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Yes. That's what makes metric expansion of spacetime elegant: it doesn't require you to suppose there's anything special about our location in the Universe. If

Yes. That's what makes metric expansion of spacetime elegant: it doesn't require you to suppose there's anything special about our location in the Universe. If the recession of galaxies were intertial, the only explanation would be that the Big Bang happened exactly where Earth is now, and we're at the exact center of the Universe and everything is moving away from us. It's the key absurdity that needed to be explained away. Further, we know from more recent evidence that the Universe is homogenous on scales larger than galactic clusters. On the scales of stars and galaxies, the Universe has large expanses of empty space sprinkled with massive galaxies. Zooming out, you can see "clusters" of up to about a thousand galaxies separated by larger areas that have fewer galaxies. But once you get above that level, on the order of 10^24 to 10^25 meters, there's simply no more structure to be found. As far as we can tell, if you take a spherical region with a diameter 10^25 meters from anywhere in the Universe, it will have roughly the same amount of mass regardless of where you take it from, and the Universe is of roughly uniform density.

boredguy8 16y ago
So then, and this is where the crazy starts to happen: when people talk about the size of the universe, I start to get really confused. While there's an observable limit from our reference point, isn't that functionally a consequence of the null cone? That is: doesn't the homogeneity irrespective of location mean that if I were on the edge of the spacetime 'bubble' from Earth's point of view, there would be a completely different spacetime bubble with different 'stuff' in it? That is, different galaxies alltogether? Maybe the question is too absurd, since we can't teleport in the way I described, but it's...I don't know...interesting. Thanks!
- lmkg 16y ago
  You are correct. There's more to the universe than we can see, but what we can't see should be basically the same as what we can. By analogy, from any place on the earth, you can only see as far as the horizon[1]. If you were to teleport (or travel) to the edge of your horizon, you would see more earth that you couldn't see before, but that new stuff would basically be similar to the earth you're seeing now. Just for kicks, let's add in that the earth is expanding like the universe. You're at point A, there's a tree at point B, and the horizon past the tree in that direction is point C. If the earth is expanding, then point B will be receding from both point A and point C, in part because point C is receding twice as fast. [1] The reason for the boundary of the observable earth is totally different, but that doesn't change the point.
- goodside 16y ago
  You're right: the "Observable Universe" is defined only relative to a point of observation. For any spot in the Universe, there's a 93-billion-light-year-wide sphere centered about that point that constitutes the OU for that point. What exactly this sphere means it somewhat complicated. The simplest way I can think to say is this: The radius of the OU is the current distance of the farthest point in space such that if a radio signal were emitted from that point 13.7 billion years ago, immediately after the Big Bang, it would be possible in principle (but by no means in practice) for us to receive that signal. Anything that is outside of the OU is causally disconnected from us as a matter of fundamental principle, and nothing that has ever happened or ever will happen at any point outside the OU can ever affect us in any way, ever. It was previously (up until a decade or so ago) an open question as to whether the Universe itself might be smaller than the OU. This sounds absurd, but consider the example of a hypothetical jet that could circle the Earth ten times without refueling. While normally the "range" of a jet plane is a circle about some point, the range of this jet exceeds the size of the Earth itself. Similarly, if the Universe were smaller than the OU, some of the distant galaxies we see would actually be repetitions of closer galaxies from an earlier time and a different angle, since the light had been "looping around" the Universe once or more before reaching us. Or, as Modest Mouse put it in one of their better songs: "The Universe is shaped exactly like the Earth / If you go straight long enough, you end up were you were." Recent evidence from the Cosmic Microwave Background has made this idea very unlikely, but it's a useful example to clear up misconceptions about the OU. So, however big the Universe actually is, it's bigger than the OU, which means that the stuff cosmologists are debating about is space that cannot, even in principle, ever be observed. It's an important question, though, since whether the Universe loops back on itself on the large scale or just keeps going forever has implications for whether gravity will win out over Hubble Expansion in the long run, determining the eventual fate of the Universe.
  - zppx 16y ago
    Whoa, I praise your deep knowledge of cosmology, anyway, to my question: > "So, however big the Universe actually is, it's bigger than the OU, which means that the stuff cosmologists are debating about is space that cannot, even in principle, ever be observed" What if we learn how to make better neutrino telescopes, them how much of the Unobservable Universe could we see? Can you recommend a paper about this?

boredguy8 16y ago

So then, and this is where the crazy starts to happen: when people talk about the size of the universe, I start to get really confused. While there's an observable limit from our reference point, isn't that functionally a consequence of the null cone? That is: doesn't the homogeneity irrespective of location mean that if I were on the edge of the spacetime 'bubble' from Earth's point of view, there would be a completely different spacetime bubble with different 'stuff' in it? That is, different galaxies alltogether? Maybe the question is too absurd, since we can't teleport in the way I described, but it's...I don't know...interesting. Thanks!

lmkg 16y ago
You are correct. There's more to the universe than we can see, but what we can't see should be basically the same as what we can. By analogy, from any place on the earth, you can only see as far as the horizon[1]. If you were to teleport (or travel) to the edge of your horizon, you would see more earth that you couldn't see before, but that new stuff would basically be similar to the earth you're seeing now. Just for kicks, let's add in that the earth is expanding like the universe. You're at point A, there's a tree at point B, and the horizon past the tree in that direction is point C. If the earth is expanding, then point B will be receding from both point A and point C, in part because point C is receding twice as fast. [1] The reason for the boundary of the observable earth is totally different, but that doesn't change the point.
goodside 16y ago
You're right: the "Observable Universe" is defined only relative to a point of observation. For any spot in the Universe, there's a 93-billion-light-year-wide sphere centered about that point that constitutes the OU for that point. What exactly this sphere means it somewhat complicated. The simplest way I can think to say is this: The radius of the OU is the current distance of the farthest point in space such that if a radio signal were emitted from that point 13.7 billion years ago, immediately after the Big Bang, it would be possible in principle (but by no means in practice) for us to receive that signal. Anything that is outside of the OU is causally disconnected from us as a matter of fundamental principle, and nothing that has ever happened or ever will happen at any point outside the OU can ever affect us in any way, ever. It was previously (up until a decade or so ago) an open question as to whether the Universe itself might be smaller than the OU. This sounds absurd, but consider the example of a hypothetical jet that could circle the Earth ten times without refueling. While normally the "range" of a jet plane is a circle about some point, the range of this jet exceeds the size of the Earth itself. Similarly, if the Universe were smaller than the OU, some of the distant galaxies we see would actually be repetitions of closer galaxies from an earlier time and a different angle, since the light had been "looping around" the Universe once or more before reaching us. Or, as Modest Mouse put it in one of their better songs: "The Universe is shaped exactly like the Earth / If you go straight long enough, you end up were you were." Recent evidence from the Cosmic Microwave Background has made this idea very unlikely, but it's a useful example to clear up misconceptions about the OU. So, however big the Universe actually is, it's bigger than the OU, which means that the stuff cosmologists are debating about is space that cannot, even in principle, ever be observed. It's an important question, though, since whether the Universe loops back on itself on the large scale or just keeps going forever has implications for whether gravity will win out over Hubble Expansion in the long run, determining the eventual fate of the Universe.
- zppx 16y ago
  Whoa, I praise your deep knowledge of cosmology, anyway, to my question: > "So, however big the Universe actually is, it's bigger than the OU, which means that the stuff cosmologists are debating about is space that cannot, even in principle, ever be observed" What if we learn how to make better neutrino telescopes, them how much of the Unobservable Universe could we see? Can you recommend a paper about this?