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I'm wondering what is behind that barrier.

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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)

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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
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Inflation implies either 2 or 3, as far as I know. – David Zaslavsky♦ Mar 13 '12 at 20:18

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