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Olbers' Paradox, the CMB and the observable universe

  1. Dec 6, 2008 #1
    From Olbers' paradox, the night sky is dark because the universe is finite in age, so some stars' light hasn't had time to reach us. BUT, we see the CMB which occured some 13 billion years ago (14 billion maybe?) So, if light from the cmb has had time to reach us, shouldn't every object's light have had time to reach us? If all the stars we see in the night sky are within our galaxy, which is only 75000 lyr across, shouldn't we be able to see all the stars if we can see the cmb 13 billion lyr ago. I'm not considering dust extinction or anything like that that would prevent us from seeing the stars, just the light travel time and the observable universe.
     
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  3. Dec 6, 2008 #2

    marcus

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    Nice question. I have to go out briefly but will check back later.

    Olber says nightsky is dark because light from only a finite part of the universe is reaching us.

    (Finite age implies only a finite chunk of the universe has gotten its light to us)

    The matter that emitted the CMB is part of that finite chunk. There is stuff outside that, even, whose light has still not had time to get here.

    I may be able to say that more clearly later. If you have questions though, keep asking. Someone will step in. Keep questioning until you are sure you understand.
     
    Last edited: Dec 6, 2008
  4. Dec 6, 2008 #3

    Jonathan Scott

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    The sky is dark not just because the universe is finite in age, but also because the observable universe is finite in extent and hence the number of stars in the observable universe is finite. Also, the density of those stars is low enough that if you consider all possible viewing directions out to the limit of the observable universe, only a small fraction of them hit a star. (There are also other factors such as the fact that even if the universe were closed in a circular way, so that a line in a given direction would eventually loop right round, the finite age means that we can't see far enough into the past to see an infinite numbers of copies of the same stars).
     
  5. Dec 6, 2008 #4
    Ok, but if there are stars who are younger than the cmb, which I'm assuming all stars are, then shouldn't we be able to see all of them? Saying that there are objects whose light hasn't reached us means they are older than the cmb? Or, stated another way, can we see everything withing the observable universe or are there objects withing the observable universe whose light hasn't reached us?
     
  6. Dec 6, 2008 #5
    the observable universe is finite in extent, but we can see out to the cmb. Isn't that the very first few minutes after the big bang. Outward from the cmb (back in time) there was coupled light/matter so we wouldn't be able to see it anyway. But if we see the cmb, and the cmb defines the uncoupled universe, then we should be able to see the entire uncoupled universe? If so, how can we argue that there are objects whose light hasn't had time to reach us?
     
  7. Dec 6, 2008 #6

    marcus

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    Not older, just farther away (than the matter we are currently getting CMB from.)

    Astronomers should use more words and be more careful when they explain what they mean by the observable universe. They mean the stuff whose emitted light has already had time to reach us.

    Material outside that range, whose light hasn't yet reached us, might be potentially observable, but is not considered part of the observable universe. Just a definition issue.
    =========================

    Cosmogrl,
    I see you are getting excellent answers from Jonathan Scott. Our posts have been coinciding. If you have questions about what I've said, please quote me when you ask. Otherwise, since Jonanthan is responding, I'll drop out.

    If you want to read up on the CMB there's probably something useful at Einsten-online---URL in my sig.
     
    Last edited: Dec 6, 2008
  8. Dec 6, 2008 #7
     
  9. Dec 6, 2008 #8

    Jonathan Scott

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    The things we can't see are mainly because they are too young, not too old. We can't see stars which were created sufficiently recently at such a distance that the light hasn't reached us yet (and in some cosmological models might never reach us).
     
    Last edited: Dec 6, 2008
  10. Dec 6, 2008 #9
    Aha! That's it. Thank you so much. That's the bit of info my feable mind was missing!
     
  11. Dec 6, 2008 #10

    marcus

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    Good! We have closure, so to speak. Thanks Jonathan, from me as well.

    Cosmogrl, as long as your curiosity about Olber, CMB, and observable universe is satisfied would you mind if I continued the thread and said a few more things? it's your thread, so you are rightfully in charge.
    What I'd want to comment on is the question you raised here:
     
  12. Dec 6, 2008 #11
    Please do carry on. I always want to know more! I certainly still want to know more, always.
     
  13. Dec 6, 2008 #12

    russ_watters

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    Maybe I missed someone else posting it, but I believe the explanation is simpler than what people are saying.

    There is a certain density of stars and the expansion of the universe makes only some of them visible, which is why not every line of sight ends up on a star. Perhaps that density is .00000000000000000000000001% (made-up number), which makes for a very dark night sky. But the CMB is a remnant of the Big Bang and the surface of last scattering - a time when the universe was 100% opaque. So that energy has always permeated the entire universe.
     
  14. Dec 7, 2008 #13

    marcus

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    Cosmogrl, we could take a cue from Russ, who mentioned the surface of last scattering, and talk about that, if you want.

    It's an important visual concept, something to be able to picture in your mind that helps understand the CMB.

    You will have encountered the SoLS (temporary abbreviation) if you glanced at Wikipedia CMB
    http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation
    the SoLS is mentioned in 3rd paragraph of the CMB Features section. They put it in bold type to make it easy to spot.

    The SoLS is also mentioned in the Wikipedia Observable Universe in the Size section.
    http://en.wikipedia.org/wiki/Observable_universe#Size

    Wikipedia is not 100percent reliable as an authority (!) but it's fair to conclude that since these two articles highlight the SoLS idea it could be a useful thing to visualize. If you read the Wikipedia CMB, or the Features section, you will probably have questions, like e.g. how can it be 45 or 46 billion lightyears away, and we just now getting light emitted by the matter at the SoLS? Or something else. Ask if something puzzles you or if you question it.

    Since Russ mentioned it, he might want to elaborate. Or you may already have encountered the idea.

    ============================
    I have a brief comment on the SoLS too. I picture the matter out there as it is today. It has formed stars and galaxies and some of the stars have planets. And creatures that observe and ask, like us. And our matter was what was once hot and emitted what they receive as CMB. We are at their surface of last scattering, just like they are at ours.

    Everybody's matter was once hot and uniformly distributed like a glowing fog, just at that moment slowly becoming transparent as it cooled to below 3000 kelvin. So everybody's matter was the source of somebody else's CMB. It's only a question of getting the travel-time for the light right. The people 45 billion LY from us are the ones now receiving the glow from our patch of hot fog. And we are receiving theirs, because of how long it's been since the moment of clearing, when transparency occurred. Namely it has been 13.7 billion years, and therefore the present distance of separation has to be 45 or 46 billion LY just to get the timing right. That's my comment. Anything raise questions for you or is it clear? Or is it something trivial or that you already had gone thru?

    =====================

    Cosmogrl, my understanding is that the Olber dark sky business has now been adequately handled. Is that right? So we are now moving on to other questions your posts raised, about CMB in particular.
     
    Last edited: Dec 7, 2008
  15. Dec 7, 2008 #14

    russ_watters

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    Well.... early in it's life, the universe was completely filled with dense, hot plasma, like being inside the sun. As the universe expanded, it became transparent and that radiation was left zooming throughout the universe, today redshifted as the CMB. Even the point in space currently occupied by the tip of your nose emitted light 14 billion years ago. That light has had 14 billion years to fly across the universe....to a place where 14 billion years ago light was also emitted, which zoomed across the universe for 14 billion years to get to you today.
     
  16. Dec 7, 2008 #15

    Chronos

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    Neutrino telescope, anyone? They could peer beyond the surface of last scattering wall.
     
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