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LQC beginners question

  1. Jun 30, 2011 #1
    In LQC, the bounce can avoid the singularity by applying plancke scale quantum gravitational effects to effectively cause a "bounce" - this simultaneously avoids the classical singularity while transitioning into classical GR once certain scales are reached. Essentially singularity of BB can be resolved, by a modification on the EFE which only has an effect in extreme conditions.

    For LQC to cause a bounce, the common model would require a previous classical spacetime with a contraction phase leading to
    "the density and curvature enter the Planck scale quantum geometry effects become dominant
    creating an effective repulsive force which rises very quickly, overwhelms the classical gravi-
    tational attraction, and causes a bounce thereby resolving the big bang singularity." Extract from http://arxiv.org/PS_cache/arxiv/pdf/1005/1005.5491v1.pdf

    My question is this: If LQC requires a classical spacetime - but a contracting one, in order for an initial "bounce", then does this indicate that rather than the idea of multiple identical "bounces", that the current "bounce" of the Universe is a distinct phase, ruled by certain physical laws (These laws being GR but initially with modified quantum geometry effects at Planck scale to allow for resolution of singularities)? Does this imply that the previous phase of the Universe, the phase prior to the current "bounce" predicted by LQC, was ruled by different physical laws?

    What does LQC say in regards to continuity - if a contracting classical space-time caused the current bounce, what caused that classical spacetime to contract. For some reason intuition is screaming at the back of my head saying that the matter dominance of the Universe has a role to play?

    As most knowledgeable people who post on here know and as my name implies I am a novice, so any misonceptions I am happy for people to point out.

  2. jcsd
  3. Jun 30, 2011 #2


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    These are good questions, and the paper you cite of Ashtekar is a good place to start.
    http://arxiv.org/abs/1005.5491 , in case anyone just wants to view the abstract.

    I don't think different physical laws are needed. There are solutions to classical GR which have an infinitely long contraction ending in a crunch. The only difference with LQC is in that case the crunch would turn into a bounce.

    LQC does not address the great existential questions like why does existence exist, the goal is relatively practical and modest. People look back and see evidence of a start of expansion from a very dense state. So let's ask how could that very dense state have arisen? How could expansion have started?

    It just pushes ordinary cause and effect and mathematical modeling back past a snag where the earlier analysis got stuck. It is incremental in that sense, not complete. And hopefully testable! There should be features in the CMB-polarization map that they can look for and which if they don't find would rule the theory out.

    Two useful research literature searches:
    (Search aimed at all types of QC, 2009 or later)
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]
    (Search aimed at early universe phenomenology concerned with testing LQC, 2009 or later)
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]
    Last edited by a moderator: May 5, 2017
  4. Jun 30, 2011 #3
    I think you misunderstand my point. I will try to explain myself a little bit better, this is a very difficult concept for and my first post on this forum was when I started looking at anything to do with space, physics and cosmology. I understand that the prior contraction phase ended in a bounce, the bounce being an effect of quantam gravity, which I think the paper shows can modify classical GR that at current spacetime architecture has liffle effect, but essentially at the planck scales becomes an extreme force, this is a slightly modified version of classical GR.

    My problem with this is, to "bounce", requires a classical contraction phase, this is the opposite of the current model of thinking of expansion of classical GR. I accept that the solution is elegant, and solves the horizon problem neatly BUT it requires a priori. That is not to say that in my opinion a priori contradicts the theory, in fact I can accept the idea the Universe has always existed.

    Yes I am in no way looking for a meaning for existence. I think the obvious answer to a dense state arising are pertubations in a priori contracting spacetime. I just do not think we can assume a classical contraction pre bounce.

    Ok so LQC assumes a pre bounce;

    "once the density and curvature enter the Planck scale quantum geometry effects become dominant creating an effective repulsive force which rises very quickly, overwhelms the classical gravi-tational attraction, and causes a bounce thereby resolving the big bang singularity." - http://arxiv.org/PS_cache/arxiv/pdf/1005/1005.5491v1.pdf

    which falls in line with the second law of thermodynamics.

    I suspect (please PF forgive my speculation) that priori spacetime could contain infinitely dense points in their spacetime, these points in priori spacetime could bounce, with the local spacetime curvature of "infinite" objects and convergence of world lines, you would expect this bounce to be internal to the closed manifold of the "infintely" dense point in priori spacetime. (I have quotemarked infinite as in LQC it seems to me a measure of maximum density, as essentially a bounce predicts a certain matter density until quantum effect geometry becomes dominant)

    A localised classical contraction in a priori spacetime would essentially allow the pre-bounce to be absolutely in line with GR - assuming the gravitational quantum effect modifactions to GR were added.

    This could also mean a priori spacetime could have multiple matter dense pertubations that eventually form infinitely dense classically contracting points in a priori spacetime that bounce. This could be extended to say that currently infinitely dense classically contracting points in spacetime are "bouncing" internally to their manifold. It would also be possible that given that there may be a matter density limit before quantum effects of gravity become overwhelming there may be a stage in matter density where t=almost infinitely slow - in this line of thinking the bounce would not need to be internal to the manifold.

    Ok so please do not shoot me for those comments anyone, I am in now way promoting them as a theory, they are just something I thought and wanted to share.

    Empirical testing is nice, it would be nice if it could be proved, as the onus is on the claimant!
    I found the link very good so thankyou for that, I am going to do a bit more research on the subject and think I will re-read the article a few times.
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