  In general relativity, a gravitational horizon (more commonly known as the
"apparent horizon") is an imaginary surface beyond which all null geodesics
recede from the observer. The Universe has an apparent (gravitational) horizon,
but unlike its counterpart in the Schwarzschild and Kerr metrics, it is not
static. It may eventually turn into an event horizon---an asymptotically
defined membrane that forever separates causally connected events from those
that are not---depending on the equation of state of the cosmic fluid. In this
paper, we examine how and why an apparent (gravitational) horizon is manifested
in the Friedmann-Robertson-Walker metric, and why it is becoming so pivotal to
our correct interpretation of the cosmological data. We discuss its
observational signature and demonstrate how it alone defines the proper size of
our visible Universe. In so doing, we affirm its physical reality and its
impact on cosmological models.
