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 $\Lambda$
cold dark matter ($\Lambda$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.
