= Solution
For an expanding decelerating power law $a\propto t^\alpha$ with $0<\alpha<1$, the particle horizon is $ct/(1-\alpha)$, and the comoving Hubble radius is $c/(aH)\propto t^{1-\alpha}$. It increases with time, so regions that are mutually visible today can have been out of causal contact at early times. The nearly uniform early thermal conditions inferred across such regions give the <horizon problem>.
During <cosmic inflation>, accelerated expansion makes the comoving Hubble radius decrease. A patch initially within one causal communication scale can expand into the region now observed, after which its separated parts are temporarily outside the Hubble radius and later re-enter during deceleration. Sufficient duration before the hot radiation era permits a common causal origin; accelerated expansion is not itself a claim that signals propagate faster than light locally.
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