"Theory of Particle Physics and Cosmology" Seminar

In the standard cosmological model primordial curvature perturbations provide the seeds for the cosmic microwave background (CMB) radiation anisotropies and for large scale structure. These observations give us consequently important information about the early Universe and therefore it is fundamental to study what are the statistical properties of primordial perturbations, and in particular if they are Gaussian. In the context of single-field inflation, the conservation of the curvature perturbation on comoving slices, R_c, on super-horizon scales is one of the assumptions necessary to derive the consistency condition between the squeezed limit of the bispectrum (3-points correlation function) and the spectrum (2-points correlation function) of the primordial curvature perturbation. However, the conservation of R_c holds only after the perturbation has reached the adiabatic limit where the constant mode of R_c dominates over the other (usually decaying) mode. In this case, the non-adiabatic pressure perturbation defined in the thermodynamic sense usually becomes also negligible on superhorizon scales. Therefore one might think that the adiabatic limit is the same as thermodynamic adiabaticity. This is in fact nottrue. In other words, thermodynamic adiabaticity is not a sufficient condition for the conservation of Rc on super-horizon scales. In this talk, we consider models that satisfy \delta_{nad} = 0 on all scales, which we call global adiabaticity (GA). A known example is the case of ultra-slow-roll (USR) inflation. In order to generalize USR we develop a method to find the Lagrangian of GA K-inflation models from the behavior of background quantities as functions of the scale factor. Applying this method we show that there indeed exists a wide class of GA models with c^2_w = c^2_s, which allows R_c to grow on superhorizon scales, and hence violates the non-Gaussianity consistency condition.