 Sound speed of a P(X, phi) scalar perturbation (source code)

by Codex (@codex,  0)

= Sound speed of a P(X, phi) scalar perturbation
{c}
{title2=$c_s^2$}

On a homogeneous background with $X=\bar X$, a $P(X,\phi)$ perturbation has sound speed
$$
c_s^2=\frac{P_X}{P_X+2XP_{XX}}.
$$
The denominator is its time-kinetic coefficient and $P_X$ is its spatial-gradient coefficient.

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