Inflation may have involved more than a single scalar degree of freedom. Spectator scalar fields, while subdominant in the background evolution, can acquire nontrivial fluctuations and leave observable imprints long after inflation ends. In this talk I will discuss how such fields can generate blue-tilted isocurvature perturbations during inflation, remain compatible with large scale CMB bounds, and become strongly enhanced on smaller scales. I will then emphasize their gravitational wave phenomenology: spectator fluctuations can source stochastic gravitational wave backgrounds both indirectly, through enhanced scalar and isocurvature perturbations, and more directly through post-inflationary matter dynamics during reheating. Interactions with the inflaton, decay channels, and resonance effects can substantially reshape the resulting signal, affecting both the amplitude and frequency range of the spectrum. Altogether, spectator fields provide a broad framework in which inflationary physics, reheating dynamics, and gravitational wave observables are closely connected.
Sarunas Verner, KCIP Fellow, University of Chicago
Host: Adrian Thompson
Joan West
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