We consider a spinning test particle around a rotating black hole and compare the Mathisson-Papapetrou-Dixon (MPD) formalism under the Tulczyjew-Dixon spin supplementary condition to the test-mass limit of the effective-one-body (EOB) Hamiltonian of [T. Damour and A. Nagar, New effective-one-body description of coalescing nonprecessing spinning black-hole binaries, Phys. Rev. D 90, 044018 (2014)] with enhanced spin-orbit sector. We focus on circular equatorial orbits: we first compare the constants of motion at their linear in secondary spin $\sigma$ approximation and then we compute the gravitational-wave (GW) fluxes using a frequency domain Teukolsky equation solver. We find no difference between the EOB and MPD fluxes when the background spacetime is Schwarzschild, while the difference for a Kerr background is maximum for large, positive spins. Our work could be considered as a first step to improve the radiation reaction of the EOB model, in view of the needs of the next-generation of GW detectors.

We derive the analog of the Tolman-Oppenheimer-Volkoff equation in conformal Killing gravity in a static spherically symmetric spacetime, sourced by anisotropic fluid matter. It differs from the original equation by new dark terms associated to a conformal Killing tensor. The formulation of gravity as an Einstein equation augmented by a conserved conformal Killing tensor enables to implement the junction conditions for a sphere of anisotropic fluid with the conformal Killing vacuum. The equations are solved for a perfect fluid sphere with uniform matter density, in the Harada vacuum. The extension of Buchdahl’s equation for the critical radius–mass density is obtained.

We have investigated the polarized images of synchrotron emission from a magnetically charged, spherically symmetric regular Bronnikov black hole (in general relativity coupled to nonlinear electrodynamics) and the singular Reissner-Nordström black hole (in general relativity coupled to Maxwell electrodynamics). By taking into account the fact that, within the framework of nonlinear electrodynamics, the propagation of the light ray is governed not by the null geodesics of the original spacetime, but rather by the null geodesics associated with the effective metric, we have examined synchrotron radiation characteristics near these black holes at different inclination angles. Our calculations reveal that intensity differences between these two black holes are significant, especially at high values of magnetic charges and large tilt angles. Polarization quantities, such as the electric vector polarization angle and normalized intensity difference ($\mathrm{\Delta }I/I$) show similar trends, driven by lensing, Doppler effects, and redshift, offering insights into black hole metrics.

We show that gravity field equations based on a tensor with rank greater than 2 have consistency problems in the sense that integration constants in the solutions, such as the parameter $m$ in the Schwarzschild metric, do not allow for an interpretation in terms of conserved quantities in the theory. The recently introduced conformal Killing gravity, an interesting extension of general relativity that inherits all the solutions of the latter, and defined with a rank-3 tensor field equation that does not arise from a diffeomorphism-invariant action, is plagued with this problem. In this theory, it is not clear at all how one can define the energy and angular momentum for black hole solutions, or define the analogs of the formulas, such as the quadrupole formula, in the weak field limit for gravitational waves emitted by compact sources.

Singular regular points often arise in differential equations describing physical phenomena such as fluid dynamics, electromagnetism, and gravitation. Traditional numerical techniques often fail or become unstable near these points, requiring the use of semianalytical tools, such as series expansions and perturbative methods, in combination with numerical algorithms; or to invoke more sophisticated methods. In this work, we take an alternative route and leverage the power of machine learning to exploit physics informed neural networks (PINNs) as a modern approach to solving ordinary differential equations with singular points. PINNs utilize deep learning architectures to approximate solutions by embedding the differential equations into the loss function of the neural network. We discuss the advantages of PINNs in handling singularities, particularly their ability to bypass traditional grid-based methods and provide smooth approximations across irregular regions. Techniques for enhancing the accuracy of PINNs near singular points, such as adaptive loss weighting, are used in order to achieve high efficiency in the training of the network. We exemplify our results by studying four differential equations of interest in mathematics and gravitation—the Legendre equation, the hypergeometric equation, the solution for black hole space-times in theories of Lorentz violating gravity, and the spherical accretion of a perfect fluid in a Schwarzschild geometry.

The dynamical realization of the equation of state $p+\rho =0$ is studied. A nonpathological dynamics for the perturbations of such a system mimicking a dynamical cosmological constant (DCC) requires to go beyond the perfect fluid paradigm. It is shown that an anisotropic stress must be always present. The Hamiltonian of the system in isolation resembles the one of a Pais-Uhlenbeck oscillator and linear stability requires that it cannot be positive definite. The dynamics of linear cosmological perturbations in a DCC dominated Universe is studied in detail showing that when DCC is minimally coupled to gravity no dramatic instability is present. In contrast to what happens in a cosmological constant dominated Universe, the nonrelativistic matter contrast is no longer constant and exhibits an oscillator behavior at small scales while it grows weakly at large scales. In the gravitational waves sector, at small scales, the amplitude is still suppressed as the inverse power of the scale factor while it grows logarithmically at large scales. Also the vector modes propagate, though no growing mode is found.

