arXiv subjects

Justin L. Ripley

Publications and source records attributed to Justin L. Ripley.

28 records · Page 2Linked to original sources

arXiv2021

Dynamics of spontaneous black hole scalarization and mergers in Einstein-scalar-Gauss-Bonnet gravity

We study the dynamics of black holes in Einstein-scalar-Gauss-Bonnet theories that exhibit spontaneous black hole scalarization using recently introduced methods for solving the full, non-perturbative equations of motion. For one sign of the coupling parameter, non-spinning vacuum black holes are unstable to developing scalar hair, while for the other, instability only sets in for black holes with sufficiently large spin. We study scalarization in both cases, demonstrating that there is a range of parameter space where the theory maintains hyperbolic evolution and for which the instability saturates in a scalarized black hole that is stable without symmetry assumptions. However, this parameter space range is significantly smaller than the range for which stationary scalarized black hole solutions exist. We show how different choices for the subleading behavior of the Gauss-Bonnet coupling affect the dynamics of the instability and the final state, or lack thereof. Finally, we present mergers of binary black holes and demonstrate the imprint of the scalar hair in the gravitational radiation.

gr-qc↗

arXiv2021

A symmetric hyperbolic formulation of the vacuum Einstein equations in affine-null coordinates

We present a symmetric hyperbolic formulation of the Einstein equations in affine-null coordinates. Giannakopoulos et. al. (arXiv:2007.06419) recently showed that the most commonly numerically implemented formulations of the Einstein equations in affine-null coordinates (and other single-null coordinate systems) are only weakly-but not strongly-hyperbolic. By making use of the tetrad-based Newman-Penrose formalism, our formulation avoids the hyperbolicity problems of the formulations investigated by Giannakopoulos et. al. We discuss a potential application of our formulation for studying gravitational wave scattering.

gr-qc↗

arXiv2021

Second Order Perturbations of Kerr Black Holes: Reconstruction of the Metric

Motivated by gravitational wave observations of binary black hole mergers, we present a procedure to compute the leading order nonlinear gravitational wave interactions around a Kerr black hole. We describe the formalism used to derive the equations for second order perturbations. We develop a procedure that allows us to reconstruct the first order metric perturbation solely from knowledge of the solution to the first order Teukolsky equation, without the need of Hertz potentials. Finally, we illustrate this metric reconstruction procedure in the asymptotic limit for the first order quasi-normal modes of Kerr. In a companion paper, we present a numerical implementation of these ideas.

gr-qc↗

arXiv2021

Numerical computation of second order vacuum perturbations of Kerr black holes

Motivated by the desire to understand the leading order nonlinear gravitational wave interactions around arbitrarily rapidly rotating Kerr black holes, we describe a numerical code designed to compute second order vacuum perturbations on such spacetimes. A general discussion of the formalism we use is presented in (arXiv:2008.11770); here we show how we numerically implement that formalism with a particular choice of coordinates and tetrad conditions, and give example results for black holes with dimensionless spin parameters $a=0.7$ and $a=0.998$. We first solve the Teukolsky equation for the linearly perturbed Weyl scalar $Ψ_4^{(1)}$, followed by direct reconstruction of the spacetime metric from $Ψ_4^{(1)}$, and then solve for the dynamics of the second order perturbed Weyl scalar $Ψ_4^{(2)}$. This code is a first step toward a more general purpose second order code, and we outline how our basic approach could be further developed to address current questions of interest, including extending the analysis of ringdown in black hole mergers to before the linear regime, exploring gravitational wave "turbulence" around near-extremal Kerr black holes, and studying the physics of extreme mass ratio inspiral.

gr-qc↗

arXiv2021

Evolution of Einstein-scalar-Gauss-Bonnet gravity using a modified harmonic formulation

We present numerical solutions of several spacetimes of physical interest, including binary black hole mergers, in shift-symmetric Einstein-scalar-Gauss-Bonnet (ESGB) gravity, and describe our methods for solving the full equations of motion, without approximation, for general spacetimes. While we concentrate on the specific example of shift-symmetric ESGB, our methods, which make use of a recently proposed modification to the generalized harmonic formulation, should be generally applicable to all Horndeski theories of gravity (including general relativity). We demonstrate that these methods can stably follow the formation of scalar clouds about initially vacuum non-spinning and spinning black holes for values of the Gauss-Bonnet coupling approaching the maximum value above which the hyperbolicity of the theory breaks down in spherical symmetry. We study the collision of black holes with scalar hair, finding that the theory remains hyperbolic in the spacetime region exterior to the black hole horizons in a similar regime, which includes cases where the deviations from general relativity in the gravitational radiation is appreciable. Finally, we demonstrate that these methods can be used to follow the inspiral and merger of binary black holes in full ESGB gravity. This allows for making predictions for Horndeski theories of gravity in the strong-field and non-perturbative regime, which can confronted with gravitational wave observations, and compared to approximate treatments of modifications to general relativity.

