Gauge dependence of scalar-induced gravitational waves from isocurvature perturbations: Analytical results

Abstract

We analytically study the gauge dependence of scalar-induced gravitational waves (SIGWs) sourced by primordial isocurvature perturbations during radiation domination (RD), working across nine gauges. Through analytical integrations of the kernels supported by graphical comparison we identify a clear dichotomy. We find that in some gauges viz. the uniform-density (UD), total-matter (TM), uniform-curvature (UC), comoving-orthogonal (CO) and transverse-traceless (TT) gauges the energy density grows polynomially in conformal time $\eta^n$, where $n$ varies from $2$ to $8$. While in rest of the gauges viz. the longitudinal (Long.), uniform-expansion (UE), Newtonian-motion (Nm), and N-body (Nb) gauges the late-time energy spectrum converges, and SIGWs behave as radiation. For subhorizon modes ($ k\eta \gg 1 $), the divergence becomes severe, showing that SIGWs are gauge-dependent observables in this regime. We resolve it through a kernel projection that isolates the luminal, freely propagating gravitational wave components (oscillating as $\sin(k\eta)$ and $\cos(k\eta)$), eliminating spurious contributions. The resulting kernel decays as $ (k\eta)^{-1} $ and yields a finite, gauge-independent late-time spectrum, confirming that only luminal modes represent physical SIGWs.