Gravitational Waves from Isocurvature Perturbations of Spectator Scalar Fields

We present a mechanism for gravitational wave (GW) production from isocurvature perturbations in spectator scalar fields during inflation. These energetically subdominant fields develop blue-tilted power spectra through inflationary dynamics, generating second-order scalar perturbations that source a stochastic GW background. The mechanism naturally satisfies CMB constraints at large scales while producing enhanced signals at smaller scales across a broad frequency range $10^{-20} - 1$ Hz. We perform comprehensive numerical and analytical calculations of the complete isocurvature spectrum evolution, including gravitational particle production, reheating dynamics, and scalar-induced GW generation. For spectator fields with effective masses $0.5 \lesssim m_{\chi,\mathrm{eff}}/H_I$, the resulting GW energy density reaches $\Omega_{\text{GW}} h^2 \sim 10^{-20}$-$10^{-12}$, accessible to pulsar timing arrays, space-based interferometers, and next-generation CMB experiments. Our analysis reveals that GW-induced constraints exceed current isocurvature bounds. We examine both unstable (curvaton-like) and stable (dark matter) spectator fields, demonstrating strong sensitivity to reheating temperature, inflaton-spectator coupling, and decay dynamics. This framework establishes isocurvature-sourced GWs as a powerful probe of early universe physics, enabling simultaneous constraints on inflationary dynamics, dark matter production, and reheating through coordinated multi-frequency GW observations.