[Eric A. Taylor (OP)]

It is thought that dark energy is responsible for the accelerating expansion of the universe. That it has a repulsive force of gravity. We know that mass and energy are the same thing and that both have attractive gravity. If dark energy is repulsive would it have a temperature less than 0 Kalvin?

[PhysBang]

Mass and energy are not precisely the same thing, though they can be interchangeable.

There are two important differences between dark energy and other forms of energy.

The first is that dark energy isn't made up of particles, so it doesn't have a temperature. What it does have is something analogous to pressure, and this is negative.

The second is that dark energy sits in a different place in the Einstein Field equation, so it can act differently from ordinary energy and be repulsive.

(The analogy to pressure has to be negative because the energy has to be the same in a vacuum state that looks the same regardless of the choices that we make in constructing a set of coordinates. Regardless of what frame of reference we use for motion, an empty area of space has to have the same amount of dark energy. The only way for this to work out given the underlying mathematics of general relativity is if this analogy to pressure is negative.)

Ned Wright has a good overview. http://www.astro.ucla.edu/~wright/cosmo_constant.html

[Geezer]

It is thought that dark energy is responsible for the accelerating expansion of the universe. That it has a repulsive force of gravity. We know that mass and energy are the same thing and that both have attractive gravity. If dark energy is repulsive would it have a temperature less than 0 Kalvin?

Don't trust that Kalvin character.

http://www.gocomics.com/calvinandhobbes/

[Eric A. Taylor (OP)]

Mass and energy are not precisely the same thing, though they can be interchangeable.

There are two important differences between dark energy and other forms of energy.

The first is that dark energy isn't made up of particles, so it doesn't have a temperature. What it does have is something analogous to pressure, and this is negative.

The second is that dark energy sits in a different place in the Einstein Field equation, so it can act differently from ordinary energy and be repulsive.

(The analogy to pressure has to be negative because the energy has to be the same in a vacuum state that looks the same regardless of the choices that we make in constructing a set of coordinates. Regardless of what frame of reference we use for motion, an empty area of space has to have the same amount of dark energy. The only way for this to work out given the underlying mathematics of general relativity is if this analogy to pressure is negative.)

Ned Wright has a good overview. http://www.astro.ucla.edu/~wright/cosmo_constant.html

This is very confusing. I was very surprised to learn that Relativity has never been tested on the extremely large scale. While the theory conforms to observation to several decimal places (as many as we can add with our technology) on the scale of our solar system this does not give us a large enough data set to test to exact numbers (say ten to the one-hundredth decimal places). Imagine a ski race that measured only to the tenth of a second. I saw one race in the last Olympics where the time difference of the first 5 finishers was less than .09 seconds. Had we not measured to the hundredths, they would have had to award 5 gold metals!!!

Is it very likely that the problems for which dark matter and dark energy were created lie not with the universe but rather with our understanding? It would not be the first time this has happened. Relativity itself was created to solve problems with Newton's Laws. I'm not saying Relativity is wrong 9any more than Newton was wrong), clearly it is a very correct theory. But maybe we need to do for Relativity what Relativity did for Newton. Make it more accurate.

[PhysBang]

This is very confusing. I was very surprised to learn that Relativity has never been tested on the extremely large scale.

How would you test it?

Is it very likely that the problems for which dark matter and dark energy were created lie not with the universe but rather with our understanding?

It is not very likely, if only because this would require that alternatives influence our observations in different types of research projects in different ways that just happen to coincidentally give us the same readings for dark matter and dark energy that we get in other projects. Any alternative would have to just luckily work out perfectly to fool us in three or four different ways. (Ned Wright describes many of those research projects.)

[Eric A. Taylor (OP)]

This is very confusing. I was very surprised to learn that Relativity has never been tested on the extremely large scale.

How would you test it?

Is it very likely that the problems for which dark matter and dark energy were created lie not with the universe but rather with our understanding?

It is not very likely, if only because this would require that alternatives influence our observations in different types of research projects in different ways that just happen to coincidentally give us the same readings for dark matter and dark energy that we get in other projects. Any alternative would have to just luckily work out perfectly to fool us in three or four different ways. (Ned Wright describes many of those research projects.)

It's interesting you should say that. Relativity "just happens" to match EXACTLY the observational precession of Mercury that Newton predicted should not be.

This can only go two ways. We will discover what dark energy/matter really is or we will discover a new theory that makes it irrelevant. Personally, judging from history, my money is on the second.