We study the degrees of freedom in new general relativity—a flat and metric compatible family of theories—around the Minkowski background in a gauge-invariant manner. First, we confirm the decoupling case, in which the theory reduces to linearized gravity plus a massless Kalm-Ramond field. We then show that, unless they vanish, the vector modes of this theory will be unstable. In addition, we find two new branches of the theories, which are instability-free and propagate linearly two tensor modes and in one of the cases also a massless scalar field. This shows that while the generic theory is ill behaved, there are three possible realizations of instability-free cases, in contradiction to the previous literature, which states that there is only one healthy theory in addition to general relativity.

The Laser Interferometer Space Antenna (LISA) is designed to detect a variety of gravitational-wave events, including mergers of massive black hole binaries, stellar-mass black hole inspirals, and extreme mass-ratio inspirals. LISA’s capability to observe signals with high signal-to-noise ratios raises concerns about waveform accuracy. Additionally, its ability to observe long-duration signals will raise the computational cost of Bayesian inference, making it challenging to use costly and novel models with standard stochastic sampling methods without incorporating likelihood and waveform acceleration techniques. In this work, we present our attempt to tackle these issues. We adapt rift for LISA to take advantage of its embarrassingly parallel architecture, enabling efficient analysis of large datasets with costly gravitational wave models without relying on likelihood or waveform acceleration. We demonstrate that our open-source code can accurately infer parameters of massive black hole binary signals by carrying out a zero-noise injection recovery using the numerical relativity surrogate model NRHybSur3dq8. By utilizing all available $m\ne 0$ modes in the inference, we study the impact of higher modes on LISA data analysis. We study the impact of multiple massive black hole binary signals in a dataset on the inference of a single signal, showing that the selected source’s inference remains largely unaffected. Furthermore, we analyze the LDC-1A and blind LDC-2A datasets from the Radler and Sangria challenge of the LISA data challenges. When eschewing specialized hardware, we find NRHybSur3dq8 injection-recovery takes approximately 20 h to complete, while the analysis of Sangria and Radler datasets takes about 10 h to complete.

Recent evidence for stochastic gravitational waves reported by pulsar timing array (PTA) Collaborations might open a new window for studying cosmology and astrophysical phenomena. In addition to signals from gravitational waves, there is motivation to explore residual signals from oscillating dark matter, which might partially comprise the galactic halo. We investigate fluctuations in pulsar timing originating from the coherent oscillation of scalar dark matter up to the subleading-order correction of $\mathcal{O}(k/m)$, as well as from acoustic oscillations of nonadiabatic perfect fluid dark matter. Both types of dark matter can induce the Newtonian potential and curvature perturbations, thereby affecting pulsar timing. We show distinctive signatures in pulsar timing residuals and angular correlations in the PTA frequency band and considering the known distances of pulsars. For scalar dark matter, both the timing residuals and the angular correlation are sensitive to small variations in the distance, $\delta L$, due to the subleading-order correction of $\mathcal{O}(k/m)$. In contrast, for perfect fluid dark matter, it is insensitive to the $\delta L$. For deterministic sources from scalar dark matter, the distance of a pulsar has influence on the degree of directional dependence of timing residuals, significantly. For stochastic sources from perfect fluid dark matter, the angular correlation tends to a constant and enhances only when the pulsar pair is very close to each other. In this sense, perfect fluid dark matter is shown to be a more suitable physical origin for monopolar signals in angular correlations compared to the scalar dark matter.

We investigate how resourceful gravitational cat states are to preserve quantum correlations. In this regard, we explore the dynamics of gravitational cat states under different situations such as thermal, classical stochastic, general decaying, and power-law noisy fields. In particular, the one-way steerability, Bell nonlocality, entanglement, and purity in two qubits are our main focus. We also address the weak measurement protocol on the dynamics of quantum correlations and purity of the state. Our results show that the gravitational cat states have a reliable and better capacity to preserve quantum correlations and remain one of the good resources for the deployment of quantum information processing protocols. Additionally, two independent channels are also employed, and it is observed that only the weaker coupling regimes are effective in preserving quantum correlations. Notably, in terms of non-Markovian dynamics implication, quantum correlations are found to be longer preserved because of the information feedback phenomenon between the system and environment. Finally, we present a brief analysis to extend our gravitational model to include the electrostatic notion, providing insight into the key differences between the considered configurations.

