Polarizations of Gravitational Waves in the Bumblebee Gravity Model

TL;DR

This paper investigates how Lorentz violation in the bumblebee gravity model affects gravitational wave polarizations, revealing multiple polarization states and their dependence on the background vector field and propagation direction.

Contribution

It provides a detailed analysis of GW polarizations in the bumblebee gravity model, highlighting novel polarization phenomena and their relation to Lorentz violation.

Findings

Five independent degrees of freedom for a purely timelike background.

Preferred spatial direction affects polarization content.

Mixture of all six polarizations possible in certain propagation directions.

Abstract

Lorentz violation modifies the dispersion relation of gravitational waves (GWs), and induces birefringence and anisotropy in propagation. Our study shows that Lorentz violation can also activate multiple polarizations of GWs. We use the gauge invariants to investigate the polarizations of GWs in the bumblebee gravity model, and obtain the following results. (i) For a vector background $b^\mu$ with only a nonzero temporal component $b^t$, there are five independent propagating degrees of freedom (DOFs), which is simlar to the Einstein-aether theory. (ii) The presence of a spatial component in the background defines a preferred spatial direction which breaks rotational symmetry. We denote $\hat{\bf b}$ as the direction of the spatial part of the background and $b_s$ as its length. If GWs propagate along $\hat{\bf b}$, the polarization content is similar to the purely timelike case. (iii) If the propagation direction of GWs is separated by an angle $\beta$ to $\hat{\bf b}$, and $\beta=\arccos(b^t/b_s)$, there are only two tensor polarizations. (iv) If $\beta\neq \arccos(b^t/b_s)$, there are only two independent DOFs, and the vector and scalar modes degenerate with the tensor modes. The tensor perturbations can activate a mixture of all six polarizations simultaneously. Finally, we point out the difference in GWs between the bumblebee gravity model and the minimal Standard-Model Extension framework in the linearized regime. Current observations have placed stringent constraints on the anisotropy induced by the background, while our theoretical study still reveals some novel phenomena and provides more understanding about the interaction between the Lorentz-violating vector field and gravity.