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New Ambjorn et al. paper (AJL follow-up)

  1. Nov 16, 2004 #1


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    Semiclassical Universe from First Principles
    Authors: J. Ambjorn, J. Jurkiewicz, R. Loll
    15 pages, 4 figures

    "Causal Dynamical Triangulations in four dimensions provide a background-independent definition of the sum over space-time geometries in nonperturbative quantum gravity. We show that the macroscopic four-dimensional world which emerges in the Euclidean sector of this theory is a bounce which satisfies a semiclassical equation. After integrating out all degrees of freedom except for a global scale factor, we obtain the ground state wave function of the universe as a function of this scale factor."
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  3. Nov 16, 2004 #2


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    "background independent" is a key point here.
    In their previous paper
    Emergence of a 4D World from Causal Quantum Gravity

    it was not clear (to me at least) that background independence had been achieved.

    This is a long-awaited paper-----have been waiting some six months for the follow-on to the previous one, which caused a lot of excitement at the Marseille LQG conference in May

    another keyword here is bounce

    in a growing number of papers on cosmology the picture is of the world beginning in a bounce (which has replaced the classical singularity).

    the quantum version of the big bang is the "big bounce" apparently

    so it is confirming that the AJL model has this feature too

    AJL do computer runs, simulating the geometry of the universe.
    this is cool. they have a model that is tractable enough to compute with
    and when they run lowerdimensional version you get computer-generated pictures of spacetime evolving

    In this paper their Figure 1 is shows their universe (in one of their Monte Carlo runs) expanding and contracting---it is a computer graphic of the size parameter evolving thru time.

    anyway, I guess this paper is a must-print
    I will want to have hardcopy around to scribble-on, highlight etc.
    will do that and be back shortly
    Last edited: Nov 16, 2004
  4. Nov 16, 2004 #3


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    Here's a photo of Renate Loll (at the LQG conference in May):

    Here's a shot of her conversing with Thomas Thiemann:

    this is Renate out for a stroll with Julian Barbour and Don Marolf:

    this is the quantum gravity semiclassical limit.
    the classical or semiclassical limit has been a goal in QG for a long time.
    so it could turn out that Renate Loll is part of an historically significant turn of events (and besides she is prettier than Jan Ambjorn)

    John Baez, who has been a friend of Loll's since the early 90s,
    indicated this might be the case in his This Week's Finds #207
    May 2004 which reported highlights of the May conference.
    It was somewhat unexpected then, because Ambjorn et al had
    been trying Simplicial models of spacetime----aka Dynamical Triangulations---
    ---or Simplicial Gravity----for 10 years or more
    and couldnt seem to get it to work right.

    So the previous paper, "Emergence of a 4D world" took people a
    little by surprise. This paper looks like more of a followup,
    firming up and nailing down the results.

    In the meantime, Lee Smolin has (last month) posted a paper on
    the Dynamical Triangulations approach. I wonder if further down
    the road AJL will cite it.
    Last edited: Nov 16, 2004
  5. Nov 16, 2004 #4


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    I want to get clear how they extend this to Lorentzian case

    on page 6 they refer to still another AJL paper "to appear"
    which is their reference [18]

    also they refer to the forthcoming [18] in the conclusions paragraph
    on page 13.

    Interesting mention of the cosmological constant at bottom of page 11:

    "Note that Lambda in (22) is the real cosmological constant and no longer a Lagrange multiplier. We have thus calculated the wave function of the universe from first principles up to prefactors and corrections to the semiclassical approximation."

    Again, in the conclusions:
    "In this letter we showed that the scale factor characterizing the macroscopic shape of this ground state of geometry is well described by an effective action similar to that of the simplest minisuperspace model used in quantum cosmology. However, in our case such a result has for the first time – we believe – been derived from first principles."
  6. Nov 17, 2004 #5


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    Also notice that in their forthcoming paper (cited as 18), they are going to take up questions of Asymptotic Safety, which means the Renormalization Group fixed points. We have seen other work on this (analytical), so this represents another convergence created by the AJL approach.
  7. Nov 17, 2004 #6


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    Again you are seeing something I didnt and responding in a way that really adds. Have come to count on it! thanks.

    right now the interesting thing catching at my attention is that
    their references [1] and [2] are to hawking-style quantum gravity papers.

