  This paper may solve the dark-energy problem because our universe is not
alone, and the multiverse is a powerful part of the cosmos. The decay of our
aging universe is reviewed first. The accelerated expansion takes the decay
debris into the inter-universal medium (IUM) of the multiverse for
conservation. A prominent component of the debris and of the IUM is the
enormous number of old cold photons from decaying universes. When a small
central volume (apparently 6.4 percent of the total mass) of our proto-universe
reached proton density, the old photons and protons became fully re-energized.
Outside of that volume, the large numbers of remaining old photons continued
their acceleration and the expansion of our universe. The accretion and
expansion are described a second time with what we know of dark energy,
particularly its acceleration of the expansion of our universe. Identical
results are obtained; in fact, the two descriptions are complementary, and the
conclusion is therefore made that dark energy is the acceleration energy of old
photons. The model is supported by 30 observations and considerations for
future work.
