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My possibly mistaken understanding is that dark energy changes with time, whereas a cosmological constant is, well, constant. What about gravitational clumping? Detecting relative motion?

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Dark energy is a catch all term for whatever is accelerating the expansion. The simplest form of dark energy consistent with general relativity is a cosmological constant, and this is just an energy density per unit volume of space.

The cosmological constant is time independant (and position independant) by definition, that is if it were changing in time or space it wouldn't be called a cosmological constant. You can replace the cosmological constant with a scalar field that is allowed to vary in space and/or time, and this gives rise to other models such as quintessence. It's interesting to speculate about ideas like this, but at the moment there is no evidence that the dark energy is changing, and therefore a simple cosmological constant is consistent with observations.

You need to bear in mind that the cosmological constant and quintessence are just mathematical models that fit the observed data. At the moment there is no theoretical reason to recomend the cosmological constant, quintessence or indeed anything else.

The Wikipedia article you linked discusses the various forms of dark energy. Can you expand a bit on what issues the article leaves you unsure about.

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Actually, I didn't supply those wikipedia links, Qmechanic did, thank you very much. I have read the links and they support both the time constancy and the gravitational clumping as possible differences, (but below today's level of sensitivity.) I found nothing about relative motion however. We have detected our motion relative to the CMB. If we were not at rest relative to the quintessence field, could we detect that, (assuming we could detect the quintessence field)? –  Jim Graber Sep 2 '12 at 17:41
Good question. Motion makes no difference to the cosmological constant, but I don't know if it would affect quintessence. I imagine it would depend on the form it takes. –  John Rennie Sep 2 '12 at 18:24
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My view is that dark energy is like hot air in a balloon. It's density does not stay constant, but decreases as the universe expands, like every other form of energy.

Like other forms of energy, it would also be subject to the second law of thermodynamics and would cool down, slow down, and get sucked into black holes, converting it's pushing effect, into a pulling effect of gravity.

There's no reason to think it behaves completely differently to all other forms of energy we know about.

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There is reason to believe it behaves differently. See for instance my answer to your question physics.stackexchange.com/questions/105533/… for some ideas to get you thinking. –  Kyle Mar 28 at 21:06
Maybe it's quintessence that I'm talking about, I don't really know, but nobody has explained how dark energy is not the same energy that makes us up, made of a certain number of sub-atomic particles or strings, which could not increase in number, unless defying the first law of thermodynamics. Observations do not suggest that this basic law needs to be broken, it's to do with calculations which make more sense if it is a constant density, even though I can't see how that's possible. Einstein suggested the constant to explain a static non expanding universe. –  rowanman28 Apr 3 at 6:13
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