Separately conserved cosmological fluids
= Separately conserved cosmological fluids
A noninteracting <perfect fluid in general relativity> has $\nabla_\mu T_i^{\mu\nu}=0$, so $\dot\rho_i+3H(\rho_i+P_i)=0$. If energy is transferred between species, source terms $Q_i$ appear and only their sum vanishes. The <Friedmann equation> still involves the sum of all component stress tensors because all components share one spacetime metric.