We May Finally Know Why the Universe Is Racing Apart

(via PBS Space Time) We've known that the universe is expanding since 1929, and that its expansion is accelerating since 1998. The culprit behind the acceleration is unknown, so we live with a stand-in term "dark energy". Our modern cosmological model assumes that dark energy has a constant density--always the same amount of the outward-shoving stuff per volume. But there's recent evidence to the contrary--which may be why our primary efforts to measure the expansion rate of the universe disagree with each other.