Friedmann-Robertson-Walker models do not require zero active mass

Abstract

The Rh = ct cosmological model has received considerable attention in recent years owing to claims that it is favoured over the standard Λ cold dark mater (ΛCDM) model by most observational data. A key feature of the Rh = ct model is that the zero active mass condition ρ + 3p = 0 holds at all epochs. Most recently, Melia has claimed that this condition is a requirement of the symmetries of the Friedmann-Robertson-Walker spacetime. We demonstrate that this claim is false and results from a flaw in the logic of Melia's argument.