Inflationary gravitational wave background as a tail effect

Abstract

The free propagator of a massless mode in an expanding universe can be written as a sum of two terms, a light cone and a tail part. The latter describes a subluminal (timelike) signal. We show that the inflationary gravitational wave background, influencing cosmic microwave background polarization and routinely used for constraining inflationary models through the so-called r ratio, originates exclusively from the tail part.