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Expansion of the Universe

  1. Jul 21, 2008 #1
    How can the fabric of spacetime expand at a certain acceleration? Isn't speed the rate of space divided by time itself? How can you measure spacetime only using variables that are dependant on spacetime? It's like trying to measure the length of a stick using a stick."Kind of like saying an eye to see. The eye can't see unless it has a pair of eyes to see itself because it's an observer obtaining and using the eye that is doing the seeing, not the eye itself." But what really is expanding? The space in and between objects or everything in reality itself? And expanding relative to what?
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  3. Jul 21, 2008 #2
    Spacetime does certainly not expand! In certain general relativity models spatial distances between objects, as measured using a specific coordinate chart, increase which is often called expansion of space.
  4. Jul 21, 2008 #3


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    It's the space between galaxies that's expanding relative to e.g. those sticks you mentioned. Sticks don't expand. They stay the same length in local inertial frames, and that means that the proper lengths between their endpoints are constant.

    Galaxies on the other hand stay at constant spatial coordinates in the specific coordinate system that MeJennifer mentioned. That, combined with the properties of that coordinate system, implies that the proper distances between galaxies are increasing.

    Galaxies aren't expanding either. Well, they are, but at a rate that's much less than the cosmological expansion. That's why the expansion is often explained by suggesting that you imagine coins glued to a ballon (the coins representing galaxies). The reason why galaxies don't expand is that the large-scale expansion is a mathematical consequence of homogeneity and isotropy of space, and space isn't homogeneous or isotropic at the scale of galaxies.
  5. Jul 21, 2008 #4
    G'day from the land of ozzzzzzzz

    Fredrik said

    What you say and what they said is disputed. To begin with, space and time cannot change, expanded ofr contracted. The only parts that can do that is matter in its varies phases.

    Galaxies tend to cluster and merge, observations of thousands of galaxies indicate this.

    I think many people are lost in that ballon. Too much maths and not enough observation would put anybody off track.

    You are right galaxies do not expand, once sec. How did they form? They must have expanded to that form and via observation most contract back to the centre, where black holes eject matter via jets and reform via expansion of matter.
  6. Jul 21, 2008 #5


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    I don't understand the first sentence. I'm not sure what you mean by the second either. If you mean that spacetime doesn't change, then I agree, but it's definitely the case that if we slice up spacetime into a one-parameter family of (homogeneous and isotropic) spacelike hypersurfaces that can represent space at different times, those hypersurfaces are going to be different from each other. (They all contain a factor that's a fuction of the parameter that labels the different hypersurfaces). So "space" is changing with time.

    That's true, but the only relevant consequence of that (in this discussion) is that we have to look at even larger scales to see why it makes sense to describe the universe as homogeneous and isotropic.

    Maybe, but it's impossible to understand the expansion without understanding FLRW solutions. A FLRW solution is what you get if you assume that space is homogeneous and isotropic. There are three of them, and they all describe an expanding universe.

    Inhomogeneities in the initial matter distribution caused the formation of stars and galaxies. In other words, small inhomogeneities led to much bigger inhomogeneities, and as the inhomogeneities grew larger, those regions of space looked less and less like a region of "space" in a FLRW spacetime. That's why they don't expand at the rate of the cosmological expansion.
  7. Jul 22, 2008 #6
    G'day from the land of ozzzzz

    Rather than trying to explain the non-expanding universe.

    It is better to try to understand the formation of the stars and their varies changes.

    Than trying to understand the evolution of the different forms of galaxies.

    By doing that, by observation one can deduce the workings of the ongoing universe.

    Lets look at the link:

    Hubble sees galaxies galore
    http://www.spacetelescope.org/images/html/heic0406a.html [Broken]

    What do you think of the information?
    Last edited by a moderator: May 3, 2017
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