According to cosmological constant, dark energy should have a constant density but since the universe is expanding so is the volume of the universe and so should the mass of dark energy.
But how does dark energy gain mass or energy?
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4$\begingroup$ Whoever could answer this, would have a Nobel prize in physics. $\endgroup$– zephyrCommented Apr 25, 2017 at 13:41
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$\begingroup$ @zephyr Are there any proposed theories or models that explain this? I guess slim chances. $\endgroup$– kingW3Commented Apr 25, 2017 at 17:00
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$\begingroup$ For all you string theorists out there, I wonder if string theory has any incite into why this "constant density energy" would be a PROPERTY of space and why it would be constant? As for the models of it not being constant, one would have to explain why it changes. $\endgroup$– Jack R. WoodsCommented May 12, 2017 at 18:34
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3 Answers
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Not all models of dark energy assume it constant.
However, when this is assumed true, dark energy is usually compared to the energy density of the vacuum. In other words, while the Universe expands, the dark energy increases proportionally, so its ratio with the Universe volume (the density) stays constant.
Or, better, we generally believe those models where the dark energy is a constant because they fit better our observations of the Universe, in particular the Big Bang model, which describes how our Universe evolves.
If you like some math, you can have it here.
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$\begingroup$ I do like math :) so +1,anyway I wanted to focus more on the part how dark energy gain energy/mass then how does it remain constant,since as you said there are models which predict that the density is not constant $\endgroup$– kingW3Commented Apr 26, 2017 at 10:23
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$\begingroup$ It does not gain energy. It is the wrong way to look at it. Perhaps we could say that its amount grows. Is this any close to what you mean? $\endgroup$– Py-serCommented Apr 26, 2017 at 14:11
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You're missing the point: it's called "dark energy" not because it's dark or because it's energy. The name is the cosmological equivalent of the "black box" in a laboratory setting. Until we figure out what the heck it is, there's no reason for it to follow our known laws.
answered Apr 25, 2017 at 13:53
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1$\begingroup$ I would say it is called dark because we can't see it and energy because it contains energy. Anyway why would there be no reason for it to follow our laws? Even if it doesn't there are still theories and models which give different explanations, like pretty much everything in astronomy. $\endgroup$– kingW3Commented Apr 25, 2017 at 16:56
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1$\begingroup$ You're missing the point: S/he didn't ask this because of the term's name, but because of its suspected properties. $\endgroup$– HelenCommented Apr 25, 2017 at 17:04
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$\begingroup$ @kingW3 really - how do you know it contains energy? All we know is that there's something which appears to make things more energetic than we expected. $\endgroup$ Commented Apr 25, 2017 at 18:15
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$\begingroup$ I don't know whether it contains energy or not, I just wanted to point out that the name dark energy does have its meaning,and as Helen said I'm not interested in the terminology.I understand your point,and you are probably correct though I don't think this answers my question. $\endgroup$– kingW3Commented Apr 26, 2017 at 10:35
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$\begingroup$ @CarlWitthoft From wikipedia: dark energy is an unknown form of energy. Obviously, given that we don't know what it is, you could claim it's purple fairy dust, but the fact of the matter is that current theory models it as a form of (unknown) energy. That's how it appears in the Friedmann equations that forms the basis of current cosmological theory. As such it is called dark energy because we it is dark (e.g., we can't see it) and it is energy (or at least, we model it as if it is). $\endgroup$– zephyrCommented Apr 26, 2017 at 21:13
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The energy of universe increases, that energy is contributed to dark energy. In order for accelerated expansion of universe, dark energy would have to have negative pressure.
According to first law of thermodynamics $\mathrm{d}U=\mathrm{d}Q-P\mathrm{d}V$,since the universe is an isolated system and the heat doesn't change $\mathrm{d}Q=0$, so it's expected by thermodynamics that the universe gains energy which is attributed to dark energy.