In the extended scalar-tensor-Gauss-Bonnet (GB) theory, spontaneous scalarization in the ${\mathrm{GB}}^{-}$ regime typically occurs only in rotating black holes, while it is absent in spherically symmetric black holes, a phenomenon known as spin-induced scalarization. However, we find that when the spacetime is permeated by perfect fluid dark matter, spontaneous scalarization can also be induced by dark matter in the ${\mathrm{GB}}^{-}$ regime. Analytical calculations reveal that this scalarization occurs when the dark matter parameter $b/M$ exceeds a critical value $(b/M{)}_{\mathrm{crit}}\simeq 1.86287$, a threshold determined by the lower boundary of the unstable region for scalar perturbations as the coupling constant approaches negative infinity. Additionally, we verified these findings through numerical analysis of the time evolution of scalar perturbations, identifying the unstable parameter region. The results show that when coupling constant $-\lambda /M^2$ is small, spontaneous scalarization only occurs near the extremal black hole limit. As $-\lambda /M^2$ increases, the scalarization region expands; however, its lower boundary remains above $b/M\simeq 1.86287$, consistent with theoretical predictions.

Regular black holes without curvature singularity can arise in Einstein gravity with appropriate matter energy-momentum tensor. We show that these regular solutions represent only a special case of a much broader family of black holes with a free mass parameter. The regularity is achieved only at a specific mass value, and any deviation from the fine-tuned parameter inevitably results in curvature singularity. As a concrete example, we consider nonlinear electrodynamics (NLED) as matter sources. A new NLED theory is proposed that is a generalization of the Bardeen class and the Hayward class. New regular black holes and their singular counterparts are obtained. Significant distinctions between regular black holes and their singular counterparts are analyzed. These findings provide new insights into regular black holes.

We investigate the pseudospectrum of a Schwarzschild-like spacetime within the framework of black hole perturbation theory to analyze a counterintuitive assertion regarding the instability of quasinormal modes. Recent findings suggest that random perturbations to the effective potential associated with gravitational waves may enhance the stability of the underlying wave operator, thereby yielding a stable spectrum of randomly displaced quasinormal modes. Given the unphysical nature of such random perturbations, this work examines these findings within a spacetime that inherently exhibits a perturbed quasinormal spectrum. We find that, in contrast to the quasinormal mode spectrum of the Schwarzschild spacetime under random perturbations, the quasinormal spectrum of a Schwarzschild-like black hole deformed through a physically motivated implementation of the Rezzolla-Zhidenko parametrization is unstable. In particular, we show that the pseudospectra of these Schwarzschild-like black holes do not display the typical features associated with wave operators that yield stable quasinormal spectra. We corroborate our findings by computing the quasinormal spectra when additional (ad hoc) deformations are added to the effective potential of the Rezzolla-Zhidenko black hole. We also argue that, when multiple perturbation sources are present, identifying the origin of the instability may be difficult.

We compute the angular momentum flux from a noncircular nonspinning binary system of compact objects in massless scalar-tensor theories up to one and a half post-Newtonian (1.5PN) order using multipole moments. The angular momentum flux in scalar-tensor theories involves both a tensorial and a scalar contribution, which can be further decomposed as instantaneous, tail, and nonlinear memory effects up to 1.5PN order. We compute the explicit expressions of tail effects using the Fourier decomposition of tensorial and scalar multipole moments, and nonlinear memory effects using the Newtonian order quasi-Keplerian representation of elliptic orbits in scalar-tensor theories. This work is important to construct the radiation-reaction force and hence the waveform templates for eccentric binaries in scalar-tensor theories of gravity.

Motivated by understanding the nonlinear gravitational dynamics of spacetimes admitting stably trapped null geodesics, such as ultracompact objects and black string solutions to general relativity, we explore the dynamics of nonlinear scalar waves on a simple (fixed) model geometry with stable trapping. More specifically, we consider the time evolution of solutions to the cubic (defocusing) wave equation on a four-dimensional static, spherically symmetric, and asymptotically flat (horizonless) spacetime admitting a stable photon sphere. Unlike the known results for scalar waves on spacetimes with unstable trapping, our study shows fundamental differences between linear and nonlinear scalar dynamics. The local energy, as well as all local higher-order energies, of solutions to the linear wave equation on our model spacetime can be rigorously proven to remain uniformly bounded and to decay uniformly in time. However, due to the presence of stable trapping, the uniform decay rate is slow. To help elucidate how the slow linear decay affects solutions to the nonlinear wave equation considered, we examine numerical solutions of the latter, restricting to axisymmetric initial data in this work. In contrast to the linear dynamics, we exhibit a family of nonlinear solutions with turbulent behavior. Within the region of stable trapping, the slow linear decay allows local higher-order energies of the nonlinear solution to grow over the time interval that we numerically evolve. The growth is induced by a direct energy cascade: Beginning with initial data containing a small number of low-order multipole modes, a spectrum of high-order multipole modes are populated in time by the nonlinear interactions and eventually dominate over the low-modes in the evolution. That the system exhibits a direct cascade limits the time over which our numerical scheme can provide convergent solutions for the short-wavelength structure that develops, and hence we can only speculate what this intermediate-time dynamics implies for the nonlinear stability of the motivating spacetimes in general relativity. Nevertheless, we provide a heuristic argument suggesting that, if a similar behavior occurs for gravitational wave perturbations of these spacetimes, it would likely not generically lead to black hole or singularity formation.

