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I've been studying astrophysics and I read that our universe is still expanding because the gravity is not enough compared to the dark energy present. And right now we are not sure whether are universe is flat, open or closed because we don't know the critical mass.

However what I don't understand is that while this universe is expanding, distance between the particles is increasing, doesn't that mean that their pull on each other i.e. is gravitational force is reducing? So shouldn't that mean that our universe is going to expand forever?

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Yes, our current understanding is that on cosmological scales dark energy will drive a perpetual expansion of space at an accelerating rate. However, the galaxies in each galaxy cluster are gravitationally bound to one another, and there is no (widely accepted) evidence that the gravitational constant changes over time, so they will not disperse. Instead, the galaxies in the Local Supercluster, for example, are expected to merge into one giant galaxy over a timescale of $10^{11}$ to $10^{12}$ years. Meanwhile light from other galaxy clusters will have become so red-shifted that they are effective no longer visible.

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  • $\begingroup$ but as their distance is increasing shouldnt the force reduce? Regardless of G $\endgroup$ – Megan mars Mar 26 at 13:37
  • $\begingroup$ No, because dark energy creates a negative pressure that is (as far as we know) homogenous, i.e. the same everywhere. $\endgroup$ – gandalf61 Mar 26 at 13:53
  • $\begingroup$ Megan yes as masses get far the gravitational force reduces. But they can be bound and reverse the motion, simply proceed further, or do so in accelerated manner. You can see the first two case launching something up in simple mechanics (escape speed etc). Dark energy is required because accelerated expansion is observed. $\endgroup$ – Alchimista Mar 26 at 14:22

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