= Negative-w barotropic-fluid gradient instability
{title2=$c_s^2=w<0$}
A propagating adiabatic fluid with the constant closure $p=w\rho$ has $c_s^2=dp/d\rho=w$. For $-1<w<0$, a short-wavelength Fourier perturbation obeys $\omega^2\simeq wk_{\rm phys}^2<0$, producing gradient growth. This does not exclude a negative-pressure perfect-fluid stress tensor with nonadiabatic pressure response, or a canonical scalar with rest-frame <sound speed> one. A <cosmological constant> has no independent propagating fluid density mode.
// Target: cosmology.bigb
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