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

LQC and inflation

  1. Mar 21, 2014 #1


    User Avatar
    Gold Member

    This is a thread for discussion about papers on LQC and what was published regarding inflation (not counting super inflation), and its relation to the supposedly observed gravitational waves by BICEPS and others experiments.

    I will post just a few papers here:


    Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves

    Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau
    (Submitted on 24 Mar 2010 (v1), last revised 16 Jun 2010 (this version, v2))
    Loop quantum cosmology provides an efficient framework to study the evolution of the Universe beyond the classical Big Bang paradigm. Because of holonomy corrections, the singularity is replaced by a "bounce". The dynamics of the background is investigated into the details, as a function of the parameters of the model. In particular, the conditions required for inflation to occur are carefully considered and are shown to be generically met. The propagation of gravitational waves is then investigated in this framework. By both numerical and analytical approaches, the primordial tensor power spectrum is computed for a wide range of parameters. Several interesting features could be observationally probed.


    Loop quantum cosmology and slow roll inflation

    Abhay Ashtekar, David Sloan
    (Submitted on 21 Dec 2009 (v1), last revised 2 Oct 2010 (this version, v2))
    In loop quantum cosmology (LQC) the big bang is replaced by a quantum bounce which is followed by a robust phase of super-inflation. Rather than growing unboundedly in the past, the Hubble parameter \emph{vanishes} at the bounce and attains a \emph{finite universal maximum} at the end of super-inflation. These novel features lead to an unforeseen implication: in presence of suitable potentials all LQC dynamical trajectories are funneled to conditions which virtually guarantee slow roll inflation with more than 68 e-foldings, {without any input from the pre-big bang regime}. This is in striking contrast to certain results in general relativity, where it is argued that the a priori probability of obtaining a slow roll with 68 or more e-foldings is suppressed by a factor e−204.


    Inflation and Loop Quantum Cosmology

    Aurelien Barrau
    (Submitted on 24 Nov 2010)
    On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a "naturalness" problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC.
  2. jcsd
  3. Mar 21, 2014 #2


    User Avatar
    Science Advisor

    As far as I can see, the problem with Loop quantum cosmology, is that it is somewhat like multifield or multi bout models of inflation. There is the first 'super' bounce, and then afterwards a second bout of standard slowroll inflation kicks in.

    The problem with that, is that the second bout will wipe out almost all physical traces of the first round and so you are looking at a prediction which is merely a small departure from the prediction of the second (in one of Barrau's papers he claims there is a small departure at low multipoles).

    Of course since the inflaton itself is an arbitrary parameter here, you don't really have a falsifiable model b/c you can always move the goalpost and simply say 'oh well it was simply that we picked the wrong inflaton'.

    I was unable to find any prediction for any CMB quantities in any of their papers and as far as I can see, they don't have enough control over the theory itself to really be able to pin down unambiguous values.
  4. Mar 21, 2014 #3


    User Avatar
    Gold Member

    I wrote that I was not considering the superinflation. Plus, there are many citations for these papers. So, I am asking to see if it is possible to find something among them. This is why I was asking for more papers.
  5. Mar 21, 2014 #4


    User Avatar
    Science Advisor

    I did look through them, but again they don't really put constraints on the form of the inflaton perse. Instead they take something simple, like phi^2 or phi^4 inflation and then analyze what LQC does to the spectrum. And one of the paper claims that it modifies it at low multipoles in a small way (see the first Barrau paper). For what its worth, I don't see that type of departure in the power spectrum (instead the data hugs the r=.2 line pretty nicely) but then the sensitivity is probably nowhere near enough to really pick out a LQC effect.