We present a systematic analysis of the weak cosmic censorship conjecture (WCCC) through gedanken experiments involving black hole perturbations induced by test particles. Our approach allows for the calculation of perturbations to any order for a general class of black holes that admit the zero-temperature extremal limit. We find that the WCCC for extremal and near-extremal black holes is hinged upon the positive sign of only one quantity, namely $W={(\frac{\partial S}{\partial T})}_{Q_{\alpha };T=0}$, which is indeed positive for all the well-known black holes.

Exact plane gravitational radiation fields are presented within the framework of general relativity and their properties are described. The physics of nonlinear elliptically polarized plane gravitational waves is developed in close analogy with electromagnetic waves. The motion of free test particles in the dynamic gravitational fields of elliptically polarized plane waves is investigated. In particular, we demonstrate the cosmic jet property of these spacetimes, namely, most timelike geodesics tend to line up in the direction of wave propagation and produce a cosmic jet whose speed asymptotically approaches the speed of light.

We present and discuss the possibility, derived from work carried out 20 years ago, that the equations of motion for compact binary neutron stars at the third post-Newtonian (3PN) order in general relativity might actually depend on the internal structure of the bodies. These effects involve integrals over the density and internal gravitational potentials of the bodies that are independent of the mass and radius of the bodies but dependent on their equations of state. These effects could alter the coefficients in the 3PN equations derived using “point mass” methods by as much as 100 %. They were found in independent calculations done at Washington University using the direct integration of the relaxed Einstein equations (DIRE) approach and at the Institut d’Astrophysique de Paris using the multipolar post-Minkowskian (MPPM) approach. Neither calculation was completed because of the enormous complexity of the algebraic computations and the limitations of software of the day (Maple, Mathematica) and because of an assumption that the effects would somehow cancel or be removable by some transformation. This assumption was rooted in the strong equivalence principle (SEP), which would suppress such effects up to the stage where tidal interactions become important, effectively 5PN order for compact bodies. SEP was well supported at lower PN orders and in special examples. We argue that this assumption needs to be verified by actual calculations. If the results show that these structure-dependent terms exactly cancel or can be absorbed into renormalized masses or shifted positions of each body, this would provide remarkable support for the strong equivalence principle of general relativity, but if they do not cancel and are not incorporated into gravitational waveforms, they could severely impact efforts using next-generation gravitational-wave interferometers to extract information about the equation of state for neutron star matter from gravitational-wave signals from binary neutron star or black hole-neutron star mergers.

The iconic, deep-MOND-limit (DML) relation between acceleration and mass, $a\sim (M{\mathcal{A}}_0{)}^{1/2}/r$, implies that, in modified Newtonian dynamics (MOND), accelerations cannot be linear in the mass distribution (${\mathcal{A}}_0\equiv G{a}_0$ is the DML constant, and $a_0$ the MOND acceleration). This leads to important idiosyncracies of MOND, such as a breakdown of the strong equivalence principle, and the resulting “external-field effect” (EFE). I show that the DML axioms are, in themselves, consistent with a, possibly unique, nonrelativistic, action-based, linear formulation of the DML: For a test particle in the gravitational field of a mass distribution $\rho (\mathbf{r})$, the equation of motion is $d^4\mathbf{r}/d{t}^4=\overrightarrow{\nabla }\psi$, where $\psi (\mathbf{r})=-\frac{{\mathcal{A}}_0}{2}\int \frac{\rho ({\mathbf{r}}^{\prime })d^3{\mathbf{r}}^{\prime }}{|\mathbf{r}-{\mathbf{r}}^{\prime }{|}^2}$. This model suffers from important drawbacks, which may make it unacceptable as a basis for a full-fledged MOND theory, which may imply that the answer to the title question is “yes.” The model is unique among MOND theories propounded to date not only in being linear—hence not exhibiting an EFE, for example—but in constituting a modification of both Newtonian inertia and Newtonian gravity. This linear and time-local model inspires and begets several one-parameter families of models. One family employs nonlinear, time-nonlocal kinetic terms, but still linear gravitational-field equations. Other families generalize the DMLs of Aquadratic Lagrangian and quasilinear MOND, modifying gravity as well as inertia. All families employ fractional time derivatives and possibly fractional Laplacians. While, at present, I cannot base some acceptable MOND theory on these models—for example, I cannot offer a sensible umbrella theory that interpolates between these DML models and Newtonian dynamics—they are useful in elucidating various fundamental aspects of MOND. For example, they help to understand which predictions follow from only the basic tenets of MOND—so-called primary predictions—and which are secondary, i.e., theory dependent. For example, they demonstrate how the EFE can be strongly theory dependent, not only in its strength but in its very nature. The models may also help show the way to a wider class of MOND theories. Finally, I describe another family of models that are all linear, but non-MONDian, having Newtonian dynamics at one end and the linear DML model at the other.

