Friction in Gravitational Waves: a test for early-time modified gravity

Understand

Modified gravity theories predict in general a non standard equation for the propagation of gravitational waves.

• Here we discuss the impact of modified friction and speed of tensor modes on cosmic microwave polarization B modes.
• We show that the non standard friction term, parametrized by $\alpha_{M}$, is degenerate with the tensor-to-scalar ratio $r$, so that small values of $r$ can be compensated by negative constant values of $\alpha_M$.
• We quantify this degeneracy and its dependence on the epoch at which $\alpha_{M}$ is different from the standard, zero, value and on the speed of gravitational waves $c_{T}$.