis. We investigate the relative contributions of instrument noise, lensing, and dust, as well as astrophysical and instrumental systematics. We also explore the constraining power of different angle estimators, depending on analysis choices. We establish that the BICEP3 2-year dataset (2017–2018) has an on-sky sensitivity to the cosmic birefringence angle of ${\sigma }_{\alpha }=0.078°$, which could be improved to ${\sigma }_{\alpha }=0.055°$ by adding all of the existing BICEP3 data (through 2023). Furthermore, we emphasize the possibility of using the BICEP3 sky patch as a polarization calibration source for CMB experiments, which with the present data could reach a precision of 0.035°. Finally, in the context of inflation searches, we investigate the impact of detector-to-detector variations in polarization angles as they may bias the tensor-to-scalar ratio $r$. We show that while the effect is expected to remain subdominant to other sources of systematic uncertainty, it can be reliably calibrated using polarization angle measurements such as the ones we present in this paper.