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The currently accepted theory of the expansion of the universe would suggest that for any point in the universe, there is a 'boundary' (somewhat like the Schwarzschild radius) beyond which it cannot see the stars that may be there. (since beyond that radius the expansion must appear to be faster than light). However this also implies that stars that are currently visible should disappear when the expansion takes them across that boundary (as some must since the expansion is ongoing). Now even allowing for a very long delay in this being observed from earth say, it still means that after billions of years we must be seeing stars "dropping" out of our visibility radius. Is this an observed phenomenon? or am I misunderstanding something?

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I believe this case is known as the "Big Rip" –  user2963 Apr 1 '12 at 16:40
The stars we see in the night with the naked eye are all within the Milky Way. The distance between stars within a galaxy does not increase over time because of their gravitational attraction to one another; the expansion of the universe refer to the distances between galaxies increasing over time. I think the question makes more sense if one replaces "star" with "galaxy." –  Noah Apr 1 '12 at 16:43

2 Answers 2

This happens when the universe has accelerated expansion, or when the cosmological horizon goes away slower than the speed of light, so that it is at a fixed location. Right now, the cosmological horizon is expanding outward in such a way that new things are coming in, new galaxies, into our field of vision.

In the future, when the universe is deSitter (and also in the past, when inflation was happening) the cosmological horizon was sitting still and stuff just fell into it. This is the situation you describe, and it doesn't describe the universe today, although we are in the transition period from a big-bang universe, where stuff is coming in, to a separating universe where stuff comes out.

The important thing is that there is an interplay between how fast the universe expands and how fast the horizon moves away, an interplay which is determined by the ratio of ordinary matter to cosmological constant, and right now the ordinary matter is about 30%, while the cosmological constant is 70%.

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For a detailed discussion see "Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe" by Davis and Lineweaver:


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Hi Leos, and welcome to Physics stack exchange! Unfortunately, in its current format, your answer isn't quite up to the mark--we require a lowdown of the answer even when posting a link. This should be easy to fix: just write a bit explaining the gist of the matter. Thanks! –  Manishearth Apr 2 '12 at 12:17

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