5. Heat Death
Hey, how about some fun with thermodynamics? In a heat death scenario, a closed system will obey the laws of entropy, meaning that the disorder in the universe will only ever increase. This, in terms of matter, means that everything eventually breaks down. In terms of the universe it means that there will come a time where all of the energy in the universe is dissipated to the point where no more work can be extracted from the universe. This doesn't actually refer to a specific temperature, but it simply means that the energy in the universe will be distributed so evenly that it ceases to flow from one point to another. This is a little bit like when you put a hot water bottle in your bed, the heat energy will "flow" from the hot water bottle to the bed until they are both the same temperature, at which point the exchange just ... stops. This is what would happen to the universe. Everything ... just ... stops. It becomes a big, lukewarm bowl of cosmic soup. Granted, this one isn't so much a method by which one can destroy the universe, it's more of a matter of patience. Well, it would be if the theory of universal heat death hadn't been roundly condemned as a load of dingo's kidneys by many of today's physicists. Just thought we should air this one out as it is a popular one to bring up when discussing the destruction of the universe (as you do). Moving on.