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A question of universal expandsion

  1. Dec 22, 2004 #1
    If the universe is an enormous black hole as some have suggested, will it also loose it's mass energy due to Hawkins radiation. And assuming that it does, where does the mass energy go to. Observations have shown that our universe is expanding at an incredible rate. Is this expansion not inconsistent with the behavior of a black hole. Or just maybe, the compression of matter due to tidal forces is causing the illusion of expansion. Think about it?
     
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  3. Dec 22, 2004 #2

    pervect

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    Firstly, I don't see how the universe can be compared to a black hole as far as hawking radiation goes. It just doesn't have the right structure (one where things can go in, but not out).

    Secondly, there's nothing paradoxical about Hawking radiation - the process conserves energy (as much as GR allows energy to be conserved).

    Thirdly of course there is the whole issue of energy conservation in GR - I would suggest reading the sci.physics.faq on the topic.
     
  4. Dec 22, 2004 #3
    If the universe is closed, then it most certainly is a black hole. And being such would be subject to the same principles that could be applied to any other black hole. My question has more to do with the apparent expansion of our universe. Is it really expanding or is this an illusion?
     
  5. Dec 22, 2004 #4

    Garth

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    It depends on how you measure the expansion, i.e. how you measure length.
    If you have fixed atomic rulers then the universe expands, but if your ruler is the wavelength of a photon sampled from the CMB then, because it expands with the universe, there is no expansion. Conformal gravity theories such as Self Creation Cosmology have such rulers. In such a conformal frame of measurement (SCC's Jordan frame) the expansion of the universe is an illusion; the unverse is static with 'shrinking rulers'.

    Garth
     
  6. Dec 23, 2004 #5
    Hubble Expansion does not postulate that everything expands uniformly - gravitationally bound objects are not affected in the conventional interpretation of Hubble cosmology. A closed universe does appear to have the correct mass and size to comport with the black hole formula; we can look upon our place in the interior thereof as a large black-hole research lab.
     
  7. Dec 23, 2004 #6
    Garth and Yogi; I'm greatfull for the intelligent response that you both have rendered. I am familiar with both points of view, so let's build upon the information we have and attempt a brave new exercise. The next question I have would be; what concrete data do we possess that proves an acceleration for universal expansion. When we consider the vastly outdated evidence coming to us from distant quasars, is it not possible that universal expandsion is slowing down. What are your thoughts?
     
  8. Dec 23, 2004 #7

    Garth

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    The only evidence for acceleration of expansion is the data from distant Type Ia supernovae which are fainter than expected. This predicted expectation of their apparent magnitude depends on a number of factors: their distance that itself is determined from their red shift and Hubble's constant, the geometry of intervening space-time, e.g. is the universe open or closed, any intervening absorption, and the intrinsic Absolute magnitude of such distant objects. Any of these factors may also be varied in competing explanations of the data.

    The interpretation of acceleration is debatable because the normal understanding is that the universe is decelerating in its expansion because of the mutual gravitational attraction of its contents. Acceleration requires the introduction of mysterious Dark Energy, however this is a convenient invention that can also be used to make the universe's total density up to the critical density and 'close the universe'.

    Are these deductions valid? I would be happier if DE could be identified otherwise it seems that modern cosmology requires the continual addition of ‘epicycles’ to make the standard model work.

    I personal favour the freely coasting model in which the universe is neither accelerating nor decelerating but expanding strictly linearly with time.
    R = ct
    Its advantage is that it does not require Inflation or Dark Matter either.
    The model does, however, require a mechanism to explain the linear expansion, one theory, Self Creation Cosmology, delivers this.

    Garth
     
  9. Dec 23, 2004 #8

    russ_watters

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    That's a contradiction: the universe is closed and a black hole is not.
     
  10. Dec 23, 2004 #9
    Please explain why a black hole is not closed.
     
  11. Dec 23, 2004 #10
    I believe you are thinking of the ability for a black hole to absorb surrounding matter. This is true, however the reference to being closed speaks of the density being such that no information can escape, therefore both a closed universe and a black hole share this same characteristic.
     
  12. Dec 23, 2004 #11
    This brings us back to my original question, can the universe loose mass through Hawkins radiation and is the universe truly expanding?
     
