Cosmological Horizons

Horizons are frontiers between things observable and things unobservable. Even if such horizons exist we may extrapolate from observable to non-observable regions of the universe by (a) using the laws of physics which tell us how a presently observable galaxy will evolve even when we can no longer observe it, and (b) using the cosmological principle, which tells us that the physical situation beyond the horizon is by and large the same as the physical situation within the horizon. It would be paradoxical to define the universe in terms of objects or events within a horizon because it is only in virtue of information about objects and events outside the horizon that we have reason to believe that the horizon exists.