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B Do we live in a privileged time

  1. Jul 4, 2016 #1

    wolram

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    According to this paper we do, which sounds very interesting.

    arXiv:1607.00002 [pdf, other]
    The dimensionless age of the Universe: a riddle for our time
    Arturo Avelino (1), Robert P. Kirshner (1 and 2) ((1) Harvard University, (2) Gordon and Betty Moore Foundation)
    Comments: 14 pages, 12 figures. Accepted for publication in the Astrophysical Journal (ApJ)
    Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

    We present the interesting coincidence of cosmology and astrophysics that points toward a dimensionless age of the universe H_0*t_0 that is close to one. Despite cosmic deceleration for 9 Gyr and acceleration since then, we find H_0t*_0 = 0.96 +/- 0.01 for the LCDM model that fits SN Ia data from Pan-STARRS, CMB power spectra, and baryon acoustic oscillations. Similarly, astrophysical measures of stellar ages and the Hubble constant derived from redshifts and distances point to H_0*t ~ 1.0 +/- 0.1$. The wide range of possible values for H_0*t_0 realized during comic evolution means that we live at what appears to be a special time. This "synchronicity problem" is not precisely the same as the usual Coincidence problem because there are combinations of Omega_Matter and Omega_Lambda for which the usual coincidence problem holds but for which H_0*t_0 is not close to 1.
     
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  3. Jul 4, 2016 #2

    Jorrie

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    This is a coincidence known for quite some time and I do not think most cosmologists find it too special. With the normalized Hubble value starting out very high and is decreasing to a quite low value, normalized H*t must cross unity at some stage. With the observed cosmic parameters, it was in the range 0.5 to 2 from time some 2000 years until double the present cosmic time.
     
  4. Jul 4, 2016 #3
    Wouldn't homogeneity and isotropy pertain to the time domain of the universe as well?
     
  5. Jul 4, 2016 #4

    Jorrie

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    What exactly do you mean by "homogeneity and isotropy in the time domain"?
     
  6. Jul 4, 2016 #5
    I mean isn't the universe homogeneous and isotropic throughout time? If so, then no time is special just as no position in the universe is special.
     
  7. Jul 4, 2016 #6

    Jorrie

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    One can argue that the hot-dense state (the 'BB') must have been special. And so may be a heat death in the 'infinite future'.
    If we just mean that if the universe is spatially homogeneous and isotropic now, has it been and must it always be the case, then the answer is yes, approximately.
     
  8. Jul 4, 2016 #7

    PeterDonis

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    No.

    "Homogeneous" in time would mean that the universe looks the same at every moment of time. Obviously that's false since it is expanding.

    "Isotropic" in time would mean that the universe looks the same in the past direction as in the future direction. Obviously this is also false since the universe is hot and dense in the past and more and more cool and diffuse in the future.
     
  9. Jul 4, 2016 #8

    Jorrie

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    I agree as the original question was stated, but to the OP's clarification "I mean isn't the universe homogeneous and isotropic throughout time?", I think the answer is a conditional 'yes'.
     
  10. Jul 4, 2016 #9

    PeterDonis

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    With your interpretation as given in post #6, yes. But we cannot draw from that interpretation the deduction that "no time in the universe is special", because the universe does change in time, and particular things of importance, such as the Big Bang, or the CMB emission, happened at particular times. So your interpretation still says "no" to the implied inference in post #5.
     
  11. Jul 5, 2016 #10
    Thanks Peter, that makes sense. The universe would appear different in time depending on the phase changes since the BB.
     
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