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Abstract The ‘ringdown’ radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave (GW) data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based GW detectors. 

Abstract Gravitational memory effects and the BMS freedoms exhibited at future null infinity have recently been resolved and utilized in numerical relativity simulations. With this, gravitational wave models and our understanding of the fundamental nature of general relativity have been vastly improved. In this paper, we review the history and intuition behind memory effects and BMS symmetries, how they manifest in gravitational waves, and how controlling the infinite number of BMS freedoms of numerical relativity simulations can crucially improve the waveform models that are used by gravitational wave detectors. We reiterate the fact that, with memory effects and BMS symmetries, not only can these next-generation numerical waveforms be used to observe never-before-seen physics, but they can also be used to test GR and learn new astrophysical information about our Universe. 

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 We present a major update to the Simulating eXtreme Spacetimes (SXSs) Collaboration’s catalog of binary black hole (BBH) simulations. Using highly efficient spectral methods implemented in the Spectral Einstein Code (SpEC), we have nearly doubled the total number of binary configurations from 2018 to 3756. The catalog now more densely covers the parameter space with precessing simulations up to mass ratioq = 8 and dimensionless spins up to $|\overrightarrow{\chi }|\text{⩽}0.8$ with near-zero eccentricity. The catalog also includes some simulations at higher mass ratios with moderate spin and more than 250 eccentric simulations. We have also deprecated and rerun some simulations from our previous catalog (e.g. simulations run with a much older version ofSpECor that had anomalously high errors in the waveform). The median waveform difference (which is similar to the mismatch) between resolutions over the simulations in the catalog is $4\times {10}^{-4}$ . The simulations have a median of 22 orbits, while the longest simulation has 148 orbits. We have corrected each waveform in the catalog to be in the binary’s center-of-mass frame and exhibit gravitational-wave memory. We estimate the total CPU cost of all simulations in the catalog to be 480 000 000 core-hours. We find that using spectral methods for BBH simulations is over 1000 times more efficient than previously published finite-difference simulations. The full catalog is publicly available through thesxsPython package and athttps://data.black-holes.org . 

Abstract In this short Note, I answer the titular question: yes, a radiation gauge can be horizon-locking. Radiation gauges are very common in black hole perturbation theory. It’s also very convenient if a gauge choice is horizon-locking, i.e. the location of the horizon is not moved by a linear metric perturbation. Therefore it is doubly convenient that a radiation gauge can be horizon-locking, when some simple criteria are satisfied. Though the calculation is straightforward, it seemed useful enough to warrant writing this Note. Finally I show an example: the $\ell$ vector of the Hartle–Hawking tetrad in Kerr satisfies all the conditions for ingoing radiation gauge to keep the future horizon fixed. 

Abstract One of the important targets for the future space-based gravitational wave observatory Laser Interferometer Space Antenna is extreme mass ratio inspirals (EMRIs), where long and accurate waveform modeling is necessary for detection and characterization. Modeling EMRI dynamics requires accounting for effects such as the ones induced by an external tidal field, which can break integrability at resonances and cause significant dephasing. In this paper, we use a Newtonian analogue of a Kerr black hole to study the effect of an external tidal field on the dynamics and the gravitational waveform. We have developed a numerical framework that takes advantage of the integrability of the background system to evolve it with a symplectic splitting integrator, and compute approximate gravitational waveforms to estimate the timescale over which the perturbation affects the dynamics. Comparing this timescale with the characteristic time under radiation reaction at resonance, we introduce a tool for quantifying the regime in which tidal effects might be included when modeling EMRI gravitational waves. As an application of this framework, we perform a detailed analysis of the dynamics at one resonance to show how different entry points into the resonance in phase-space can produce substantially different dynamics, and how one can estimate bounds for the parameter space where tidal effects may become dominant. Such bounds will scale as $\epsilon \gtrsim Cq$ , whereɛmeasures the strength of the external tidal field,qis the mass ratio, andCis a number which depends on the resonance and the shape of the tide. We demonstrate how to estimateCusing our framework for the 2:3 radial to polar frequency resonance in our model system. This framework can serve as a proxy for proper modeling of the tidal perturbation in the fully relativistic case. 

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. 

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.