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Message: <blockquote data-quote="freyar" data-source="post: 6258857" data-attributes="member: 40227">Hubble isn't really the right instrument to do a search for a finite size universe/wrap-around effect.  The main problem is that Hubble only looks very far away in a very small area of the sky, and you've got to look in pretty much every direction if you want to see a wrap-around effect.  Think of it this way, if there is one galaxy we are seeing duplicated, we should see it both to the right and to the left.  You wouldn't see both duplicates in the same spot to the right, for example.But you can look for wrap-around in the cosmic microwave background, which is the light left over from the end of the Big Bang and is the farthest away thing we can see (or ever will see, as the universe was opaque at earlier times).  This light is like a picture of the universe almost 14 billion years ago, and it tells us the universe was not quite the same temperature or density at every point.  So what people have done is look for evidence of repeating patterns in the hot and cold spots which could be explained by a wrap-around effect.  There's no credible evidence of any, which means the universe can't be smaller than 14 billion lightyears across (actually even a bit larger if I understand the results correctly).  I can't remember if there have been checks done for every possible shape (see below), so I suppose there might be some wiggle room, but I don't think so.  If you think about the usual theories of the early universe, it would be kind of weird for the universe to be just the size we see today anyway.There are two sorts of shape issues here.  One has to do with whether space is curved.  In 2D, you can think of the three basic curvatures as being flat like a piece of paper, curved like a sphere, or curved like a Pringle chip.  Measurements (again from the cosmic microwave background, among others) tell us that our universe is flat (within error).  But the other question is if the universe is infinite or finite.  In 2D again, the old video game Asteroids is a good example of a flat but finite universe with wrap-around.  In fact, there aren't many other possibilities in 2D.  In 3D, there are a lot more choices.  I'm not a geometer, but my understanding is that the options more or less correspond to how you can take a regular polyhedron and glue the faces together.  What I'm not sure about in the cosmological searches if they've got a search algorithm that can handle all possibilities at once of if they've only been able to check a few.  I just don't remember, as it's not something I work on actively.</blockquote>

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