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  1. #1
    pe51c's Avatar
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    Default Understanding of space by amateur astronomer



    Consider the space between galaxies where we find close to true vacuum. Matter density is few atoms per cubic meter. There are also a few billion photons per second flying every which way in this one cubic meter volume.. Seems like a quiet party.

    Astronomy books and magazines teach us about space expansion, basically galaxies are receding from each other faster proportionally to the distance between them. Seems like a space is being stretched or expanded between galaxies so to speak rather than actual motion of galaxies.

    There are several ways to visualize it, one popular example is marker dots getting further away from each other on expanding balloon. Example is helpful but we cannot shake the idea of stretching. Instead of space stretching I prefer to understand it as gain in space volume.

    I like to visualize it as a 2D Flatlanders being pushed away from each other by somebody pouring water between them at multiple points. As the water enters Flatland, it looses one dimension (assumes 2D) and acts repulsively between Flatlanders.

    Is new space volume appearing from another dimension and what process is facilitating that?

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  3. #2
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    Quote Originally Posted by pe51c View Post
    There are several ways to visualize it, one popular example is marker dots getting further away from each other on expanding balloon. Example is helpful but we cannot shake the idea of stretching. Instead of space stretching I prefer to understand it as gain in space volume.

    I like to visualize it as a 2D Flatlanders being pushed away from each other by somebody pouring water between them at multiple points. As the water enters Flatland, it looses one dimension (assumes 2D) and acts repulsively between Flatlanders.

    Is new space volume appearing from another dimension and what process is facilitating that?
    No. The universe is not changing in number of dimensions. The balloon with dots is as good an analogy as any I ca think of. Space is basically stretching. If you'd like, I'll give you a mathematical description:

    Do you remember how to compute the distance between two points (x1, y1, z1) and (x2, y2, z2) in Eucledian space? The formula is:

    distance = sqrt( (x1-x2)^2 + (y1-y2)^2 + (z1-z2)^2 )

    The current cosmological model is that we can choose a coordinate system where each galaxy is fixed at a given coordinate (x,y,z). Here we are ignoring the local proper motions of the galaxies, which on a cosmological scale are not important anyway. Then the model is that the distance between two galaxies (1) and (2) changes with time:


    distance(t) = a(t) * sqrt( (x1-x2)^2 + (y1-y2)^2 + (z1-z2)^2 )

    Where "a(t)" is the universal scale parameter and it is an increasing function of time. The exact function depends on the physics of the universe. For example, in a radiation-dominated universe a(t) = sqrt(t). The exact formula for a(t) is not important. What is important is that it increases with time, so the distance between galaxies increases with time even though their coordinates do not change.

    This type of universe is a solution to the Einstein field equations under some assumptions such as the universe being homogeneous and isotropic ("isotropic" means that it looks the same in all directions).

    This equation is what astronomers mean when they talk about an expanding universe.

    Hope this helps.
    Astronomer. Lund Observatory

    Current project: Formation of terrestrial planets.
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    The astronomers at Caltech did a nice video about this--here:

    Ask an Astronomer Video Segment number 2009-001
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    Daniel C, Joe Lalumia


    what's confusing me is trying to combine idea of expansion with the ideas in the book I read recently: Lightness of Being by Frank Wilczek.

    Frank Wilczek on "Core" Physics: The Analytic Theory of Matter

    Wilczek explains space as multi layered, transparent,superconducting framework he calls The Grid. The Grid seem to permeate the whole of space or is integrated with space providing scaffold of reality.
    I just wonder if already existing space (including The Grid) is stretching or appearing new altogether ?


    BTW link didn't work but it's OK. I'll look it up on Youtube

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    What is important is that it increases with time, so the distance between galaxies increases with time even though their coordinates do not change.


    Daniel C,

    would you please elaborate on this????? this thick skull of mine just does not understand what this means. I try and try. it just does not compute.

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    I have heard of this person but I am not exactly sure what to make of the title of the book.

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    Quote Originally Posted by pe51c View Post
    Daniel C, Joe Lalumia


    what's confusing me is trying to combine idea of expansion with the ideas in the book I read recently: Lightness of Being by Frank Wilczek.

    Frank Wilczek on "Core" Physics: The Analytic Theory of Matter

    Wilczek explains space as multi layered, transparent,superconducting framework he calls The Grid. The Grid seem to permeate the whole of space or is integrated with space providing scaffold of reality.
    I just wonder if already existing space (including The Grid) is stretching or appearing new altogether ?


    BTW link didn't work but it's OK. I'll look it up on Youtube

    I have never heard of this theory. I have no idea what he means.
    Astronomer. Lund Observatory

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    Maybe the confusing part is that expansion does not jibe with what the book is advocating?

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    Quote Originally Posted by chas53 View Post
    What is important is that it increases with time, so the distance between galaxies increases with time even though their coordinates do not change.


    Daniel C,

    would you please elaborate on this????? this thick skull of mine just does not understand what this means. I try and try. it just does not compute.
    I think the balloon with dots is a very good analogy. The dots are not "moving" within the balloon, it is the balloon that is stretching. That's the best explanation I can think of. I know it is very weird, but it is also the truth. Space itself is what expands.

    However, this might help a little: You have heard of length contraction and time dilation. Observers moving at different speeds will measure time and length differently. You also know that gravity makes time dilate as well, and clearly, if the speed of light is going to be constant, then gravity must affect space correspondingly.

    If we have already accepted that gravity can stretch space, that the notion of "length" and "time" are flexible and can be altered by gravitational fields, then maybe it is not so weird that the space between galaxies might also stretch.
    Astronomer. Lund Observatory

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    Well, the guy is a Nobel Laureate, so at least we know he's not a moron.

    FRANK WILCZEK - MIT Department of Physics

    I still have never heard of his theory, so my instinct is to take it as some musings or hypothesis from a scientist. Scientists some times hypothesize and ponder, and that's fine. Roger Penrose has some strange ideas about the end of the universe looking like the Big Bang on a different scale, and Lee Smolin has an idea about black holes giving baby universes and universes evolving through darwinian evolution. These are all just interesting ideas. They are not scientific theories since there isn't currently any way to test them, some of those might one day become theories if someone develops them enough to make them testable. Strange hypothesis are fine in science as long as people know that that's what they are. I suspect that Wilczek's idea might fall into this category, but I cannot give an opinion because I don't know anything about his field.
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