2 ms·

Not in an expanding universe.

• tejtm 5y ago
  This has to be a question someone here might be able to answer. Is there some mass-distance(-velocity?) function that defines the alternative fates? Where below some threshold, things are fated to merge and beyond which ...not so much. I am pretty confident all the mass on the earth would be expected to end up together, and would be comfortable imagining that theoretical planets on opposite sides of the observable universe may never merge but what is the transition point? the solar system? the milky way? the local group?
  • pdonis 5y ago
    Systems that are gravitationally bound will stay that way, yes. (At least as long as a "big rip" scenario does not happen; our best current observations say it won't, but the error bars don't completely rule it out.) Generally speaking, the boundary in our universe seems to be around the galaxy cluster level: galaxy clusters are the largest systems that are generally gravitationally bound (although often quite loosely, so that objects near the outer edge of the cluster might fairly easily acquire escape velocity through random encounters with other objects). That corresponds roughly to a distance scale of a few tens of millions of light-years.