[Bill S (OP)]

Ethan Siegal https://medium.com/starts-with-a-bang/ask-ethan-58-what-is-dark-energy-61db04945b3d points out that “…as the Universe expands and more and more space appears between the galaxies, the energy density of dark energy doesn’t decrease,….”

This must mean that not only is space constantly being created, but so is dark energy.

How can this be justified in terms of conservation of energy?

[JohnDuffield]

I don't think it can. Conservation of energy is just about the number one law of physics. But I'm afraid some bloggers / cosmologists / physicists give glib non-explanations that don't make much sense, and they're evasive when it comes to awkward questions. It's like they want to be seen as the expert, and they don't want anybody challenging what they say. But having said that, see at the bottom of Ethan's article where it says this:

Leave your comments at the Starts With A Bang forum on Scienceblogs!

Follow the link to that, and you can see comments. One of them is mine. Ethan didn't acknowledge it or reply to it, but anyway, this is what I said:

As for Ethan’s article, IMHO dark energy is indeed “the energy inherent to space itself”. There’s an energy-pressure diagonal in the stress-energy tensor, and you can think of space expanding like a stress-ball when you open your fist. But I don’t see how the spatial energy density can remain constant. That’s creation ex-nihilo, it’s in breach of conservation of energy. I think one has to look again at the balloon analogy, which Ethan depicted. The balloon is in vacuum, the pressure within is counterbalanced by the tension in the skin, and there’s two ways to make it bigger. You can increase the pressure by blowing air into it, which is in breach of conservation of energy. Or you reduce the tension by making the skin weaker. Like it’s a bubblegum balloon. The skin gets weaker so the balloon expands, so the skin gets weaker, and so on.

Replace 2D skin with 3D space, and it isn’t dark energy per se causing the increasing expansion, it’s the reducing “strength of space”. This is maybe unfamiliar to some, but it’s related to the strong force. See http://arxiv.org/abs/0912.2678 where you can read this on page 5:

“We see that the modification of GR entailed by MOND does not enter here by modifying the ‘elasticity’ of spacetime (except perhaps its strength), as is done in f(R) theories and the like.”

I'm pretty sure this is right for my own reasons. I ought to write a paper on it I suppose. Ideally with somebody riding shotgun because I'm just an amateur.

[PmbPhy]

Ethan Siegal https://medium.com/starts-with-a-bang/ask-ethan-58-what-is-dark-energy-61db04945b3d points out that “…as the Universe expands and more and more space appears between the galaxies, the energy density of dark energy doesn’t decrease,….”

This must mean that not only is space constantly being created, but so is dark energy.

How can this be justified in terms of conservation of energy?

The gravitational field energy is decreasing by the same amount leaving the total energy constant.

[Bill S (OP)]

The gravitational field energy is decreasing by the same amount leaving the total energy constant.

Could you say a bit more about how that works, please.

[jeffreyH]

IMHO If the gravitational energy decreases, which I now don't believe BTW, then this is counterbalanced by an increase in dark energy.

[PmbPhy]

Could you say a bit more about how that works, please.

As dark energy accelerates matter apart from matter and thus increases the distances between particles the gravitational potential increases.

That was my original thought. However it could be wrong. Let me get more information on this and get back to you.

[yor_on]

This one is nice, discussing the concepts. Why Does Dark Energy Make the Universe Accelerate?

Gravity has one sign, 'attractive' as I gather. So, 'dark energy', have then either the opposite, 'expanding', or it isn't there at all. If it's not then what we think us see should be a property of SpaceTime, should it not? If it is a property then? What the f* is 'energy'?

[JohnDuffield]

What the f* is 'energy'?

Space.

Sounds naff I know. But look at the stress-energy-momentum tensor on Wikipedia. It "describes the density and flux of energy and momentum in spacetime". See the pressure diagonal an the shear stress?



Space is like this gin-clear ghostly elastic. Imagine you could slide in a hypodermic and inject more space. You'd have a pressure gradient, wouldn't you? 

[yor_on]

Don't read this wrong please. Einstein was perfectly correct and the only thing I'm doing is using the idea of 'locality'. I'm expanding on it, but then I'm just human, not perfect in any way. So 'energy' belongs to his idea, and whatever I believe, it has to fit it. So 'energy' is here, but so is love, and thoughts. Why do you live?

[acsinuk]

If the dark energy is magnetic energy then it is massless has no gravity but exists and can expand if it is pushed outwards to create a larger volume

[PmbPhy]

If the dark energy is magnetic energy then it is massless has no gravity but exists and can expand if it is pushed outwards to create a larger volume

Magnetic energy also has gravitational mass but it's attractive rather than repulsive.

[acsinuk]

I was thinking more of two magnets with their north poles facing each other or in the same direction which will repel.     BTW. I consider there is a huge difference between electro-magnetic energy which is associated with matter and the pure fundamental magnetic flux energy which is able to move at the speed of light.  CliveS