I'm wondering what is behind the cosmological horizon barrier?
Well - there are a couple of possibilities:
- Nothing: the universe could actually be the size we can see, with the edge about 46 billion light-years away
- Lots more universe, similar to what we can see
- An infinite universe
It doesn't really matter which, though, as nothing beyond that horizon can effect us or be affected by us (the possible exception being objects around the same distance as the horizon, which may pop into view, due to variation in the Hubble parameter)
The optical horizon is calculated to be about 46 Gyrs away if we presume a flat Universe, from the simple relationship R_h = 3ct, where R_h is the optical horizon and t time. This relationship can be found in P.J.E.Peebles book, Principles of Physical Cosmology, and in the book by J.B. Hartle, Gravity. So the 46 Gyrs mentioned above is consistent with a t = 15 Gyrs, and our visible Universe is about 90 Gyrs across.
Now if the Universe is not flat, then things really go wild quickly and that value grows very quickly with increased curvature. Same if the accelerating universe turns out to be true.