    I also printed out their reference [8]
    Vilenkin "Quantum cosmology and eternal inflation"
    Last edited: Nov 17, 2004
  8. Nov 17, 2004 #7


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    Lubos knocks AJL's latest

    After reading the paper, I was afraid that Lubos would pick up and criticise their restriction on the path integrals, that they be nice and causal. He was criticising the subsetting of path integrals by LQG theorists yesterday, on the grounds that to be valid, path integration has to include everything, and hence be non-differentiable almost everywhere, as well as non-physical(FTL, etc.). Well I was right, he has included just that criticism in his latest review of hep-th papers on sps: http://groups.google.com/groups?hl=...411162202230.21626-100000@feynman.harvard.edu. I don't know what to make of this; his strictures seem valid to me, but I would really like to see a response by a quantum gravity pro.
  9. Nov 17, 2004 #8


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    Hi selfAdjoint

    It seems to me pretty clear that AJL are making progress and Lubos comment could have some interesting angles on it---hostile or dismissive tho the attempt be. So we might do well just to copy it here so all can take a gander at it.

    I omit whatever looks like a general condemnation of "discrete gravity people", and which we have heard several times already, and include only what seems aimed at the AJL paper specifically.

    There may be mistakes here so read at your own risk (don't necessarily take it for gospel :) )

    ---quote Lubos SP.strings comment on AJL paper---

    * http://www.arxiv.org/abs/hep-th/0411152 - Triangulated gravity

    These colleagues first repeat a lot of the commercials about "Causal
    Dynamical Triangulations" that they've already written in many previous
    papers. The starting points are very obvious and sort of naive: try to
    define the path integral of quantum gravity in a discretized form. (It's
    like spin foams in loop quantum gravity, but you don't necessarily require
    that the details will agree.) OK, so how can you discretize a geometry?
    You triangulate it into simplices, and you imagine that every simplex has
    a region of flat Minkowski spacetime in it.

    (That's not like loop quantum gravity - the latter assumes that there is
    no geometry "inside" the spin foam simplices - the geometry is
    concentrated at the singular points and edges of the spin foam.)

    Then you write down the Einstein-Hilbert action many times and you
    emphasize that it is discretized. There are many other differences from
    loop quantum gravity: while the minimal positive distance in loop quantum
    gravity is sort of Planckian, in the present case they want to send the
    size of the simplices to zero and the regulator should be unphysical. Of
    course that if you do it, you formally get quantized general relativity
    with all of its problems: as soon as the resolution becomes strongly
    subPlanckian, the fluctuation of the metric tensor becomes large. The path
    integral will be dominated by heavily fluctuating configurations where the
    topology changes a lot and where the causal relations are totally obscured
    - and the results of these path integrals will be non-renormalizably
    divergent - at least if you expand them perturbatively. But this is simply
    what a correct, authentic quantization of pure gravity gives you.

    These authors are doing something different in one essential aspect. They
    don't want to sum over all configurations, all metrics - the objects that
    you encounter in the foamy GR path integral above. They don't do it
    because they sort of know that pure GR at subPlanckian distances is
    rubbish. Instead, they truncate the path integral to contain "nice and
    smooth" configurations only. The allowed configurations they include must
    be not only nice, but they must have the trivial causal diagram as well as
    a fixed topology - namely S3 x R in their main example. Well, if you
    restrict your path integral to configurations that look nice, it's not
    surprising that your final pictures will look nice and similar to flat
    space, too. But it by no means implies that you have found a physical

