Table of Contents
Universe, Volume 4, Issue 8 (August 2018)
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Cover Story (view full-size image) The detection of gravitational waves (GWs) by LIGO/Virgo collaboration provides a new probe into [...] Read more. The detection of gravitational waves (GWs) by LIGO/Virgo collaboration provides a new probe into the nature of gravity. In General Relativity, GWs have two polarizations. In a generic metric theory of gravity, there are up to six polarizations. Therefore, the measurement of the polarizations is the best way to probe the nature of gravity. We analyzed the polarizations of the Horndeski theory and f(R) gravity. We found out that, besides the plus and cross polarizations, the massless Horndeski theory predicts the breathing polarization, and the longitudinal polarization always accompanies the breathing one as a mixed single state in the massive Horndeski theory and f(R) gravity. The extra polarizations may be detected by pulsar timing arrays. It is also shown that the classification of polarizations using Newman-Penrose variables cannot be applied to massive modes or the local Lorentz violating theories. View this paper