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Many cosmological measurements today suggest that the Universe is expandingat a constant rate. This is inferred from the observed age versus redshiftrelationship and various distance indicators, all of which point to a cosmicequation of state (EoS) p=-rho/3, where rho and p are, respectively, the totalenergy density and pressure of the cosmic fluid. It has recently been shownthat this result is not a coincidence and simply confirms the fact that thesymmetries in the Friedmann-Robertson-Walker (FRW) metric appear to be viableonly for a medium with zero active mass, i.e., rho+3p=0. In their latest paper,however, Kim, Lasenby and Hobson have provided what they believe to be acounter argument to this conclusion. Here, we show that these authors aremerely repeating the conventional mistake of incorrectly placing the observersimultaneously in a comoving frame, where the lapse function g_tt is coordinatedependent when rho+3p is not 0, and a supposedly different, free-falling frame,in which g_tt=1, implying no time dilation. We demonstrate that the Hubble flowis not inertial when rho+3p is not 0, so the comoving frame is generally not infree fall, even though in FRW, the comoving and free-falling frames aresupposed to be identical at every spacetime point. So this confusion of framesnot only constitutes an inconsistency with the fundamental tenets of generalrelativity but, additionally, there is no possibility of using a gaugetransformation to select a set of coordinates for which g_tt=1 when rho+3p isnot 0.

The Zero Active Mass Condition in Friedmann-Robertson-Walker Cosmologies