Ask Ethan: Where Does The ‘Energy’ For Dark Energy Come From?
“[T]he total energy of the universe is increasing such that the energy inherent of space-time is kept constant as the universe expands. It is like, in order to build an extra cubic kilometer of space-time you need this quanta of energy. No more and no less. This energy has to come from somewhere. In everything else I know of, energy (including matter via E = mc2), cannot just appear from nowhere. So something must be giving energy into our universe to cause it to expand. […] Will it ever stop?”
Think about it: the Universe is expanding, and it’s full of matter (both normal and dark), radiation, and dark energy. As it expands, the matter density drops, since density is mass (which is fixed) divided by volume (which increases). Radiation density drops even faster, since individual photons also redshift, losing energy. But dark energy is the energy inherent to space itself. As the Universe expands, the dark energy density remains constant. Therefore, as the volume of the Universe increases, so does the total energy within it.