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A beginner's question re: size of universe

  1. Jun 9, 2009 #1
    Perhaps you'll find these questions child's play. My excuse is I have no training in the field - only curiosity. I suspect there is a single answer to both questions.

    Two questions:
    1. If, when we examine the distant edges of the universe, what we're really doing is looking back in time roughly 13.7 billion years then why has it taken 13.7 billion years for that light to reach us considering that at the moment that photon began its trip the universe was quite young and small?
    2. If, say, the most distant body in the universe from our current position is 13.7 billion light years away will the most distant body 1 billion years hence be 14.7 billion light years away? If so, does that imply the both the earth and that most distant body are traveling away from the center of the universe at 1/2 the speed of light?

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  3. Jun 9, 2009 #2


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    This is something to get straight. The presentday distance to the furthest material we can see is 45 billion lightyears. That is, if you could freeze expansion, light would take 45 billion years to get from that material to us.

    I am talking about the matter which is the source of microwave background radiation, the "oldest light" we can see. It has a redshift of 1090 (its wavelength has been extended by about that factor.)

    The travel time has been nearly the entire expansion age---13 some billion years.

    When the light from that material was emitted and started out on its way here, that material was about 41 million lightyears from us (or the material that eventually condensed to form us). If expansion could have been frozen at that point. The light would only have needed 41 million years to get here.

    Distance then: 41 million LY
    Distance now: 45 billion LY
    Travel time: 13-some billion years.

    You see expansion gets in there and confuses the relation between travel time and actual now distance. At first, when the light sets out on its way, expansion seems to make travel harder because it is extending the distance the light has to go. Later it seems to have facilitated travel because it increases the distance the light has already traveled.

    Because the spatial distance has expanded at different rates at different times in history, there is no regular relation between light travel time and actual distance. Maybe now the answers to your questions are obvious.

    I already explained, using the CMB (cosmic microwave background) light as an example.

    According to the standard cosmo model that nearly all professionals use, there is no "center of the universe". The main standard of motion and rest that people use is the CMB itself, according to which none of the galaxies are moving significantly, they have negligible individual motion compared with the rates that distances are expanding.

    To get the picture, google "wright balloon model" and watch it for a while. In that toy model of the universe all existence is concentrated on the 2D surface of an expanding sphere---there is no inside or outside surrounding--the whole world is 2D and on that surface. Galaxies are the white dots. They stay always at the same latitude and longitude, which means fixed location relative to CMB. They don't move although they do get farther apart. The other things, the little wigglers, do move. They are the photons of light. They change latitude and longitude. You can also see them gradually get their wavelength stretched out. You can watch the redshift happen to them.

    Ned Wright is a prof at UCLA who teaches cosmology. That one little computer animation movie of his is worth a lot of words.

    When all the distances are expanding, nobody is traveling because there is no destination you or anyone else is getting closer to. So expansion is not ordinary motion as we know it. It is geometry changing (according to the Einstein rules for geometry change that were set out in 1915, called gen. rel.) In a world governed by those rules you don't expect either geometry or distance to always remain fixed. Small percentage increases of largescale (extragalactic) distances occur in accordance with gen. rel.

    I'll google "wright balloon model" for you to make it easy:

    Remember that is a 2D toy model. The corresponding model of space would be a 3D hypersphere. The 3D analog. There would be no center of expansion in the 3D hypersphere, just as there is no center of expansion on the balloon surface. Or if you like to think of it that way, every galaxy is the center of expansion because all the others are getting farther from it. The center in the case of the toy model is not in the 2D universe. The 2D creatures imagined living on the balloon surface would have no notion of it. Likewise for us there is no center of expansion anywhere in 3D space. No direction we can point and say "it is over there". The existence of a center in higher dimension is entirely conjectural and supported by no empirical evidence, so it is a non-issue.

    The point should be made that both the actual distance and the light travel time are derived quantities which depend on the parameters of the model.
    The redshift is what one measures and then one appeals to a model (which has been fitted to other data) to calculate an estimate of the time the light has been traveling and the presentday distance to source.

    One is as "speculative" as the other, which is to say that neither presentday distance or light travel time are speculative, they simply are sensitive to the parameters of the model (like the value of the Hubble rate, something that much effort goes into getting right.)
    Last edited: Jun 9, 2009
  4. Jun 9, 2009 #3


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    Light travel time is just that . . . the time it took for photons to reach earth and be observed by our telescopes. Hence, the term 'observable universe'. How 'far' away these objects may be 'right now' is purely speculative [and unprovable] IMO.
  5. Jun 10, 2009 #4
    Marcus posted:

    This takes some time to absorb... and even more time to be able to explain in understandable language.

    In other words, the universe is about 1090 times bigger right now than when the oldest light was emitted.
  6. Jun 10, 2009 #5
    Ok, the stuff your posting right now is confusing me. Sorry to just jump in here but I didn't feel it would be necessary to create a new post.

    I understand that the light we observe right now isn't in that location because of many different reasons like expansion and gravity. These numbers 41 million LY and 45 million LY are confusing the heck out of me.

    Marcus, did you mean to say when the light began to travel here it was 13.7 million LY away and by the time it reached us the object was infact 41 million LY away? And now present day the object should be 45 million LY away.

    I don't see how when the object initially sent light to us it was 41 million LY away (which is what I'm getting from your post) if we can only see light that was 13 million LY old, that 41 million LY object would not have reached us yet... If my understanding is completely wrong please correct me. :)

  7. Jun 10, 2009 #6


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    The numbers are 41 million and 45 billion. The second number is about 1090 times larger.

    Your understanding is obstructed by your tendency to confuse million with billion and to confuse light travel time with actual distance (what is technically called "proper" distance)

    What I was describing was the example of CMB light. It has been traveling 13-some billion years, almost the whole expansion age of the universe.
    But you should avoid saying "13.7 billion light years". The 13-some billion year travel time is a time. A light year is a distance.

    Distance is related to travel time in a non-linear way so it is better to keep them separate. It will confuse you if you mix them up.

    When the material that emitted the CMB light emitted the light, that material was 41 million LY from here. That is a very small distance in universe terms. It is not even as far as the Virgo cluster (the nearest large cluster of galaxies).
    The actual distance to that same material is now 1090 times farther.
    It is now about 45 billion lightyears.

    And the light has taken 13-some billion years to get here.

    The reason 45 is bigger than 13 is the same reason why if you save money in the bank you end up with more money than you put in. Each year the light travels another lightyear of distance and as soon as it has done so that distance starts to expand like money you put in your savings account, by a certain percentage each year, like compound interest.

    Imagine measuring actual proper distance by freezing expansion and then seeing how long a flash of light would take to travel the distance. According to the standard cosmo model, if you could freeze expansion right now then it would take 45 billion years for light to travel between us and the material that emitted the CMB light that we are receiving. In other words that material's presentday distance from us is 45 billion lightyears.

    (There are other measures of distance like based on luminosity or brightness, and also on angular size. The most confusing of these other measures is, I think, the unfrozen light travel time. Because it mixes in the expansion effect in a rather obscure way :biggrin:)
    Last edited: Jun 10, 2009
  8. Jun 11, 2009 #7


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