Orateur
Description
Cosmology usually assumes quantum perturbations on top of a mostly classical background. In quantum cosmology, one quantizes the background as well and then write the full quantum state as $|\Psi_\text{tot}^{(\text{BO})}\rangle = |\Psi_\text{bckg} (a,t)\rangle \otimes |\Psi_\text{pert}[v(x^i,t);a(t)\rangle$, in which the unique background state $|\Psi_\text{bckg} (a,t)\rangle$ solves the zeroth-order Schrödinger equation and provides, often by means of taking an expectation value $\langle a \rangle$,a time dependent scale factor $a(t)$ to be plugged back into the second order perturbation equations. This so-called Born-Oppenheimer approximation is only an approximation, and going beyond it naturally induces the appearance of non-gaussianities, in practice a nonvanishing trispectrum with a universal shape.
