I read this article a while ago that said the worst possible threat to the universe had something to do with the partial vacuum of space being sucked into a purer vacuum, thus destroying everything that exists now instantaneously. But for the life of me I can’t remember what this is called or how it could theoretically happen. Any ideas?
Entropy.
I was going to answer “time”, but since the passage of time just increases the amount of entropy, wilie beat me to it.
As for vaccuum sucking universal death, I’m sure our elected officials have looked into this problem and are dealing with it accordingly.
As an avid reader of SF and a completely unqualified amateur in the realms of theoretical physics, my completely unfounded WAG is: a collision or near miss between universes (within whatever hyper-dimensional embedding space that may or may not exist) could turn out to be quite nasty as it could destroy or distort not only space, but time too (and the universe as we know it might not just cease to exist, but cease to have ever existed), on the other hand, collision or near miss between two such objects might actually be responsible for the phenomenon that we call ‘the universe’. Take your pick.
No, there apparently is a thing called “True Vacuum”. To quote from an on-line definition I found: ** A stable state in which a quantum field is zero and the corresponding potential is also zero; i.e., the vacuum energy density is zero. **
In layman’s terms, apparently even vacuum has some energy and that state is unstable. It’s possible that an area could be created where there was no energy (energy’s not the right word here, but it gets the idea across) which is far more stable. My (very, VERY) limited understanding is that if such a state were created somehow, a bubble of “true vacuum” would begin spreading at lightspeed, “eating” anything it touched.
About 4 science fiction authors latched onto the idea at just about the same time (Spider Robinson in the utterly dreadful Callahan’s Key was one which hinged what little plot that book had on the idea). I first read about it in a non-Science Fiction context in Paul Davis’s The Last Three Minutes
Fenris
That would be ‘heat death’. The end result of entropy over time. Eventually, the universe will achieve a constant energy density, and no work can possibly be done.
A false vacuum hypothesis scared some people. Link:
http://www.parascope.com/articles/slips/fs28_2.htm
As for non-universe ending events, gamma ray bursters are scary:
this explains how things function at my job 
physics are great for everything
The reset button.
Pauly Shore?
A General Protection Fault?
The actual process would be similar to phase changes like crystallization. If you take liquid water, for instance, and very carefully cool it below zero Celsius, without disturbing it, it’ll stay liquid (this is called a supercooled liquid). It’s unstable, though: drop so much as a single snowflake into a supercooled bathtub of water, and it’ll all freeze solid almost instantaneously. You can get similar effects with solutions, where you add a “seed crystal” to a supersaturated solution (this is how one makes rock candy). Similarly, our Universe might be in some sort of supercritical state, just waiting for some seed of true vacuum to “crystallize” to the more stable state. Fortunately, it’s very difficult to get such a seed into the Universe: Some theories suppose that the final stages of an evaporating black hole might do the trick, but we’ve never yet seen that occuring, and for most black holes, it won’t happen until long, long after everything in the Universe is already dead.