How the expansion of the universe doesn't violate the principle of the conservation of energy?
If we would put a spring between two objects, the distance would get bigger between them, so would the potential of the spring. And with objects with mass, the same goes for the gravitational potential energy between them...
What am I missing here? Where does this energy come from?
The initial energy to effectively fuel this expansion came from the Big Bang. Energy is not then continuously required to fuel further expansion. The temperature of the universe may decrease due to expansion, but the rate of expansion is accelerating due to currently unobserved dark energy, according to common theory. If the repulsive forces from the dark energy are not considered, the expansion would have probably stopped, come to a negligible rate, or the universe may have started contracting.
