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Is this a fair analogy?

  1. Oct 30, 2006 #1
    Its confusing to think that the Universe is 78 billion light years across considering that we think its only 13.7 billion light years old. So I tried to construct an analogy that I could tell to kids. Can you tell me if this analogy is ok?

    I'm in the back of a pickup truck facing backwards and you are 5 miles behind me standing still. My truck is moving away from you with every second that passes by, so after a while we are 6 miles, then 7 miles, 8 miles, 20 miles apart, etc. Think of my pickup truck as the edge of the Universe and you standing still, as the Earth. I have a gun aimed at you 20 miles away. As I fire the gun, the bullet takes a while to travel the 20 mile distance between us before it hits you. But when you feel the bullet hit you, my truck is now much further away than 20 miles because the whole time the bullet was travelling towards you, my truck was travelling away from you. Think of the bullet as light.

    So this means that even though here on Earth we see light that is 13.7 billion years old (really more like 13.4 billion years old, but we don't need to get into it), this light has been travelling towards us for 13.7 billion years, while AT THE SAME TIME, the Universe's fringes have been expanding away from us for 13.7 billion years. Well then, we would expect the Universe to have a radius of 13.7 billion + 13.7 billion years = 27.4 billion years. Multiply this by two to get the Universe's diameter of 54.8 billion years. 'BUT WAIT!' you say. I thought the Universe's diameter was 78 billion years across? Well, the Universe WOULD be 54.8 billion years in diameter, EXCEPT that its expansion rate is accelerating. AKA. The truck is accelerating faster and faster away from you.

    Astronomers have seen light from Quasars that are 13.4 billion light years away. The crazy thing is that the fringes of the Universe are expanding at a speed faster than the speed of light (its important to know that the fabric of the Universe itself can expand at speeds greater than light speed). THIS MEANS that every year Quasars we could see last year, suddenly blip out into darkness, WE CANNOT SEE THEM ANYMORE because they are receding from us faster than the speed of the light that they send at us.

    Just imagine if the truck sped away from you faster than the bullet I shot towards you, the bullet would never hit you!
  2. jcsd
  3. Oct 31, 2006 #2


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    Your analogy seams appropiate to me. It seams useful to visualize how objects can be farther away than the distance traveled by light that was emited by them. But...

    The universe is actually about 13.7 billion years old and about 92 billion light-years in diameter.

    This is incorrect. The fact that something emiting light now cannot be seen in future is related to the existence of a cosmological event horizon. Event horizons exist only in cosmological models with accelerated expansion of space. Superluminal expansion exists in every cosmological model with a Hubble law.
  4. Oct 31, 2006 #3
    Here is another analogy.

    Have as a large circlular sheet of latex with numerous ants crawling over it all with the same constant speed. Now have 20 people, evenly spread around, pull the latex sheet with an accelerating speed.

    Now think of the latex sheet as the expanding space and the ants as the speed of light. :smile:
  5. Oct 31, 2006 #4

    Ok, so its 92 billion across, gotcha. Have a good link for this?

    'This is incorrect. The fact that something emiting light now cannot be seen in future is related to the existence of a cosmological event horizon. Event horizons exist only in cosmological models with accelerated expansion of space. Superluminal expansion exists in every cosmological model with a Hubble law.'

    I'm not sure you are talking about what I was talking about. I'm saying that light from a quasar 13.4 billion ly away, that was emitted 13.4 billion years ago, will reach our eyes today. But tommorrow, any light that this quasar happened to emit 13.4 billion years ago plus a day, will never reach us because this quasar is receding from us at superluminal speeds.
  6. Oct 31, 2006 #5
    Note that it is not necessary for two objects to separate at superluminal speeds for a light signals to never reach the other objects.

    If both accelerate away from each other at a constant or increasing rate and have a certain distance between them they will never see the other object's light signals.
  7. Oct 31, 2006 #6

    Yes, I know. Can you speak to my last question to hellfire Jen?
  8. Nov 1, 2006 #7


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    Ned Wright's cosmological calculator, or mine (see my signature).

    You are right that it will not. But this is not only due to the fact that it recedes with the speed of light, because in cosmological models with decelerated or constant expansion the light of objects receding at speeds greater than c would indeed reach us. In our universe there is a cosmological event horizon that is more or less located at the Hubble sphere (where objects recede at the speed of light) and this is the reason for the emitted light to not reach us.
    Last edited: Nov 1, 2006
  9. Nov 1, 2006 #8


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    Once the light of an object 'touches' us, it never lets go. Remote objects merely redshift [and time dilate] into oblivion as the universe expands . . . at least until they run out of fuel.
  10. Nov 1, 2006 #9
    No. It wasn't 13.7 billion light years away 13.7 billion years ago. In fact, the galaxy whose light takes 13.7 billion light years to get here was actually closer to us than 13.7 billion light years at the beginning:


    See "angular diameter distance" and how it relates to the past distance of galaxies (i.e. at the time when the light was emitted).
  11. Nov 2, 2006 #10
    I urge you to reconsider what you have just said.
  12. Nov 4, 2006 #11
    Space expands between then an now (according to the big bang). Galaxies that used to be a few billion light years away are now much further (if they still exist). A galaxy that used to be 2 billion light years away can be much further than 5 billion away. See the chart below where is says [itex]D_A[/itex]. That's the distance the light was away at the time it was emitted.

    The distance between the galaxies now > The distance light travels > The original distance between the galaxies.


    Attached Files:

    Last edited: Nov 4, 2006
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