usual QFT. Still, there are many differences, and in this work, we look at some of these differences. The interactions of metric perturbations cause corrections to cosmological correlations calculated in linear perturbation theory. This can cause not only primordial non-Gaussianity [@Maldacena-2003], but also, higher order corrections to e.g. the two-point function [@SZ_all_loops].

Just like in the usual QFT, in cosmological perturbation theory too, while calculating correlations at sufficiently high order, we encounter expressions which are ultraviolet (UV) divergent. In usual QFT, we cancel these infinities by (i) collecting the divergent terms with similar dependence on external momenta, (ii) finding which terms (allowed by the symmetries of the theory) in the action can lead to the terms in the correlations with the said dependence on external momenta, and finally, (iii) adding these counterterms to the old action (which is now called the renormalized action while the sum of the renormalized action and the counterterm is the bare action) in order to get correlations which are UV finite at every step of the calculation.

Does the same procedure work in cosmological perturbation theory too? As the analysis of this work illustrates, there are subtleties associated with this, one thus has to be 