Gravitational waves in modified gravity

Gravitational waves (GWs) in Modified Gravity are discussed and compared with General Relativity in view of possible detection of new modes. After a summary of some classes of modified theories, we recall GWs properties in General Relativity as the basic standard by which to compare any modified theory. Then, we discuss GWs in Extended Theories like f(R) gravity, scalar-tensor gravity, Gauss-Bonnet gravity and other gauge theories, as well as Teleparallel Equivalent General Relativity (TEGR), taking into account also torsion instead of curvature, to describe the gravitational field. As a general remark, all these theories lead to new polarizations in addition to the two standard modes of General Relativity. It is possible to show that, in 4 dimensions, the polarizations are at most 6. Theoretical and experimental implications of new modes are briefly discussed.