Generic searches for alternative gravitational wave polarizations with networks of interferometric detectors

Abstract

The detection of gravitational wave signals by Advanced LIGO and Advanced Virgo enables us to probe the polarization content of gravitational waves. In general relativity, only tensor modes are present, while in a variety of alternative theories one can also have vector or scalar modes. Recently test were performed which compared Bayesian evidences for the hypotheses that either purely tensor, purely vector, or purely scalar polarizations were present. Indeed, with only three detectors in a network and allowing for mixtures of tensor polarizations and alternative polarization states, it is not possible to identify precisely which nonstandard polarizations might be in the signal and by what amounts. However, we demonstrate that one can still infer whether, in addition to tensor polarizations, alternative polarizations are present in the first place, irrespective of the detailed polarization content. We develop two methods to do this for sources with electromagnetic counterparts, both based on the so-called null stream. Apart from being able to detect mixtures of tensor and alternative polarizations, these have the added advantage that no waveform models are needed, and signals from any kind of transient source with known sky position can be used. Both formalisms allow us to combine information from multiple sources so as to arrive at increasingly more stringent bounds. For now we apply these on the binary neutron star signal GW170817, showing consistency with the tensor-only hypothesis with p-values of 0.315 and 0.790 for the two methods.

Tests of general relativity with binary black holes from the second LIGO-Virgo gravitational-wave transient catalog

“…The only exception is the polarization analysis (Sec. VIII), which relies on null-stream projections of the data [87,88]. In GR, the GW signal from a BBH on a quasicircular orbit is fully characterized by 15 parameters [89].…”

“…The damped-sinusoid ringdown analysis is carried out with the PYRING pipeline [122,123]. The polarization analysis is carried out with the BANTAM pipeline [88].…”

“…Here we probe the same extreme polarization hypotheses as in previous studies, but through a different technique that does not rely on specific waveform models. This nullstream based polarization test is a Bayesian implementation of the null stream construct proposed in [269], generalized to vector and scalar antenna patterns [88,264]. A null stream is a linear combination of the data streams from different detectors that is known to be free of true GWs with a given helicity and sky location, irrespective of the GW waveform.…”

“…By marginalizing over the source location, we may obtain the evidences of different polarization hypotheses and compute Bayes factors comparing them. We take a uniform distribution over the celestial sphere as our sky-location prior and compute evidences through an extended version of the BANTAM pipeline presented in [88].…”

Searching for modified gravity in the astrophysical gravitational wave background: Application to ground-based interferometers

“…Current detectors are not sensitive enough to probe the Universe at cosmological distances, not more than z ≲ 1. But, certainly, some promising projects like ET, CE and LISA will be able to detect GW events with great accuracy at cosmological distances and provide information for powerful cosmological tests [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70].…”