# Is dark energy frame-dependent?

Let's assume that dark energy is uniformly distributed through space (which is one theory) and that dark energy is quantized (as everything else seems to be). We can visualize that dark energy consists of packets of equal size and distance between the packets, so there would be three-dimensional grid of dark energy packets spread through universe.

Let's imagine an observer at rest relative to grid and another observer moving. If the observers could measure the distance between packets, the moving observer would measure smaller distance (at the direction of motion) due to length contraction. By comparing these, they would know that one observer is moving relative to universe and other is not, so the question of "who is moving" wouldn't be totally relative anymore?

measuring dark energy directly is like saying we measure gravity directly. We can't do either. We can only measure their influence on matter distributions. Think of it this way, treat gravity as a positive pressure influence. Dark energy as the negative pressure influence. Can you measure directly pressure? Or do you measure its influence on matter. Answering that, why would the laws of relativity change if you could directly measure pressure compared to measuring matter? It simply wouldn't, not that doing such is possible.

edit: as an aside note the energy density of the cosmological constant is incredibly small per cubic meter. A rough figure based on an older calculation is roughly 6 *10-10 joules per m3. Its total energy budget is large simply because of the tremendous volume of space.

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Drakkith
Staff Emeritus
You are associating this "grid" of dark energy as being "the" preferred frame of reference for the universe. It would not be.

You are associating this "grid" of dark energy as being "the" preferred frame of reference for the universe. It would not be.

Thanks for both answers so far. Yes the "grid" is associated as preferred frame, but I don't necessarily believe that it is such in the end. I just don't see what is wrong. Can you elaborate why "it would not be"?

Drakkith
Staff Emeritus
Thanks for both answers so far. Yes the "grid" is associated as preferred frame, but I don't necessarily believe that it is such in the end. I just don't see what is wrong. Can you elaborate why "it would not be"?

The idea of reference frames is that the laws of physics are invariant. That is, in any inertial frame of reference (a non-accelerating frame of reference), there is no experiment you could perform that could tell you whether you were moving relative to some "absolute" frame. You can accelerate and decelerate to any velocity relative to another inertial frame but the laws of physics always remain the same. You have to compare yourself to another frame of reference and then state your motion relative to them.

If there were some "preferred" frame of reference for the universe, then moving relative to that frame would alter the laws of physics in your frame in such a way that you would be able to tell if you were the one "really" moving or someone else were.

In relativity this is not the case. No inertial frame of reference is any more valid than another. It is equally valid to say that you are moving while I am stationary, or that I am moving and you are the one stationary.

pervect
Staff Emeritus
Let's assume that dark energy is uniformly distributed through space (which is one theory) and that dark energy is quantized (as everything else seems to be). We can visualize that dark energy consists of packets of equal size and distance between the packets, so there would be three-dimensional grid of dark energy packets spread through universe.

Let's imagine an observer at rest relative to grid and another observer moving. If the observers could measure the distance between packets, the moving observer would measure smaller distance (at the direction of motion) due to length contraction. By comparing these, they would know that one observer is moving relative to universe and other is not, so the question of "who is moving" wouldn't be totally relative anymore?

You don't need the hard-to-measure dark energy to do this. Or quantum mechanics. You can use the cosmic microwave background radiation, as measured in the WMAP experiment.

What you get via this experiment is that there is a set of observers to whom the CMBR appears remarkably isotropic. This is sometimes called the "Hubble flow". Unfortunately, these observers don't form a traditional "frame" of reference, because they are moving apart relative to each other - i.e. the universe is expanding. Your proposed model of a "grid" will be misleading unless you incorporate cosmological expansion into your grid.

The ability to measure velocity relative to the CMBR is interesting, but it in no way violates relativity. One way of simply describing the principle of relativity is that there is no way, in a closed box, of determining one's state of absolute motion. This remains true, CMBR observations are not done in a "closed box".