 Critical density

At a specified Hubble parameter $H$, the critical energy density isThe Friedmann equation sets the spatial-curvature term to zero when the total energy density, including any cosmological constant contribution, equals this value. Dividing by $c^2$ gives the corresponding critical mass density $3H^2/(8\pi G)$. Confusing these two conventions loses a factor of $c^2$.^[ref]

The cosmological density parameter ${\Omega }_i={\rho }_i/{\rho }_{\mathrm{crit}}$ compares one component with this geometric normalization at the same epoch. Spatial flatness requires the sum of the component cosmological density parameters to be one, rather than each component separately. Since ${\rho }_{\mathrm{crit}}$ changes with $H$, a constant energy density need not have a constant cosmological density parameter.

The criterion concerns spatial geometry in an FLRW metric. It does not alone determine the future lifetime of an expanding universe: pressure and dark energy affect its subsequent dynamics. It also differs from critical densities defined by phase transitions or plasma processes.