  Many cosmological measurements today suggest that the Universe is expanding
at a constant rate. This is inferred from the observed age versus redshift
relationship and various distance indicators, all of which point to a cosmic
equation of state (EoS) p=-rho/3, where rho and p are, respectively, the total
energy density and pressure of the cosmic fluid. It has recently been shown
that this result is not a coincidence and simply confirms the fact that the
symmetries in the Friedmann-Robertson-Walker (FRW) metric appear to be viable
only 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 a
counter argument to this conclusion. Here, we show that these authors are
merely repeating the conventional mistake of incorrectly placing the observer
simultaneously in a comoving frame, where the lapse function g_tt is coordinate
dependent 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 flow
is not inertial when rho+3p is not 0, so the comoving frame is generally not in
free fall, even though in FRW, the comoving and free-falling frames are
supposed to be identical at every spacetime point. So this confusion of frames
not only constitutes an inconsistency with the fundamental tenets of general
relativity but, additionally, there is no possibility of using a gauge
transformation to select a set of coordinates for which g_tt=1 when rho+3p is
not 0.
