  The R_h=ct Universe is a Friedmann-Robertson-Walker (FRW) cosmology which,
like LCDM, assumes the presence of dark energy in addition to (baryonic and
non-luminous) matter and radiation. Unlike LCDM, however, it is also
constrained by the equation of state (EOS) p=-rho/3, in terms of the total
pressure p and energy density rho. One-on-one comparative tests between R_h=ct
and LCDM have been carried out using over 14 different cosmological
measurements and observations. In every case, the data have favoured R_h=ct
over the standard model, with model selection tools yielding a likelihood
~90-95% that the former is correct, versus only ~5-10% for the latter. In other
words, the standard model without the EOS p=-rho/3 does not appear to be the
optimal description of nature. Yet in spite of these successes---or perhaps
because of them---several concerns have been published recently regarding the
fundamental basis of the theory itself. The latest paper on this subject even
claims---quite remarkably---that R_h=ct is a vacuum solution, though quite
evidently rho is not 0. Here, we address these concerns and demonstrate that
all criticisms leveled thus far against R_h=ct, including the supposed vacuum
condition, are unwarranted. They all appear to be based on incorrect
assumptions or basic theoretical errors. Nevertheless, continued scrutiny such
as this will be critical to establishing R_h=ct as the correct description of
nature.
