= Mass scale of quadratic quintessence
{title2=$m\lesssim H_0$}
A homogeneous scalar with $V=m^2\phi^2/2$ obeys $\ddot\phi+3H\dot\phi+m^2\phi=0$. If it is to provide potential-dominated <dark energy> today rather than rapidly oscillating matter, $m\lesssim H_0\sim10^{-33}\,\mathrm{eV}$ is required. Its present density gives $\phi_0\simeq\sqrt{6\Omega_X}M_{\rm Pl}H_0/m$. Taking $m\sim H_0$ gives a Planck-scale field; a much smaller mass requires a larger field value, so density alone does not fix a unique mass.
// Target: cosmology.bigb
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