I just want to state that for anyone reading this in disbelief thinking "the universe has always been here" - we already know that the universe is not eternal. There was a definite start. We know this by the quantity called entropy.

Entropy is a macro state variable that describes the number of micro states a system can take on. Entropy (S) is (delta)S = Q/T with Q = the amount of heat added to a system and T = the temperature at which the heat is added and S = k ln (omega) with k = constant times the number of states the system can take on (assuming all of the states are equally probable). The second law of thermodynamics says that when any process is undergone the entropy of the universe is always greater than or equal to zero.

We know that the universe has finite energy in it. We know that the universe is not eternal because if it was, all of the energy and 'orderedness' in the universe would already have been used up and dissolved at some inexplicable point in time, so to speak. It would be cold and lifeless. We could not exist.

Entropy is referred to as 'the arrow of time.' A kind of Zelda-ish sounding name.