(QFT) are some of the most universally applicable techniques in all of physics. The same techniques apply to (among other things) elementary particle physics [@quantum], to statistical and condensed matter physics [@tsvelik2007quantum], to the non-linear theory of structure formation in the universe [@2002PhR...367....1B; @2013arXiv1301.7182P] , to the theory of turbulence in fluids and plasmas [@Zhou20101], to the calculation of spectrum of gravitational waves from inspiralling binaries [@2006IJMPD..15.2293G], to quantum optics [@gerry2005introductory], to black hole thermodynamics [@2010arXiv1011.5875K] as well as to the calculation of correlations of primordial metric perturbations in inflationary cosmology [@2009arXiv0907.5424B]. In this light, it is not surprising that the details of calculations in these varied scenarios, though similar, are not exactly identical. Thus, it is important to find out which ideas apply universally to all these problems and which ones are specific to the applications we are most familiar with (e.g. scattering problems). To aid the discussion, in this work, the familiar version of QFT shall be referred to as usual  QFT.

It is a well known fact that inflationary cosmological perturbation theory is almost like a QFT of fluctuations around a time-dependent background solution [@Maldacena-2003]. Consequently, cosmological perturbation theory derives most of its calculational machinery from the 