In a preceding paper [M A. M. Smith, V. Paschalidis, and G. Bozzola, High-energy interactions of charged black holes in full general relativity. I. Zoom-whirl orbits and universality with the irreducible mass, Phys. Rev. D 111, 104031 (2025)], we initiated a study of high-energy interactions of charged binary black holes near the scattering threshold, focusing on zoom-whirl orbits. In this second paper in our series, we focus on merger remnant properties and energetics with new simulations of equal-mass, equal-charge, nonspinning binary black holes with variable impact parameter. We find near-extremal merger remnants with Kerr-Newman parameter reaching ${\mathrm{\Upsilon }}_f=0.97$, and find evidence that the maximum ${\mathrm{\Upsilon }}_f$ increases monotonically with $\lambda$ for a fixed initial Lorentz factor. We find that binaries with larger $\lambda$ radiate less total energy despite having stronger electromagnetic emission. The maximum energy radiated by a binary in our study is 31% of its gravitational mass. Increasing $\lambda$ has little effect on the maximum angular momentum radiated, which was $\approx 72\%$ of the spacetime total angular momentum for each $\lambda$ explored here. Lastly, we provide additional evidence for the universality with the irreducible mass at fixed initial linear momenta that we discovered in [M A. M. Smith, V. Paschalidis, and G. Bozzola, High-energy interactions of charged black holes in full general relativity I: Zoom-whirl orbits and universality with the irreducible mass, Phys. Rev. D 111, 104031 (2025)]. The black hole horizon areal radius may determine a fundamental, gauge-invariant length scale governing black hole interactions near the scattering threshold.

We simulate high-energy scattering of equal-mass, nonspinning black holes endowed with like charges in full general relativity while varying the impact parameter $b$. We show that electrodynamics does not suppress zoom-whirl orbits at least for charge-to-mass ratios $\lambda =0.1$, 0.4, 0.6. However, we find that as $\lambda$ increases, the immediate merger and scattering thresholds defining the zoom-whirl regime move to smaller impact parameter $b/M_{\mathrm{ADM}}$, with $M_{\mathrm{ADM}}$ designating the binary black hole gravitational mass. This demonstrates that in the zoom whirl regime charge can leave observable imprints in key properties in high energy interactions. By contrast, charge has no significant influence in head-on collisions at the same energy scales. Additionally, we find evidence that the threshold impact parameters for immediate merger and scattering are universal, i.e., charge-independent, when we normalize $b$ by the sum of the initial black hole irreducible masses in the binary ($b/M_{\mathrm{irr}}$). This indicates that the irreducible mass, which is proportional to the black hole areal radius, may define a fundamental gauge-invariant length scale governing horizon scale scattering events in the strong-field, dynamical spacetime regime.

Differentially rotating scalarized neutron stars can possess an enormous amount of angular momentum larger than what could possibly be sustained in a neutron star in general relativity by about one order of magnitude. A natural question to ask is whether these solutions are stable and thus can be realized in a binary coalescence. With this motivation in mind, we examine the criterion of dynamical stability against axisymmetric perturbations for these ultra-rotators by numerically tracking their nonlinear evolution in an axisymmetric setup. We demonstrate that the turning-point criterion still serves as a sufficient condition for axisymmetric stability within the accuracy of the performed simulations. Our findings open an interesting question of whether the merger of two scalarized neutron stars can produce (possibly short-lived) ultra-highly rotating merger remnants.

