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Thread: Expansion theory/space-time

  1. #1
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    Expansion theory/space-time

    Well I was going to post this as a reply to a thread but it was slightly off topic so here is my own thread

    My question has to do with someones post kind of wondering what space-time was and someone responded that space-time was really a mathematical representation of the universe and is not a physical entity. When I read the reply it made me think of the expansion theory that states that space-time expanded at a rate faster then the speed of light shortly after the big bang.

    My question is if space-time is only a mathematical representation of the universe and not it's own entity how can we state that it's expanding?

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  3. #3
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    Quote Originally Posted by Opaqueuphony View Post

    My question is if space-time is only a mathematical representation of the universe and not it's own entity how can we state that it's expanding?
    That's an important question, and the answer is basically that stating that "space itself" expands as the universe ages is actually not a physical assertion by some theory, it is a picture of how to imagine the physical assertions of some theory (the Big Bang theory, using general relativity). In other words, it is not intended to be taken as literally or as seriously as many people take it. Had the theory of general relativity afforded space with physical properties (which it does not), then we could say we have a "theory of space" that asserts "space expands". But space is just a way to talk about distance, and distance is just a coordinate for talking about invariant separation. However, the cosmological principle suggests such an obvious coordinatization, one in which distances increase with time, that it is quite natural to rely so heavily on those coordinates that we say space is expanding.

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    Quote Originally Posted by Opaqueuphony View Post
    My question has to do with someones post kind of wondering what space-time was and someone responded that space-time was really a mathematical representation of the universe and is not a physical entity. When I read the reply it made me think of the expansion theory that states that space-time expanded at a rate faster then the speed of light shortly after the big bang.

    My question is if space-time is only a mathematical representation of the universe and not it's own entity how can we state that it's expanding?
    Welcome, Opaqueuphony.
    The maths we use to describe the Universe involves a matrix of coordinates which expands, with stuff distributed "at rest" within that expanding matrix. Things move further apart; the "expanding space" comes from the maths of general relativity.

    You might be interested in a couple of points arising from your question:
    1) When we talk about the expansion of the Universe, it's "space" that expands, not "spacetime": the maths describe a change in space which develops over time.
    2) Space is still expanding faster than the speed of light. We can see objects which were moving away from us faster than the speed of light when they emitted the light we're currently seeing, and which are still moving away from us faster than light at the moment (although we need to be careful how we define "at the moment" with regard to the whole Universe).
    For more on point two, you could take a look at Lineweaver and Davis's Scientific American article, Misconceptions About The Big Bang (440KB pdf).

    Grant Hutchison

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    Indeed, the "space expands" picture is so useful that you can literally imagine that the universe is a giant rubber chess board. It then has two spatial dimensions instead of three, but that's easier to picture anyway. Then imagine that there are chess pieces on the board, and they are the gravitationally bound objects (like galaxies and even clusters of galaxies). Then imagine that there are ants crawling on the board at a constant speed in a straight line-- they are the photons. Now imagine the rubber board is being stretched, at a rate that is faster the farther you go from the center (every point on the board can imagine it is the center, as we have never seen any edges and there is nothing external to the board to use as a reference). There is also no limit to the stretching of the board that has anything to do with the speed that the ants can crawl (the latter being the speed of light). Finally, imagine that a chess game is being played on this board, which obeys dynamical laws like the pieces can never move from square to square any faster than the ants can walk, and if the pieces are just stretching with the board, they are not being moved in the chess game at all.

    Now, you have a picture that is essentially exactly the same as the Big Bang model. But the board is something physically real, with its own properties-- space does not have properties like that, so the picture is just a useful analogy, it should not be taken to mean there is literally a chess board like that. Unlike with the board, the "squares" are not actually drawn, they are just in our minds-- a coordinate choice.

  6. #6
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    Thanks guys, I think what was throwing me off was the word expands. I was thinking of the meaning of the word and how it typically applies to a physical entity but I think what is happening is we use the word to explain the expansion of the space because we really don't have any other word that describes the theory since the English language wasn't ready for non physical entities "expanding"

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    I think the 2D rubber sheet analogy confuses a lot of people. We should refrain from using it altogther. An inflatable Nerf basketball, though, works nicely.

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    So when we say that, space is expanding, really we are just talking about a mathematical model. In a physical sense, we don't really no what space is. Is that correct ?
    Far away is close at hand in images of elsewhere...

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