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Proton decay is a theoretical possibility, as it is predicted to happen by some GUTs, so the proton may not be stable and could decay into lighter particles considering enormous time scales.

If proton decays, neutrons would be free, therefore they would be no longer stable and would suffer beta decay

The electrons from the atoms would be free. Some would annihilate with the positrons that the proton may produce when decaying. The surviving ones, would be inevitably repelled by the electromagnetic force (which would be stronger than the force of gravity attracting them)

Neutrinos are relativistic so they would not be gravitationally bound to anything. They could suffer a "redshift" due to the expansion of the universe (which would reduce their momentum: In a similar way to how photons increase in wavelength with the expansion of space, neutrinos and other massive particles would also see their speed reduced since their momentum would suffer a "redshift" such that p∝1/a where "a" is the scale factor) and I though that far enough into the future, they could lose enough velocity to be gravitationally bound to matter overdensities, such as superclusters. However, I have spoken with several physicists about the topic and they say they would not become gravitationally bound in any case (not even relic neutrinos) and that this would be impossible

Then, if we have no other stable known particles, and we don't even know if dark matter exists (so I don't consider it) will then all particles be diluted by the cosmic expansion of the universe as none of them would be gravitationally bound to anything?

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  • $\begingroup$ Consider that the proton may decay to a still massive but lighter particles, these could still be charged by charge conservation and perhaps still form neutral gravitationally bound matter. $\endgroup$
    – Triatticus
    Commented Aug 20 at 1:57
  • $\begingroup$ if the expansion is fast enough, such as in the "big rip" version, eventually no bound states will exist, unless they are black holes, and even in this case. something funny happens when the event horizon reaches the size of the cosmic horizon $\endgroup$
    – Pato Galmarini
    Commented Aug 20 at 2:03

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Yes. You don't even need other rapid expansion scenarios with big $a$. Given very large time scales matter will annihilate and dilute and no significant gravitational clusters could form. The same is generally true for dark matter assuming a particulate nature.

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  • $\begingroup$ but the thing is that the expansion of spacetime cannot affect gravitationally bound structures @Mike $\endgroup$
    – vengaq
    Commented Aug 20 at 12:49
  • $\begingroup$ Hmm you might be right! But the thing is I think OP is basically asking if on large enough scales that matter which would be very sparse and be in the form of free floating leptons and photons,then with big $t$, will matter be diluted to basically nothing? Which is why I answered in the way I did. So yes expansion doesn't affect gravitationally bound stuff but in the future there's basically going to be no gravitationally bound structures. $\endgroup$
    – Mike
    Commented Aug 20 at 13:06
  • $\begingroup$ But how can a large gravitationally bound structure like a supercluster stop being gravitationally bound? @Mike $\endgroup$
    – vengaq
    Commented Aug 20 at 14:10
  • $\begingroup$ If you're going to have annihilation into light leptons (almost entirely electrons) and photons, you can't have some large scale gas of $e^-$$γ$ so why would you expect a gravitational clusters? $\endgroup$
    – Mike
    Commented Aug 20 at 20:30

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