Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A finite universe: what does it mean for QG?

  1. Feb 12, 2005 #1

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Last year the best available (WMAP etc.) data said omega was
    1.01 +/- 0.01, so univ. could be infinite and perfectly flat
    or it might be slightly curved and finite. (other possibilities but those were the main two people considered)

    now a new study says 1.01 +/- 0.009
    which narrows errorbar down to omega range 1.001 to 1.019
    in other words if we take this new study seriously we can still consider maybe univ. is infinite but more apt to be finite and just slightly curved. it has some kind of radius of curvature which might be an interesting distance to determine

    So what implications might this have for Quantum Gravity?

    There are some QG models which are intrinsically finite in character, or at least appear so at first glance. Like the "dynamical triangulation" modeling by Ambjorn and Loll and Jurkiewicz where they set up the geometry of the universe in a computer and create paths of evolution for the shape of the universe, and randomize. their monte carlo simulations are all finite. maybe they could handle the infinite case by taking a limit and maybe this is just a trivial observation. But my hunch is that if the idea of finiteness is accepted it will tend to favor some lines of QG thinking. I don't know which. this thread could be a place to explore that, if we can.

    Here is the paper:
    http://arxiv.org/abs/astro-ph/0501171
    Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies

    It has 48 authors and BTW Max Tegmark who wrote the SciAm "Multiverse" article is one of them
     
  2. jcsd
  3. Feb 12, 2005 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Ned Wright featured this article in his "News of the Universe" for 11 January 2005:
    http://www.astro.ucla.edu/~wright/cosmolog.htm
    http://www.astro.ucla.edu/~wright/cosmolog.htm#News

    he references astro-ph/0501171, and has this to say

    "Cosmic Ripples Seen by Galaxy Surveys

    11 Jan 2005 - Both the Sloan Digital Sky Survey and the 2 Degree Field Galaxy Redshift Survey reported the discovery of features in the distribution of nearby galaxies that correspond to the oscillations seen in the anisotropy of the Cosmic Microwave Background for several years. The overall statistical significance of this result is good but not great: 3.5 standard deviations. But observations of these ripples provide two valuable new constraints on cosmological models, and verify the current Lambda-CDM model of the Universe. The detection of these ripples is shown at right in a version of Figure 3 from a technical paper describing these results. It gives a matter density in gm/cc that agrees with the value found by WMAP. Both WMAP and the SDSS measure this density to a precison of 8% and their values agree to within 5%. Combining the CMB and SDSS data gives an improved limit on the total density of the Universe: Omegatot = 1.01 +/- 0.009. If Omegatot = 1, the Universe is flat; if Omegatot > 1 the Universe is closed; while if Omegatot < 1 the Universe is open."

    I cant provide a copy of the "Figure 3" which Wright reproduced but it is on
    page 5 of the paper.

    BTW when Wright says "if Omegatot > 1 the Universe is closed" he doesn't mean by closed that it eventually collapses in a Big Crunch. (that used to be the way people thought, but no longer)
    a closed universe with positive cosm. const. can continue expanding.
    what closed means in this context is spatially finite.
    it is essentially synonymous with saying that Omega is greater than one.
     
    Last edited: Feb 12, 2005
  4. Feb 12, 2005 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Smolin CNS

    another Quantum Gravity-related idea that this shift toward finite univ. seems to favor is Smolin CNS (cosmic natural selection) principle

    which is a model that makes predictions about the laws of physics and the fundamental physical constants

    it would be interesting if this model could be proven wrong by making some finer measurements of some physical constants and discovering that they are NOT optimal for black hole formation

    this model is not something Smolin is PROMOTING or asking people to believe in, at present, but something he has offered for testing. It may have already been shot down by observing neutron stars, or it may not have. the idea is extremely interesting and deserves rigorous testing to see if it will hold up.

    it makes the radical assumption that our universe arose from a black hole (bounce singularity) in a prior universe-----and concludes that sets of physical constants optimal for BH formation are overwhelmingly more likely than suboptimal ones.

    one requirement for CNS is that our universe is finite. LQG eliminates the singularities of both black holes and big bangs so that one can evolve smoothly into the other.

    Omega > 1 would be good news for CNS. (also I suspect for LQG as a whole but am not sure about that)
     
  5. Feb 13, 2005 #4

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Well, I've been scouting for that sort of thing as you know. So far I've not had any compelling findings. The most recent candidate:
    http://arxiv.org/abs/astro-ph/0410151
    CNS still appears to be on the table. The finding that Omega slightly exceeds 1 could be very important. But I still believe it is exactly 1. And the authors of that paper concede certain observational error bars are not very firm. Omega remains the most elusive empirical value in the observed universe.

    It still seems odd Omega is so close to being exactly 1. I can't shake the feeling there must be a more fundamental explanation. I can accept the possibility some of the basic constants in the universe may just be what they are and cannot be derived from first principles: although I think their relationships to one another are driven at a deeper level.
     
    Last edited: Feb 13, 2005
  6. Feb 13, 2005 #5
    This is interesting information.

    On the other hand, if I've understood the paper correctly, the +/- .009 means that the standard deviation (of the estimate that came out as 1.01) is 0.009. That would disconfirm exact flatness very, very weakly (if at all); results that are one standard deviation away from the mean are pretty typical.

    (In this case, if omega is exactly one, the estimate came out at about the 85th percentile of its probability distribution (assuming it's normal) -- hardly extreme.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?