The Rh = ct universe without inflation

Context. The horizon problem in the standard model of cosmology (Lambda DCM) 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. Aims. In this paper, we demonstrate that the horizon problem only emerges for a subset of Friedmann-Robertson-Walker (FRW) cosmologies, such as Lambda CDM, that include an early phase of rapid deceleration. Methods. The origin of the problem is examined by considering photon propagation through a FRW spacetime at a more fundamental level than has been attempted before. Results. 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 this cosmology, 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) s to 10(-32) s was not needed to fix this particular problem, though it may still provide benefits to cosmology for other reasons.