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Quoting Stephen Hawking:

  1. Jul 16, 2006 #1
    Quoting Stephen Hawking: "....

    In "Universe in a Nutshell", if in english sounds like that, Hawking said:

    "universe cannot be infinite because...bla bla...if it would be, every view line will approach to a star, and the sky would appear to us brighter than the sun; obviusly this does not agree with our experience."

    but in a infinite universe the light of the most far stars would not have the time to reach us, so all will appear as it appears now. or not?
     
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  3. Jul 16, 2006 #2

    Labguy

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    Do a search on "Olber's Paradox", on PF or Google.
     
  4. Jul 16, 2006 #3

    marcus

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    hello Labguy,
    glad to see you.
    I thought you were going to Arizona but it looks like the weapons-ranging business has taken you to Iowa instead. hope the nights are clear and seeing conditions good.

    Let's see if we can help this guy Born2. His problem seems to be with Hawking. It sounds like Hawking said something wrong, or else confusing.

    that wouldn't be so surprising. books for popular audience can be have mistakes or be unclear in places, no matter who writes them

    and Hawking might actually have said something else, he might have meant IF THE UNIVERSE WERE SPATIALLY INFINITE AND ALSO wasnt expanding and WENT INFINITELY FAR BACK IN TIME

    I don't like Hawking's popular writing from what I've sampled, so I avoid his books. But maybe someone has seen this Nutshell book and can explain what has confused Born2.
    ====================

    Born2,
    basically what Labguy says is right. Do a google on Olber's paradox. whatever you learn is probably right and Hawking must have said something unclearly.
    the universe does NOT have to be spatially finite. It CAN be spatially infinite as long as you make other assumptions that would allow the light not to have reached us yet

    or other assumptions that make it so that a lot of the light that reaches us is so stretched out (by expansion) that it is redshifted into total harmlessness. stretching out light, to have very long wavelength, deprives it of its energy, so it cannot dazzle or cook you.

    there IS light coming from all directions in the sky, but it is harmless: it is called the CMB. if you could see CMB waves, then the night sky would NOT be dark.
     
  5. Jul 16, 2006 #4

    SpaceTiger

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    For some subjects, I would agree with this, but Olber's paradox is very often mistreated on the web. One of the most common solutions you'll see is "dust obscuration". However, even if we took all of the dust and absorbing gas away, the night sky would still not be uniformly bright. Furthermore, in an infinitely old and infinitely large universe, the dust would eventually reach equilibrium with the background radiation and begin shining with the rest of the night sky.

    The best explanation, I think, is the first one you gave -- the universe is not infinitely old, so only so much light could have reached us since the big bang.
     
  6. Jul 16, 2006 #5

    marcus

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    Yeah, come to think of it that is right. In fact I suspect unselective googling is always something of gamble! It is good to be warned in general (and SpaceTiger says especially about web treatment of Olber's paradox in particular).
    Wikipedia is good about a lot of things, but even that is not infallible.
     
  7. Aug 3, 2006 #6
    I think the reason that the sky appears dark to us at night, rather than lit up like the day time, is because of the Big Bang. Matter is so sparsely distributed in the universe that we just don't see a lot of the light coming from the stars. It is, as was previously stated, stretched out so much and cooled down so much, that it appears only as faint cosmic rays. Possibly tachyons?:bugeye:
    CMB is also given as evidence of the Big Bang. With minor fluctuations, the temperature of space everywhere is roughly 4 deg K.
     
  8. Aug 4, 2006 #7
    *Picks up handy-dandy copy of "The Universe in a Nutshell"*

    I knew there was a reason I kept this thing on my bookshelf (and not just for the pretty pictures).

    Marcus and SpaceTiger have it exactly right. What Hawking was saying here is that the fact that the sky is not uniformly bright tells us that it can't have existed for an infinite period of time. In fact, this is how he introduces his section on the expanding universe.
     
  9. Aug 4, 2006 #8

    SpaceTiger

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    baryon's answer would seem to be relevant as well. After all, there was a time (before recombination) when the night sky would have been uniformly bright. The reasons for this, however, are unrelated to what Olbers was puzzling about. He was more concerned with question of how, given an infinite and eternal universe, the current night sky was not uniformly bright. It doesn't matter how diffusely matter is dispersed in the universe, we still expect a uniformly bright sky in an infinite and eternal universe. Nowadays, although it may still be infinite in size, we know it is not eternal, so there is no longer a paradox.
     
  10. Aug 4, 2006 #9

    marcus

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    Personally I don't believe I know this. I think that eternal is a reasonable possibility that cannot, as yet, be excluded on experimental grounds.

    You may have meant something different, that I am missing. :smile:
     
  11. Aug 4, 2006 #10

    SpaceTiger

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    The universe as concerns Olbers' Paradox certainly isn't eternal. Light was not free to travel before recombination and all other particles are limited by the end of inflation. This is the sense in which I meant it.

