[yor_on]

Could you explain a little more how you think here Graham?
"A vacuum is not a vacuum if it has particles in it with a temperature that is not at zero Kelvin. That means there is a pressure."

Do you mean that any vacuum with a particle in it isn't a real 'perfect (free) vacuum'
Or that it just isn't a vacuum?
Or that only when the temperature (heat?)of those particles is at zero K will it be a vacuum.

And how you see that proving a pressure?
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If I would guess I think you mean that as long as there are 'real particles' with a nonzero energy there will be a 'pressure'? Is that it?

And then you see this 'energy' as undefined in space perhaps?
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I'm wrong heh

[graham.d]

The atmoshere, as a gas, has a particular measurable pressure. If you surround a cubic metre of it by a box there will be no net force on the box walls, but that does not mean you cannot speak of the pressure of the gas. Even without the box, the volume of gas has a pressure which it exerts on the surrounding gas (and vice versa). If you take the box of gas up a mountain and into a very large sealed room (say), the pressure will exert a net force on the walls of the container. If released it will disperse into the room and the room pressure will go up and the box pressure go down until they are equalised. The same would be true if you took the box into space although the pressure change in space would be rather small :-) In the milky way galaxy there is a measurable density of hydrogen ions and electrons (and some other particles) buzzing about and they have a pressure. In the galaxy the distances of pulsars are estimated by how much this gas density spreads the emitted sharp pulses by virtue of the finite refractive index that the gas has.

Yes, real particles with energy produce positive pressure. The mysterious thing about the idea of dark energy is that it must produce negative pressure to give the observed effect. I don't know enough about the QM side of this to see how this happens. When the particles appear and then annihilate they are, for their period of existence, real particles and should have positive pressure and kinetic energy (I think). Maybe there is something that can be said about the virtual particle (the ones that have potential to exist) that more than balance this by having negative energy and pressure, but I don't understand this. Maybe someone can explain.

[yor_on]

Thanks Graham, I can see how you thought now, and it makes perfect sense to me. Well as much as anything can do those days But it makes me wonder if there can be any 'free (perfect) space' considering the pressure you describe here?

That is assuming that the pressures behavior resembles waves?
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And, can we measure that pressures influence on invariant mass if so?
Like heavenly objects?

(Nope, not Marilyn Monroe, but it was a near guess:)

[Farsight]

It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands. 

[PhysBang]

It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands.

This may be good pseudoscience, but it has nothing to do with the relevant science.

To properly assign an energy density in General Relativity, one uses a 4X4 tensor, T^μν, the 00 component of which is the energy density proper. Under certain general conditions allowable in cosmology, we can remove the off-diagonal elements of the tensor, but we cannot get rid of the diagonal elements. These are all equivalent, T¹¹ = T²² = T³³ = p  and they are a function of density and the behaviour of the particles or field in phase space. This function agrees with the usual definition of pressure in its use with ideal gases, but its role is as a source of gravity, not a pressure as classically used. This is especially important because the expansion of the universe is a gravitational effect, not one of force imparted through some other mechanism.

If we assign an energy density ρ_Λ to the vacuum, then we must assign it in such a way that the energy density is associated with Lorentz invariant tensor. This is so because the properties of the vacuum itself don't transform along with a change in coordinates. The only Lorentz invariant tensor is η^μν, that tensor with 1 in the 00 position, -1 on the other diagonal positions, and 0 on all off-diagonal positions. So we assign the vacuum energy tensor as T_Λ^μν = ρ_Λ η^μν.

Combine these two requirements, and we see that the "pressure", p, associated with the energy density of the vacuum must be -ρ_Λ. Thus if the energy density of the vacuum is positive, its pressure term, such as it is, is negative.

(The above adapted from pp. 130-132 of James Rich, Fundamentals of Cosmology, Springer 2001.)

[graham.d]

As I have said before, it is very hard to explain some of the less intuitive results of GR without some knowledge of differential geometry.

To address your point and ball analogy, farsight, it is not that the universe is expanding that is suggestive of dark energy (or any sort of pressure), but the acceleration of the expansion.

[Soul Surfer]

Many people who wish to propose alternatives to expansion to explain the red shift forget some of the more subtle aspects of observations of this in the form or indirect observations of the many other gas clouds that light has passed through at different distances.  This caused what is known as the "Lyman alpha forest" on the Lyman alpha emissions from very remote active galaxies.

[yor_on]

It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands. 

Farsight, look here for an explanation of how it seems to be thought to work.
Negative energy