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Dumb Questions?

  1. Nov 9, 2009 #1
    I have a few questions that I'd love to have someone with specific knowledge of the field to answer. Forgive my simplicity.

    1. If the observable sphere of the universe is 13.5 billion light years in radius, this means that when looking at the farthest observable objects one is looking back in the past 13.5 billion years. Since we can only see that far, we say that the universe is that old and at least that large (apx 27 billion light years in diameter) and yet where are those distant objects *now* after all that time? Would they not be now located however much farther away an additional 13.5 billion years of expansion would take them? So why cannot we estimate where they are now and include this in our estimation of the 'diameter' of the universe?

    2. (Don't laugh please) If the universe is expanding faster than we'd estimated and we credit this fact to the presence of a 'dark energy' that produces repulsion instead of attraction between objects provided that the objects have enough space between them, could not this 'dark energy' be analogous to something like vapor pressure, which causes a gas to expand to fill it's container? So that when the distances cause the gravitic force to be insufficient to cancel it, it takes over and causes a gas-like expansion?
    I'm really sure this one is a dumb question, but what the heck.

    3. If Gravity is the curvature of space-time, which I get, then why do physicists speak of it as a force and of it's speed of propagation and even of 'gravitons' or particles of gravity?
    Last edited: Nov 9, 2009
  2. jcsd
  3. Nov 9, 2009 #2
    1. The observable universe I believe is around 93bly in diameter. (last number I've seen not sure if it's up-to-date)

    I'm sure marcus will come and give his run down of why this is as well as provide you with many current resources.

    2. I don't know much about dark matter/dark energy. I am quite sure that the way dark energy works though is by negative pressure. I won't say anymore about that though and someone will soon make a more informed post for sure.

    ++The only dumb question is the one that you don't ask. ;)
  4. Nov 9, 2009 #3
    Sorry 'Sorry,' I edited my original post there after you'd responded to it apparently. There's another question there now.

    Thanks for your answers.

    That figure sounds huge to me. Really? 93bly? How so?
  5. Nov 9, 2009 #4


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    These are three different models of the same thing.

    The Einsteinian theory of General Relativity models gravity as curved space-time. In this model gravity is always everywhere, however changes in the curvature propogate at a fixed speed (like waves across a pond).

    The Newonian theory of gravity models gravity as a force. Space and time in the Newtonian universe are fixed "background" against which everything else happens. It is outdated and has been replaced by Einsteinian GR, but it is very useful in the practical world.

    The Quantum theory of gravity models gravity as a particle (because quantum theory requires everthing to be discrete identiities rather than fields).

    All three all useful within their scope.
  6. Nov 9, 2009 #5


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    Far flung parts of our universe are receding faster than the speed of light.
  7. Nov 9, 2009 #6
    Because there is a vast amount of distance between the 'edges' of the observable universe they are travelling away from each other at speeds greater than c; this is from general relativity.
  8. Nov 9, 2009 #7
    Far flung parts of our universe are receding faster than the speed of light.

    That makes sense. Thank you.
    Can you point to any papers/information regarding this? I don't want to belabor you with incessant queries. The justification of that figure, I mean. I'd love to know how we arrive at that huge distance.
  9. Nov 9, 2009 #8


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    Simply PF-searching or Googling or wikiing the terms "size of the universe" or "superluminal expansion" will give you a start.
  10. Nov 9, 2009 #9
    Interesting. I googled it.
    Once not long ago I had the idea that, if space itself was expanding, that this would negate in some way the redshift of the light coming from distant objects being used as an accurate way to judge their distance from us, and this is actually a similar concept. The reason that we can see the light from objects so far away that they're moving at more than C relative to us, I mean.
    Still, what I saw seems to say that very distant objects that we can resolve with the Hubble for instance, are not apx 13 bly away, but 93 bly? That's how it reads on Wiki.
    This seems wrong to me somehow, or at least utterly different from the way that I pictured it.
  11. Nov 9, 2009 #10
    Does it mean that the distant objects *were* 13 bly away at the *time* when that light was emitted that Hubble sees now, but by this time are estimated to be much more distant, at 93 bly distant? That's an 80 bly difference... Or is it half of the 93 bly distance to the most distant objects, if the 93 blys is the estimated diameter and not the radius? Still about a 32 bly difference...
    I'm so confused.
  12. Nov 9, 2009 #11
    (In my defense, I did warn you that they were stupid questions)
  13. Nov 9, 2009 #12
    The age of the universe is not how distant objects are from us. Something important to think about when attempting to understand cosmology. If you look for the user marus on these forums and check the links in his signature it may clear up a few details for you. :)
  14. Nov 9, 2009 #13
    Thank you.
  15. Nov 9, 2009 #14
    So my next question is, if the expansion of space is the reason that we can even see distant objects that are moving away from us at more than C, then why do the objects show a red-shift at all? I mean, in a way they're not really moving away from us; the intervening space is expanding. Wouldn't the expansion of the space through which it passes negate the red-shift of an apparently 'receding' object, because isn't the wavelength of light tied to the structure of the space through which it is passing? It must be, if that's the very reason that we can see objects so distant that their velocity of recession exceeds C.

    I know I've got this wrong, but I don't know how exactly. I'll understand if you guys blow me off this time. But if anybody's bored, please do take a crack at it for me.
  16. Nov 10, 2009 #15


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    The expansion of space also causes the wavelength of the light in transit to expand on its way to us.
  17. Nov 10, 2009 #16
    Yes Saint Brian, the expansion of space is the reason for the read shifting o light, as Chalnoth told you.

    But no one answer you about dark energy - gas analogy. I am interested too about this idea. Especially after I read about the two-fluid model of vacuum with absolute ground state (or superfluid vacuum) in papers of Sinha and Preparata. The basic idea is that cosmic vacuum can be seen having superfluid properties and the pressure of this "fluid" can produce the negative force of expansion. But is this assumption a true one?
  18. Nov 10, 2009 #17


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    Well, basically, if there exists an energy density of the vacuum that is independent of the expansion, then it by definition has negative pressure. There are also other proposals (none well-motivated) that would produce a similar effect based upon various hypothetical quantum mechanical fields.
  19. Nov 10, 2009 #18
    The expansion of space also causes the wavelength of the light in transit to expand on its way to us.
    That's what I've always thought. And that is what is obvious, given the Doppler effect. But that wasn't what I was saying.

    Never mind, I'm probably way off anyhow.
  20. Nov 10, 2009 #19
    If a star used to be 13 bly away and the light from there to here is only getting here now after travelling for 13 billion years, but by now said star is not 13 bly away but 93 bly away due to spatial expansion, how does this not violate C? It has moved 80 bly farther away relative to us in only 13 billion years.

    My questions seem to have spawned more questions.
  21. Nov 10, 2009 #20


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    SR prohibits local velocities equal to or greater than c; it does not prohibit distant points receding from each other faster than c due to the expansion of space.
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