= Mukhanov-Sasaki variable
{c}
{title2=$v=z\mathcal R$}
= Canonical curvature perturbation
{synonym}
For single-field adiabatic inflation, the Mukhanov-Sasaki variable rescales the <comoving curvature perturbation> by $z=a\sqrt{2\epsilon}M_{\rm Pl}$, where $M_{\rm Pl}$ is the <reduced Planck mass>. Its quadratic bulk <action> is $\tfrac12\int[(v')^2-(\nabla v)^2+(z''/z)v^2]$, after a boundary term is removed. A <Fourier transform> mode obeys $v_k''+(k^2-z''/z)v_k=0$ and has canonical <Wronskian normalization>. The mass scale and sound-speed factors depend on the underlying scalar theory; this definition assumes the canonical single-field action and positive slow-roll kinetic coefficient.
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