gr-qc↗

arXiv2020

Dynamics of a Z2 symmetric EdGB gravity in spherical symmetry

We report on a numerical investigation of black hole evolution in an Einstein dilaton Gauss-Bonnet (EdGB) gravity theory where the Gauss-Bonnet coupling and scalar (dilaton) field potential are symmetric under a global change in sign of the scalar field (a 'Z2' symmetry). We find that for sufficiently small Gauss-Bonnet couplings Schwarzschild black holes are stable to radial scalar field perturbations, and are unstable to such perturbations for sufficiently large couplings. For the latter case, we provide numerical evidence that there is a band of coupling parameters and black hole masses where the end states are stable scalarized black hole solutions, in general agreement with the results of Macedo et al (2019 Phys. Rev. D 99 104041). For Gauss-Bonnet couplings larger than those in the stable band, we find that an 'elliptic region' forms outside of the black hole horizon, indicating the theory does not possess a well-posed initial value formulation in that regime.

gr-qc↗

arXiv2020

Scalarized Black Hole dynamics in Einstein dilaton Gauss-Bonnet Gravity

We report on a numerical investigation of the stability of scalarized black holes in Einstein dilaton Gauss-Bonnet (EdGB) gravity in the full dynamical theory, though restricted to spherical symmetry. We find evidence that for sufficiently small curvature-couplings the resulting scalarized black hole solutions are nonlinearly stable. For such small couplings, we show that an elliptic region forms inside these EdGB black hole spacetimes (prior to any curvature singularity), and give evidence that this region remains censored from asymptotic view. However, for coupling values "superextremal" relative to a given black hole mass, an elliptic region forms exterior to the horizon, implying the exterior Cauchy problem is ill-posed in this regime.

gr-qc↗

arXiv2019

Excision and avoiding the use of boundary conditions in numerical relativity

A procedure for evolving hyperbolic systems of equations on compact computational domains with no boundary conditions was recently described in [arXiv:1905.08657]. In that proposal, the computational grid is expanded in spacelike directions with respect to the outermost characteristic and initial data is imposed on the expanded grid boundary. We discuss a related method that removes the need for imposing boundary conditions: the computational domain is excised along a direction spacelike with respect to the innermost going characteristic. We compare the two methods, and provide example evolutions from a code that implements the excision method: evolution of a massless self-gravitating scalar field in spherical symmetry.

gr-qc↗

arXiv2017

Black hole perturbation under 2 + 2 decomposition in the action

Black hole perturbation theory is useful for studying the stability of black holes and calculating ringdown gravitational waves after the collision of two black holes. Most previous calculations were carried out at the level of the field equations instead of the action. In this work, we compute the Einstein-Hilbert action to quadratic order in linear metric perturbations about a spherically symmetric vacuum background in Regge-Wheeler gauge. Using a 2+2 splitting of spacetime, we expand the metric perturbations into a sum over scalar, vector, and tensor spherical harmonics, and dimensionally reduce the action to two dimensions by integrating over the two sphere. We find that the axial perturbation degree of freedom is described by a two dimensional massive vector action, and that the polar perturbation degree of freedom is described by a two dimensional dilaton massive gravity action. Varying the dimensionally reduced actions, we rederive covariant and gauge-invariant master equations for the axial and polar degrees of freedom. Thus, the two dimensional massive vector and massive gravity actions we derive by dimensionally reducing the perturbed Einstein-Hilbert action describe the dynamics of a well studied physical system: the metric perturbations of a static black hole. The $2+2$ formalism we present can be generalized to $m+n$ dimensional spacetime splittings, which may be useful in more generic situations, such as expanding metric perturbations in higher dimensional gravity. We provide a self-contained presentation of $m+n$ formalism for vacuum spacetime splittings.

gr-qc↗

arXiv2013

X-ray Decay Lines from Heavy Nuclei in Supernova Remnants as a Probe of the r-Process Origin and the Birth Periods of Magnetars

The origin of rapid neutron capture (r-process) nuclei remains one of the longest standing mysteries in nuclear astrophysics. Core collapse supernovae (SNe) and neutron star binary mergers are likely r-process sites, but little evidence yet exists for their in situ formation in such environments. Motivated by the advent of sensitive new or planned X-ray telescopes such as the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Large Observatory for X-ray Timing (LOFT), we revisit the prospects for the detection of X-ray decay lines from r-process nuclei in young or nearby supernova remnants. For all remnants planned to be observed by NuSTAR (and several others), we conclude that r-process nuclei are detectable only if the remnant possesses a large overabundance O > 1e3 relative to the average yield per SN. Prospects are better for the next Galactic SN (assumed age of 3 years and distance of 10 kpc), for which an average r-process yield is detectable via the 10.7(9.2) keV line complexes of Os194 by LOFT at 6 sigma (5 sigma) confidence; the 27.3 keV line complex of Sb125 is detectable by NuSTAR at 2 sigma for O > 2. We also consider X-rays lines from the remnants of Galactic magnetars, motivated by the much higher r-process yields of the magneto-rotationally driven SNe predicted to birth magnetars. The ~ 3.6-3.9 keV lines of Sn126 are potentially detectable in the remnants of the magnetars 1E1547.0-5408 and 1E2259+586 by LOFT for an assumed r-process yield predicted by recent simulations. The (non-)detection of these lines can thus probe whether magnetars are indeed born with millisecond periods. Finally, we consider a blind survey of the Galactic plane with LOFT for r-process lines from the most recent binary neutron star merger remnant, concluding that a detection is unlikely without additional information on the merger location.

astro-ph.HE↗