  The horizon problem in the standard model of cosmology (LDCM) arises from the
observed uniformity of the cosmic microwave background radiation, which has the
same temperature everywhere (except for tiny, stochastic fluctuations), even in
regions on opposite sides of the sky, which appear to lie outside of each
other's causal horizon. Since no physical process propagating at or below
lightspeed could have brought them into thermal equilibrium, it appears that
the universe in its infancy required highly improbable initial conditions. In
this paper, we examine this well-known problem by considering photon
propagation through a Friedmann-Robertson-Walker (FRW) spacetime at a more
fundamental level than has been attempted before, demonstrating that the
horizon problem only emerges for a subset of FRW cosmologies, such as LCDM,
that include an early phase of rapid deceleration. We show that the horizon
problem is nonexistent for the recently introduced R_h=ct universe, obviating
the principal motivation for the inclusion of inflation. We demonstrate through
direct calculation that, in the R_h=ct universe, even opposite sides of the
cosmos have remained causally connected to us - and to each other - from the
very first moments in the universe's expansion. Therefore, within the context
of the R_h=ct universe, the hypothesized inflationary epoch from t=10^{-35}
seconds to 10^{-32} seconds was not needed to fix this particular "problem",
though it may still provide benefits to cosmology for other reasons.