    Obviously, there are models of the early universe that are eternal (e.g. eternal inflation, cyclic universe), but this is well beyond the scope of the thread. Perhaps you'd like to discuss it in another one.
     
  12. Aug 4, 2006 #11
    If all space were expanding, then there is more of it to come. And if there is room for more, then it cannot be infinite in size. If it is not infinite in size and has always been expanding, then it must have had a beginning. That beginning may have been with exponentially smaller size with time. It may be infinitely old, but only with ever smaller size. Right?
     
  13. Aug 4, 2006 #12
     
  14. Aug 4, 2006 #13

    marcus

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    Good! LIGHT is not eternal and PARTICLES are not eternal. (regardless of whether spacetime is or not.)

    All the light coming to us had a beginning a finite time ago and all the particles we are made of and encounter began a finite time ago.

    This is a good point to make.

    Actually you know one of the useful things about this kind of discussion is just refining the language one uses----finding clearer more precise ways to say things. Sometimes we can have the same mental picture and use different words, so that there is an illusion of disagreement.

    You mentioned various models where the universe past is not obviously bounded----the past extends back indefinitely. I would say that Loop Cosmology bounce scenarios are an example. You mentioned some others.

    An important point to make about these (possibly "eternal" but in any case) unbounded-past models is that regardless of whether or not the universe is eternal, the light and particles we see only go back so far. This is what i get from your post, and I agree.:smile:
     
    Last edited: Aug 4, 2006
  15. Aug 5, 2006 #14
    Hello All

    The Universe is endless and ageless. If you exposed your eyes for 1 year the amount of stars that you would see would be blinding.

    I think some of you above need to read up on alot of things.

    The nutshell size is determined by the knowledge of information.

    As for the Big Bang, what is it that holds you to it.
    Look at this simple calculation.

    We see 13.2 billion light years deep field in one direction and 13.2 billion in the opposite direction and we see existing galaxies over 10 billion years old.. This gives us 26.4 lbillion light years travel and 10 billion years ,all up 36.4 billion years. We have the Big Bangers tell us that the universe is 13.7 Billion years old. The BB's will use fantasy and have matter travelling at 100 times the speed of light just to make their model correct.

    I'm just sick and tied of being sick and tied of a model that has become a standard model and for people to have followed such a fantasy. This will be the greatest laughter of the 20th century.

    I can give you a list of hundreds of cosmologists and scientists who think the same.

    If someone can give me actual evidence for the BBT without fantasy i would be very happy.
     
    Last edited: Aug 5, 2006
  16. Aug 5, 2006 #15
    Blinding, perhaps; but, the sky would still not be truly uniformly bright in all directions. If it were, exposure time would be irrelevant.

    The observational evidence, I should think. Let's start with the fact that almost every single galaxy in the universe is moving away from us. Add in the cosmic microwave background radiation. And, consider that the relative abundances of light elements (those existing before the first stars) fit perfectly with the parameters which properly describe the other two effects.

    What is it you think would have to have travelled 26.4 light years? Certainly no light that we see has. And, light from the most distant objects we see in one direction has certainly not reached the most distant object we see in the opposite direction.

    To be very clear, in Big Bang cosmology there are no violations of known physics. In fact, the class of cosmological models comes directly out of the general theory of relativity.

    Specifically, these models do not allow objects to travel through space faster than the speed of light. However, the recession rate of an object due to the expansion of the space between us and it is not constrained in any such way, nor should it be, since the rate of expansion is the same everywhere.

    Ah, but then we would have to respond with a list of hundreds of scientists named Steve who think that the current model is well supported. :tongue:

    This board has quite a bit of it; and there are quite a few resources on the web which do as well.
     
  17. Aug 6, 2006 #16

    Chronos

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    Olber's paradox is one of the profound examples of scientific thinking. It asserts the universe cannot both be infinitely old and infinitely populated by luminous objects [stars]. The logical consequences are obvious - the entire sky would be lit up like the surface of a pretty average star [like our own sun]. And as others have pointed out, intervening dust grains would be glowing like stars after being struck by an infinite number of background photons.
     
  18. Aug 10, 2006 #17



    Fallacious logic, sir. There does not have to be "more of it to come." It can be like an inner tube that is being inflated with nothing in the middle.
     
  19. Aug 11, 2006 #18
    I equate expansion to "more of it to come". If it is expanding everywhere, no matter what it is, then it is not infinite yet. Name me one thing that is expanding and inifinite... besides the universe.
     
  20. Aug 12, 2006 #19
    Mike2, what do you mean "not infinite yet"?
     
  21. Aug 12, 2006 #20

    SpaceTiger

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    The simple flat or open FRW models are spatially infinite and they are expanding. Keep in mind, however, that these are just toy models that rely on universal validity of the cosmological principle. The models well describe the observable universe but have not been shown to be valid for the universe at large. It's possible that we will never know how to describe the universe at large, since our observations are all limited (by, for example, the CMB) in the cosmological scales they can probe.
     
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