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Expansion of the Universe Question

  1. Jan 16, 2015 #1
    1: does the universe expand uniformly?
    2: does galaxies closer to the calculated center expand faster or slower then galaxies further away?

    im asking because im wondering on what drives the accelleration of the universe.

    there are many factors involved like dark matter, dark energy, photons, gravity etc that could factor in on this.
    if for instance you have the center "C" and a galaxy closer to the center "G1" and a galaxy further away "G2"

    Gravity: C ---- G1 ---- G2 if only gravity was at work then G1 would be accellerating faster due to the pull from G2.
    G2 would be dragged backwards due to the gravity of G1.

    Photons: C ---- G1---- G2 if only photons would be the source then G2 would accellerate faster while G1 would slow down.

    these are just 2 examples but im looking for the answer to question 1 and 2 to figure this out.
    identifying all possible energies working in concert to either pull or push the galaxies would be a massive undertaking, especially the math but it would also allow us to figure out if accelleration will continue indefinately or if it will stop? right or wrong?
  2. jcsd
  3. Jan 16, 2015 #2
    With photons i mean the effect of trillions upon trillions of photons hitting a galaxy from 1 side over the course of billions of years. surely it must have some effect.
    if our sun can accellerate a solar sail spacecraft in theory then the amount of photons hitting G2 from G1 over billions of years must surely accellerate it atleast a bit.
  4. Jan 16, 2015 #3


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    What reason do you have to think that there would be more stars one side of a given galaxy than another?
  5. Jan 16, 2015 #4


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    As far as is known, yes.

    What "calculated center" are you talking about?

    You seem to have the wrong idea about the expansion- it is not a matter of some force pushing or pulling galaxies apart, it is the space between galaxies that is expanding.
  6. Jan 16, 2015 #5


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    1) no, it expands according to the Hubble law - the farther the faster. (@HallsofIvy the OP seems to be asking whether velocities are uniform everywhere)
    2) there's no centre, apart from the perceived centre - every observer wherever he's standing in the universe will see himself as the "centre" with all galaxies receding away from him with increased velocity the farther they are.

    The various contributions to the expansion have been identified almost a century ago. Read up on FLRW metric - a solution of Einstein's General Relativity equations:

    Turns out pressure is actually contributing to deceleration. Read the "interpretation" section of the above link.
  7. Jan 16, 2015 #6


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    There is no such thing, any more than there is a "center" of an infinite plane or the surface of a sphere. The universe is either spatially infinite, or finite but unbounded (we're pretty sure it's the former, but the numbers still leave an outside chance of the latter). In either case it has no center.
  8. Jan 16, 2015 #7


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    orionis, I suggest you read the page linked to in my signature. It should clear up some of your misconceptions.
  9. Jan 17, 2015 #8


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    There have been some confusing replies here.

    No, the expansion isn't perfectly uniform. No, the expansion does not change with distance (it is recession velocity that changes with distance, not expansion).

    The expansion is a large-scale effect. On large scales on average, things get further away from one another. On small scales, they don't (e.g. the Earth is in a stable orbit around the Sun and isn't getting further away by any significant amount). When our universe has expanded by a factor of two, on average any two objects in our universe will be twice as far away from one another.
  10. Jan 17, 2015 #9
    im trying to understand how it works but im struggling, from what i have read of the responses here im thinking our universe is like an amoeba where the galaxies and nebulaes would be the equivallent of the foodstuff and other bits and pieces inside the amoeba, while the space between the galaxies would be the cytoplasm.

    basically as the cytoplasm grows the distance between the foodstuffs and other bits would get larger, as the space between galaxies gets larger as the universe grows.
    good analogy or horribly wrong?
  11. Jan 17, 2015 #10


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    What do you find lacking in the balloon analogy, as explained in my link? Why the need for a different one?
  12. Jan 17, 2015 #11
    well, the penny on the surface of the baloon to me suggest space is a flat surface being bulged by matter entering in the middle. it gives no room for depth between galaxies and suggest a vast volume of space where there is no matter whatsoever in the center.
    im looking for an analogy of how the universe actually behaves and not oversimplified example.

    a Quote from wikipedia
    "According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter is estimated to constitute 84.5% of the total matter in the Universe, while dark energy plus dark matter constitute 95.1% of the total content of the Universe."

    how does that fit the pennies analogy?
    maybe if the ballon was filled with gelatine and crumbs or somesuch.
    im bad at this i know but i dont really see the balloon analogy as a good one.
  13. Jan 17, 2015 #12


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    I take it then that you did not actually read the page I linked to. You should. It is not what you seem to think it is.
  14. Jan 17, 2015 #13
    sorry about that phinds, i did indeed not read everything. i was on my lunch break offshore when i read through the replies, only had 45 mins to eat and read so i took a shortcut and read the synapsis.
    wont happen again
  15. Jan 17, 2015 #14


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    Not terrible. The main problem with that analogy is that an amoeba is extremely non-uniform, while our universe is very uniform on large scales.

    A better 3-dimensional analogy that I've heard is a raisin cake rising in the oven: the raisins represent galaxies, and as it expands in all directions as it rises, the raisins within the cake are all getting further away from one another.
  16. Jan 17, 2015 #15
    I think it is worth emphasizing, that space is only part of the picture and story

    Whenever you hear SPACE ... Look for the TIME part of the tale

    Because space is only a 3D slice of ( curved ) SPACE-TIME

    Taking the two together, the actual fabric of SPACE AND ALSO TIME...

    Is NOT expanding ...

    As time passes, the quantum wave functions of all of our particles and atoms and molecules...

    Evolve into a larger SPACE SLICE of the fabric of SPACE AND ALSO TIME

    So as time passes our quantum wave function ACTORS wind up on a larger space STAGE ...

    And so wind up farther apart...

  17. Jan 17, 2015 #16
    the whole entire fabric of SPACE AND ALSO TIME is " not doing anything " for want of worthier words

    The expansion of space is us evolving forward in time, into larger space slices, of the fabric of SPACE AND TIME TOO

    I've tried to upload the best picture I could find
  18. Jan 17, 2015 #17
  19. Jan 17, 2015 #18
  20. Jan 27, 2015 #19

    You may find the below linked article both enjoyable and edifying:


    This is an article by Charles Lineweaver and Tamara Davis entitled _Misconceptions about the Big Bang_ that appeared in the March 2005 issue of Scientific American.

    It addresses your question, as well as others that may be burbling below the surface, and I suspect it will offer a pretty good answer to what is being modeled as well as providing one of the most illuminating descriptions of the _Big Bang_ model of cosmology offered to a general readership. It is not a polemical article, but rather outlines what the model in question does and does not express.

    Highly recommended reading all around, and may well go a good distance to answer many other of your, as yet, unasked questions as well.

    Last edited: Jan 27, 2015
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