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If the universe is in a constant quest to reach an equilibrium, and lets say it finally does reach this point, what will happen?
Question Date: 2016-09-28
Answer 1:

The notion that the universe will reach equilibrium is referred to as the heat death of the universe. The second law of thermodynamics states that entropy increases over time in a closed system. Therefore, given sufficient time, there should reach a state where energy in the universe is evenly distributed. If this were to occur, there would be no temperature differences exploitable to do, perform work. Some speculate that this would begin a dark period with little energy and things occurring over very long time scales. Basically, a universe with very few types of particles and not a lot of activity.

This is indeed a controversial topic among physicists. I should also note that it's a bit tricky and controversial to apply what we know about thermodynamics to the universe as a whole for a few reasons. One is that in thermodynamics, we talk about the system and the surroundings. For the universe, it does not make sense to call the universe the system as we cannot define the surroundings, and therefore should not be treated as a closed thermodynamic system. Another reason is that it is tricky to apply ideas of thermodynamic equilibrium to constituents of the universe that have never been in thermodynamic equilibrium. It comes down to the following: not everyone agrees on how to define entropy for a nonequilibrium system. It is definitely something to ponder, but not worry about. The decay time of a supermassive black hole is about 10100 years, meaning that it will take this long for heat death to occur.

Answer 2:

Good question. For one thing, life would not be possible, since life depends on gradients that would not exist if the universe were in perfect equilibrium.

The thing that we don't know is that the rate of expansion of the universe is increasing, which is a non-equilibrium process that we don't understand. If the expansion continues forever (if!), and if protons decay (theorized but never observed), then all particles of the universe will be drawn to infinity in all directions to the point where no particle will ever interact with another again, except maybe for black holes. If black holes evaporate (which they are thought to), then it depends on what they evaporate into.

Both of these possibilities are so far in the future that it is quite possible that the expansion of the universe will be doing something else then.

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