Can the Universe's Expansion Allow Us to Escape Its Event Horizon?

[daveb]

Not sure if this is the best place to post this, but here goes anyway.
While musing about various weirdness recently, I wondered about the following:
Last I heard, cosmologists still refere to the universe has having a size (i.e., light years across). After the Big Bang, and at some point afterwards, the universe's "radius" was less than twice its mass, so the event horizon (so to speak) was outside the confines of the universe. Eventually, the universe expanded, and this "radius" exceeded twice the mass (haven't done any calculations to see when or even if this has occurred yet). So that means one could theoretically escape the event horizon for the universe (to who knows where) on the assumption one could overtake the Hubble expansion.
I know there's something funadmentally wrong with this, but I can't quite put my finger on it.

 

[PeterDonis]

The fundamental issue is that the concept of "event horizon" you are using is the boundary of a region of spacetime that cannot send light signals to infinity. But this concept requires the spacetime to have an "infinity", i.e., that the spacetime must be asymptotically flat. Heuristically, this means the spacetime has an isolated region occupied by "matter", surrounded by empty space. But the spacetime of our universe as a whole is not like that: it has roughly the same average density of matter everywhere, and is not asymptotically flat. So the concept of "event horizon" (and "black hole") that you are using doesn't even make sense for the spacetime of our universe as a whole.

More information in this Insights article:

https://www.physicsforums.com/insights/universe-black-hole/