Testing general relativity with gravitational-wave catalogs: the insidious nature of waveform systematics

Abstract

Gravitational-wave observations of binary black holes allow new tests of general relativity to be performed on strong, dynamical gravitational fields. These tests require accurate waveform models of the gravitational-wave signal, otherwise waveform errors can erroneously suggest evidence for new physics. Existing waveforms are generally thought to be accurate enough for current observations, and each of the events observed to date appears to be individually consistent with general relativity. In the near future, with larger gravitational-wave catalogs, it will be possible to perform more stringent tests of gravity by analyzing large numbers of events together. However, there is a danger that waveform errors can accumulate among events: even if the waveform model is accurate enough for each individual event, it can still yield erroneous evidence for new physics when applied to a large catalog. This paper presents a simple linearised analysis, in the style of a Fisher matrix calculation, that reveals the conditions under which the apparent evidence for new physics due to waveform errors grows as the catalog size increases. We estimate that, in the worst-case scenario, evidence for a deviation from general relativity might appear in some tests using a catalog containing as few as 10-30 events above a signal-to-noise ratio of 20. This is close to the size of current catalogs and highlights the need for caution when performing these sorts of experiments.

Improved gravitational-wave constraints on higher-order curvature theories of gravity

“…This result is not believed to be a real deviation from GR, but rather a consequence of waveform systematics and covariances between parameters. While one could ignore these deviations and calculate upper limits on √ α EdGB regardless, the obvious impact of waveform systematics is of note [93]. This stark difference in the two posteriors along with the issues of waveform systematics, in a context where neither interpretation of the event GW190814 is unquestionable, led us to omit this source from the main body of this work.…”

Tight constraints on Einstein-dilation-Gauss-Bonnet gravity from GW190412 and GW190814

“…We further carry out Bayesian analysis on GW190814, assuming it is a BBH event, which gives better constraints on both √ α dCS and √ α EdGB parameters, through the posteriors of √ α dCS are still not informative enough to place meaningful constraint on it. Since the modification of dCS gravity appears at higher PN order, the contribution of spin effect to the GW phase may be comparable with the additional phase produced by deviation of GR [38]. Therefore, √ α dCS and spin parameters will be coupling with each other in match filtering, which may lead to biased constraints on these parameters.…”

CLASS_GWB: robust modeling of the astrophysical gravitational wave background anisotropies

“…Subtracting a signal with not well determined parameters introduces systematics in the data analysis[111], hence probably the best approach is to remove that portion of the data stream. Recent works[112] also suggest that incorrect waveforms can hide New Physics signals, hence they can impact also the analysis presented in this work 13. Computing the observed SNR of an event is non-trivial because of the effects of cosmological perturbations on GW propagation.…”