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Expansion rate of the universe

  1. Apr 7, 2006 #1
    I am not an expert on this matter but it got really interested in the physics of the expanding universe, I believe that the universe is expanding. My question is how could science know if this expansion rate is increasing or decreasing, and how can science ever measure such a rate.
    Also I would like to know what science says about the idea of the universe contracting.
     
  2. jcsd
  3. Apr 7, 2006 #2

    SpaceTiger

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    Remember that the more distant the galaxy or quasar we're looking at, the further we're looking back in time. The recession rate of nearby galaxies give us an idea of the current expansion rate because we're not looking very far back in time (relative to the age of the universe) when we look at them. On the other hand, when I look at a galaxy with a redshift of 1 or 2, I'm looking at it when the universe was much younger and had a different expansion rate.


    The universe might contract in the future, but it's hard to say. The favorite model at the moment has it expanding indefinitely (at an ever increasing rate), but there are other models that fit the data which predict a coming contraction phase.
     
  4. Apr 7, 2006 #3
    Thanks spacetiger, im also wondering that when the expansion rate increases doesnt this mean that all time is actually faster, and isnt time related somehow to the expansion rate.
    I just need a little more info about how time is affected by the expansion and the rate of expansion and it would be great if you can direct me to some resources about these topics.
    I am also interested in the idea of time reversal as a result of universe contraction, and if there is any information out there about this topic it would be great.
     
  5. Apr 7, 2006 #4

    SpaceTiger

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    Cosmic time is defined by the clock of an observer at rest with respect to the Hubble flow or CMB. There is a relative time dilation between us and the distant galaxy (and it's considered when interpreting the observations), but we wouldn't experience time any differently now than 10 billion years ago.
     
  6. Apr 11, 2006 #5

    Chronos

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    General relativity is the short answer. Distant [high z] objects age more slowly than more nearby objects from our point of view. Sandard candles [i.e., supernova] is what convinced most scientists exansion started accelerating a while back. Figuring out why is a horse of a different color.
     
  7. Apr 12, 2006 #6
    Fine, but can one explain WHY is the universe EXPANDING?
     
  8. Apr 12, 2006 #7

    Chronos

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    When Einstein worked out his field equations, he realized his model of the universe was unstable. Thus, he added the 'cosmological constant' to force a steady state universe [which as all the rage in those days]. But he quickly realized this was an untenable solution. His steady state universe was literally balanced upon the point of his pencil. He cheerfully abandoned that idea when Hubble found evidence the universe was exanding. Although Hubble was never entirely satisfied with that conclusion, current evidence suggests he was correct.
     
  9. Apr 12, 2006 #8
    So, is it the cosmological constant the reason of expansion? If yes, what is than the cosmological constant?
     
  10. Apr 12, 2006 #9
    If I'm right, the cosmological constant was introduced to oppose the rate of expansion exactly opposite - so the universe was in this steady, non-expanding or contracting, state.

    I think what you are after is the Hubble constant, which is the rate the universe is expanding at. But I believe that is just a value and I'm not sure how or why the universe is expanding at such a speed.

    [Please don't be harsh if I'm wrong...I've only currently started to take up my own learning of this subject as my school doesn't tell me anything about it!]
     
  11. Apr 13, 2006 #10

    Chronos

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    Welcome to PF, TEOTW! I think you grasp the essentials. Einstein's motivation for inserting the CC was to explain why the universe does not collapse. Like Newton, he realized gravity posed a serious problem in an infinitely old universe.
     
  12. Apr 17, 2006 #11
    Could the Universe be expanding simply because it is in a giant vacuum? When the universe was an infinitely small point. It was in some sort of space. A giant void? Infinite void? So after the BB the Universe is steadily expanding to fill the vacuum of the void. So then wouldnt it expand faster as it went because as matter pulls apart there is less gravitational pull between the matter. So there becomes less and less force slowing down the expansion?
     
  13. Apr 17, 2006 #12
    Not quite. A vacuum would also be considered part of the universe. It is very ahrd to grasp logically, but essentially what happens is that it just expands, it doesn't expand into some space or into a void, it just expands.
     
  14. Apr 18, 2006 #13
    How do we know this? How do we know that what we consider to be the universe. Isnt just an explosion in a great void. The universe is steadily expanding and taking up more and more space (area) what space is it taking up?
     
  15. Apr 18, 2006 #14

    Garth

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    GR describes the expansion internally to the universe itself, within the fabric of space-tme. Empty space (the void?) itself expands. Objects within the universe move further apart, even if the universe is infinite in extent. We can say nothing about what happens beyond the universe, or though there are conjectures that other universes may exist.

