Mr. H. V. Ragavendra

The primordial scalar power spectrum is well constrained by the cosmological data on large scales, primarily from the observations of the anisotropies in the cosmic microwave background (CMB). Over the last few years, it has been recognized that a sharp rise in power on small scales will lead to enhanced formation of primordial black holes (PBHs) and also generate secondary gravitational waves (GWs) of higher and, possibly, detectable amplitudes. It is well understood that scalar power spectra with COBE normalized amplitude on the CMB scales and enhanced amplitudes on smaller scales can be generated due to deviations from slow roll in single, canonical scalar field models of inflation. In fact, an epoch of so-called ultra slow roll inflation can lead to the desired amplification. It is found that scenarios that lead to ultra slow roll can be broadly classified into two types, one wherein there is a brief departure from inflation (a scenario referred to as punctuated inflation) and another wherein such a departure does not arise. In this talk, I shall consider a set of single field inflationary models involving the canonical scalar field that lead to ultra slow roll and punctuated inflation and examine the formation of PBHs as well as the generation of secondary GWs in these models. Apart from considering specific models, I shall illustrate the reconstruction of potentials from certain functional choices of the first slow roll parameter leading to ultra slow roll and punctuated inflation and investigate their observational signatures. In addition to the secondary tensor power spectrum, I shall discuss the secondary tensor bispectrum generated (in the equilateral limit) in these scenarios. Lastly, I shall describe the computation of the inflationary scalar bispectrum that arises in all the cases and discuss the imprints of the scalar non-Gaussianities on the extent of PBHs formed and the amplitude of the secondary GWs generated.