Infrared Behavior of Induced Gravitational Waves from Isocurvature Perturbations

Abstract

Induced gravitational waves provide a powerful probe of primordial perturbations in the early universe through their distinctive spectral properties. We analyze the spectral energy density $\Omega_{\text{GW}}$ of gravitational waves induced by isocurvature scalar perturbations. In the infrared regime, we find that the spectral slope $n_{\text{GW}} \equiv \text{d} \ln\Omega_\mathrm{GW}/\text{d}\ln k$ takes the log-dependent form $3-4/ \ln (\tilde{k}_*^2 / 6k^2)$, where $\tilde{k}_*$ represents the effective peak scale of the primordial scalar power spectrum. This characteristic behavior differs markedly from that of adiabatic-induced gravitational waves, establishing a robust observational discriminant between isocurvature and adiabatic primordial perturbation modes.