Jeans length
= Jeans length
{c}
{title2=$\lambda_J=c_s\sqrt{\pi/(G\rho)}$}
With physical wavelength $2\pi/k$, the <Jeans length> is $2\pi/k_J$, where the <Jeans wavenumber> is $\sqrt{4\pi G\rho}/c_s$. Linear pressure support dominates gravity below this length, whereas longer wavelengths can grow. In an expanding background the growth also depends on the Hubble damping term, so the static threshold does not by itself give the time dependence.