Gauge dependence of gravitational waves induced by primordial isocurvature fluctuations

Primordial isocurvature perturbations, which can arise from various sources in the early Universe, have the potential to leave observable imprints on the gravitational-wave background and provide insights into the nature of primordial fluctuations. In this study, we investigate the gauge dependence of induced gravitational waves (IGWs) sourced by these isocurvature perturbations during radiation dominated era and the kination period in the early Universe. We analyze the energy density spectra of IGWs in three different gauges: synchronous, Newtonian, and uniform curvature gauges. To facilitate this analysis, we derive analytical solutions for the perturbations that contribute to the IGW spectra and a general gauge transformation from Newtonian gauge to an arbitrary gauge. Our results reveal significant differences in the energy spectra across these gauges. We find that the energy density of IGWs during radiation domination increases with conformal time as ${\ensuremath{\eta}}^{8}$ and ${\ensuremath{\eta}}^{4}$ for synchronous and uniform curvature gauges, respectively, while it converges in the Newtonian gauge. These findings highlight the importance of gauge choice in calculating IGWs and have implications for the interpretation of future observations of the gravitational-wave background.