Coasting fluid
= Coasting fluid
{title2=$w=-\tfrac13,\quad\rho\propto a^{-2}$}
A <coasting fluid> has pressure $p=-\rho/3$. The <cosmological perfect-fluid continuity equation> makes its density scale as $a^{-2}$. If it alone fills a flat expanding universe, the <scale factor> grows linearly in physical time. Its background density has the same scale-factor dependence as a spatial-curvature term, although the fluid does not itself create spatial curvature.