When heat death finally arrives

Originally posted by @fabianfnas
I still don't understand how the matter would be evenly dispersed, as if gravity wouldn't be there anymore.
Even if the atomic nuclei isn't holding together anymore, of some reason, gravity would anyway bring particles with mass together in lumps.

Gravity would still be there, as it is a property of mass-in-space. However, gravity is not what holds subatomic particles together; gravity is much weaker than the subatomic forces which hold particles together.

There are two distinct but related phenomena involved in so-called heat death. One is the expansion of space, the other is the cooling of matter.

1. Assuming that all suns eventually die and eject waste elements, and that these elements recede from all other elements due to expansion of space, the matter in the universe will eventually be so dispersed that lumps no longer form. There will just be particles receding from each other, isotropically (in all directions at once). The effect of gravity decreases with distance; ergo, at some distance it is negligible. It is, in any case, weaker than subatomic forces. Now see point two below:

2. As the temperature approaches absolute zero, chemical processes stop, molecules fall apart.

The theoretical conclusion is that at some distance time, matter will be so tiny and so dispersed that lumps no longer form, the universe will be big, but vastly, overwhelmingly empty, and cold and dark.

@fabianfnas said
I still don't understand how the matter would be evenly dispersed, as if gravity wouldn't be there anymore.
Even if the atomic nuclei isn't holding together anymore, of some reason, gravity would anyway bring particles with mass together in lumps.

But if the universe was say a trillion light years across and particles are dispersed so each particle is say a meter apart there would be no local density variations since say, an electron a meter from its nearest neighbor would not have enough gravitational attraction to clump together and besides that if all its buddies were electrons the electric field would keep them apart so there would be no chance of those particles getting any closer so the universe would end up a static field of particles unable to interact.