# A Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background

## TL;DR

This paper searches for different polarization modes in the stochastic gravitational-wave background using LIGO data, setting the first direct bounds on vector and scalar contributions, and finds no evidence for non-tensor polarizations.

## Contribution

It provides the first direct constraints on vector and scalar polarizations in the stochastic gravitational-wave background, expanding tests of gravitational theories beyond general relativity.

## Key findings

- No evidence for non-tensor polarizations was found.
- Upper bounds were placed on the energy densities of vector and scalar modes.
- Constraints improve understanding of gravitational wave polarization properties.

## Abstract

The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually-unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically-polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy-densities of tensor, vector, and scalar modes at 95% credibility to $\Omega^T_0 < 5.6 \times 10^{-8}$, $\Omega^V_0 < 6.4\times 10^{-8}$, and $\Omega^S_0 < 1.1\times 10^{-7}$ at a reference frequency $f_0 = 25$ Hz.