The presence of the cosmological constant and matter density affect the propagation of gravitational waves (GW) beyond the usual cosmological redshift. Altough small, these effects might be observable in pulsar timing arrays (PTA). The initial analyses were done at leading order in the Hubble constant H_0, which is proportional to the square root of the various cosmological fluid densities. At the next-to-leading order the corrections do not appear in the form of H_0, thus lifting the degeneracy among the various cosmological components. We first investigate the problem in Schwarzchild-de Sitter (SdS) like coordinates. When transformed into comoving coordinates, the ones used in cosmological measurements, this SdS solution does satisfy the perturbation equations in a FLRW metric up to and including Λ^(3/2) terms. This analysis is then extended to the other cosmological fluids, allowing us to consider GW sources virtually at any range. Finally, we investigate the influence of these corrections in PTA
Martín González Alonso