As next-generation gravitational-wave (GW) observatories approach unprecedented sensitivities, the need for robust methods to analyze increasingly complex, overlapping signals becomes ever more pressing. Existing matched-filtering approaches and deep-learning techniques can typically handle only one or two concurrent signals, offering limited adaptability to more varied and intricate superimposed waveforms. To overcome these constraints, we present the UnMixFormer, an attention-based architecture that not only identifies the unknown number of concurrent compact binary coalescence GW events but also disentangles their individual waveforms through a multidecoder architecture, even when confronted with five overlapping signals. Our UnMixFormer is capable of capturing both short- and long-range dependencies by modeling them in a dual-path manner, while also enhancing periodic feature representation by incorporating Fourier analysis networks. Our approach adeptly processes binary black hole, binary neutron star, and neutron star–black hole systems over extended time series data (16,384 samples). When evaluating on synthetic data with signal-to-noise ratios (SNR) ranging from 10 to 50, our method achieves 99.89% counting accuracy, a mean overlap of 0.9831 between separated waveforms and templates, and robust generalization ability to waveforms with spin precession, orbital eccentricity, and higher modes, marking a substantial advance in the precision and versatility of GW data analysis.

We construct the canonical ensemble of a $d$-dimensional Reissner-Nordström black hole spacetime in a cavity surrounded by a heat reservoir through the Euclidean path integral formalism. The cavity radius $R$ is fixed, and the heat reservoir is at a fixed temperature $T$ and fixed electric charge $Q$. We use York’s approach to find the reduced action by imposing the Hamiltonian and Gauss constraints and the appropriate conditions to the Euclideanized Einstein-Maxwell action with boundary terms, and then perform a zero loop approximation so that the paths that minimize the action contribute to the partition function. We find that, for an electric charge smaller or equal than a critical saddle electric charge $Q_s$, there are three solutions $r_{+1}$, $r_{+2}$, and $r_{+3}$, such that $r_{+1}<r_{+2}<r_{+3}$. The solutions $r_{+1}$ and $r_{+3}$ are stable within the ensemble, while $r_{+2}$ is unstable. For an electric charge equal to $Q_s$, the solution $r_{+2}$ merges with $r_{+1}$ and $r_{+3}$ at a given specific temperature. For an electric charge larger than $Q_s$, there is only one solution $r_{+4}$, which can be seen as the merging of the $r_{+1}$ and $r_{+3}$ solutions, with $r_{+4}$ being stable. Since the partition function is directly related to the free energy in the canonical ensemble, we read off the free energy and calculate the thermodynamic variables, namely the entropy, the thermodynamic electric potential, the thermodynamic pressure, and the mean energy. We investigate thermodynamic stability, which is controlled by the positivity of the heat capacity at constant area and electric charge, and show that the heat capacity is discontinuous at the electric charge $Q_s$, signaling a turning point. We analyze the favorable states, examining the free energies of the stable black hole solutions and the free energy of electrically charged hot flat space, in order to check for possible first and second order phase transitions between the possible states. For instance, the two stable black hole solutions $r_{+1}$ and $r_{+3}$ are in competition between themselves, more specifically, for certain ensemble parameters there exists a first order phase transition from one solution to the other, and at the critical charge $Q_s$ this transition turns into a second order phase transition. We also compare the thermodynamic radius of zero free energy with the generalized Buchdahl bound radius, which do not match, and comment on the physical implications, such as the possibility of total gravitational collapse of the thermodynamic system. We study the limit of infinite cavity radius and find two possibilities, the Davies and the Rindler solutions. The Davies thermodynamic solution of electrically charged black holes in $d=4$ dimensions is recovered from the general $d$-dimensional canonical ensemble analysis. We obtain, in particular, the heat capacity given by Davies and the Davies point. The Rindler solution describes the black hole horizon as a Rindler horizon, and the boundary, which is at fixed temperature $T$ provided by the reservoir, must have the necessary acceleration to reproduce the corresponding Unruh temperature. Going back to a cavity with finite radius we find that the three solutions mentioned above are related to the original York two Schwarzschild black hole solutions and to the two Davies solutions, with the middle unstable solution $r_{+2}$ belonging simultaneously to the two sets of solutions. In this sense, York’s and Davies’ formalisms have been unified in our approach. In all instances we mention carefully the four-dimensional case, $d=4$, for which we accomplish new results, and study in detail all aspects of the five dimensional case, $d=5$.

We adopt the Reall-Santos method to obtain the quadratic curvature corrections to the thermodynamics of Myers-Perry rotating black holes in diverse dimensions. We consider the corrections in canonical and grand canonical ensembles, and also for fixed mass and angular momenta. We find that there exist inevitable divergences for generic rotation parameters so that the perturbative approach breaks down. We present a simple and universal formula to determine where these divergences could arise, and argue, using an explicit example, that these divergences could be an artefact of perturbation and might not exist in the full nonlinear theory.

