  The free propagator of a massless mode in an expanding universe can be
written as a sum of two terms, a lightcone and a tail part. The latter
describes a subluminal (time-like) 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.
