Gravitational Wave Signals from Spectator Scalar Fields

In this talk, I will present a novel mechanism for gravitational wave generation from spectator scalar fields in the early universe. Spectator scalar fields with masses below the inflationary scale can produce significant isocurvature power spectra in the ultraviolet region, generating substantial curvature perturbations at small scales. These perturbations result in a stochastic gravitational wave background (SGWB) across a very broad range of frequencies. Notably, these signals fall within detection ranges of planned gravitational wave observatories. I will demonstrate that these gravitational waves impose new, more stringent constraints on spectator field masses than those derived from isocurvature perturbations alone. Finally, I will explore enhancement mechanisms for the isocurvature power spectrum through direct coupling between the inflaton and spectator fields, highlighting how such interactions modify the resulting gravitational wave signatures.