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Accelerating expansion of universe is an illusion

  1. Oct 1, 2011 #1
    I'm not sure how strong the evidence is for this but I found it interesting:

    "Now, a new theory suggests that the accelerating expansion of the universe is merely an illusion, akin to a mirage in the desert. The false impression results from the way our particular region of the cosmos is drifting through the rest of space, said Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece. Our relative motion makes it look like the universe as a whole is expanding faster and faster, while in actuality, its expansion is slowing down — just as would be expected from what we know about gravity.

    If Tsagas' theory is correct, it would rid cosmology of its biggest headache, dark energy, and it might also save the universe from its harrowing fate: the Big Rip. Instead of ripping it to bits, the universe as Tsagas space-time envisions it would just roll to a standstill, then slowly start shrinking."

    http://www.astro.auth.gr/~tsagas/Publications/Journals/PRD/PRD14.pdf
    http://www.msnbc.msn.com/id/44690771/ns/technology_and_science-science/
     
  2. jcsd
  3. Oct 1, 2011 #2

    Chronos

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    The author made good points favoring a dark flow interpretation, but, there remains plenty of observational work to confirm this is a viable alternative to dark energy. The basis for including a cosmological constant [dark energy] in GR is mathematically sound. QM provides a mechanism. The real puzzle is why it is so weak.
     
  4. Oct 2, 2011 #3

    Chalnoth

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    I have a really really hard time buying that this could both explain the acceleration and be consistent with our current observations of the isotropy of our universe.
     
  5. Oct 2, 2011 #4

    RUTA

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    You need a lot of coincidences to be fooled in this fashion. If the universe has conspired against our best science to the extent that we can't figure out what's really going on, we might as well give up trying.
     
  6. Oct 4, 2011 #5

    DevilsAvocado

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    https://www.physicsforums.com/showthread.php?t=536522" :smile:


    (Hi RUTA, nice to have you back!)
     
    Last edited by a moderator: Apr 26, 2017
  7. Oct 4, 2011 #6

    RUTA

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    I was away while I was working on this very problem, actually. I bailed on all discussion forums during this time to focus on the problem at hand. I do believe the Nobel citation should not have stated "for the discovery of the accelerating expansion of the Universe," since that conclusion follows from their data only in the context of a particular theory of cosmology. They certainly deserve the prize, but the citation should have said something like, "for observational techniques associated with type Ia supernovae." Then, the Nobel committee doesn't have to worry about changes to our theoretical cosmology that change the interpretation of the data.
     
    Last edited by a moderator: Apr 26, 2017
  8. Oct 4, 2011 #7

    DevilsAvocado

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    Cool, I had some remorse about not recapturing the discussion on simplices manifold/non-separability/RBW... a lot of 'messy things' in world politics got between, and then you were gone...

    Anyhow, I’m glad you’re back, and maybe sometime we could continue... I have bunch of 'topics' in pipeline that I’ll try to get posted... but maybe later?

    The comment on the Nobel citation is interesting. It’s not the first time those Nobel guys made a "mistake", and of course – not one single human has seen DE, or know what it is, or how it works.

    But as you said - they certainly deserve the prize – they proved the accelerating expansion beyond any doubts, even if we don’t know the 'mechanism'.
     
  9. Oct 4, 2011 #8

    RUTA

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    Thanks, I'm glad to be done with this last calculation. It was nasty :devil:

    To clarify on this topic, what everyone agrees these guys did was obtain distance moduli (μ) out to redshift z ~ 1.5 using type Ia supernovae. Whether or not this data provides evidence that the universe is expanding depends on your cosmology model. Using inhomogeneous spacetime models, for example, the data does not show accelerating expansion (arXiv:gr-qc/0605088v2). Likewise, the PRD paper that started this conversation shows the accelerated expansion could be an illusion. However, no one is disputing the validity of the μ versus z data these Nobel recipients obtained, and that process took them years to perfect and employ.
     
  10. Oct 4, 2011 #9
    Begging patience for an imbecile....

    If a supernova far, far away is moving very, very fast does that mean the universe long, long ago was moving faster than today?
     
