And subsequently how long will it be until we can never see them again?
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4$\begingroup$ Can someone explain the close votes? I thought this question was quite clear in what it was asking. $\endgroup$– zephyrCommented Mar 30, 2017 at 12:50
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1$\begingroup$ I suspect there are several issues. One is "faster than light", since two objects can't have a speed greater than light when measured in the same frame of reference. The second is that the answer to this question depends greatly on the nature of dark energy. There may be no settled answer to the questions. Thirdly, the timescales may be longer than the stelliferous period of the universe, so the galaxies will already be dark. I also remember a similar question before. $\endgroup$– James KCommented Mar 30, 2017 at 17:23
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$\begingroup$ @JamesK But there are currently galaxies which are receding faster than light right now. The galaxies which we see at the edge of the observable universe are currently receding at $3c$. And your other points don't deserve a close vote, merely the proper explanation of these points in the answer. $\endgroup$– zephyrCommented Mar 30, 2017 at 21:49
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$\begingroup$ fwiw I didn't vote to close. $\endgroup$– James KCommented Mar 30, 2017 at 22:00
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$\begingroup$ What do you mean by local? Gravitationally bound entities may be unaffected. $\endgroup$– ProfRobCommented Mar 31, 2017 at 6:22
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1 Answer
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This will take about 2 trillion years.
Given everything we know now, it's not too hard to predict the eventual fate of the universe. Of course there may be some changes as our knowledge advances, but I think the general course of events will occur as we expect them to. Based on our current knowledge, if you run through the math, you find that after about 2 trillion years from now, the expansion of the universe will have made it so that all by the closest galaxies will no longer be visible to us. You do ask two separate questions, but they're more or less the same question. They might not occur precisely at the same time, but predictions of the future aren't precise enough to distinguish such a small time difference.
By the way, you can read more about the future of the universe on Wikipedia, and specifically this section as it relates to your question.
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2$\begingroup$ I really think you should at least sketch out the maths you're using, rather than just stating the result. $\endgroup$ Commented Mar 30, 2017 at 20:14
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$\begingroup$ Given that we have currently no idea what Λ physically means/is and whether it may change with time, no solid predictions beyond at best a few Gyr can be made. Note also that we have no fundamental understanding of gravity, including regarding the constancy (or lack thereof) of G. $\endgroup$– WalterCommented Mar 31, 2017 at 17:28