# What would happen to the universe if dark energy started "disappearing"?

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.

• Gravity would take over, the universe will shrink, and everything will eventually end with a Big Crunch Commented Jan 10, 2015 at 1:47
• 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. Commented Jan 10, 2015 at 1:53
• Anyways, your question is imho good, so you got an up. Commented Jan 10, 2015 at 1:56
• @PeterHorvath, energy from where? On cosmological scales I am only aware of Dark Energy and Gravity operating. Commented Jan 10, 2015 at 1:57
• @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. Commented Jan 10, 2015 at 15:01

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".

• 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! Commented Jan 10, 2015 at 15:07
• @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. Commented Jan 10, 2015 at 15:11
• @PhotonicBoom does $\frac{1}{x^3}$ ever reach zero? Same thing with rate of expansion
– Jim
Commented Jan 10, 2015 at 15:22
• @RobJeffries, I see. I stand corrected. +1 Commented Jan 10, 2015 at 15:28
• @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?) Commented Jan 10, 2015 at 19:10