  11. Oct 4, 2011 #10

    RUTA

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    Consider the relationship between the Milky Way and a galaxy that is far away today, for example. In the flat, matter-dominated (pressureless dust) GR cosmology model (Einstein-deSitter, EdS for short) that galaxy was moving away from us at time of emission at a much faster speed than it is today (time of reception). The rate of recession between the two galaxies will continue to diminish, slowing to v = 0 at t = ∞. In the Lambda CDM model (EdS + cosmological constant), the recession rate slowed at first, but then started to speed up again (when cosmological constant outward pressure started to dominate attraction of matter). But, in both models, the galaxies today are still not receding faster than they were at time of emission, although eventually in LCDM they will be.
     
  12. Oct 4, 2011 #11

    DevilsAvocado

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    Congrats!

    Oops... I took it for granted this was some 'cranky solution'...

    A lot of questions arise:
    • How could a contracting universe look like an expanding universe? We will not get the same 'setup', playing 'the movie' backwards, right?

    • How could we observe the (extreme) red-shift in the CMB?

    • Why do we observe red-shift at all?

    • When did the universe swap 'direction'? BB did happen, right?

    • Are all data from Type II Supernovae wrong?

    500px-HST_SN_1987A_20th_anniversary.jpg
     
    Last edited: Oct 4, 2011
  13. Oct 4, 2011 #12

    DevilsAvocado

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    According to the Lambda-CDM model the answer is: At early inflation, yes, later, no.

    700px-CMB_Timeline300_no_WMAP.jpg
     
  14. Oct 4, 2011 #13

    RUTA

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    Oops, I'm sorry, I've given the wrong impression. The universe is expanding, no one disputes that. It's just a question of whether or not the expansion is speeding up or slowing down. In LCDM it was slowing down until the cosmological constant started dominating the matter density at which point it started speeding up. This demarcation occurs in the data at about z = 0.752 (arXiv:1105.3470).
     
  15. Oct 4, 2011 #14

    DevilsAvocado

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    Phew, big relief!

    Thanks

    (:smile:)
     
  16. Oct 4, 2011 #15
    If you think of space and time as opposite directions, you would know that looking out in space is looking back in time to the same point.
     
  17. Oct 4, 2011 #16

    DevilsAvocado

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    Thanks, I understand, but I was thinking more on the second law of thermodynamics, the entropy of the universe, and the arrow of time.

    AFAICT, you will not end up with an "exact copy" of the early universe, if you tried to reverse the current expansion for 13.75 billion years...
     
  18. Oct 4, 2011 #17

    Chalnoth

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    Why not? If you have the current configuration of our universe exactly, then in running the clock backward it will necessarily reach an identical state.
     
  19. Oct 4, 2011 #18

    RUTA

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    As an aside, in answer to this question, it is possible in theory to see increased redshift with increased distance in a collapsing universe. The reason is that the redshift only tells you the universe scale factor is bigger at reception than at emission. Thus, z is independent of what happens dynamically between emission and reception. So, if a photon is emitted during an expanding phase and received shortly after the collapsing phase begins, as in the closed model, then you will see increased redshift with increased distance even though the universe is now collapsing. I explained this in a paper some years ago (“Kinematics between Comoving, Photon Exchangers in the Closed Matter-dominated Universe,” W.M. Stuckey, American Journal of Physics 60, No. 6, 554 - 560 (1992)).
     
  20. Oct 4, 2011 #19

    Chalnoth

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    Well, that's not really a contracting universe looking like an expanding one, though. The redshift combined with a distance measure still captures accurately the expansion history. The recent start of the collapse would also be captured in the relationship between redshift and distance (though if the collapse was recent enough it could only be extrapolated).
     
  21. Oct 4, 2011 #20

    DevilsAvocado

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    True, but it would be a terrible bad "illusion", because of the change of the direction of time; watching the omelet jumping out of the pan to 'regenerate' into 4 complete eggs... we would just know that there’s something 'fishy' going on... :smile:

    Or?
     
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