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Given a nearly infinite timeline, would the expansion of space eventually cause individual particles to become "unobservable" from one another due to the vast distance and increasing rate of expansion with that distance.

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Maybe someone can give chapter and verse on this, but my understanding is that bound objects are unaffected by the expansion for the case of dark energy represented by a simple cosmological constant.

But a "big rip" will occur in a finite time if the dark energy equation of state $P = w \rho$ is characterised by $w < -1$. In this scenario, the expansion accelerates so quickly that the scale factor becomes infinite in a finite time and structures must disintegrate when the size of the observable universe becomes smaller than that structure.

The present state of play is that $w$ appears to be very close to -1 (a cosmological constant). But given that dark energy is not understood, who can really say whether it might change in the future...

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  • $\begingroup$ I'm guessing (hoping :-) ) that "individual particles" here refers to stable items, i.e. not quarks. So, does "bound objects" extend to molecules as well as atoms? $\endgroup$ – Carl Witthoft Apr 6 '17 at 11:25
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    $\begingroup$ @CarlWitthoft for a cosmological constant scenario, "bound objects" could extend all the way up to galactic clusters or superclusters. As long as the attractive forces overpower the expansive forces, the object is bound. This is certainly true for molecular bonds vs. dark energy. $\endgroup$ – Asher Apr 8 '17 at 22:11

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