4 ms·

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 observa

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?
  - goodside 16y ago
    The stuff outside the OU cannot be seen in principle. Its size is fully independent of our current level of technology, and no advancement of technology will let you see beyond it. (Seeing beyond it would mean you can send signals faster than light, which implies you can build a time machine, in which case all bets are off.) In practice, we cannot see anywhere near as far as the radius of the OU, since after a certain distance you're looking so far into the past that space is opaque to microwave radiation. If you can somehow see neutrinos you get to see more, but that's still all inside the boundary of the OU.
    - zppx 16y ago
      Yes, I know that they cannot move faster than the speed of the light, and that the universe was opaque to visible light before the decoupling of radiation. But there're a group of neutrinos that decoupled before the decoupling of radiation, even before this gravitational waves were at large in the universe, they could in principle say something about regions that are outside the Observable Universe, this ignoring the fact that they must be absolutely difficult to observe and that both neutrinos and gravitation waves are generated by new events and that we can in fact extract some information about these regions from them in the same way we can from light. This was what I found from a paper, "Detection of gravitational waves with resonant antennas" from Francesco Ronga, earlier today: "Gravitational wave and neutrino astronomy will increase the amount of observable universe, because they will investigate places that are completely inaccessible to the electromagnetic radiation and probably will change our knowledge of the universe evolution"

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?
- goodside 16y ago
  The stuff outside the OU cannot be seen in principle. Its size is fully independent of our current level of technology, and no advancement of technology will let you see beyond it. (Seeing beyond it would mean you can send signals faster than light, which implies you can build a time machine, in which case all bets are off.) In practice, we cannot see anywhere near as far as the radius of the OU, since after a certain distance you're looking so far into the past that space is opaque to microwave radiation. If you can somehow see neutrinos you get to see more, but that's still all inside the boundary of the OU.
  - zppx 16y ago
    Yes, I know that they cannot move faster than the speed of the light, and that the universe was opaque to visible light before the decoupling of radiation. But there're a group of neutrinos that decoupled before the decoupling of radiation, even before this gravitational waves were at large in the universe, they could in principle say something about regions that are outside the Observable Universe, this ignoring the fact that they must be absolutely difficult to observe and that both neutrinos and gravitation waves are generated by new events and that we can in fact extract some information about these regions from them in the same way we can from light. This was what I found from a paper, "Detection of gravitational waves with resonant antennas" from Francesco Ronga, earlier today: "Gravitational wave and neutrino astronomy will increase the amount of observable universe, because they will investigate places that are completely inaccessible to the electromagnetic radiation and probably will change our knowledge of the universe evolution"