Size of the observable Universe

Discussion in 'Science' started by WarpSpider74, Jan 21, 2009.

  1. WarpSpider74

    WarpSpider74 Member

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    I'm wondering if someone can help me with a problem I'm having conceptualising the universe. Before I start; firstly my scientific background is as a biologist, not a physicist - my training in physics is fairly low so I may be missing something in my general knowledge base that a lot of you guys may take for granted. Secondly, I'm an amateur astronomer.

    Now, the problem. I was watching some crappy doco on cable in my hotel room in Kalgoorlie that was about the size of the universe. Lots of Sagan references. Lots of repetition of the famous Hubble Ultra Deep Field image. Lots of numbers too big for normal human comprehension. You probably know the type of doco.

    Anyways, a couple of numbers were postulated that just don't add up to me, they're both numbers I'm familiar with, but their relation (or lack thereof) hasn't twigged with me before. Firstly, the age of the universe: commonly quoted as 13.7 billion years or thereabouts. No problem. Next, the size of the observable universe: they quoted in the region of 45 billion lightyears. Again, seems plausible from observable evidence, I guess.

    Now, how can the universe be so far from edge to edge when the maximum velocity anything (with mass) can travel at is the speed of light? My logic is trying to tell me that with an age of 13.7 billion years, and assuming a start from a singularity and even expansion in all directions, the maximum size of the universe is 27.4 billion lightyears. How can the expansion be faster than the speed of light?
     
  2. Ashpool

    Ashpool Member

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    Ripped from teh wiki
    http://en.wikipedia.org/wiki/Observable_universe
     
  3. Arbite

    Arbite Member

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    my brain :wired:
     
  4. Whisper

    Whisper Member

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    I wish somebody could explain what the hell all this means to us simple minded people. :o
     
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    WarpSpider74

    WarpSpider74 Member

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    And there lies my problem. I kinda expected the "space itself is expanding" explanation, as it's been put to me before in a different argument. But how do we measure that? Or is it just a convenient "let's sweep all the logic under the carpet" argument?

    My head struggles with this one.
     
  6. LeadAccelerator

    LeadAccelerator Member

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    "In electro-kinetic theory, space expands to accommodate the time necessary to encompass it's dimensions"
    From the original Doctor Who movie (before the first series in the 1960's).
    An explanation of how the TARDIS is bigger on the inside than the outside.

    Maybe it actually has relevance to the universe?
     
  7. eXponential

    eXponential Member

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    Nothing is swept under the carpet. Evidence for the expanding universe is based heavily of the redshift of objects.

    If we measure the emission spectrum of, say, hydrogen in the lab we'll get a characteristic emission wavelength. If you look at a star 10 light years away, the wavelength will have shifted slightly. At 1 billion light years it will have shifted even further. The further away an object is, the greater the redshift. This can only be explained if the universe is expanding.
     
  8. hlokk

    hlokk Member

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    Imagine the furtherest object we can see. The light travels around 13.7 billion light years, but the universe is expanding, so during those 13.7 billion light years, that object has been moving away from us. So when the light finally gets to us, the actual distance to that object is 46.5 billion years (in technical terms, this is called the comoving distance)


    As for stuff expanding faster than the speed of light:
    The big bang was not an explosion. Matter is not flying through space from an initial starting point. Rather it is the space itself that is expanding.

    Imagine our universe as a balloon that is being expanded. Now, we have two points on the balloon. Say we communicate by sending ants with messages. If the balloon wasnt expanding, we could send ant messages anywhere. Now, if the balloons expanding, as that ant walks between places, the ground is stretching underneath it. If two places were far enough away, the ant could never get there as the ground would be stretching too much under its feet (though the ant will keep walking). The limit where the ants can just keep up would be the equivalent of the observable universe.

    To explain that simpler, imagine a rubber band (roughtly 1D object). If i expand it at a certain speed, two points close together will only move slowly apart, but two points further apart will move faster relative to each other. The local stretching doesnt disobey a maximum 'speed limit', but if you went some certain speed, because the rubber band was being stretched underneath, you wouldnt ever get to a point really far away (e.g. 10% an hour, for a point 10mm away would mean a new dist of 11mm, but at 1km, theres an extra 100m. If you're travelling 100m/h, you'll never get to that point)


    edit: changed one wrong 13.7 to the correct 46.5
     
    Last edited: Jan 22, 2009
  9. Maldark

    Maldark Member

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    Very well explained, I tip my hat to you good sir.
     