    But my point still stands, we don't have any independent way of knowing what the shape of the inflaton is to begin with, so there is no way to actually come up with a model independant prediction.
    This is typical of multi-bout models. The first bout fixes a problem (like how to start inflation) and the second bout is responsible for the observable consequences.
  6. Mar 21, 2014 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Here's another I thought was interesting. It is by one of the top people (Barrau) whose papers you already mentioned, but it is more recent than any of those you listed:

    Duration of inflation and conditions at the bounce as a prediction of effective isotropic loop quantum cosmology
    Linda Linsefors, Aurelien Barrau
    (Submitted on 7 Jan 2013)
    Loop quantum cosmology with a scalar field is known to be closely linked with an inflationary phase. In this article, we study probabilistic predictions for the duration of slow-roll inflation, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "prebounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is consistent with the most favored minimum values. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture of inflation, superinflation, deflation, and superdeflation is also much clarified in the framework of bouncing cosmologies.
    Comments: 7 pages, 7 figures

    This was published in Physical Review D in 2013. Barrau is a phenomenologist who has studied how to test various QC including stringy. IOW not exclusively concentrating on Loop. Lately he shows increasing interest in Loop QC phenomenology.

    This is interesting, too, because there is no necessity to include "multiverse" in the picture. There is simply ONE spacetime which collapses and rebounds, and causes ADEQUATE inflation with very high probability (peaked around 145 e-folds.)

    My impression is that LQC accommodates inflation in a way that is high-probability, simple, generic, adequate. There is no "measure problem". It doesn't make the kind of difficulties with starting and stopping that cause people to imagine a huge variety of "bubbles" of inflation and then start worrying whether we are "in" a high-likelihood "bubble". :biggrin:
  7. Mar 21, 2014 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Another Barrau (and friends) paper. It seems to be along the lines of what you were asking for, MTd2:

    Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
    J. Grain, A. Barrau, T. Cailleteau, J. Mielczarek
    (Submitted on 8 Nov 2010)
    Cosmological models where the standard big bang is replaced by a bounce have been studied for decades. The situation has, however, dramatically changed in the past years for two reasons: first, because new ways to probe the early Universe have emerged, in particular, thanks to the cosmic microwave background, and second, because some well grounded theories -especially loop quantum cosmology- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode cosmic micorwave background experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental loop quantum cosmology parameters. This establishes the early Universe as an invaluable quantum gravity laboratory.
    Comments: 12 pages, 5 figures. Published in Physical Review D

    Here's the Inspire record for it (27 cites so far):
    Here is the Inspire author profile for Aurelien Barrau:
  8. Mar 21, 2014 #7
    Thanks a lot for sharing Marcus! I'm making a compilation on anything bounce! ^^
  9. Mar 25, 2014 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You're very welcome! You might be interested in this LQC bounce paper that came out SINCE the BICEP2 report, with its increased estimate of the tensor/scalar ratio r.
    Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
    Jaume de Haro, Jaume Amorós
    (Submitted on 25 Mar 2014)
    The CMB map provided by the Planck project constrains the value of the ratio of tensor-to-scalar perturbations, namely r, to be smaller than 0.11 (95% CL). This bound rules out the simplest models of inflation. However, recent data from BICEP2 is in strong tension with this constrain, as it finds a value r=0.20+0.07−0.05 with r=0 disfavored at 7.0σ, which allows these simplest inflationary models to survive. The remarkable fact is that, even though the BICEP2 experiment was conceived to search for evidence of inflation, its experimental data matches correctly theoretical results coming from the matter bounce scenario (the alternative model to the inflationary paradigm). More precisely, most bouncing cosmologies do not pass Planck's constrains due to the smallness of the value of the tensor/scalar ratio r≤0.11, but with new BICEP2 data some of them fit well with experimental data. This is the case with the matter bounce scenario in the teleparallel version of Loop Quantum Cosmology.
    4 pages, 1 figure

    We have a Jaume de Haro paper on the teleparallel version of LQC as a candidate in the 4th quarter 2013 MIP ("most interesting/important paper") poll.

    I'm using the 4th quarter MIP poll thread as a staging area to gather possible candidates for the 1st quarter 2014 poll. So I'll add this one.
  10. Mar 26, 2014 #9
    Great and thanks! I'm still fairly new to non-singular bounce and just started reading and studying the Weitzenbock space-time-global system of orthonormal vector fields. It's a possible alternative to slow-roll inflation. I also like how they came up with an scale invariant spectrum and the non-singular point for tensor perturbation to avoid BB singularity and a bit of modification on the Friedman equation(critical density). Interesting.. ^^
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook