# Yet another Dark Energy question

1. Sep 17, 2010

### diggy

I apologize, I know this all has to be answered in other threads, but when reading through I'm still missing "it"...

The whole concept of Dark Energy not conserving energy is of course bothersome. My simple mind has only considered ~3 mechanisms that would give rise to this. Would someone be kind enough to clear up the thought-direction/problems with the lines of thinking.

1) Dark energy comes from the vacuum. Problem is that this approach is essentially creating energy out of nothing, I don't see how an argument like this doesn't break conservation of energy. But it would be uniform in time/space. Of course there is the additional problem that calculations never comes close to matching the expected DE value, but ignore that to start.

2) Dark energy comes from heavier particles that decay, giving off "new" energy. This would require that the energy is stored as mass (which should be noticeable for GR), and would be time dependent like radioactive decay, so 2 strikes right there. So although energy no longer comes out of nothing, I'd assume this is out of the question from the get go.

3) Dark energy comes from our universe expanding into a void. Like a bubble rising through the water. I never really hear this argument made as a possibility, if anything people say that our models don't favor an "outer" void. But this seems to avoid some of the problems of (1) and (2). So its not clear to me why this thought direction is disliked. This is off hand the argument I'd favor, so some clarity on this one would be really appreciated.

Best Regards.

Last edited: Sep 17, 2010
2. Sep 17, 2010

### ClamShell

I thought this type of guessing would be popular
on PF; I think it is the wrong question. Not what,
but why. Why is probably (opinion), to patch an
ailing "big-bang" model of cosmology. First on the
list of things to rule-out is that "dark matter", "dark
energy" and now "dark flow" do not exist. They could
possibly be "fudge factors" to make expansion fit
observations better. Perhaps, a better model of the
cosmos would not need undetected(dark) inputs.

3. Sep 17, 2010

### diggy

Of course I don't disagree with you. But I'd still like to here what the current thoughts on this are. Back in grad school I saw some talks on method (1) but they weren't converging.

4. Sep 18, 2010

### Chalnoth

Not really. Energy just isn't conserved in an expanding universe, using the most common formulation of General Relativity (which has no gravitational potential energy). One way to see this is that in General Relativity, conservation of energy is not a law. Rather, what is conserved is the stress-energy tensor. And under specific situations, conservation of the stress-energy tensor forces energy to not be conserved.

One example of this is radiation. If we imagine a gas of photons in an expanding universe, as the universe expands, the number of photons in an expanding volume stays the same, though the volume gets bigger. So the density of photons decreases as the universe expands. But the photons themselves also undergo a redshift as the universe expands, which means that the energy of each individual photon drops as $1/a$. This means that the total energy within the comoving volume also drops as $1/a$.

So you don't need dark energy to see non-conservation of energy in the universe. This is just a general property of how an expanding universe behaves.

Another way of potentially understanding this is to consider where conservation of energy comes from: Noether's Theorem. Noether's Theorem states that when you have a symmetry of the system in question, then there is a conserved quantity that goes along with that symmetry. The symmetry that leads to conservation of energy is time invariance: if certain properties of your system don't change in time, then energy is conserved. However, the expansion of the universe breaks this time symmetry, so Noether's theorem doesn't hold for the universe.

5. Sep 18, 2010

### diggy

Reading a little between the lines, was I misinterpreting the DE to be the energy gain of the accelerating universe, when it is actually a larger energy pool basically just sitting there (like the CMB) that just happens to be driving the acceleration of the universe.

6. Sep 18, 2010

### Chalnoth

Dark energy definitely increases in energy per expanding volume. You can interpret this as due to gravitational potential energy becoming more negative, or just note that energy conservation is not a rule that General Relativity obeys.