    Garth
     
  16. Apr 18, 2006 #15
    The second law of thermodynamics says that a system will move from order to disorder. In the case of the expansion of the universe if a quantity of matter is surrounded by nothing it will expand to fill the void, unless it has sufficient gravity to haul itself into a single lump. Because we don't really know the size of the universe we cannot really ascertain if it will expand forever or collapse. The consensus is that there is not enough gravity to collapse the universe, but there are enough mysteries to be solved that there may yet be something we have not discovered that might totally change this view.
     
  17. Apr 18, 2006 #16

    Garth

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    Welcome to these Forums Tzemach!

    The second law certainly applies to a closed system, whether the whole universe constitutes such a system or not is debatable.

    The expansion of the universe may itself be increasing the total number of states available and thus explain where order has come from.

    The standard understanding is that the universe of matter is not increasing in a vacuum, but that rather the vacuum in which the matter is situated is increasing in volume.

    The latest results from WMAP3 suggest that, if anything, there is enough total energy-density in the universe for its gravitation to 'close' the universe. However, this requires 23% to be unknown Dark Matter and another 73% to be even more unknown Dark Energy.

    Whether the universe will recollapse or not depends not only on the density but also on how that DE behaves.

    Garth
     
    Last edited: Apr 19, 2006
  18. Apr 19, 2006 #17
    Does it depend on what someone considers the Universe to be?

    To make this less confusing I will make up a couple terms.

    SMALL UNIVERSE= All matter was in an infinitely small space then exploded and started expanding. So that the Universe is steadily getting bigger, as the explosion/matter expands. Basically the Small Universe is just the space that the explosion from the BB is taking up. There is nothing beyond that. This is what is generally concidered "The Universe" right?

    Big Universe= Or is "The Universe" the explosion and also everything beyond the space the explosion/matter is taking up? So that if we had the ability we could fly out of the explosion and beyond all matter. Into a great empty void that has no matter at all. Like as if the Small Universe was just a firecracker exploding inside a vacuum ( The Big Universe).

    How could we prove either one is true or not?

    To me it seems to be more likely that the Small Universe (what is I think generally concidered "The Universe") is just an explosion inside of a giant vacuum "The Big Universe". That would more easily explain why the matter from the BB is accelerating. It is expanding to fill the void. As it expands matter spreads out, thus having less and less gravitational attraction to other matter. So having less and less resistance to the expansion to fill the void/vacuum. So that would mean that the Universe will continue to expand forever.

    If the universe is just the area that the BB explosion is taking up( The Small Universe). Then you have to add in hypothetical things like dark matter to explain the accelerating expansion of the Universe.

    What do you think?
     
  19. Apr 19, 2006 #18

    Garth

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    The modern (post 1915) theory of cosmology is based on General Relativity. General Relativity describes how the space-time continuum itself is shaped by matter and energy.

    When applied to the cosmological solution in which the distribution of matter and energy is isotropic and homogeneous, GR predicts expanding or contracting space. Hubble's discovery of the (nearly) linear relationship between the distance and red shift of galaxies is accepted as evidence that the universe is actually expanding, thus confirming that prediction of GR.

    The theory predicts that, within a 'block' space-time, the hyper-surfaces of space expand as time progresses. This means that if we consider dust particles suspended in a vacuum, representative of the galaxies in the cosmos, then my statement
    means the dust particles all move away from each other, the whole volume expands over time.

    I hope this helps,

    Garth
     
    Last edited: Apr 19, 2006
  20. Apr 20, 2006 #19
    Another way to say my theory is that. Maybe the Universe always existed. Just all the matter in it was compressed into an infinitely or very small space. With the BB the matter exploded out, and continues to expand to fill the void it is in. So what is normally thought of as the Universe is just the area the matter from the BB is taking up. But really the Universe is much larger than that. We could see the distant matter at the edge of the explosion, but there would be nothing to see beyond that. Because beyond that is just an empty void. Is there any reason that this couldnt be true?
     
  21. Apr 20, 2006 #20

    Garth

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    We certainly cannot be sure about what we cannot see, lying beyond our particle horizon.

    I was just questionning your idea of the matter in the universe expanding into a void. If by void you mean empty space beyond the matter universe then I was pointing out in GR it is the space itself that expands and carries the matter with it.

    As far as we can see the universe is more or less isotropic and homogeneous. - No really big lumps!

    Garth
     
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