Why does dark energy have a constant density?

It's constant because we observe it to be constant. More precisely, we think it's constant because the best scientific model we have is one where the dark energy density is constant. This is like asking why the electron has electric charge. read more

It's constant because we observe it to be constant. More precisely, we think it's constant because the best scientific model we have is one where the dark energy density is constant. This is like asking why the electron has electric charge. A model in which electrons don't have electric charge will not match what we see in experiments. read more

If [math]w=-1[/math] for the dark energy, which could be modeled as a cosmological constant, then its energy density is constant as the universe expands. If [math]w<-1[/math] for the dark energy, then its energy density increases as the universe expands, which would eventually lead to a Big Rip. read more