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The short answer is nobody has a plausible upper bound on the size of the entire Universe, and there's no firm consensus on whether it's literally infinite. If

The short answer is nobody has a plausible upper bound on the size of the entire Universe, and there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. It's analogous to how if you stood on the ground of featureless planet, the land you could inhabit is finite, but it has no borders and no point where you could stick a flag in the ground and say "this is the center of the planet's land". Most of the time when people give hard numbers for the size of the Universe, they either mean A) the portion of the Universe we can see, B) the portion of the Universe we could ever see in principle, or C) the portion of the Universe which could, in principle, be causally influenced by early particle interactions that could have causally influenced us. If you suspect someone might be talking out of their ass about cosmology, ask them how big the Universe is -- if they give you a clean multiple of 14.6 billion light years, they're an idiot. Technically inclined readers will find this more enlightening than the article above: http://arxiv.org/pdf/astro-ph/0310808.pdf http://arxiv.org/pdf/astro-ph/0310808.pdf

peterwaller 14y ago
> [...] there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. My understanding is that for your interpretation here of "finite" to be true, there must be some curvature. If you only consider the geometry of the universe, then what we see with our instruments does allow us to put a bound on the size of the curvature. If you want to say something along the lines of "outside of the observable universe is probably cheese whiz" then that's your perogative, and no-one can refute that. But (from the article) the geometry of the universe that we have measured is consistent with an infinite universe, and gives a minimum bound on its size which is many multiples of the observable universe.
- Strilanc 14y ago
  It's mathematically simple for a space to loop without being curved locally, although I am under the impression that there are physics reasons to infer we don't exist in such a space. The pac-man universe acts euclidean until you have universe-sized effects. For example, a circle will satisfy circumference = diameter * pi until its diameter is greater than the span of the universe. Basically the universe could 'hide' the fact that it is a torus from pac-man by simply being really large.
- cafebabe 14y ago
  The problem is Time. You would have to save (or predict) the state of the universe for measuring the distance somehow. And the universe changes constantly over time. Lets say the universe evolves by the speed of light, that would make it impossible to measure it at all. This would mean to everything inside the universe, that it is infinite.
scott_s 14y ago
I think that confusing A, B or C with "the Universe, all of it" is hardly grounds to call someone an idiot.
- goodside 14y ago
  There isn't anything of importance in cosmology that's a clean multiple of 14.6 billion LY. Anything under discussion at that scale would by influenced by the Hubble expansion. But, yeah, being misinformed about cosmology doesn't make you an idiot. Hyperbole.

peterwaller 14y ago

> [...] there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. My understanding is that for your interpretation here of "finite" to be true, there must be some curvature. If you only consider the geometry of the universe, then what we see with our instruments does allow us to put a bound on the size of the curvature. If you want to say something along the lines of "outside of the observable universe is probably cheese whiz" then that's your perogative, and no-one can refute that. But (from the article) the geometry of the universe that we have measured is consistent with an infinite universe, and gives a minimum bound on its size which is many multiples of the observable universe.

Strilanc 14y ago
It's mathematically simple for a space to loop without being curved locally, although I am under the impression that there are physics reasons to infer we don't exist in such a space. The pac-man universe acts euclidean until you have universe-sized effects. For example, a circle will satisfy circumference = diameter * pi until its diameter is greater than the span of the universe. Basically the universe could 'hide' the fact that it is a torus from pac-man by simply being really large.
cafebabe 14y ago
The problem is Time. You would have to save (or predict) the state of the universe for measuring the distance somehow. And the universe changes constantly over time. Lets say the universe evolves by the speed of light, that would make it impossible to measure it at all. This would mean to everything inside the universe, that it is infinite.

scott_s 14y ago

I think that confusing A, B or C with "the Universe, all of it" is hardly grounds to call someone an idiot.

goodside 14y ago
There isn't anything of importance in cosmology that's a clean multiple of 14.6 billion LY. Anything under discussion at that scale would by influenced by the Hubble expansion. But, yeah, being misinformed about cosmology doesn't make you an idiot. Hyperbole.