Terribly naive question, I know. Obviously, not simply "disappearing", but if it could, theoretically, be absorbed or "used" somehow, what would happen to the universe? Would it stop expanding, would it cause chaos, what? Also, please be gentle, I'm just a science fiction nerd asking the real experts here. Would love to be educated rather than chastised.
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1Gravity would take over, the universe will shrink, and everything will eventually end with a Big Crunch – PhotonBoom Jan 10 '15 at 01:47
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I think it could be calculated by using the Friedman-model in this point, but without the gravitational constant. Afaik, there weren't big crunch, only the expansion of the universe changed back to deceleration again. – peterh Jan 10 '15 at 01:53
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Anyways, your question is imho good, so you got an up. – peterh Jan 10 '15 at 01:56
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1@PeterHorvath, energy from where? On cosmological scales I am only aware of Dark Energy and Gravity operating. – PhotonBoom Jan 10 '15 at 01:57
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@Peter and PhotonicBoom - Thanks guys. I have another question, could it ever be possible to live in a static universe? Or would any hint of inconsistency expand or contract us? In other words, is it even possible for the universe to ever be in a state of stable equilibrium? – Tamale Fox Jan 10 '15 at 02:03
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@TamaleFox, I believe its possible. I don't think the universe expanding, contracting or being static plays any role in star formation or life formation and evolution. All we need for that is gravity.. and a lot of luck :) – PhotonBoom Jan 10 '15 at 02:11
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@PhotonicBoom - Excellant, thank you for answering! – Tamale Fox Jan 10 '15 at 02:26
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@TamaleFox Ask this in a different question, this is my suggestion. – peterh Jan 10 '15 at 02:52
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@PhotonicBoom Sorry, I misthinked :) But I think, it is not trivial, did you ever heard from the Friedmann-equations? – peterh Jan 10 '15 at 02:53
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1@PhotonicBoom Gravity would not take over. The matter density presently in the universe is insufficient to bring everything back together in a big crunch. In fact it would continue to expand almost at its present rate. – ProfRob Jan 10 '15 at 15:01
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Dear Fox, if you are satisfied with an answer, you can accept that by clicking the pipe icon on the left. This is a reward to the answering person (). – peterh Jan 12 '15 at 03:17
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The answer depends also on the process by which dark matter would be "used" or absorbed. The process would probably not just remove the dark matter particles from the universe: that would be violation of the energy conservation. The dark matter particles would be most likely transformed into other particles, so the total amount of energy in the given volume of space would stay unchanged. Thus also the gravitational pull would not change at all. – mpv Oct 29 '15 at 18:21
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If we take the present-day universe, adopt the so-called concordance cosmology, or Lambda-CDM model, accept GR and the Robertson-Walker metric, but then allow the dark energy content to be suddenly zero, then the universe will no longer be flat. Instead it will have an open geometry and the energy density within it would be dominated by dark matter. However, this energy density would have insufficient gravitational influence to halt the current expansion. As the density universe get bigger, the density falls and the influence of gravity becomes even weaker.
$\Omega_M \simeq 0.32$, where $\Omega_M$ is defined as the ratio of the matter density to that required in order to make the universe flat. Thus, although the expansion would decelerate, matter (including dark matter) is short by a factor of three of being able to halt the expansion at any point in the future. Only if $\Omega_M \geq 1$ does the expansion come to a halt. These solutions were explored by Friedmann in 1922. The plot below (taken from this wikipedia page) shows solutions with $\Omega_M = 0.3$, which can be contrasted with the concordance model of $\Omega_M \simeq 0.3$, $\Omega_{\Lambda} \simeq 0.7$, which is also shown as an accelerating expansion.
To give any other answer would require you to be a bit more definite on what you mean by "not simply disappearing".
ProfRob
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How can the expansion decelerate without eventually halting? I don't understand this bit. However weak gravity is, if there is deceleration, wouldn't the universe eventually roll back on itself? It might take forever, but isn't this statement true? If so you might want to expand on it a bit! – PhotonBoom Jan 10 '15 at 15:07
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@PhotonicBoom Because the deceleration decreases with time as the density of the universe decreases. i.e. if $\Omega_M < 1$ now, then it will be even smaller in the future. I'll add a picture. – ProfRob Jan 10 '15 at 15:11
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@PhotonicBoom does $\frac{1}{x^3}$ ever reach zero? Same thing with rate of expansion – Jim Jan 10 '15 at 15:22
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@RobJeffries - Thank you for answering! So just so I understand fully here, nothing can really stop the universe from expanding even with less dark energy? Ratio of matter density would have to be greater than 1 which it is not (btw, what exactly does this mean, master density? quantity, volume, how much other matter there is i.e real matter?) – Tamale Fox Jan 10 '15 at 19:10
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@TamaleFox Matter density can be measured in kg/m$^3$. The critical density to "close" the universe (in the absence of dark energy), is equal to $3H^2/8\pi G$, where $H$ is the Hubble "constant", and is currently about $8\times10^{-27}$ kg/m$^{3}$. The measured mass density of the universe is about one third of this and is mostly in the form of dark matter. So an absence of dark energy would not be sufficient to arrest the expansion of the universe. – ProfRob Jan 10 '15 at 19:54
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@RobJeffries - Alright, I'm slowly getting it. Btw, where does the current theoretical extending to the ΛCDM model that David Wands and others have made fit in to this? Where dark matter is allowed to decay and create dark energy? I assume it's the same outcome? – Tamale Fox Jan 10 '15 at 21:31
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@RobJeffries - Also, second part of question, in a Big Freeze scenario where gasses needed for star formation and life are exhausted, what happens to dark matter and dark energy? – Tamale Fox Jan 10 '15 at 21:52
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@TamaleFox I am unfamiliar with either of the speculative ideas you mention. As we know of no coupling between dark matter, dark energy and normal matter beyond gravitation, I don't see what difference the state of normal matter makes. – ProfRob Jan 10 '15 at 22:06
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@RobJeffries - Sorry, should of posted some info on what I was referring to. See here. And here. – Tamale Fox Jan 10 '15 at 22:17