    Any path integral that more or less works simply must be dominated by
    configurations that are non-differentiable almost everywhere, by the very
    nature of functional integration and by the uncertainty principle. One can
    often show that the path integral localizes, but that's just a result of
    theorems and calculations. One cannot define the path integral to include
    smooth and causal histories only. Such a definition simply violates the
    uncertainty principle as well as locality, if you make some global
    constraints on the way how your 3-geometry can look like. Consequently, it
    also violates general covariance, and you won't decouple the unphysical
    polarizations. If you also make global constraints about the allowed
    shapes as functions of time that cannot be derived from local constraints,
    you will also violate unitarity.
    [edit: the rest is not about AJL specifically and is stuff we've heard before]
    At any rate, they show that these strange rules of the game admit some
    big-bang big-crunch cosmological solution described by some collective
    coordinates (a nice picture animates in front of your eyes), and they
    construct or propose a wave function of the Universe that depends on the
    observable representing the "3-volume of the Universe".

    posted by Lumo at 8:17 PM
    ---end quote from Lubos---

    ---quote Robert Helling's reply (Lubos riposte later)---
    - Triangulated gravity
    > These colleagues first repeat a lot of the commercials about "Causal
    > Dynamical Triangulations" that they've already written in many previous
    > papers.

    Once again, Lubos is much faster than me and I make my comments
    without having ready anything of the papers than the abstract. And I
    agree, when I saw this paper on the arxive, my reaction was "another
    one of those. how for into the abstract do I have to read to find the
    new stuff?" as was yours.

    However, again once again, I am a bit less critical than you are. OK,
    it seems they beat the publicity drum a lot but I think this is fair
    if you are a small group that wants to be noticed in the stringy
    atmosphere of hep-th. And I should mention that Jan Ambjorn has worked
    on many different things including matrix models (the old ones),
    lattice theories and string theory.

    So let me try to say a couple of words in defence of their approach:
    This stuff obviously has its background in the matrix model
    literature and the realization of 2d gravity in terms of dynamical
    triangulations (dual to matrices) was one of the successes of the
    80s. But you are right, the Euclidean path integral is not only
    dominated by but also seems to localise in non-smooth geometries.
    So they try to cure this problem by changing the rules of their path
    It is probably fair to divide geometry into different levels of
    structure. One possible distinction is

    0) differentiable structure
    1) topology
    2) causal structure
    3) conformal structure
    4) metric structure

    It is up to discussion at which of these levels you start varying in
    your path integral and which parts you keep fixed.
    I guess, Lubos wants to vary 1-4 while keeping 0 fixed, Ambjorn and
    friends only vary 3-4 and I had long discussions with Hendryk Pfeiffer in
    which he tried to convince me that one should vary all 0--4.
    Nobody has done a really convincing 'sum over geometries' yet, so I
    think it should be allowed to try all these approaches.
    What Ambjorn etal find is that again in 2d you can solve this model
    exactly (ie compute the partition function with sources) and it agrees
    with expectations (whatever those would be). Second the typical
    configurations look much smoother (something they haven't put in, they
    only demand causality and global topology) than in the Euclidean case.
    Of course, in 2D gravity is not typical, all the dynamics is in the
    cosmological constant and its conjugate variable, the volume,
    respectively. And in higher dimensions it is not possible to solve the
    problem analytically, you can only run in on your computer.
    Another success that they claim is that they break the 'c=1 barrier'. OK,
    I have no idea what that really is because I try to stay away from all
    this old matrix model technology but Matthias Staudacher, who was around
    in those days, says this is quite non-trivial: In these models, you do not
    have to restrict yourself to pure gravity, you can couple matter to it:
    For example you can add an Ising spin degree of freedom to all your
    triangles and sum over it as well.

    As, you say, in all these models you have to take the continuum limit
    and then you get a conformal field theory. In the old days, it was
    observed that whatever you did matter wise or matrix wise, you could
    only get models with central charge <1. But with causal triangulations
    coupled to matter you can break this barrier.

