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I am not a physicist but this is a question I've been trying to find the answer to for years and no answer I've been given has satisfied me.

It's my understanding that gravity affects all matter all the time forever. There is no distance, size, nor amount of time that can change the fact that, however slightly, gravity is pulling all atoms towards one another.

Based on this idea, isn't the only possible end for the universe a return to the same state it existed in right before the big bang? Eventually the energy dispersed by the big bang, which as I understand is finite, (and is still accelerating the universe apart at the given moment) will be less than the force of gravity pulling things together.

At that point, the universe would begin to shrink into itself and given infinite time will inevitably reach the singularity once more...Perhaps only to create a new big bang and start the process again.

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  • $\begingroup$ Big Crunch. Not impossible, not likely. $\endgroup$
    – Rob
    Commented Apr 25, 2018 at 17:30
  • $\begingroup$ "Based on this idea, isn't the only possible end for the universe a return to the same state it existed in right before the big bang?" No, this is not the only possible end. $\endgroup$
    – Rodney Dunning
    Commented Apr 25, 2018 at 17:44
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    $\begingroup$ In addition to the points made in Javier's answer, there are several conceptual mistakes in the question. (1) Current models do not allow us to talk about time before the big bang, so we can't talk about the state that the universe "existed in right before the big bang." (2) General relativity also doesn't allow us to define the total energy of the universe (regardless of whether the universe is finite or infinite), so re "the energy dispersed by the big bang, which as I understand is finite," no, it's not even well-defined, so we can't talk about whether it's finite or infinite. [...] $\endgroup$
    – user4552
    Commented Apr 25, 2018 at 18:27
  • $\begingroup$ [...] (3) "the energy dispersed by the big bang... is still accelerating the universe apart at the given moment" No, the acceleration is caused by dark energy. Without dark energy, we would have a deceleration. $\endgroup$
    – user4552
    Commented Apr 25, 2018 at 18:28

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No, this is not the only possibility, because there are two wrong premises:

  • Gravity pulls everything but that doesn't mean everything must collapse. If I throw a rock upwards at more than 11 km/s, that rock will leave Earth and never come back. Earth's gravity is always pulling back, but the rock's energy is enough to escape its pull. Similarly, if the universe expands sufficiently fast (and this depends on its matter content), gravity could simply not be strong enough.

  • Gravity doesn't always pull. A constant energy density will in fact have a repulsive effect. We know that the expansion of the universe is actually accelerating, and since that must (probably) be due to a uniform energy filling space we call it "dark energy", though we don't know what it is or where it comes from.

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  • $\begingroup$ I know it has been a while but I find myself thinking about this again - Can you expand on "Gravity doesn't always pull. A constant energy density will in fact have a repulsive effect."? Also, you argue that the rock will never come back in your first example - I argue that given enough time, yes it will. If gravity even exerts 1x10^-(some arbitrarily large number) force as acceleration, eventually it will win out. $\endgroup$
    – Ethan The Brave
    Commented Apr 26, 2019 at 14:33
  • $\begingroup$ @Ethan: the first point is not easy to explain, but the basic idea is that if the density is constant in time (instead of decreasing as the universe expands), the pressure must be negative. And this pressure, which also generates gravity, overwhelms the density and creates a net repulsive effect. And you can argue all you want, but I can solve the equations of motion and show that if the speed is large enough, the rock literally never comes back, because gravity decreases with distance. $\endgroup$
    – Javier
    Commented Apr 26, 2019 at 14:48
  • $\begingroup$ that actually clears up a lot for me, even some of the other comments on the question itself. I suppose if the rate of, let's say, 'gravity decay' is high enough per-second that it could outpace the speed decay of the object, it would go from 11.0 km/s to 10.9-something km/s and possibly never even see 10.8. Thank you! $\endgroup$
    – Ethan The Brave
    Commented Apr 26, 2019 at 19:19

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