Quantum mechanical explanation of the expansion of the universe? All the explanations for the expansion of the universe that I have heard about are usually based off Einstein's General Relativity, and many quote the proposed all permeating Dark Energy as the cause of the acceleration of the expansion. Quantum mechanics seems to play no role in the expansion of the universe. My understanding is that many processes, such as gravity, electromagnetism, and the weak and strong forces, can all be explained through quantum mechanics, if this is the case then does the expansion of the universe have a quantum mechanical explanation? And if there is no QM explanation currently available, is it necessary that it should have a quantum mechanical explanation, or do we just accept that quantum mechanics plays no role in the expansion of the universe?
 A: The large scale structure of the universe can be described with general relativity. Whether the structure of general relativity can be described with quantum mechanics is an unsolved problem. Many people certainly think so and have spent a considerable amount of effort on trying to formulate such a description. 
For the second part of your question, I would opine (and that is all it is) that there is no requirement to have such a description. It may very well be that macroscopic gravity and the large scale structure are the result of a thermodynamic process that acts on top of a, yet unknown, quantum mechanical fundamental level. Magnetism is a nice example for that. All elementary magnets (be they entire atoms or electron or nuclear spins) are quantum mechanical in nature, but the description of a macroscopic magnetic phase at high temperature (i.e. magnetized iron) can be understood in terms of thermodynamic averages over these elementary magnets. On the other hand there are also macroscopic magnets that derive directly from a thermodynamically not averaged quantum state: superconductors. So we have both examples in a different phenomenon. It may very well be that gravity gives us a similar scenario. Dark energy and weak gravity may both be thermodynamic remnants, while the strong fields near the singularities (or at the event horizon) of black holes are actually quantum phenomena (that would be my naive take on it, but it might be the other way around or completely different, altogether). My main point is that, at this point, until someone delivers a really strong argument, the question should be treated as open. 