We consider the effects of quartic self-interactions on the dynamics of a binary system due to a (nearly) massless scalar field conformally coupled to matter. We investigate the deviations from general relativity at the conservative level and put a bound on the self-coupling $\lambda \lesssim ({\beta }^2{G}_N{M}_{\odot }^2{)}^{-1}$ where $\beta$ is the conformal coupling of the scalar to matter. We also consider the radiative sector where we use the Schwinger-Keldysh formalism to find the tail interactions which couple the multipoles of the binary system and induce a small advance of the periastron.

Hotspots, often characterized as pointlike emissions, frequently appear near black holes with significantly enhanced luminosity compared to the surrounding accretion flow. Notably, such hotspots are regularly observed near the black hole at the center of the Milky Way. Light rays emitted from these sources follow complex trajectories around the black hole before reaching distinct locations on the observer’s image plane. Precisely resolving both direct emissions and their higher-order images—despite the latter’s intensity suppression—is essential for extracting detailed spacetime information, including the black hole’s mass, spin, and inclination angle. To improve the accuracy and efficiency of hotspot modeling, we develop a forward ray tracing method based on the analytic integral solution of Kerr geodesics, leveraging conserved quantities. Our approach traces geodesics from a given emission point near the black hole to a distant observer, effectively capturing multiple images with a tailored parametrization scheme for root-finding. By perturbing these geodesics, we map finite-size emissions to distinct regions on the image plane, enabling the quantification of image shapes and amplification rates. This method not only enhances the identification of strongly lensed photons from black holes but also enables efficient spacetime tomography and hotspot localization, leveraging observations from the Event Horizon Telescope and its upcoming next-generation upgrades.

Current gravitational wave (GW) detection pipelines for compact binary coalescence based on matched filtering have reported over 90 confident detections during the first three observing runs of the LIGO-Virgo-KAGRA (LVK) detector network. Decreasing the latency of detection, in particular, for future detectors anticipated to have high detection rates, remains an ongoing effort. In this paper, we develop and test a sparse dictionary learning (SDL) algorithm for the rapid detection of GWs. We evaluate the algorithms biases and estimate its GW detection rate for an astrophysical population of binary black holes. The SDL algorithm is assessed using both simulated data injected into the proposed $\mathrm{A}+\mathrm{detector}$ sensitivity and real data containing confident detections from the third LVK observing run. We find that our SDL algorithm can reconstruct a single binary black hole signal in less than 1 s. This suggests that SDL could be regarded as a promising approach for rapid, efficient GW detection in future observing runs of ground-based detectors.

The so-called Buchdahl limit is not yet fully understood in the context of theories of gravity beyond the Einsteinian framework. In this investigation, we generalize this limit for the case of static, spherically symmetric, relativistic compact stars in $f(R)$ theories of gravity within the metric formalism. We present a comprehensive analysis that ensures regularity, thermodynamic stability, fulfillment of all required junction conditions, recovery of the Newtonian potential at long distances and a correct extraction of the asymptotic mass. Our results are exemplified for the $f(R)=R+\alpha R^2$ Starobinsky model and several realistic equations of state describing neutron-star matter. We also compare these results with the case of compact stars immersed ad hoc in an Schwarzschild exterior vacuum, although this scenario does not fulfill all necessary $f(R)$ junction conditions. To test the validity of viable $f(R)$ models, we show that such stars can indeed host additional energetic content, so their gravitational redshift can be greater than 2, which is prohibited in general relativity.

We investigate the dominant physical effects of superhorizon fluctuations in a flat Friedmann-Lemaitre-Robertson-Walker universe, focusing on whether the combined evolution of scalar and tensor adiabatic modes in the near-horizon regime could lead to geometries beyond those predicted by the conventional separate-universe approach. Assuming a matter-dominated universe and working to first order in perturbations but second order in a gradient expansion, we identify modes that are either pure gauge or unsourced, making them observationally irrelevant. This allows us to derive an effective metric that preserves the spatial symmetries of three well-known Bianchi cosmologies, namely, types I, V, and IX. In this framework, scalar perturbations induce spatial curvature, while the shear arises from long-wavelength tensor perturbations.

