Description
Multifield inflation models involving ultra-light fields such as moduli and axions are a generic outcome of string and supergravity cosmology. In this talk we consider under what conditions their primordial perturbation spectra may be consistent with observations on the largest (CMB) scales. The usual route is to stabilise the light fields so as to recover single-field inflation. Another possibility is if massless, frozen isocurvature perturbations can source adiabatic perturbations efficiently over many e-folds. I will discuss a third option, that has emerged more recently, in which the isocurvature perturbations are tachyonic (unstable) but the instability is shut off by an exponentially decaying turning rate. We call this regime “ultra-slow-turn” (by analogy with the ultra-slow-roll regime where the first slow roll parameter decays exponentially).
[Based on arxiv 2603.03204 with P. Christodoulidis, J-O. Gong and O. Iarygina.]