    Finally these people claim their model has a well behaved continuum
    limit and I see no reason to doubt it.
    But in the end this is only gravity if you end up in the correct
    universality class. That is, all your weird rules you make up to construct
    your discretised space-times correspond only to irrelevant operators that
    go away in this limit. And to show this is of course the hard part.


    ---end quote---
    LM#1:[Moderator's note: Well, I understand. That's what I criticize.
    Every path integral in a quantum theory is dominated by
    non-differentiable configurations because this is necessary
    for the uncertainty principle. A classical configuration has
    sharp, well-defined values of the fields like X(t) or PHI(x,t)
    or g_{12}(x,t), and by the uncertainty principle, the uncertainty
    of the canonical momentum must therefore be infinite, which is
    reflected in the path integral by the fact that the |derivative|
    of the field is typically infinite, i.e. the non-differentiable
    configurations dominate. Do you agree that you could not get
    quantum mechanics if your path integral only summed over
    differentiable paths? If you succeeded to define this "truncated"
    integral in quantum mechanics, it would violate unitarity
    and the rule U(t1,t2)U(t2,t3)=U(t1,t3) because the different
    intervals would disagree "how much differentiable" the functions
    must be. LM]
    LM#2:[Moderator's note: It is fair to divide geometries, but it is never
    fair to "cut" some configurations from a path integral, I think.
    We've had a recent debate on sci.physics.strings about the overcritical
    electric field which was exactly about this issue - did you agree with
    our conclusion that you can't ever omit "unwanted" configurations? LM]
    LM#3:[Moderator's note: It is fair to divide geometries, but it is never
    fair to "cut" some configurations from a path integral, I think.
    We've had a recent debate on sci.physics.strings about the overcritical
    electric field which was exactly about this issue - did you agree with
    our conclusion that you can't ever omit "unwanted" configurations? LM]
    LM#4:[Moderator's note: Nobody has found a really convincing luminiferous
    aether theory, so should all of us divide to different approaches how to
    construct aether? Actually I think that these two questions are more
    similar, even in details, than you might think. ;-) LM]
    LM#5:[Moderator's note: I am not getting this point at all. What's exactly
    the difference between the input and output? Typical configurations
    in the gravity path integral have strongly oscillating topology, both
    in the Minkowski and the Euclidean case, and in the Minkowski case,
    they have also a highly nontrivial and chaotic causal diagram.
    If you unphysically cut the "ugly" configurations, of course, you will
    end up with the "nice" ones, and because you made more constraints
    about the allowed configurations in the Minkowski case, you will
    get even nicer configurations than in the Euclidean space at the end. But
    that's not a result, that's your assumption. And it's an assumption
    that contradicts quantum mechanics. LM]
    [Moderator's note: Right, 2D and 3D gravity don't really have gravitons
    as local degrees of freedom. All of us know how to compute 2D gravity
    as a path integral over "nice topologies" of two-dimensional spacetime:
    it's called the stringy worldsheet. But the conformal structure on
    the worldsheet is only "nice" because *any* configuration in 2D
    can be mapped to the "standard ones" by diff x Weyl transformations.
    Analogous things hold for 2D string theory - one really wants to
    calculate the path integral over the scalar fields in spacetime
    and their effects. Moreover, my arguments above that talk about the
    uncertainty principle for g_{12} and its time derivative can break down
    in d<4 because there is no such a physical degree of freedom. LM]
    LM#7:[Moderator's note: in the case of the present paper, I don't have
    difficulties with the word "continuum limit" but rather with the word
    "model". You can define some set of rules that gives you a *classical*
    theory in some limit, but it by no means implies that your rules,
    before you take the limit, define a meaningful quantum theory, does it?
    For example, you should always ask whether your rules can lead to a
    unitary S-matrix, which path integrals should, and the answer will
    be NO in the 4D case, I think. LM]
    LM#8:[Modeator's note: That may be a different way to say the same thing.
    You're simply not sure whether the "restricted path integral" has
    anything whatsoever in common with the real path integral. LM]

    ----end quotes---

    Last edited: Nov 18, 2004
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