Search

We study the late-time relaxation of a perturbed Schwarzschild black hole, driven by a source term representing an infalling particle in generic orbits. We consider quasi-circular and eccentric binaries, dynamical captures and radial infalls, with orbital dynamics driven by an highly accurate analytical radiation reaction. After reviewing the description of the late-time behaviour as an integral over the whole inspiral history, we derive an analytical expression exactly reproducing the slow relaxation observed in our numerical evolutions, obtained with a hyperboloidal compactified grid, for a given particle trajectory. We find this signal to be a superposition of an infinite number of power-laws, the slowest decaying term being Price's law. Next, we use our model to explain the several orders-of-magnitude enhancement of tail terms for binaries in non-circular orbits, shedding light on recent unexpected results obtained in numerical evolutions. In particular, we show the dominant terms controlling the enhancement to be activated when the particle is far from the BH, with small tangential and radial velocities soon before the plunge. As we corroborate with semi-analytical calculations, this implies that for large eccentricities the tail amplitude can be correctly extracted even when starting to evolve only from the last apastron before merger. We discuss the implications of these findings on the extraction of late-time tail terms in non-linear evolutions, and possible observational consequences. We also briefly comment on the scattering scenario, and on the connection with the soft graviton theorem., Comment: 27 pages, 18 figures

Stationary, asymptotically flat, black hole solutions of the vacuum field equations of General Relativity belong to the Kerr family. But how does one approach this state, dynamically? Linearized fluctuations decay at late times, at fixed spatial position, as a Price power law for generic initial conditions. However, little attention was paid to forced and nonlinear spacetimes, where matter and nonlinearities play a role. We uncover a new, source-driven tail governing waves generated by pointlike matter and nonlinearities, which can dominate over Price's decay., Comment: 13 pages, to appear in Physical Review D Letters

Black hole superradiance has proven being very valuable in several realms of gravitational physics, and holds a promising discovery potential. In this paper, we consider the superradiant instability of magnetically-charged, rotating black holes and find a number of important differences with respect to neutral ones. Considering massive charged bosonic fields, we find that the instability timescale is much shorter, and this is true even if the black hole contains an order-one number of magnetic monopoles, or merely a single one, and possesses either low, moderate or large values of angular momentum. In particular, the instability is drastically faster than the radiative decay time of charged pions, potentially making it physically relevant. Furthermore, our analysis identifies the most unstable modes as a class of monopole spheroidal harmonics, that we dub north and south monopole modes, whose morphology is markedly different from the ones in standard superradiance since they extend along the rotational axis. For completeness, we also study the quasinormal mode spectrum and amplification factors of charged massless fields, finding no evidence of instabilities in that case., Comment: v1: 5+pages, 4+figures. v2: Discussions improved, results unchanged. Matches published version in PRD

We study nonlinear evolutions of binary black hole mergers, uncovering power-law contributions generated by the long-range behaviour of the highly-curved dynamical spacetime. The result is achieved by exploiting the strong increase of the tail relevance due to binary eccentricity, recently observed in perturbative evolutions of a small-mass-ratio binary under accurate radiation reaction by Albanesi and collaborators [PhysRevD.108.084037]. We demonstrate the presence of this enhancement even in the nonlinear case of comparable-mass binary mergers in eccentric orbits, using the public RIT waveform catalog. The instantaneous frequency of the simulations displays large oscillations at intermediate to late-times, due to interference terms in the transition between a fast-decaying, constant-frequency quasinormal-mode driven regime, and a power-law, slowly-decaying one. The power-law exponent displays broad convergence with perturbative predictions, although longer and more accurate simulations will be needed to pinpoint the asymptotic value. Our results offer yet another confirmation of the highly predictive power of black hole perturbation theory in the presence of a source, even when applied to nonlinear solutions. The magnitude of the tail signal is within reach of gravitational-wave detectors measurements, unlocking the possibility of observationally investigating an additional set of general relativistic predictions on the long-range dynamics of relaxing compact objects., Comment: Includes Supplemental Material, 6+3 pages, 3+3 figures, 1 table

As the only gravity theory with quadratic curvature terms and second-order field equations, Einstein-dilaton-Gauss-Bonnet gravity is a natural testbed to probe the high-curvature regime beyond General Relativity in a fully nonperturbative way. Due to nonperturbative effects of the dilatonic coupling, black holes in this theory have a minimum mass which separates a stable branch from an unstable one. The minimum mass solution is a double point in the phase diagram of the theory, wherein the critical black hole and a wormhole solution coexist. We perform extensive nonlinear simulations of the spherical collapse onto black holes with scalar hair in this theory, especially focusing on the region near the minimum mass. We study the nonlinear transition from the unstable to the stable branch and assess the nonlinear stability of the latter. Furthermore, motivated by modeling the mass loss induced by Hawking radiation near the minimum mass at the classical level, we study the collapse of a phantom field onto the black hole. When the black-hole mass decreases past the critical value, the apparent horizon shrinks significantly, eventually unveiling a high-curvature elliptic region. We argue that evaporation in this theory is bound to either violate the weak cosmic censorship or produce horizonless remnants. Addressing the end-state might require a different evolution scheme., Comment: 27 pages, 23 figures, companion paper of arXiv:2205.13006 [gr-qc] . v2: added references and comparison between event horizon and apparent horizon. v3: extended discussion with new simulations of a pair of positive- and negative- energy wave packets to mimic the Hawking quantum pair. Results and conclusions unchanged. v4: minor changes to match version to appear in PRD

During the final stages of black hole evaporation, ultraviolet deviations from General Relativity eventually become dramatic, potentially affecting the end-state. We explore this problem by performing nonlinear simulations of wave packets in Einstein-dilaton-Gauss-Bonnet gravity, the only gravity theory with quadratic curvature terms which can be studied at fully nonperturbative level. Black holes in this theory have a minimum mass but also a nonvanishing temperature. This poses a puzzle concerning the final fate of Hawking evaporation in the presence of high-curvature nonperturbative effects. By simulating the mass loss induced by evaporation at the classical level using an auxiliary phantom field, we study the nonlinear evolution of black holes past the minimum mass. We observe a runaway shrink of the horizon (a nonperturbative effect forbidden in General Relativity) which eventually unveils a high-curvature elliptic region. While this might hint to the formation of a naked singularity (and hence to a violation of the weak cosmic censorship) or of a pathological spacetime region, a different numerical formulation of the initial-value problem in this theory might be required to rule out other possibilities, including the transition from the critical black hole to a stable horizonless remnant. Our study is relevant in the context of the information-loss paradox, dark-matter remnants, and for constraints on microscopic primordial black holes., Comment: 6 pages, 3 figures. v2: extended discussion and clarification. Results and conclusions unchanged. v3: minor changes to match version to appear in PRL. Companion paper available at arXiv:2205.13007

The aim of this study is to synthesize all existing empirical evidence on the effects of Cooperative Learning on inter-ethnic relations in school settings. The review is: (1) Systematic including published and unpublished research; (2) Up-to-date; (3) Inclusive of all school going age groups (4 to 18 years of age); and (4) Inclusive of only school based, valid cooperative learning strategies.