  13. Dec 23, 2004 #12

    Garth

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    James - The Black Hole - Schwarzschild - solution is not the same as the cosmological solution of Einstein's field equation, though there may be similarities.
    If the universe is to be treated as a black hole and Hawking radiation postulated, where would it radiate to?

    Garth
     
  14. Dec 23, 2004 #13
    Anyone living within a black hole large enough to give them time might ask the same question.
     
  15. Dec 23, 2004 #14

    Chronos

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    The idea of the universe as a black hole has been kicking around for quite awhile. The evidence, however, suggests the universe is quite the opposite. The GR solutions that apply to a black hole singularity are much different from those that apply to the Big Bang singularity. The Friedmann-Robertson-Walker solution [the currently favored model] predicts a universe that expands from infinite density into an increasingly diffuse state, as is observed. In effect, the universe is a time reversed black hole. The equivalent of the black hole event horizon in this universe is the big bang itself, and only existed at t=0. What came afterwards was a universe that has always been spatially unbounded, and may or may not be spatially infinite. Nothing can escape the universe because there is nowhere to escape to. Similarily, nothing new can enter the universe because there is no 'outside' from which to enter. At least this the prevailing opinion of most scientists based on current observations and theory.

    This is entirely consistent with the laws of thermodynamics which say energy cannot be created or destroyed [and Einstein himself said the laws of themodynamics were the most inviolate in all of science]. Were this not true, the universe would be even more badly behaved than it already is: objects could suddenly start and stop moving, fires could erupt and go out, and physical entities could simply vanish or appear out of thin air for no explainable reason as energy leaked in and out of the universe. These bizarre events would not just occur at some remote 'event horizon', they would occur everywhere in the universe... because everywhere is the 'event horizon' in this universe.

    Footnote: same thing Garth said, just more detailed
     
    Last edited: Dec 23, 2004
  16. Dec 23, 2004 #15

    pervect

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    "If the universe is closed" is a pretty big if, since the universe not only appears to be open, but accelerating in its expansion.

    As far as whether or not the universe is "really expanding", that depends a lot on how your definitions, as Garth has pointed out.
     
  17. Dec 23, 2004 #16
    Thanks Chronos; it does cause one to wonder. If our universe is a time reversed black hole, then at some later date our universe might halt it's expansion and become a time reversed universe. Personaly I'm with Garth, it does seem that we are balanced between an open and a closed situation here. But forever is along time, can the universe expand forever?
     
  18. Dec 23, 2004 #17

    pervect

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    I should have remembered the sci.physics.faq Is the big bang a black hole

    The standard FRW cosmology is not a black hole, nor a time reversed black hole (white hole).

    A white hole model based on a time-reversed presureless contraction is indistinguishable from the standard FRW cosmologies initially. This means that one could always argue that the apparent FRW cosmology will change into a white hole cosmology if one "waits longer". But there's no evidence that this is the case.

    White holes cannot turn into black holes, nor vice versa IIRC. Thus I don't think it's possible for the universe to be a white hole and turn into a black hole.
     
  19. Dec 23, 2004 #18
    I will have to agree with Garth on all his points - the only experimental evidence for acceleration is the 1a supernova data - which assumes they are standard candles. But I really have some serious doubts as to whether the constants that are involved in the triggering of the SN event were the same billions of years ago - There is a critical equation of Chandaraskra (wrong spelling) but it limits the size of the mass to a very narrow range - and since G is a factor in that equation - if G were greater in the past, the supernova event would involve less mass and therefore be fainter, ergo these so called standard candles appear fainter not because they are further away, but because they are less energetic.
     
  20. Dec 24, 2004 #19

    Chronos

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    You could argue as to whether SN1a is the only data, but that is not really relevant. A smooth gradient in G over time does not explain why the SN1a data is not linear. Accelerated expansion has more observational and theoretical support.

    Footnote: Agreed, pervect. I was using the FRW model as an analogous way to illustrate the differences.
     
    Last edited: Dec 24, 2004
  21. Dec 24, 2004 #20
    Chronos - The Chandrasekhar limit equation is not linear in G - moreover, the data is not that precise - but what data there is suggests that fainter events are also shorter in duration - this would correspond to less energy rather than greater distance.
     
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