= Adiabatic initial conditions
= Adiabatic cosmological perturbation
{synonym}
Adiabatic initial conditions perturb every species by the same local time shift, so their relative number-density ratios are initially unperturbed. Photon acoustic modes then begin predominantly as displacement modes with cosine phase.
Equivalently, $\delta\rho_i/\dot{\bar\rho}_i$ is independent of the component $i$. The <non-adiabatic pressure perturbation> then vanishes, and the <comoving curvature perturbation> is conserved on <superhorizon scales> when anisotropic stress and gradient terms can be neglected.
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