Gravitational waves from spectator scalar fields

This is the authors' abstract. We don't add key points for this paper.

We propose a mechanism for gravitational wave (GW) production sourced by spectator scalar fields during inflation. These fields, while not driving cosmic expansion, generate blue-tilted isocurvature fluctuations that naturally satisfy current cosmic microwave background (CMB) constraints at large scales while producing enhanced power spectra at smaller scales accessible to GW detectors. The resulting GW spectrum spans an exceptionally broad frequency range from 10 − 20 to 1 Hz, with amplitudes ranging from Ω GW h 2 ∼ 10 − 19 to 10 − 14 depending on the reheating temperature and spectator field mass. Observable signals across multiple detector bands are possible in a restricted region of parameter space, m χ / H I within the isocurvature- and Δ N eff -allowed window and T reh , such that the spectator either provides all of the dark matter (stable case) or decays near matter-radiation equality (unstable case). Our analysis reveals multiple complementary constraints on spectator field parameters. For a high reheating temperature of T reh > 10 10 GeV , GW-induced limits on the effective number of relativistic species ( Δ N eff ) require m χ ≳ 0.59 H I , stronger than CMB isocurvature bounds alone ( m χ ≳ 0.54 H I ). The nonobservation of primordial B-modes by provides stronger constraints, m χ ≳ 0.64 H I , with projected LiteBIRD sensitivity potentially reaching m χ ≳ 0.67 H I . This mechanism enables a unique multimessenger probe of beyond the standard model physics during inflation, providing simultaneous constraints on inflationary dynamics, dark matter production, and reheating through current and next-generation GW experiments.