According to the heat death of the universe theory which holds true if Einstein's cosmological constant is positive, every atomic / sub-atomic particle will eventually be in its own “observable universe”, spaced out enough that even moving at the speed of light away from any particle you would never reach another particle.
> if Einstein's cosmological constant is positive, every atomic / sub-atomic particle will eventually be in its own “observable universe”
This is not correct. Gravitationally bound systems will remain bound in a universe with a positive cosmological constant. A "Big Rip" scenario, where gravitationally bound systems get ripped apart and eventually every elementary particle is beyond the cosmological horizon of every other, requires "phantom energy", i.e., a density of dark energy that increases with time.
