# vacuum energy density

Phil Hill
Science 30 Aug '12 14:30
1. 30 Aug '12 14:301 edit
The energy density of the vacuum is a constant value. As the universe expands the density of matter dilutes and the value decreases. It would seem the energy density of the vacuum would dilute and decrease as space expands yet it remains a constant. This would seem to be a violation of the first law of thermodynamics. Can someone clarify this apparent problem please.
2. 30 Aug '12 14:52
As far as I know the additional vacuum energy is compensated by cooling of the universe. But this is not really my field, and from what I gather by browsing through some Wikipedia articles, vacuum energy is not very well-understood.
3. 30 Aug '12 14:561 edit
Originally posted by KazetNagorra
As far as I know the additional vacuum energy is compensated by cooling of the universe. But this is not really my field, and from what I gather by browsing through some Wikipedia articles, vacuum energy is not very well-understood.
That is pretty much why I am asking. One really interesting idea I heard had something to do with the negative pressure which was 1/3 the energy density of the vacuum. But as far as I recall,
radiation has a pressure of 1/3 it's density.

edit - the negative pressure is what causes the expansion.
4. 30 Aug '12 17:25
Originally posted by Phil Hill
This would seem to be a violation of the first law of thermodynamics.
I don't know if the laws of thermodynamics are maintained over expansion of space. Expansion doesn't happen fast enough for it to be easily tested.

I know light and other magnetic radiation is redshifted by expansion. Does it loose energy as a result?

Also, as the universe expands, surely there is potential energy being created?
5. sonhouse
Fast and Curious
31 Aug '12 00:06
Originally posted by twhitehead
I don't know if the laws of thermodynamics are maintained over expansion of space. Expansion doesn't happen fast enough for it to be easily tested.

I know light and other magnetic radiation is redshifted by expansion. Does it loose energy as a result?

Also, as the universe expands, surely there is potential energy being created?
I thought the total energy of the universe is constant, so the lengthening wavelength of EM would seem to me to represent a loss of energy but it must have been made up somewhere.
6. 31 Aug '12 05:31
Originally posted by sonhouse
I thought the total energy of the universe is constant,
I think this is assumed to be the case, but there is no easy way to verify it.
7. 31 Aug '12 15:02
Originally posted by sonhouse
I thought the total energy of the universe is constant, so the lengthening wavelength of EM would seem to me to represent a loss of energy but it must have been made up somewhere.
From what I have read, distant stars are accelerating away from each other. To me this would mean that kinetic energy is increasing more as time goes on. If the energy of the vacuum was decreasing then how can this be? Either energy does not remain constant in the universe, or there is some other reason for the acceleration. Can one dimension receive one type of energy from another dimension and the whole system including all dimensions not violate thermodynamics?
8. 31 Aug '12 15:18
Originally posted by joe beyser
From what I have read, distant stars are accelerating away from each other. To me this would mean that kinetic energy is increasing more as time goes on. If the energy of the vacuum was decreasing then how can this be? Either energy does not remain constant in the universe, or there is some other reason for the acceleration. Can one dimension receive one ...[text shortened]... from another dimension and the whole system including all dimensions not violate thermodynamics?
The density of the vacuum energy is not decreasing, it is constant. You are confusing it with the density of matter in the universe.

BTW, it isn't stars that are moving, it is the space between them that is expanding. Relative to space, they are motionless (more or less).
9. 31 Aug '12 15:51
Originally posted by Phil Hill
The density of the vacuum energy is not decreasing, it is constant. You are confusing it with the density of matter in the universe.

BTW, it isn't stars that are moving, it is the space between them that is expanding. Relative to space, they are motionless (more or less).
Yes I was making the point that if vacuum energy is decreasing then what drives the expansion? I agree with your point on the vacuum energy. The part about stars not moving though I don't agree with. In the nearby space it is not. but compared to stars many light years away it is accelerating away from them. If the distance between two objects is increasing more over time there is acceleration. Kinda like the raisin bread analogy. Dem raisins are moving as the loaf expands.
10. 31 Aug '12 15:57
Originally posted by joe beyser
Yes I was making the point that if vacuum energy is decreasing then what drives the expansion? I agree with your point on the vacuum energy. The part about stars not moving though I don't agree with. In the nearby space it is not. but compared to stars many light years away it is accelerating away from them. If the distance between two objects is increasi ...[text shortened]... acceleration. Kinda like the raisin bread analogy. Dem raisins are moving as the loaf expands.
Raisin bread is an analogy, it is not a model. Believe whatever you prefer but the fact remains it is the space between stars and galaxies that is expanding not the movement of them through space.
11. 31 Aug '12 16:01
Originally posted by Phil Hill
Raisin bread is an analogy, it is not a model. Believe whatever you prefer but the fact remains it is the space between stars and galaxies that is expanding not the movement of them through space.
yes you are right about that!!! But in relation to each other there is movement. Not through space but with it.