= Mass-equivalent cosmological critical density
{title2=$\rho_{\mathrm{crit}}=3H^2/(8\pi G)$}
The mass-density convention for the cosmological <critical density>. If $u$ denotes <energy> per unit volume, $\rho=u/c^2$ and $u_{\rm crit}=3c^2H^2/(8\pi G)$. Both conventions give the same dimensionless density parameter when numerator and denominator use consistent units. This distinction avoids inserting an extra $c^2$ into a <Friedmann equation> written in terms of mass-equivalent density.
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