Friction in gravitational waves: Atest for early-time modified gravity

Modified gravity theories predict in general a non-standardequation 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). In the particular case of scalar-tensor theories, alpha(M) is constant and strongly constrained by background and scalar perturbations, 0 <= alpha(M) < 0.01 and the degeneracy with r is removed. In more general cases however such tight bounds are weakened and the B modes can provide useful constraints on early-time modified gravity. (C) 2015 The Authors. Published by Elsevier B.V.