Cosmological bulk-to-bulk propagator
= Cosmological bulk-to-bulk propagator
For a free mode $f(k,\eta)$, the right-right bulk propagator is time ordered,
$$
G_{RR}(\eta_1,\eta_2;k)=
\theta(\eta_1-\eta_2)f(\eta_1)f^*(\eta_2)
+\theta(\eta_2-\eta_1)f(\eta_2)f^*(\eta_1),
$$
whereas the mixed propagators are Wightman functions.