https://hal-insu.archives-ouvertes.fr/insu-03644931Renaux-Petel, SébastienSébastienRenaux-PetelIAP - Institut d'Astrophysique de Paris - INSU - CNRS - Institut national des sciences de l'Univers - SU - Sorbonne Université - CNRS - Centre National de la Recherche ScientifiqueUPCité - Université Paris CitéTurzyński, KrzysztofKrzysztofTurzyńskiOn reaching the adiabatic limit in multi-field inflationHAL CCSD2015Astrophysics - Cosmology and Nongalactic AstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theory[SDU] Sciences of the Universe [physics]Gestionnaire, Hal Sorbonne Université2022-04-19 14:46:142022-08-05 11:55:132022-04-19 14:46:14enJournal articles10.1088/1475-7516/2015/06/0101We calculate the scalar spectral index n<SUB>s</SUB> and the tensor-to-scalar ratio r in a class of recently proposed two-field no-scale inflationary models in supergravity. We show that, in order to obtain correct predictions, it is crucial to take into account the coupling between the curvature and the isocurvature perturbations induced by the noncanonical form of the kinetic terms. This coupling enhances the curvature perturbation and suppresses the resulting tensor-to-scalar ratio to the per mille level even for values of the slow-roll parameter epsilon ~ 0.01. Beyond these particular models, we emphasise that multifield models of inflation are a priori not predictive, unless one supplies a prescription for the post-inflationary era, or an adiabatic limit is reached before the end of inflation. We examine the conditions that enabled us to actually derive predictions in the models under study, by analysing the various contributions to the effective isocurvature mass in general two-field inflationary models. In particular, we point out a universal geometrical contribution that is important at the end of inflation, and which can be directly extracted from the inflationary Lagrangian, independently of a specific trajectory. Eventually, we point out that spectator fields can lead to oscillatory features in the time-dependent power spectra at the end of inflation. We demonstrate how these features can be model semi-analytically as well as the theoretical uncertainties they can entail.