Axions have long been considered plausible candidates for dark matter. The axion dark matter emitted from cosmic strings after the Peccei-Quinn (PQ) symmetry breaking in the early Universe was extensively simulated. In this work, we study dark matter and gravitational waves through the lattice simulation of the axion-Higgs string. We gave the dark matter overproduction and the big bang nucleosynthesis bounds on the axion decay constant $f_a$ and the axion mass $m_a$ for axionlike particles, and found that the predicted gravitational wave spectra cannot be probed by the dataset of the current pulsar timing array experiments.

Early dark energy solutions to the Hubble tension introduce an additional scalar field which is frozen at early times but becomes dynamical around matter-radiation equality. In order to alleviate the tension, the scalar’s share of the total energy density must rapidly shrink from $\sim 10\%$ at the onset of matter domination to $\ll 1\%$ by recombination. This typically requires a steep potential that is imposed ad hoc rather than emerging from a concrete particle physics model. Here, we point out an alternative possibility: a homogeneous scalar field coupled quadratically to a cosmological background of light thermal relics (such as the Standard Model neutrino) will acquire an effective potential which can reproduce the dynamics necessary to alleviate the tension. We identify the relevant parameter space for this “thermocoupled” scenario and study its unique phenomenology at the background level, including the back-reaction on the neutrino mass. Follow-up numerical work is necessary to determine the constraints placed on the model by early time measurements.

Warm dark matter has been strongly constrained in recent years as the sole component of dark matter. However, a less-explored alternative is that dark matter consists of a mixture of warm and cold dark matter (MWDM). In this work, we use observations of Milky Way satellite galaxies to constrain MWDM scenarios where the formation of small-scale structure is suppressed either by generic thermal relic warm dark matter or a sterile neutrino produced through the Shi-Fuller mechanism. To achieve this, we model satellite galaxies by combining numerical simulations with semianalytical models for the subhalo population, and use a galaxy-halo connection model to match galaxies onto dark matter subhalos. By comparing the number of satellites predicted by MWDM models to the observed satellite population from the Dark Energy Survey and Pan-STARRS1, we constrain the fraction of warm dark matter, $f_{\mathrm{WDM}}$, as a function of its mass, $m_{\mathrm{WDM}}$. We exclude dark matter being composed entirely of thermal relic warm dark matter with $m_{\mathrm{WDM}}\le 6.6\mathrm{keV}$ at a posterior ratio of $10:1$, consistent with previous works. However, we find that warm dark matter with smaller mass is allowed when mixed with cold dark matter, and that the $f_{\mathrm{WDM}}$ constraints strengthen with decreasing $m_{\mathrm{WDM}}$ until they plateau at $f_{\mathrm{WDM}}\lesssim 0.45$ for $m_{\mathrm{WDM}}\lesssim 1.5\mathrm{keV}$. Likewise, in the case of a sterile neutrino with mass of 7 keV produced through the Shi-Fuller mechanism, we exclude a fraction of $f_{{\nu }_s}\lesssim 0.45$, independent of mixing angle. Our results extend constraints on MWDM to a region of parameter space that has been relatively unconstrained with previous analysis.

We present a computational tool, tweedledee, for empirically modeling diffuse gamma-ray background emission in a 1° region of the sky, using publicly available gamma-ray data off-axis from the region of interest. This background model allows a user to perform a purely data-driven search for anomalous localized sources of gamma-ray emission, including new physics. A major application of this tool would be in searching for dark matter annihilation in newly discovered astrophysical targets. For this purpose, we derive a scaling relation for determining velocity-dependent $J$-factors using only the stellar parameters, which can be broadly applied to obtain dark matter constraints from new targets. As an application of these tools, we use tweedledee and madhatv2 to perform the first search for dark matter annihilation in the newly discovered Leo VI dwarf spheroidal galaxy, and present model constraints for a variety of choices of the annihilation channel and velocity dependence of the cross section.

In dense neutrino environments like core-collapse supernovae (CCSNe) and neutron star mergers, neutrinos can undergo fast flavor conversions when their angular distribution of neutrino electron lepton number ($\nu \mathrm{ELN}$) crosses zero along some directions. While previous studies have demonstrated the detection of axisymmetric $\nu \mathrm{ELN}$ crossings in these extreme environments, nonaxisymmetric crossings have remained elusive, mostly due to the absence of models for their angular distributions. In this study, we present a pioneering analysis of the detection of nonaxisymmetric $\nu \mathrm{ELN}$ crossings using machine learning (ML) techniques. Our ML models are trained on data from two CCSN simulations, one with rotation and one without, where nonaxisymmetric features in neutrino angular distributions play a crucial role. We demonstrate that our ML models achieve detection accuracies exceeding 90%. This is an important improvement, especially considering that a significant portion of $\nu \mathrm{ELN}$ crossings in these models eluded detection by earlier methods.