Is Dark Energy Really Just Negative Pressure?

[newton1]

i heard that dark energy is "negative preasure"!
Is it possible??

 

[mathman]

That is the current theory. If you look up arXiv and get these papers (astro-ph/0309326, astro-ph/0308418), you will get good descriptions of what is going on thses days in cosmology.

 

[marcus]

Originally posted by Newton1
i heard that dark energy is "negative preasure"!
Is it possible??

The realization that a constant energy density exerts negative pressure, which tends to promote expansion of space, goes back to Einstein 1915.

It was precisely to prevent collapse (a big crunch) that he put the famous Lambda term-----the cosmological constant, equivalent to a uniform constant energy density, the same everywhere in space and time----into his 1915 equation of general relativity.

The Lambda term was mostly ignored until 1998, when observations of supernovae (with redshifts roughly 0.5-1.5)
led to the conclusion that the expansion of space has been accelerating, not necessarily for the whole age of the universe but for at least a substantial part of the life of the solar system.
A positive Lambda term was reintroduced into the Friedmann equantions (the simplified version of the 1915 GR equation which cosmologists use) to explain this acceleration.

There are two hard-to-understand things that must be grasped.

(1) why does a constant energy density cause negative pressure?

(2) why does negative pressure promote expansion?

These two things are not merely current theory but have been known by relativists (GR specialists) for almost 90 years. They are not "intuitive" but they are basic and worth trying to understand.
You can find explanations on the web (try gurus like John Baez, Ned Wright, Charles Lineweaver, as well as the people mathman mentioned). But I can try to save you some trouble by giving the standard explantions here.

 

[marcus]

Originally posted by marcus

There are two hard-to-understand things that must be grasped.

(1) why does a constant energy density cause negative pressure?

(2) why does negative pressure promote expansion?

These two things are not merely current theory but have been known by relativists (GR specialists) for almost 90 years. They are not "intuitive" but they are basic and worth trying to understand.
You can find explanations on the web (try gurus like John Baez, Ned Wright, Charles Lineweaver, as well as the people mathman mentioned). But I can try to save you some trouble by giving the standard explantions here.

1. The measured dark energy density is half a joule per cubic kilometer, if I remember correctly. Why does this cause a negative pressure?

For easy numbers, suppose the density was 1 joule per cubic meter (much higher!).
Imagine a cylinder with sliding piston, that contains 1 cubic meter of space. For simplicity suppose the cylinder cross-section is one square meter. Take this device outside the universe for study.
Carefully pull the piston out one meter, until the volume in the cylinder is 2 cubic meters. There are now 2 joules of energy inside. You must have done one joule of work in pulling the piston outwards (or else what made the extra joule?). Therefore you exerted one Newton of force for a distance of one meter.

The pressure in the cylinder is minus one pascal (-1 Newton per square meter).

The calculation is easy and in fact shows that if the Lambda energy density is X joules per cubic meter then the pressure
it exerts is minus the same number of pascals: -X Newtons per square meter.

This proportionality (called w, or the "equation of state") is -1 in this simplest case but it is something theorists play around with to see how the model depends on it.

The other question is
(2) Why does negative pressure promote expansion?
This is a consequence of the Friedmann equation---the basic equation used in cosmology. It has a pressure term which for roughly 80 years was largely ignored because stars and galaxies and dust do not exert significant pressure.

Pressure couples to gravity, somewhat as energy does. So positive pressure (like a concentration of mass-energy) can cause collapse---it exerts gravitational attraction just as mass does.
However negative pressure does just the opposite!

This is explicitly spelled out in the Friedmann equations, which I will save for another post.