Dark energy and conservation of energy [duplicate]

Question

With accelerated expansion of universe which is same in all direction we know that dark energy increase with time because space between any two point in space time increases with time. So after some finite time we can not see nearby galaxy cluster which we can see now. So doesn't that violate conservation of energy which says energy neither can created nor can destroyed. Because with expanding universe energy in the form of dark energy increases with time so if we consider whole universe (visible + invisible) as isolated system then energy of whole universe increase means energy is created from nothing. Am I missing something over here?

Answer 1

First, in the most common model of dark energy, $\Lambda$CDM, dark energy is a constant energy density, which means that the "energy" from dark energy does increase as the universe expands. Second, the law of conservation of Energy is only valid in a static universe. Because our universe is expanding, it is no longer the same at every moment of time and so energy need not be conserved. The conserved quantity is, instead, the stress-energy.

In this way, you are quite right, energy is not conserved, but it doesn't have to be. On small scales, the conservation of energy law still applies, but on scales where dark energy becomes important, there is no such law.