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

Another thing for quantum gravity to explain

  1. Mar 17, 2006 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed


    Look on page 43, at figure 17

    the 68 % confidence interval for Omega_k is
    [-0.037, -0.008]

    this means that the 68% confidence interval for Omega_total is
    [1.008, 1.037]

    anything over 1 means postive spatial curved finite universe

    it is still "consistent" with flat infinite, but the other cant be so easily ruled out

    something to keep in mind when thinking about loop quantum cosmology and other types QG cosmology
  2. jcsd
  3. Mar 17, 2006 #2


    User Avatar
    Gold Member

  4. Mar 17, 2006 #3


    User Avatar
    Gold Member

    In any case, also flatness is unexplained, as far as I know, isn't it?
  5. Mar 17, 2006 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Please take another look at figure 17 on page 43, Alejandro.

    This IS based on the combined data from not only CMB, but also
    the Supernova, Sloan Digital Sky Survey (SDSS), and the 2dFGRS.

    what other data sets would you wish they would combine with?

    what i believe we are talking about here is best estimates based on best available data of all types.
  6. Mar 18, 2006 #5


    User Avatar
    Science Advisor
    Gold Member

    It does appear the universe has a slight positive curvature by all current measures. I wonder, however, if curvature relaxes over time. I have this nagging suspicion it tends toward zero as the universe ages.
  7. Mar 18, 2006 #6


    User Avatar
    Science Advisor

    It seams that the evolution of our universe will end in a phase of exponential expansion. In such a case the curvature tends to be fine-tuned to zero. From the Friedmann equation you can write:

    [tex]1-\Omega_T = - \frac{kc^2}{a^2 H^2}[/tex]

    If exponential expansion [itex]H[/itex] remains constant and does not tend to zero. For [itex]k = 0[/itex], [itex]\Omega_T[/itex] must tend to one, and the energy density will be the critical. This is the same mechanism that applies during inflation to reach flatness. The similarities between both phases are obvious and may be something more than a coincidence. There is some discussion about this in the last paper of T. Padmanabhan; he considers the matter and radiation phase between both exponential expansions just a transition period between two fundamental phases in the universe's evolution.
    Last edited: Mar 18, 2006
  8. Mar 18, 2006 #7
    I'm not so sure we will end in heat death. I see two creative forces at work. It seems that the origional particles emerged from the inflationary expansion phase of the universe. Some have explained this in terms similar to the Unruh effect. With very fast expansion each location in the universe is accelerating with respect to every other, and the Unruh effect states that there is a temperature associated with that acceleration, and so there is an energy density associated with that temperature and a mass density associated with that energy density. So if the universe experiences another very fast expansion rate, there might be another particle creation phase along with it.

    Not only this, but some have equated an entropy constraint on the observable universe with the area of the cosmological event horizon. With a black hole things slow down and red shifts as they approach the horizon. And the entropy inside a black hole is equated with the area of its event horizon. Similarly with a cosmological event horizon where space is receding so fast that light from that distance can no long ever reach us. Galaxies that receed to the cosmological event horizon also slow down and red shift just as with a black hole. And some have suggested that like a black hole the entropy inside the cosmological event horizon is equated with the surface area of the cosmological event horizon.

    So with the recent discovery that the universe is accelerating in its expansion rate, the cosmological event horizon is shrinking. The surface area of the cosmological event horizon is shrinking with it, and the entropy inside the cosmological event horizon is decreasing. This mean that we should expect to emerge objects of increased complexity. It is interesting to note that life appeared on earth at about the same time that the expansion of the universe began to accelerate.

    So what happens if these trends continue in the extreme? Perhaps this wil all conclude in a new creation, particles poping out of the vacuum because of the acceleration, and aranging themselves in some high order of complexity because of the shrinking cosmological event horizon.

    "..., earth and sky fed from his presence... And I saw a new heaven and earth, for the former heaven and earth had passed away"
  9. Mar 20, 2006 #8
    If space is expanding, does that mean new space is forever being created during the expansion, or is space merely stretching such that distances are elongated within space, but the overall total of space is not increased, similar to a rubberband being stretched?
  10. Mar 22, 2006 #9
    why the title? why has the answer to this questions be subject of quantum gravity?
  11. Mar 22, 2006 #10


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    would you prefer if I say "Another thing for QG to explain if it can"

    or "Another thing which QG might explain"?

    I don't have very strong opinions about this. If you would like, I will speculate a little.

    You know that Martin Reuter has written about seeing QG effects in the large-scale structure of the universe. And various people have conjectured about if some kind of MOND is true this could have something to do with QG.

    In other words, if people finally understand QG then they may get a better understanding of spacetime geometry in the LARGE, and some modifications of Gen Rel that affect largescale, as well as small.

    this is just general context. I have not said anything about the cosmological constant and about curvature.

    Remember this is just speculation. BTW I do very much like your nickname hossi. If you continue to use the signature hossi, then I will probably think of you by that name. If you don't like that and would rather be called by real name of Sabine, then please think about changing your signature so that one sees that on your posts and gets accustomed to it.

    Yes, well some speculation about curvature. I think that the curvature of the universe goes back to the time of the bigbang and to the time of inflation.

    I think that at those times, quantum geometry was dominant. Indeed as you surely know Bojowald and others have written that QG naturally produces some inflation even without needed special assumptions or fine adjustment (roughly speaking gravity repells at high density) so there are LQC papers about the naturalness of inflation.

    So I speculate that if you really want to know about why the universe is curved, and if you really want to understand very early quantum fluctuations and possible inflation, then you need to get answers from QG!

    This is why I think that largescale curvature might be something for QG to explain or say something about.

    But I am not insisting, you hossi-who-likes-to-be-called-Sabine, and I will be happy if you think up a better title!
  12. Mar 22, 2006 #11
    Hi marcus,

    I just wanted to point out the possibility that there might be other ways besides QG to some of the questions. I see your points but I think its unlikely that QG will be a miraculous cure to everything (if it exists).

    I didn't mean to pick around on the title of the thread, was just wondering.

    I actually don't care how you call me as long as I know its me. The thing is that several of my relatives are called by the name 'hossi', but since they are unlikely to show up in this forum either way is fine with me.


  13. Mar 22, 2006 #12


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed


    started a thread on recent papers by you, Magueijo, Konopka. maybe there will be some interest and people will discuss, which would be nice
    Last edited: Mar 22, 2006
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Another thing for quantum gravity to explain
  1. Quantum Gravity (Replies: 9)

  2. Quantum gravity (Replies: 2)