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

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