Inflating with a tachyonic spectator

par Margherita Putti (University of Groningen)

mardi 21 avr. 2026, 11:00 → 12:00 Europe/Paris

Salle des Sommets (LAPTh)

Inflation provides the leading explanation for the origin of cosmic structure, with its predictions now tested to high precision by CMB observations.At the same time, multifield sectors are a generic expectation in UV-complete theories and can lead to robust deviations from the predictions of simple single-field models. In this talk, I will present a model-independent mechanism in which a spectator axion universally modifies inflationary observables without requiring direct couplings to the inflaton. The spectator is initially stabilized near the hilltop of its potential during the observable phase of inflation and rolls away only after all relevant scales have crossed the horizon. During this delayed tachyonic phase, isocurvature fluctuations undergo superhorizon growth and are transferred gravitationally to the curvature perturbation, enhancing the scalar power spectrum while leaving tensors unaffected. After normalization to the observed CMB amplitude, this leads to a suppression of the tensor-to-scalar ratio, a shift in the scalar spectral index, and a characteristic primordial non-Gaussian signal. Because the instability develops only after horizon exit, the effect is intrinsically scale-independent, making it a universal correction applicable to a broad class of single-field inflationary models.