# 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?

• Sounds right to me! Commented Jul 7, 2019 at 23:23
• Possible duplicate: physics.stackexchange.com/q/76468/2451 Commented Jul 7, 2019 at 23:48
• yes, except of being 120 orders of magnitude off Commented May 30, 2023 at 5:41

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.

In my opinion they are completely one in the same thing. Quantum mechanics predicts an enormous energy density for the ZPE field. The ZPE field is most likely composed of high density bosons similar to light. In fluid mechanics any time you look at the geometry of highly perturbed media it is always expanding to fill the boundaries of the system. Think of boiling water expanding to fill it's container. However, if there are no physical boundaries in the system (such is fluid in a zero g vacuum) the fluid media continues to expand until latent perturbation energy (heat) is expended on the defusion process. If the fluid in question were a incredibly dense sea of low frequency bosons that would look exactly like what is happening when we look up at the night sky, it would look like and sound like spacetime that is filled with "dark energy".

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Dark energy is the cause for the accelerated expansion of the universe. Its observable energy density observed indirectly via the expansion of the universe cosmological constant data collected mainly by the Planck space telescope mission expressed in units (not the dimensionless DE density constant parameter $$Ω_{Λ}$$) usually in $$Joules/m^3$$ units we call vacuum energy density or else vacuum ZPE and is is of the order of $$10^{-9}$$ $$J/m^3$$.

The fact that we use two different terms is because although the vacuum energy in total is increasing with cosmological time with the expansion of the universe at a fixed density level and since the universe is mostly vacuum space and it currently consists nearly 70% of all the energy in our observable universe we don't know really why the vacuum energy keeps increasing defying energy conservation law at cosmological scales? Therefore, the name Dark Energy was chosen to emphasize our lack of understanding and the fact that we cannot observe from where this extra "free" energy originates and why it is increasing with time.

So my answer is that DE is the vacuum ZPE energy plus some possible unknown not directly observable hidden energy reservoir in our universe some also call it a 5th force but all this is for now in my opinion highly speculative although some would characterize it as an educated guess.

However, you could say and still be accurate, that vacuum energy is the observable Dark Energy via the accelerated expansion of the universe.