Suppression of scalar power on large scales and associated bispectra

Abstract

A sharp cutoff in the primordial scalar power spectrum on large scales has been known to improve the fit to the cosmic microwave background (CMB) data when compared to the more standard, nearly scale invariant power spectra that arise in slow roll inflation. Over the last couple of years, there has been resurgent interest in arriving at such power spectra in models with kinetically dominated initial conditions for the background scalar field which leads to inflation of specific duration. In an earlier work, we had numerically investigated the characteristics of the scalar bispectrum generated in such models. In this work, we compare the scenario with two other competing scenarios (viz., punctuated inflation and a model proposed by Starobinsky) which also suppress the scalar power in a roughly similar fashion on large scales. We further consider two other scenarios involving inflation of a finite duration, one wherein the scalar field begins on the inflationary attractor and another wherein the field starts with a smaller velocity and evolves toward the attractor. These scenarios too exhibit a sharp drop in power on large scales if the initial conditions on the perturbations for a range of modes are imposed on super-Hubble scales as in the kinetically dominated model. We compare the performance of all the models against the Planck CMB data at the level of scalar power spectra. The model wherein the background field always remains on the inflationary attractor is interesting for the reason that it permits analytical calculations of the scalar power and bi-spectra. We also compare the amplitudes and shapes of the scalar non-Gaussianity parameter ${f}_{\mathrm{NL}}$ in all these cases which lead to scalar power spectra of similar form. Interestingly, we find that, in the models wherein the initial conditions on the perturbations are imposed on super-Hubble scales, the consistency relation governing the scalar bispectrum is violated for the large-scale modes, whereas the relation is satisfied for all the modes in the other scenarios. These differences in the behavior of the scalar bispectra can conceivably help us observationally discriminate between the various models which lead to scalar power spectra of roughly similar shape.