  The Universe has a gravitational horizon with a radius R_h=c/H coincident
with that of the Hubble sphere. This surface separates null geodesics
approaching us from those receding, and as free-falling observers within the
Friedmann-Lemaitre-Robertson-Walker spacetime, we see it retreating at proper
speed c, giving rise to the eponymously named cosmological model R_h=ct. As of
today, this cosmology has passed over 25 observational tests, often better than
LCDM. The gravitational/Hubble radius R_h therefore appears to be highly
relevant to cosmological theory, and in this paper we begin to explore its
impact on fundamental physics. We calculate the binding energy of a mass m
within the horizon and demonstrate that it is equal to mc^2. This energy is
stored when the particle is at rest near the observer, transitioning to a
purely kinetic form equal to the particle's escape energy when it approaches
R_h. In other words, a particle's gravitational coupling to that portion of the
Universe with which it is causally connected appears to be the origin of
rest-mass energy.
