# What's the difference between “dark energy” and “vacuum energy”?

Let's see if I understood well. "Dark energy" is a term that encapsulates everything that may be accelerating the expansion of Universe, and "vacuum energy" is a kind of energy that permeates the whole Universe and may be taken into account as a contribution for the dark energy. Is this correct?

I would say it is partially correct. Dark energy is, as you said, anything that accelerates the expansion of the universe. One can show that any component with an Equation of state parameter satisfying $$\omega \equiv P/ \rho < -1/3$$ accelerates the cosmic expansion and thus it is a good dark energy candidate. The simplest model of dark energy consists in a fluid of constant density, with this density proportional to the cosmological constant $$\Lambda$$ appearing in Einsteins Equation. In this case, $$\omega=-1$$. This is generally referred as vacuum energy. In this sense, vacuum energy is just a particular type of dark energy.