The imprint of cosmic expansion history on the propagation of gravitational waves

Gravitational waves (GWs) are regarded as standard sirens for Cosmology. GWs from compact binary coalescence (CBC) can directly determine the luminosity distance but usually can not obtain information about the redshift. However, if the universe is not flat but accelerating, GWs should carry this cosmological effect. In this Letter, for the first time, we explore how the expansion of the Universe affects GW propagation by perturbing the Robertson-Walker metric. We achieve a comprehensive and rigorous formalism at the next-leading order to describe the cosmological acceleration in GWs from any kind of sources. Theoretically, this cosmological effect will obviously amplify GWs at frequencies as low as ${10}^{-12}$ Hz.