Cosmological Fluid Equation

Significance

The third member of the Friedmann trio, and the one most often left out: energy conservation for an expanding universe. It is nothing more exotic than the first law of thermodynamics — dU = −p dV — applied to a comoving volume that is growing, which is why the pressure term appears alongside the density. Any two of the three cosmological equations imply the third, so this is the closure that makes the system solvable; without it the Friedmann equations have more unknowns than constraints.