We apply the gravity-thermodynamics approach in the case of Einstein-Gauss-Bonnet theory and its corresponding Wald-Gauss-Bonnet entropy, which due to the Chern-Gauss-Bonnet theorem is related to the Euler characteristic of the Universe topology. However, we consider the realistic scenario where we have the formation and merger of black holes that lead to topology changes, which induce entropy changes in the Universe horizon. We extract the modified Friedmann equations, and we obtain an effective dark-energy sector of topological origin. We estimate the black-hole formation and merger rates starting from the observed star formation rate per redshift, which is parametrized very efficiently by the Madau-Dickinson form, and finally we obtain a dark-energy energy density that depends on the cosmic star formation rate density, on the fraction $f_{\mathrm{BH}}$ of stars forming black holes, on the fraction of black holes $f_{\text{merge}}$ that eventually merge, on the fraction $f_{\mathrm{bin}}$ of massive stars that are in binaries, on the average mass of progenitor stars that will evolve to form black holes $⟨m_{\mathrm{prog}}⟩$, and on the Gauss-Bonnet coupling constant. We investigate in detail the cosmological evolution, obtaining the usual thermal history. Concerning the dark-energy equation-of-state parameter, we show that at intermediate redshifts it exhibits phantomlike and quintessencelike behavior according to the sign of the Gauss-Bonnet coupling, while at early and late times, it tends to the cosmological constant value. Finally, we study the effect of the other model parameters, showing that for the whole allowed observationally estimated ranges the topological dark-energy equation-of-state parameter remains within its observational bounds.

The late-time limit of the power spectrum for heavy (principal series) fields in de Sitter (dS) space yields a series of polynomial terms with complex scaling dimensions. Such scaling behavior is expected to result from an associated operator with a complex dimension. In a free theory, these complex dimensions are known to match the constraints imposed by unitarity on the space of states. Yet, perturbative corrections to the scaling behavior of operators are naively inconsistent with unitary evolution of the quantum fields in dS space. This paper demonstrates how to compute one-loop corrections to the scaling dimensions that appear in the two-point function from the field theory description in terms of local operators. We first show how to evaluate these anomalous dimensions using Mellin space, which has the feature that it naturally accommodates a scaleless regulator. We then explore the consequences for the soft de Sitter effective theory (SdSET) description that emerges in the long wavelength limit. Carefully matching between the UV and SdSET descriptions requires the introduction of novel nondynamical “operators” in the effective theory. This is not only necessary to reproduce results extracted from the Källén-Lehmann representation (that use the space of unitary states directly), but it is also required by general arguments that invoke positivity.

This paper studies the dynamics of scalar particle production, focusing on the presence of multiple fields and couplings in the medium. The interplay of various fields and couplings can influence the production rate, potentially overshadowing the effect of electrons alone. The plasma mixing, which induces the resonance and screening of scalar processes, is shown to be modified by the in-medium effects depending on the type of processes and field contents. Incorporating these in-medium effects into the analysis of stellar cooling via scalar emission allows for the emergence of various features of multifield effects given in several types of scalar models.

The recently detected extended very-high-energy $\gamma$-ray emission from the microquasar V4641 Sgr reveals a puzzling 200 parsec-long jetlike structure significantly misaligned with its radio jet. We propose that this $\gamma$-ray structure is produced by high-energy cosmic-ray particles escaping from the microquasar along ordered field lines of the Galactic magnetic field and interacting with the interstellar medium. If the $\gamma$-ray emission is produced by interactions of high-energy cosmic-ray nuclei, the system is detectable by future multi-${\mathrm{k}\mathrm{m}}^{\mathrm{3}}$ neutrino detectors. We argue that $\gamma$-ray observations of jetlike features adjacent to high-energy sources in the Milky Way provide a new method to measure the regular and turbulent components of the Galactic magnetic field at different locations in the Milky Way.

In this work, we explore two classes of density dependent relativistic mean-field models, their predictions of proton fractions at high densities and neutron star structure. We have used a metamodeling approach to these relativistic density functionals. We have generated a large ensemble of models with these classes and then applied constraints from theoretical and experimental nuclear physics and astrophysical observations. We find that both models produce similar equations of state and neutron star mass-radius sequences. But, their underlying compositions, denoted by the proton fraction in this case, are vastly different. This reinstates previous findings that information on composition gets masqueraded in $\beta$-equilibrium. Additional observations of nonequilibrium phenomena are necessary to pin it down.