# What is behind the cosmological horizon barrier?

I'm wondering what is behind the cosmological horizon barrier?

• There are several measures of the cosmological horizon. For instance, there is a future horizon and a past horizon, and both of these are changing. For some cases, the answer is that we've directly seen galaxies that later fell beyond the horizon. – Alan Rominger Oct 28 '13 at 18:39

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)

• I agree that the answer fortunately doesn't matter but doesn't inflation imply option (2)? That is, the rapid expansion was so fast that causally connected regions of space became disconnected. So they're now beyond the horizon but are (or at least were) similar to the visible Universe? – Warrick Mar 12 '12 at 10:46
• Option 1 or 2 both seem more likely than 3. The tricky thing is that the models could allow any of them to be true. Inflation can work as viewed for option 1 as easily as option 2. – Rory Alsop Mar 12 '12 at 10:55
• Just checked your profile and realised you certainly know the maths in much greater depth than I - so I am more than happy to believe that 2 is more likely :-) – Rory Alsop Mar 12 '12 at 10:57
• Inflation implies either 2 or 3, as far as I know. – David Z Mar 13 '12 at 20:18

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.

• The approximation that the Particle horizon roughly equals $3\cdot c\cdot t$ is true for the early universe and the near future, while in later times this ratio will blow up, see yukterez.net/forum/viewtopic.php?p=170#p170 – Yukterez Feb 24 '16 at 1:03
• I think the best view is really...may blow up...depends if you believe in "eternal inflation" and/or dark energy, etc. A recent publication on the optical horizon can be found in MNRAS vol. 449, p 1270-1274 with plenty of references. I think most everybody agrees that wherever the optical horizon is we shall never know what is on the other side. – Mike Smith Feb 25 '16 at 4:22
• Sure we will know, we just have to wait long enough. The partice horizon grows faster than the scale factor, so every day we see a bit farther than the day before, even if what we see today is a bit more redshifted than what we saw yesterday. So we will at least know what was behind the horizon, even if we will never know what happens there right now. Also, if there was no dark energy the universe might as well collapse, then there also could be no ratio of rh:t=3:1 for all eternity. – Yukterez Feb 25 '16 at 16:19