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The Bondi–van der Burg–Metzner–Sachs (BMS) frame of gravitational waves produced by numerical relativity simulations is crucial for building accurate waveform models. A proper comparison of numerical relativity waveforms with other models requires fixing the arbitrary BMS frame. In this work we improve the center-of-mass (c.m.) frame fixing for quasicircular, nonprecessing binary systems. Past work approximated the c.m. motion with just a linear fit. We compute a post-Newtonian result of the boosted c.m. charge to also capture its physical outspiraling oscillations. We show that using the analytical results improves the robustness of the fit parameters—translation and boost vectors—to the choice of duration and time of the fitting window. Our analysis demonstrates a maximum improvement in robustness when the window is placed at the center of the inspiral. We quantified this improvement by computing the ratio of variances of fit parameters when the fit window size is varied. The largest improvement in robustness of parameters is by a factor of ∼25 for the boost vector and ∼20 for the translation vector. Finally, we incorporate this method into the BMS frame-fixing routine of the PYTHON package SCRI for waveforms produced with Cauchy-characteristic evolution. 

Abstract The Simulating eXtreme Spacetimes Collaboration’s codeSpECcan now routinely simulate binary black hole mergers undergoing $\sim 25$ orbits, with the longest simulations undergoing nearly $\sim 180$ orbits. While this sounds impressive, the mismatch between the highest resolutions for this long simulation is $O\left({10}^{-1}\right)$ . Meanwhile, the mismatch between resolutions for the more typical simulations tends to be $O\left({10}^{-4}\right)$ , despite the resolutions being similar to the long simulations’. In this note, we explain why mismatch alone gives an incomplete picture of code—and waveform—quality, especially in the context of providing waveform templates for LISA and 3G detectors, which require templates with $O\left({10}^3\right)-O\left({10}^5\right)$ orbits. We argue that to ready the GW community for the sensitivity of future detectors, numerical relativity groups must be aware of this caveat, and also run future simulations with at least three resolutions to properly assess waveform accuracy. 

Abstract Gravitational memory effects are predictions of general relativity that are characterized by an observable effect that persists after the passage of gravitational waves. In recent years, they have garnered particular interest, both due to their connection to asymptotic symmetries and soft theorems and because their observation would serve as a unique test of the nonlinear nature of general relativity. Apart from the more commonly known displacement and spin memories, however, there are other memory effects predicted by Einstein’s equations that are associated with more subleading terms in the asymptotic expansion of the Bondi-Sachs metric. In this paper, we write explicit expressions for these higher memory effects in terms of their charge and flux contributions. Further, by using a numerical relativity simulation of a binary black hole merger, we compute the magnitude and morphology of these terms and compare them to those of the displacement and spin memory. We find that, although these terms are interesting from a theoretical perspective, due to their small magnitude they will be particularly challenging to observe with current and future detectors. 

We uncover late-time gravitational-wave tails in fully nonlinear $3+1$ dimensional numerical relativity simulations of merging black holes, using the highly accurate p code. We achieve this result by exploiting the strong magnification of late-time tails due to binary eccentricity, recently observed in perturbative evolutions, and showcase here the tail presence in head-on configurations for several mass ratios close to unity. We validate the result through a large battery of numerical tests and detailed comparison with a perturbative evolution, which display striking agreement with full nonlinear ones in the ringdown regime, and very similar tail morphologies. 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 late-time tail signal is much more prominent than anticipated until recently, and possibly within reach of gravitational-wave detector measurements, unlocking observational investigations of an additional set of general relativistic predictions on the long-range gravitational dynamics.