Nonlinear studies of binary black hole mergers in Einstein-scalar-Gauss-Bonnet gravity

Abstract

Gravitational wave observations of black hole-neutron star binaries, particularly those where the black hole has a lower mass compared to other observed systems, have the potential to place strong constraints on modifications to general relativity that arise at small curvature length scales. Here we study the dynamics of black hole-neutron star mergers in shift-symmetric Einstein-scalar-Gauss-Bonnet gravity, a representative example of such a theory, by numerically evolving the full equations of motion. We consider quasi-circular binaries with different mass-ratios that are consistent with recent gravitational wave observations, including cases with and without tidal disruption of the star, and quantify the impact of varying the coupling controlling deviations from general relativity on the gravitational wave signal and scalar radiation. We find that the main effect on the late inspiral is the accelerated frequency evolution compared to general relativity, and that-even considering Gauss-Bonnet coupling values approaching those where the theory breaks down-the impact on the merger gravitational wave signal is mild, predominately manifesting as a small change in the amplitude of the ringdown. We compare our results to current post-Newtonian calculations and find consistency throughout the inspiral.

Searching for ringdown higher modes with a numerical relativity-informed post-merger model

“…Clearly, the most sensitive and accurate test would be performed using coherent IMR models in specific alternative scenarios. Recent progress has been made in constructing such models, both analytically and numerically [136][137][138][139][140]. However, the large classes of possible deviations from GR makes this a daunting task, and naturally calls for generic models and parameterisations [141][142][143][144][145][146][147][148] capable of capturing deviations from several classes of modifications.…”

Disentangling photon rings beyond General Relativity with future radio-telescope arrays

“…This requires a well-posed initial-value formulation of the equations of motion that is amenable to numerical simulations. Finding such a formulation is a significant challenge in many settings beyond GR, that has only partially been met in a small subset of theories [78][79][80][81]. In contrast, to compare a spacetime beyond Kerr to VLBI data, one does not even need to know which dynamics that spacetime is a solution to.…”

GRFolres: A code for modified gravity simulations in strong gravity

“…There is at least one private code, based on the PAMR/AMRD and HAD (East et al, 2012;Neilsen et al, 2007) infrastructure, that was used in the first works to successfully implement the modified general harmonic gauge for 4𝜕ST (Corman et al, 2023;East & Pretorius, 2022;East & Ripley, 2021b, 2021a. Since this code uses a Generalised Harmonic Coordinates (GHC) formulation, it necessitates excision of the interior of black holes, which can be difficult to implement in practice.…”

“…An important breakthrough was made in 2020 by Kov'acs and Reall, who showed that Horndeski theories are indeed well-posed in a modified version of the harmonic gauge (Kovács & Reall, 2020b) -a particular coordinate system already used in NR. Subsequently, several specific theories within these classes were probed in their highly dynamical and fully non-linear regimes (Corman et al, 2023;East & Pretorius, 2022;East & Ripley, 2021b, 2021a. The extension of the results of (Kovács & Reall, 2020b to the alternative "singularity avoiding" coordinates in (Aresté Saló et al, 2022Doneva et al, 2023) offers an alternative gauge in which to probe questions of hyperbolicity, and may offer stability advantages for certain cases such as unequal mass ratios, as studied in (Corman et al, 2023).…”

“…Subsequently, several specific theories within these classes were probed in their highly dynamical and fully non-linear regimes (Corman et al, 2023;East & Pretorius, 2022;East & Ripley, 2021b, 2021a. The extension of the results of (Kovács & Reall, 2020b to the alternative "singularity avoiding" coordinates in (Aresté Saló et al, 2022Doneva et al, 2023) offers an alternative gauge in which to probe questions of hyperbolicity, and may offer stability advantages for certain cases such as unequal mass ratios, as studied in (Corman et al, 2023). Numerical work on these theories is still in the early stages of development and many technical details on their numerical implementation need to be further investigated.…”