> How is is that dark energy density does not change with time? Because that's how a cosmological constant works. There are alternate models where there is "d

> How is is that dark energy density does not change with time? Because that's how a cosmological constant works. There are alternate models where there is "dark energy" (as in, stress-energy that causes accelerated expansion) whose density does change with time (for example, a "Big Rip" model in which the dark energy density increases with time), but such models do not match our best current data.