5 ms·

But this expansion is happening much slower than the speed of light, so technically we can travel to the edge.

joejohnson 15y ago
Isn't the universe a closed surface? I thought the metaphor was that the universe is like a globe: if you go far enough in one direction, you end up where you started...
- dmvaldman 15y ago
  All evidence currently points to the universe being infinite, and with trivial topology. So no, you will not return. This can turn out to be incorrect in the future however. The more interesting question is that if the universe is infinite, what does it mean to "expand" and "contract." Specifically, if the universe was originally localized to a small space in the time of the Big Bang, that space was actually also infinite! When you shrink infinity you still get infinity. The only difference is our rulers are way bigger. Everything will causally affect everything else in far less time (nothing is far away).
  - wbhart 15y ago
    I am no physicist, but I still feel as though this is an immensely misleading subject. The idea of an infinite universe conjures up images an infinite number of worlds out there, infinitely many of which must be similar to our own. And somehow, all that infinite matter/energy must have been compressed infinitely densely at the beginning. But isn't it the case that we have only measured the flatness of space to within about half of one percent. If so, it seems much more likely that the universe is locally very flat but still finite, though very large, due to initial inflation (the event, not the process). So the universe might be 10^100 light years across, but still finite, yet definitely flat to well within half of one percent. But let's suppose that the universe is infinite (since we have not, and may never rule it out). What does this actually mean? Does it mean an infinite number of galaxies out there somewhere at this moment in time? Somehow I doubt it. What does it even mean for a galaxy to be out there at some point at this time? Do we mean time relative to the local reference frame of that galaxy (stationary in that frame), say with respect to the big bang or time relative to our own reference frame, which we agree is moving rapidly with respect to the other? Even distance measurements vary between the two frames of reference. So my guess is that to even have a concept of what this all means, we need to be expert at understanding space-time manifolds, which we are famously hopeless at. Another obvious point: it isn't possible for an observer to travel off in one direction and return to the same point, even if the universe is curved (as I suspect it is). The reason is simply that you couldn't make that journey. There are points in the universe which lie out of causal contact with us, which means that there is no means of getting there, even theoretically. We can't even observe those points, let alone go there and see them. So we can never make the full journey in one direction and return from the other. Doing so would imply travelling beyond our cosmic horizon.
dmvaldman 15y ago
This isn't true. The further away a galaxy is, the faster it recedes relative to us, and this can amplify well beyond the speed of light. This means different regions of space have had absolutely no communication with each other, as not even light could travel from one to the other to catch up with how they were receding from each other. http://en.wikipedia.org/wiki/Faster-than-light#Universal_expansion http://en.wikipedia.org/wiki/Faster-than-light#Universal_exp...
- twiceaday 15y ago
  Ah, right. If we consider every inch of space expanding, then long enough distances WOULD expand faster than the speed of light. Doesn't this imply that there is a reverse event horizon around every point in space? That we are on the inside of an inside-out black hole?
  - T-hawk 15y ago
    Indeed it does, there's a distance from beyond which light and information can never reach us. That's the notion of "observable universe". http://en.wikipedia.org/wiki/Observable_universe http://en.wikipedia.org/wiki/Observable_universe
  - dmvaldman 15y ago
    That's an interesting idea. My answer would be that even though every volume of space would be contracting if you reversed the clock, each of these volumes don't form their own singularities, instead the near-singularities coalesce at the point of the big bang.
  - felipemnoa 15y ago
    >>Doesn't this imply that there is a reverse event horizon around every point in space? That we are on the inside of an inside-out black hole? Wow! Thanks for the new perspective! Very original and intriguing.

Isn't the universe a closed surface? I thought the metaphor was that the universe is like a globe: if you go far enough in one direction, you end up where you started...

dmvaldman 15y ago
All evidence currently points to the universe being infinite, and with trivial topology. So no, you will not return. This can turn out to be incorrect in the future however. The more interesting question is that if the universe is infinite, what does it mean to "expand" and "contract." Specifically, if the universe was originally localized to a small space in the time of the Big Bang, that space was actually also infinite! When you shrink infinity you still get infinity. The only difference is our rulers are way bigger. Everything will causally affect everything else in far less time (nothing is far away).
- wbhart 15y ago
  I am no physicist, but I still feel as though this is an immensely misleading subject. The idea of an infinite universe conjures up images an infinite number of worlds out there, infinitely many of which must be similar to our own. And somehow, all that infinite matter/energy must have been compressed infinitely densely at the beginning. But isn't it the case that we have only measured the flatness of space to within about half of one percent. If so, it seems much more likely that the universe is locally very flat but still finite, though very large, due to initial inflation (the event, not the process). So the universe might be 10^100 light years across, but still finite, yet definitely flat to well within half of one percent. But let's suppose that the universe is infinite (since we have not, and may never rule it out). What does this actually mean? Does it mean an infinite number of galaxies out there somewhere at this moment in time? Somehow I doubt it. What does it even mean for a galaxy to be out there at some point at this time? Do we mean time relative to the local reference frame of that galaxy (stationary in that frame), say with respect to the big bang or time relative to our own reference frame, which we agree is moving rapidly with respect to the other? Even distance measurements vary between the two frames of reference. So my guess is that to even have a concept of what this all means, we need to be expert at understanding space-time manifolds, which we are famously hopeless at. Another obvious point: it isn't possible for an observer to travel off in one direction and return to the same point, even if the universe is curved (as I suspect it is). The reason is simply that you couldn't make that journey. There are points in the universe which lie out of causal contact with us, which means that there is no means of getting there, even theoretically. We can't even observe those points, let alone go there and see them. So we can never make the full journey in one direction and return from the other. Doing so would imply travelling beyond our cosmic horizon.

dmvaldman 15y ago

This isn't true. The further away a galaxy is, the faster it recedes relative to us, and this can amplify well beyond the speed of light. This means different regions of space have had absolutely no communication with each other, as not even light could travel from one to the other to catch up with how they were receding from each other. http://en.wikipedia.org/wiki/Faster-than-light#Universal_expansion http://en.wikipedia.org/wiki/Faster-than-light#Universal_exp...

twiceaday 15y ago
Ah, right. If we consider every inch of space expanding, then long enough distances WOULD expand faster than the speed of light. Doesn't this imply that there is a reverse event horizon around every point in space? That we are on the inside of an inside-out black hole?
- T-hawk 15y ago
  Indeed it does, there's a distance from beyond which light and information can never reach us. That's the notion of "observable universe". http://en.wikipedia.org/wiki/Observable_universe http://en.wikipedia.org/wiki/Observable_universe
- dmvaldman 15y ago
  That's an interesting idea. My answer would be that even though every volume of space would be contracting if you reversed the clock, each of these volumes don't form their own singularities, instead the near-singularities coalesce at the point of the big bang.
- felipemnoa 15y ago
  >>Doesn't this imply that there is a reverse event horizon around every point in space? That we are on the inside of an inside-out black hole? Wow! Thanks for the new perspective! Very original and intriguing.