Title:	Loop contributions to the scalar power spectrum due to quartic order action in ultra slow roll inflation
Authors:	Maity, Suvashis Ragavendra, H.V. Sethi, Shiv K. Sriramkumar, L.
Issue Date:	8-May-2024
Publisher:	IOP Publishing Ltd.
Citation:	Journal of Cosmology and Astroparticle Physics, 2024, Vol. 2024, p046
Abstract:	The investigation of the theory of inflation beyond the linear order in perturbations is important both for theoretical consistency and potential observables. In the contemporary literature, the calculation of modifications to the inflationary scalar power spectrum due to the loops from the higher order interaction terms in the Hamiltonian have led to an interesting discussion regarding the validity of perturbation theory and the robustness of its predictions. Recently, there have been many efforts to examine the contributions to the scalar power spectrum due to the loops arising from the cubic order terms in the action describing the perturbations, specifically in inflationary scenarios that permit an epoch of ultra slow roll (USR). A brief phase of USR during inflation is known to lead to interesting features in the scalar power spectrum which in turn has significant observational consequences, such as the copious production of primordial black holes. In this work, we consider the loop contributions to the scalar power spectrum in a scenario of USR inflation arising due to the quartic order terms in the action describing the scalar perturbations. We compute the loop contributions to the scalar power spectrum due to the dominant term in the action at the quartic order in a scenario wherein a short phase of USR is sandwiched between two stages of slow roll (SR) inflation. We analyze the behaviour of the loop contributions in terms of the parameters that characterize the non-trivial inflationary dynamics, viz. the onset and duration of USR, and the smoothness of transitions between the USR and SR phases. We examine three different cases of the scenario — the late, intermediate and early epochs of USR during inflation, each of which affects the scalar power spectrum over different ranges of wave numbers. In the inflationary scenario involving a late phase of USR, for reasonable choices of the parameters, we show that the loop corrections are negligible for the entire range of wave numbers. In the intermediate case, the contributions from the loops prove to be scale invariant over large scales and, we find that these contributions can amount to 30% of the leading order (i.e. the Gaussian) power spectrum. In the case wherein USR sets in early, we find that the loop contributions could be negative and can dominate the power spectrum at the leading order, which indicates a breakdown of the validity of the perturbative expansion. We discuss the origin of the negative sign and the divergences that arise in the loop contributions to the power spectrum. We conclude with a brief summary and outlook.
Description:	Open Access
URI:	http://hdl.handle.net/2289/8305
Alternative Location:	https://arxiv.org/abs/2307.13636# https://ui.adsabs.harvard.edu/abs/2024JCAP...05..046M/abstract http://dx.doi.org/10.1088/1475-7516/2024/05/046 https://inspirehep.net/literature/2680404
Copyright:	2024 IOP Publishing Ltd
Appears in Collections:	Research Papers (A&A)

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