Is the universe bounded?

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Solution 1

"It seems the observable universe is limited by our event horizon at some 14 billion light years"

The farthest objects whose light reaches us today are some 46 billion lightyears away (the particle horizon). The event horizon only tells us the maximum distance from where light that is emitted today will be able to reach us in the infinite future. But the term "observable universe" is reserved for everything inside the particle horizon.

"If an astronomer was placed at one of the outermost visible objects would he be looking at a nearly dark sky in a direction away from earth but a star filled sky in the direction of the earth or would he see a more or less evenly lit sky as on earth?"

We assume that the universe is homogenous and isotropic, so it should roughly look the same from everywhere.

"If the latter is most likely does it not imply an infinite/unbounded universe?"

That is what we are assuming when we say "the universe is flat", "the curvature is zero" or "the total energy density equals the critical density".

Solution 2

We do know 'where' the big bang happened. It happened everywhere.

Imagine you are inside a big inflated balloon. Now ask where that balloon inflated from. The answer is, it inflated from everywhere.

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Jens
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Jens

Updated on June 14, 2022

Comments

  • Jens
    Jens less than a minute

    As I understand it nobody can pinpoint an objective "center" of the universe nor "where" the Big Bang happened. It seems the observable universe is limited by our event horizon at some 14 billion light years and my question is simply: If an astronomer was placed at one of the outermost visible objects would he be looking at a nearly dark sky in a direction away from earth but a star filled sky in the direction of the earth or would he see a more or less evenly lit sky as on earth? If the latter is most likely does it not imply an infinite/unbounded universe?

    • CuriousOne
      CuriousOne over 6 years
      We can tell you where the big bang happened: everywhere. An astronomer at the "outermost" part of the universe would be living 13.8 billion years ago. What we are "seeing" is not just distance, but the past. We can't tell "what's there today" (that's not even a statement that makes a lot of sense, to begin with). Based on the homogeneity that we are seeing around here we can assume that astronomers everywhere are seeing roughly the same things, but that's an assumption of the theory and not something one can confirm experimentally.
    • Qmechanic
      Qmechanic over 6 years
      Related: physics.stackexchange.com/q/24017/2451 and links therein. See also Olbers' paradox.
    • John Duffield
      John Duffield over 6 years
      +1 to CuriousOne for pointing out that the homogeneous isotropic universe is an assumption rather than experimental fact.
  • Jens
    Jens over 6 years
    My point is that lf the distant astronomer sees his observable universe similar to ours he could repeat the thought experiment of placing a third astronomer at the edge of "his" universe. Your answer implies that this third astronomer would probably see a similar unjverse and so on and so on ad infinitum - even if the "edge placement" was made in random directions.
  • CR Drost
    CR Drost over 6 years
    @Jens: For a while this would definitely work. Are you objecting that after billions of billions of repetitions maybe someone would be past the edge of space? That's a fair point but it's hardly germane to anything we care about here and now, no? As long as space is much bigger than the observable universe it does not matter much.
  • Jens
    Jens over 6 years
    Maybe I use "event horizon" incorrectly. So lets place the astronomer 46 billion light years away and continue the thought process. Eventually we should then end up having covered "all" space.
  • Jens
    Jens over 6 years
    By the way how can light from the furthest objects hsve reached us if they are 46 billion light years away and the Big Bang created them 14 billion years ago? Or is some kind of time dilation involved?
  • Yukterez
    Yukterez over 6 years
    1) no, still homogenous and isotrope. 2) space expands while light travels. while the light is on the way new space is created between the emitter and the signal so it is only natural to end up with more lightyears than years. en.wikipedia.org/wiki/Particle_horizon
  • Thriveth
    Thriveth over 6 years
    A Universe can be finite but unbounded. The surface of the Earth is finite but unbounded.
  • Danu
    Danu about 6 years
    "You cannot make some assumption that the universe is homogeneous and isotropic and then declare that it must therefore look the same to that distant observer. It just isn't scientific to make such claims." Indeed, which is why that doesn't happen. You're referring to the FLRW metric, and fail (with obvious intentions to misconstrue the scientific process that occurred) to mention that what people really did was: Assume the universe is homogeneous and isotropic (spatially), derive consequences of this assumption, and see if the predictions match experiment, which they do.