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Abstract

The plunge-merger stage of the binary-black-hole (BBH) coalescence, when the
bodies' velocities reach a large fraction of the speed of light and the
gravitational-wave (GW) luminosity peaks, provides a unique opportunity to
probe gravity in the dynamical and nonlinear regime. How much do the
predictions of general relativity differ from the ones in other theories of
gravity for this stage of the binary evolution? To address this question, we
develop a parametrized waveform model, within the effective-one-body formalism,
that allows for deviations from general relativity in the
plunge-merger-ringdown stage. As first step, we focus on nonprecessing-spin,
quasicircular BBHs. In comparison to previous works, for each GW mode, our
model can modify, with respect to general-relativistic predictions, the instant
at which the amplitude peaks, the instantaneous frequency at this time instant,
and the value of the peak amplitude. We use this waveform model to explore
several questions considering both synthetic-data injections and two GW
signals. In particular, we find that deviations from the peak GW amplitude and
instantaneous frequency can be constrained to about 20$\%$ with GW150914.
Alarmingly, we find that GW200129_065458 shows a strong violation of general
relativity. We interpret this result as a false violation, either due to
waveform systematics (mismodeling of spin precession) or due to data-quality
issues depending on one's interpretation of this event. This illustrates the
use of parametrized waveform models as tools to investigate systematic errors
in plain general relativity. The results with GW200129_065458 also vividly
demonstrate the importance of waveform systematics and of glitch mitigation
procedures when interpreting tests of general relativity with current GW
observations.