the 3-momentum of the external line and $\eta$ is the conformal time at which the correlator is evaluated. We thus have an additional factor involving a polynomial in $-k \eta$: it is this extra piece which determines the forms and number of counterterms needed to renormalize the theory. During inflation, we are typically interested in the ${\eta \rightarrow 0^-}$ limit of the correlations. If this limit is taken before regularizing the UV divergent momentum integrals, the information about the forms of counterterms needed and the number of counterterms needed to get rid of UV divergences gets erased. Since the number of counterterms also determines the number of observations actually needed to fix the renormalized parameters, this is a bad news. On the other hand, regularization of momentum integrals at non-zero $\eta$ is a much more involved task to perform and hence the results are much less transparent. All this illustrates that many properties of the familiar usual QFT calculations do not apply to cosmological perturbation theory. Thus, the connection between QFT and cosmological perturbation theory is quite subtle and continues to offer surprises.

While calculating the loop corrections to cosmological correlations, it is often assumed that the process of renormalization 