# Dark energy and pressure

#### kurious

How does dark energy exert pressure - what does it push against.It must push against something solid if it has pressure units like Nm^-2?

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#### pmb_phy

kurious said:
How does dark energy exert pressure - what does it push against.It must push against something solid if it has pressure units like Nm^-2?
Dark energy has negative pressure. And I don't think that dark energy exerts pressure in the classical sense of the term. It is a form of matter which, for example, may make itself know by a non-zero cosmological constant.. This means that Einstein's field equations, even in a vacuum, behave as if there is a non-vanishing stress-energy-momentum tensor which has non-zero pressure terms. Or it may make itself know by having negative values of its effective gravitational mass density, i.e.

$$\rho_{eff} = \rho + p^2/3$$

which means there'd be negative pressure. There is more on this at
http://www.space.com/scienceastronomy/astronomy/cosmic_darknrg_020115-1.html

The non-vanishing of pressure terms does not mean that something is being pushed on. For example: The pressure in the middle of the room I'm in is 1 atm. But there is nothing in the middle of the room for the air to push on.

Pete

#### kurious

pmb phy:
there is nothing in the middle of the room for the air to push on.

Kurious:
Air molecules can move objects by flowing in one direction.
Dark energy seems to move galaxies by exerting pressure equally in all directions.
Could an extra dimension of space make dark energy behave more like air so it could be said to be flowing in a particular direction - and the galaxies too?
Or would an extra dimension upset the equations which describe dark energy's presence in the universe too much?

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#### pmb_phy

kurious said:
pmb phy:
there is nothing in the middle of the room for the air to push on.

Kurious:
Air molecules can move objects by flowing in one direction.
Dark energy seems to move galaxies by exerting pressure equally in all directions.
Nope. That is not the case. You're still thinking of the pressure associated with dark energy as being of the same kind of animal as the pressure of a gas. It is not that way. Dark energy is not what is responsible for galaxies moving apart. That happens in the absence of dark energy too. Dark energy is what is causing the rate of expansion to increase. There is nothing pushing on galaxies in the normal sense of the term. Think of it as antigravity or as a repulsive gravitational force.
Could an extra dimension of space make dark energy behave more like air so it could be said to be flowing in a particular direction - and the galaxies too? Or would an extra dimension upset the equations which describe dark energy's presence in the universe too much?
No.

Pete

#### kurious

Does dark energy have units of Nm^-2?
Is its pressure = force/area?
If so, it sounds like normal matter or radiation to me.
The energy density of dark energy is the same as its pressure in magnitude:
can I write force/area = energy/ volume for dark energy?

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#### pmb_phy

kurious said:
Does dark energy have units of Nm^-2?
That seems to be a vauge question. I'm not sure what it means. But it seems that the best answer is that the units of dark energy are identical to that of energy, e.g. Joules etc.
The energy density of dark energy is the same as its pressure in magnitude:
can I write force/area = energy/ volume for dark energy?
You can do that for the components of the effective energy-momentum tensor. This tensor has components which are mass-enegy density, momentum density and stress.

Pete

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