What is the worst case scenario for our Universe? What will the final years look like if we could see it?
Worst case might a theory I’ve heard of where a more stable version of vacuum appears somewhere. This “effect” or whatever would spread through the universe at the speed of light, destroying everything in it’s path.
The vacuum thing is described a bit here:
http://www.discover.com/oct_00/featworld.html
it’s #3, “Collapse of the vacuum”
This site has a pretty accurate and concise explaination of the leading theories of what the end of the Universe will be like. Well, more accurate and concise that what I tried typing up, but that’s not saying much. 
Ranchoth
If there is enough mass to stop the universe from expanding, then it will start to implode. This will mean that time will run backwards and everything will happen again only in reverse. Just take a minute and think of things happening in reverse and having to spend a lifetime that way. IMHO that would suck worse than any other scenario.
the conditions which we require to sustain us will be well used up before any endings take place
i dont think we will have any chance of witnessing something which is beyond our involvement.
but its good to dream:)
Why would time reverse? Call me a newbie to theoretical astrophsyics.
And how much gravitational effect does the universe as a whole have on parts at the frontier of the universe, unknown hundreds or thousands of light years away, moving through a complete vacuum?
That’s already happened.
Or, from your point of view, it will happen.
Being alive at the time that happens… now that could suck.
kniz is referring to the “Big Crunch” scenario, which most physicists think is unlikely as indications are that the conditions under which this would happen are not met by our universe. However, even if they were, there wouldn’t be a reversing of time, just a reversing of the expansion of the universe.
To use the old balloon analogy, think of the spatial part of the universe as being the surface of a balloon. At the beginning, the balloon was not inflated at all. Over time, the balloon got bigger and bigger as more and more air was put inside; this is roughly akin to the expansion of the universe in the Big Bang. If there were to be a Big Crunch as well, the rough idea is that someone would be letting the air back out of the balloon. But time is still going forward.
Inasmuch as the OP did not specify a scientific-type answer, I suppose the whole universe being instantaneously unmade and all remaining persons being consigned to “Hell” might be considered a worst-case scenario. (No, I don’t believe it will happen.)
I’m just curious how you can call any of these scenarios the ‘worst case’. How much better would it be if everything collapsed into a big crunch rather than dying a heat death or something?
That’s simple: it would happen a lot sooner. 
Other than that, I would much prefer a big crunch, as the most commonly held version of that theory (as far as I’ve heard, anyway) proposes that the Big Crunch would be followed by another Big Bang. To me, a cyclical universe that’s perpetually reborn sounds much less depressing than a heat death, where we’re left with a void full of neutrons for all eternity.
Jeff
On a related note, where did the idea that time would run backwards during a Big Crunch get started? Sounds pretty silly to me. I remember reading something about it in A Brief History of Time - did Hawking start that idea?
Jeff
Here’s a cool thing to think about:
If there IS a big crunch, followed by a big bang with an initially random configuration of the universe, and this cycle repeats infinitely, then we’re all going to live again. Because given a truly infinite sequence of random initialization, every possible configuration that fits within the laws of physics WILL happen.
In fact, I could be Sam Stone #212,003,293,102.
Anyone see anything fundamentally wrong with this speculation? Any reason why it can’t be possible?
Doesn’t it have to do with the supposed inviolability of the increase of entropy? That the orientation of “time’s arrow” is determined by the expansion or contraction of universal entropy? I seem to remember the theory going along the lines of, if the Big Crunch were to occur, entropy would reverse and move from high to low, and thus “time” as we experience it would run “backwards.” No?
Hmm… I’m exceedingly dubious, although confession I am not an expert on this and I could be wrong. I don’t really see how this would happen, but maybe there’s something funny going on that I haven’t thought of. Certainly I’m not sure what the argument is that “entropy goes up” is more inviolable than “time goes forward.”
Even if entropy were going down instead of up, I don’t see any reason to think that it would go down in exactly the same WAY as it went up. That is, shattering a glass is increasing entropy, but putting it back together by running the movie in reverse, as it were, is hardly the only way to decrease the entropy.
I think I remember reading in a New York Times article that the universe was actually expanding faster and faster to the point where one day it will be the speed of light. At this point the stars would blink out. Communication and travel between worlds would be impossible because nothing travels faster then light. Each civilized planet would be alone as their planet is hurtled into the void of space at the speed of light. That’s the worst I could think of.
Sam Stone:
IMHO as an uncertified philosopher, the “thing fundamentally wrong” is the presumption (which you recognize) that what we think of as “the laws of physics” (the particular ones we have, that is) are able to persist into a “new universe” despite the utter reduction-to-nothingness of anything that would embody or instantiate those laws.
Though I DO think there is a sense in which we can affirm that some particular things (ie, “laws”) MUST exist in ANY reality, it’s dicey indeed to determine just what those fundamentals have to be…and almost any change would seem to rule out the “infinite reshuffling” scenario. If you’re interested, more discussion on the “Can God change Pi?” thread.