Stoytcho S. Yazadjiev

Gravitational-wave memory is a low-frequency, nonoscillatory component of the radiation field that provides a potentially powerful but as yet undetected probe of strong-field gravity. We present the first calculation of gravitational memory from full inspiral-merger-ringdown waveforms in a theory beyond general relativity, focusing on scalar-Gauss-Bonnet gravity as a theoretically well-motivated and numerically accessible extension of GR. Starting from the general memory formulas in Horndeski gravity, we derive explicit spin-weighted spherical-harmonic expressions for the tensor null memory in scalar-Gauss-Bonnet theory and evaluate them on existing numerical-relativity waveforms for both shift-symmetric and dynamically scalarizing binary black hole mergers. We find that the dominant effect is an indirect modification of the tensor memory through changes in the nonlinear merger dynamics, while the direct scalar contribution to the tensor memory remains suppressed by orders of magnitude for the systems considered in this work. For the largest deviations in our dataset, the final memory amplitude differs from the corresponding GR prediction by a few percent and by up to ∼ 4 % when compared to the GR template that minimizes the waveform mismatch in a detector-oriented analysis. We further show that including memory increases the mismatch between GR and scalar-Gauss-Bonnet waveforms by more than an order of magnitude, indicating that memory can provide complementary information for testing gravity with third-generation detectors, especially for low-mass binaries.

We investigate the polarization properties of the direct images of a geometrically and optically thin accretion disk around fully self-consistent models of Kerr black holes with synchronized scalar hair. The presence of a massive scalar field alters the geodesic structure of the spacetime and thus leaves an imprint on the polarization of radiation emitted near the black hole horizon. To study this effect, we employ a simple analytical model of a geometrically thin accretion disk, orbiting in the equatorial plane and emitting synchrotron radiation. The main deviation from a corresponding Kerr black hole in general relativity is found to be a dephasing in the twist of the polarization vector, which is surprisingly larger for the least scalarized solutions we consider. This behavior suggests that polarization observables are primarily sensitive to local geometric and transport effects along photon trajectories rather than to the overall scalar field strength. Furthermore, our results demonstrate that while equatorial magnetic fields produce qualitatively similar polarization patterns to Kerr black holes in general relativity, vertical magnetic fields at high observer inclinations can lead to a characteristic reversal of the twist direction of the polarization vector.

There has been a recent revival in understanding the spontaneous scalarization phenomenon in scalar-tensor gravity as a phase transition. Using the tools of the Landau theory, we now know that first-order transitions where scalarization occurs in a discontinuous manner are more prominent than what had been considered in the literature, and this might lead to novel observation channels. However, the examples so far have been restricted to specific quadratic scalar coupling terms and spherically symmetric stars. Here we explore the phase transition structure of scalarization for more general couplings, considering linear as well as quadratic terms in the conformal scaling factor of the theory. Moreover, we also investigate the effect of rotation on the scalarization phase transition. Both of these considerations are natural choices since the coupling in a scalar-tensor theory can appear at all orders, and astrophysical neutron stars commonly have angular momentum. The introduction of linear coupling leads to a complex solution space which is harder to explore. However, we demonstrate that the Landau model of scalarization enables us to systematically find the branches of scalarized solutions that are commonly overlooked in numerical searches, providing a novel tool. On the other hand, the main effect of stellar rotation is shifting the stellar masses at which the phase transition occurs to higher values, but the qualitative picture remains similar to what happens under spherical symmetry.

Within the framework of scalar-tensor theories of gravity, certain models can evade classical black hole no-hair theorems. A well-known example is Einstein-scalar-Gauss-Bonnet gravity, where black holes carrying a scalar charge can exist. We find that, within this theory, binary black holes initially described by general relativity can acquire scalar charges once they reach a critical orbital separation (“dynamical scalarization”). We develop a simple semianalytic model, based on the adiabatic conservation of the total Wald entropy, to estimate the scalar charge evolution during the binary inspiral. We also run fully nonlinear numerical-relativity simulations for different configurations, finding consistent results. The gravitational-wave phase difference between Einstein-scalar-Gauss-Bonnet and general relativity waveforms, which we use to assess detectability, is also computed. We find that dynamical scalarization might be observable in nearly equal-mass binary black hole mergers with third-generation ground-based gravitational-wave detectors, in a narrow range of the dimensional coupling of the theory.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

Within the framework of scalar-tensor theories of gravity, certain models can evade classical black hole no-hair theorems. A well-known example is Einstein-scalar-Gauss-Bonnet gravity, where black holes carrying a scalar charge can exist. We find that, within this theory, binary black holes initially described by General Relativity can acquire scalar charges once they reach a critical orbital separation ("dynamical scalarization"). We develop a simple semi-analytic model, based on the adiabatic conservation of the total Wald entropy, to estimate the scalar charge evolution during the binary inspiral. We also run fully nonlinear numerical-relativity simulations for different configurations, finding consistent results. The gravitational-wave phase difference between Einstein-scalar-Gauss-Bonnet and General Relativity waveforms, which we use to assess detectability, is also computed. We find that dynamical scalarization might be observable in nearly equal-mass binary black hole mergers with third-generation ground-based gravitational-wave detectors, in a narrow range of the dimensional coupling of the theory.

We study the influence of the plasma environment on the shadows of stationary axisymmetric wormholes. We consider a sample of several wormhole solutions and plasma distributions for which the Hamilton-Jacobi equation for the light rays is separable. This allows us to derive analytical expressions for the shadow boundary and examine the behavior of the photon regions as the plasma frequency varies. We observe that plasma profiles which depend only on radial coordinate lead to common evolution of the photon region which does not depend on the wormhole metric and is consistent with the Kerr black hole. For plasma profiles with angular dependence the evolution of the photon region is specific for every spacetime thus wormholes are observationally distinguishable. We further investigate the formation of forbidden regions in the plasma medium where light cannot propagate. They lead to the formation of plasma frequency ranges where the shadow is no longer observable and we show that this phenomenon is characteristic for all the configurations in our sample. We obtain the critical frequencies for which the shadow vanishes and demonstrate that for all the wormholes they are lower than the critical frequencies for the Kerr black hole in the same environment. This implies that there exist plasma frequency ranges in which the Kerr black hole casts a shadow but wormholes do not, creating a strong observational signature for discriminating between compact objects. In the frequency ranges where both black hole and wormhole shadows exist the wormhole shadows are consistently smaller than those for the Kerr black hole. As the plasma frequency grows the discrepancy progresses showing that plasma medium facilitates the experimental detection of wormholes. Finally we consider aberrational effects on the wormhole shadows. They further increase the deviation from black holes making wormholes easier to detect.