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The space between galaxies isn't that much more empty than what we have here. There is some curvature, enough so that it causes gravitational lensing, even enough to make us suspect there's some extra matter (dark matter) we can't detect by other means. What you're asking is directly measurable: it's the gravitational blueshift from whatever light sources ...


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While some galaxies are billions of light years away, there are hundreds of galaxies withing just a few dozen million light years away. Astronomically speaking, a few dozen million years is pretty brief. We can be fairly certain that the galaxy hasn't changed much in this time.


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We wouldn't be able to see it if a star went supernova right now because the light wouldn't have reached us yet. If a star 100 light years away from us went supernova 100 years ago we would see it now because the light would have had enough time to travel to us.


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The practical limit in terms of sky coverage is probably given by funding. The HDF, according to wikipedia, covers a 24-millionth of the entire sky. We would have to launch thousands of Hubble-like instruments to cover the entire sky at that resolution and sensitivity. Much of that area is covered by the Milky Way and and gas clouds, anyway, so not all deep ...



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