Understanding Vacuum Energy Density in Relation to Universe Expansion

[TrickyDicky]

Is the vacuum energy density, in case it exists which is the mainstream view, supposed to be continuously varying (decreasing) as the universe volume expands or is it really a constant (wich would be rather odd judging by what density and volume usually mean in physics)?

 

[BillSaltLake]

Constant, which means it doesn't act like "ordinary" energy, which decreases in density as space expands.

 

[edgepflow]

Usually modeled as constant, but there may be three possibilities:

a. Scalar field of universe is at true minimum which is non zero (i.e. it is constant).
b. Metastable false vacuum state subject to quantum tunneling (i.e. constant for now).
c. Scalar field is still slow rolling (i.e. it varies extremely slowly).

If you want consider this in more detail, see:

http://arxiv.org/abs/astro-ph/0009491

 

[TrickyDicky]

Constant, which means it doesn't act like "ordinary" energy, which decreases in density as space expands.

How curious, but if it does not act as "ordinary" energy then why call it energy? it is confusing at the very least. I mean this is like a "Deus ex machina" magical type of explanation.

 

[TrickyDicky]

Besides, according to QM the vacuum energy is and acts like "ordinary" energy, and is demanded by the uncertainty principle, and also by QED polarizable vacuum. There seems to be nothing wrong or "extraordinary" with this energy.

 

[BillSaltLake]

I'm not claiming that D.E. exists; I'm just saying that it's a weird concept.

 

[TrickyDicky]

I'm not claiming that D.E. exists; I'm just saying that it's a weird concept.

Do you refer to its negative pressure?

 

[BillSaltLake]

Yes. Neg pressure is the weirdest part.

 

[TrickyDicky]

Yes. Neg pressure is the weirdest part.

And what about the EM field, doesn't its stress-energy tensor have negative pressure too?

 

[BillSaltLake]

A group of photons in a piston (including the specific case of blackbody photons in a "cavity") will lose energy if the container is expanded. This is positive pressure.

 

[TrickyDicky]

A group of photons in a piston (including the specific case of blackbody photons in a "cavity") will lose energy if the container is expanded. This is positive pressure.

Wouldn't you say that is the piston wall reaction to change of momentum direction of the photons that averages over many photons to a positive force per area.
What I meant was this:
http://upload.wikimedia.org/math/2/d/2/2d2696993455a873494c5ac8f1884b9c.png

The diagonal components are positive for energy density and negative for pressure in the 3 coordinate directions.

 

[BillSaltLake]

I think the diagonal terms -σ_xx, -σ_yy and -σ_zz are all positive, with expectation value (for any isotropic collection of photons, such as blackbody) equal to 1/3 the average energy density. These diagonals are of course the pressure terms.

 

[TrickyDicky]

the diagonal terms -σ_xx, -σ_yy and -σ_zz are all positive

Illuminating. Still something bothers me. What would you attribute the minus sign in those diagonal terms to? Interestingly enough is not in the energy density term.

with expectation value (for any isotropic collection of photons, such as blackbody) equal to 1/3 the average energy density. These diagonals are of course the pressure terms.

This much I knew. And since the energy-stress tensor for radiation is normally considered traceless, I thought maybe that is because the pressure terms are negative and added to the positive density term the whole diagonal vanishes.

 

[marcus]

I'm not claiming that D.E. exists; I'm just saying that it's a weird concept.

Do you refer to its negative pressure?

Yes. Neg pressure is the weirdest part.

You might be interested by what the central people in Loop Gravity, and its Cosmology application, have to say about "D.E."

Carlo Rovelli at Uni Marseille is the leading Loop figure (most of the currently active researchers in the field came out of his group of PhD students and postdocs.)

He argues, I think quite convincingly, for a particular view of the cosmological constant Lambda.
====
http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
====

Many of the equations of physics have constants in them. With most constants we don't feel we have to superstitiously attach "energies" to them and "explain" them by some primitive mythology. They are simply constants that appear in equations.

Lambda appears naturally in Einstein's GR equation. Period.

We don't have to interpret it as an "energy". It belongs on the lefthand side of the equation and happens to have the physical dimensions of a curvature (the reciprocal of area).

It only appears as a fictional energy density when you move it over to the righthand side, which can be convenient to do for algebraic reasons---might help you in a calculation to think of it that way.

But we don't need to beat our heads against a wall trying to picture it as a real energy density.

Rovelli gives some arguments why it is a mistake to think of Lambda as "D.E." and equate it with the particle physicists' "vacuum energy density." You can read his explanation of that.
It is an easy paper to read. No big technical difficulties.
===============

Dark matter is something else! There is a lot of evidence that D.M. is real particles!
It collects in clouds in and around clusters of galaxies. We can map its variations in density by gravitational lensing. It helps structure form. We can run computer simulations involving D.M. and ordinary matter and get realistic results.

Just because the popular literature uses the same word "Dark" doesn't mean they are related. Dark Energy is possibly just a bogus concept--a confusing term for Lambda (a very slight curvature constant). Dark Matter (many people think) is something real.

 

[TrickyDicky]

Dark matter is something else! There is a lot of evidence that D.M. is real particles!
It collects in clouds in and around clusters of galaxies. We can map its variations in density by gravitational lensing. It helps structure form. We can run computer simulations involving D.M. and ordinary matter and get realistic results.

Just because the popular literature uses the same word "Dark" doesn't mean they are related. Dark Energy is possibly just a bogus concept--a confusing term for Lambda (a very slight curvature constant). Dark Matter (many people think) is something real.

Why would anyone bring up DM into this discussion? Nobody mentioned DM.

 

[TrickyDicky]

A group of photons in a piston (including the specific case of blackbody photons in a "cavity") will lose energy if the container is expanded. This is positive pressure.

Let's not conflate EM field and EM radiation, they are related but different.

 

[BillSaltLake]

I think it's possible to construct a situation with EM field that has negative pressure in at least one axis; however, an anisotropic group of photons (always present in the Universe) will always exert positive pressure.

 

[1dominator1]

Is the base vacuum energy density what allows for the random formation of particles and their AM counterparts that is responsible for the decay of black holes?

 

[StateOfTheEqn]

The cosmological constant acts like a 'perfect fluid' with (density)=(LAMBDA)/8(pi)G and (pressure)=-(density). The RHS in the first equation is obviously constant and so both density and pressure of the LAMBDA 'perfect fluid' are constant.

As I understand it, there is a motivation for referring to the LAMBDA 'perfect fluid' as 'dark energy' in order to solve the critical density problem with the k=0 Friedmann model. Add the dark energy density to the ordinary density plus the density of dark matter in order to get the required density for the Friedmann model with k=0.

Unfortunately for such a procedure, the constant value of the LAMBDA density prevents the combined density from holding true to the critical density since the latter is time dependent.