The Polarizations of Gravitational Waves

Understand

The direct detection of gravitational waves by LIGO/Virgo opened the possibility to test General Relativity and its alternatives in the high speed, strong field regime.

• Alternative theories of gravity generally predict more polarizations than General Relativity, so it is important to study the polarization contents of theories of gravity to reveal the nature of gravity.
• In this talk, we analyzed the polarizations contents of Horndeski theory and $f(R)$ gravity.
• We found out that in addition to the familiar plus and cross polarizations, a \emph{massless} Horndeski theory predicts an extra transverse polarization, and there is a mix of the pure longitudinal and transverse breathing polarizations in the \emph{massive} Horndeski theory and $f(R)$ gravity.