# A question on the meaning of dark energy

Tags:
1. May 15, 2014

### Tanelorn

Is dark energy really energy and is work actually being done when it causes the Universe to expand?

It seems to me that work is being done in moving all the Galaxies further apart than they were, over coming their gravitational pull. For example a lot of work would need to be done to move the earth into an orbit further from the sun.

So what is the source of this energy? If it is some kind of stored potential energy then what is its form and where is it stored?

I vaguely recall someone saying that the word energy is not a very good word for describing the effect.

2. May 16, 2014

### Mordred

No one knows what dark energy really is or understands the mechanism behind it. However we do know how it influences expansion. In the sense that it contributes to the vacuum energy-density. Keep in mind dark energy is merely one possibility of the cosmological constant. The work is done by its negative pressure relations. Whose energy-density to pressure relation is determined by its equation of state. http://en.wikipedia.org/wiki/Equation_of_state_(cosmology)

3. May 16, 2014

### Matterwave

4. May 16, 2014

### Mordred

yeah I did get that backwards, What is the status of quintessence now, last I heard its disfavored?

5. May 16, 2014

### Matterwave

Sadly, this is beyond my area of expertise. As far as I know though, all the observations are consistent with the cosmological constant. Since the cosmological constant is simpler (it's a constant of integration and not a dynamical process) perhaps applying Occam's Razor means we should favor it over more complicated models like Quintessence. However, since Quintessence predicts a dynamical acceleration, this should be, at least in principle, observable. Whether our current technology can observe these dynamics, I cannot say. So, probably, we have to wait for better data to rule out one or the other.

6. May 16, 2014

### Chalnoth

Work isn't a term that is used very often within General Relativity, and I'm not sure it can be applied in general.

But in a homogeneous, isotropic expanding universe, it works:
If you consider a hypothetical box around a co-moving volume of space, and consider the work exerted on the walls of that box by the stuff inside it, you get an answer which precisely coincides with the change in energy within that co-moving volume.

For example, consider a universe with nothing but a cosmological constant at some density $\rho_\Lambda$. Now expand that volume from $a \rightarrow a + da$ along each side . The volume increase is $3a^2da$, while the "work" on each of the six sides is $-\rho_\Lambda \cdot 1/2 a^2 da$, since $p_\Lambda = -\rho_\Lambda$ (ignoring terms with $da^2$ in each case).

So the increase in energy is $3\rho_\Lambda a^2 da$, while the work done is $-3\rho_\Lambda a^2 da$. You can imagine the dark energy as performing negative work, and gaining energy as a result.

7. May 16, 2014

### abitslow

The Law of the Conservation of Energy is problematical in an expanding Universe. Not only is the potential energy between two objects (separated by cosmological distances) reduced as the distance between them increases, but speeds and frequencies also diminish ('lose' energy) for particles or waves traveling during such expansion. If Energy is conserved (Energy/Mass/Momentum/Stress(Pressure)) then it has to GO somewhere, meaning that dark energy must be that.

8. May 16, 2014

### Matterwave

An expanding universe like the FRW metric has no time-like Killing vectors, only space-like ones. Therefore particle motion in an expanding universe does not conserve energy (material particles slow down, radiation gets red shifted). The "potential energy" of the gravitational field is a very touchy subject in General Relativity since the gravitational field is no longer a force, but a curvature in space-time.

9. May 16, 2014

### Chronos

It is a possibility [albeit, not wildly popular] that dark energy is an illusion - e.g., http://arxiv.org/abs/0712.0370, Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae.

10. May 16, 2014

### Chalnoth

11. May 16, 2014

### Chronos

12. May 17, 2014

### martinbn

13. May 17, 2014

### Islam Hassan

From 05:45 in this clip, dark energy is masterfully -and relatively simply- defined. A far better name is in fact "smooth tension":

Smooth because it is uniform throughout the whole universe.

Tension because is has negative pressure, ie it rends things apart, separating objects from each other.

Today we only know of positive pressure, eg that of particles impacting against the wall of a vessel. The energy/momentum of these particles are positive metrics. With dark enery, one is talking about negative values for these metrics.

IH

Last edited by a moderator: Sep 25, 2014
14. May 17, 2014

### phsopher

It has been my understanding that the void models are increasingly difficult to reconcile with data. The most recent paper you've cited is over two years old. It would be nice to see what the latest data have to say. I haven't been following this area myself.

Perhaps it's worth noting that there is also another alternative to dark energy that has been proposed, namely the backeaction of structure formation on the expansion rate. My understanding is that the basic idea is that voids expand faster than overdense regions and as they do their relative fraction of the volume of the universe grows, thus increasing the average expansion rate. I think it's more of an idea at this point and it's not clear whether it can be a viable alternative. Here's a recent review:

http://arxiv.org/abs/1112.5335

15. May 18, 2014

### Tanelorn

Chalnoth, is this interpretation not like the pressure of gas atoms on a container wall? If so would this approach apply to the case of a box full of galaxies?

16. May 19, 2014

### Chalnoth

I think a better way to think of it is in terms of a volume of hot gas expanding into a vacuum. If I recall, you can use the same sort of argument to get the correct cooling rate of the expanding gas.

17. May 19, 2014

### Tanelorn

Galaxies don't interact with each other like gas atoms do. Gravitational attraction is small between gas atoms and Galaxies which collide do not bounce off each other, instead they merge. So there is no net force causing them to expand unless we add in the dark energy expansion of space idea.

18. May 20, 2014