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    WarpSpider74

    WarpSpider74 Member

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    OK, thanks hlokk, I think I can wrap my head around that example.

    But, if space itself is expanding, and all matter occupies space, then isn't the space occupied by matter also expanding? In simple terms, if space is expanding then everything in the universe is expanding at the same rate with it, so all measurements remain the same?

    Using your rubber band example, two penpoints on a rubber band, when the band stretches, the area occupied by the inkspots also stretch, and the relative sizes remain the same.

    I was being facetious. Sorry.
     
  11. hlokk

    hlokk Member

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    No, because the matter is held together by forces (e.g. gravity or the nuclear forces for atoms). So for galaxies for example, they stay about the same size as the gravity keeps them together. The way to think of this with the balloon example, is to think of universes as some object on the balloon. Perhaps some kind of insect, or a sticker. I.e. their own internal forces overcome the expanding spacetime.

    Though, even if everything was expanding the same, it would still be measurable as objects further away would move away at a faster rate. On a local scale however, it might be harder. But because the atomic force or gravity doesnt change as space expands, an atom say will have the same diameter.
     
  12. daztay

    daztay Member

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    That bit requires a triple aspro to stop brain expanding or exploding, would be more correct.

    Expanding into what I suppose really hurts the head?
     
  13. Zylatis

    Zylatis Member

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    Well it doesnt have to expand into anything, by current definition (for mainstream theories) the universe includes everything in existence. There is no analog really to explain whats going on (unless it IS expanding into something, a possibility) because the human sphere of experience doesnt encompass such things. A similar example is the idea of a point particle which occupys no volume.
     
  14. hlokk

    hlokk Member

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    Its not expanding into anything. To expand into something would require spacetime outside our universe, but because our universe is spacetime, the concept is meaningless.

    Imagine it like a videogame. All objects move away from each other. The space is expanding, but its not expanding into anything. The coordinate system is just getting stretched. Our observable universe would be a sphere with the camera at the centre (say a fog or a pop-up distance). If we create any sphere, the volume will expand, but the boundary will stay the same relative to objects. I.e. if you were outside that sphere, then that sphere wouldnt start enroaching into your space. It would actually get further away. So the space isnt moving into something else.
     
  15. Whisper

    Whisper Member

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    Maybe if you could explain how we get from 13.7 billion light years to an actual distance to that object of 46.5 billion years, I would understand.
     
  16. hlokk

    hlokk Member

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    Do you mean how we calculate it, or why the distance is much further:

    I'll try and answer both

    Imagine the rubber band being stretched again. We can only communicate by ant messages. Two points close together and the ant doesnt cover much more ground than if there was no stretching. If they are far away, there ant takes a lot more extra time (because theres more total amount of stretch as there has been more time). But while that ant is moving, whoever sent it is still moving away further.
    When they refer to 46.5 billion light years as the distance, that is the comoving distance. Basically if you were to freeze the universe, or you had a measuring tape that the end could move instantly, thats what you would measure.


    Say a close by ant takes 5 minutes, during that time, the rubber band has been stretched, so now that point is further away than when the ant started. Lets say 6m away.
    If the ant took 13.7 minutes to walk from point A to us, then during those 13.7 minutes, point A was moving away from us (because it was being stretched). So, in 13.7 minutes, point A has moved to 46.5 m away. If we were to freeze the band, it would now take the ant 46.5 minutes.

    Also, if the ant took 13.7 minutes, and the band was stretching, point A must have been closer than 13.7m at t=0 minutes.



    Theres an example and animation here
     
  17. daztay

    daztay Member

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    Thanks for the explanation Hlokk.
    The trouble is my view of nothing is only 3D not 4D or should it be 10D:D
     
  18. Zylatis

    Zylatis Member

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    4D is fine for now =) String theory requires 11 but im not quite sold on that yet.
     
  19. Whisper

    Whisper Member

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    Thanks a lot
     
  20. daztay

    daztay Member

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    suppose we wait till the LHC produces results
     

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