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LQC, observational window

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  1. Jul 29, 2015 #1
    Ashtekar and Barrau have published a paper describing the possibility of testing LQC with observations. They talk about an observable window where it might be possible to distinguish between normal inflation and the LQC bounce. Can anyone explain how this works? he article is a bit too technical for me.
    http://arxiv.org/pdf/1504.07559v1.pdf
     
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  3. Jul 29, 2015 #2

    marcus

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    Exciting paper! Thanks for calling attention to it. As I read it the paper is based on some 2012 work by Fabian Schmidt et al. Their reference [2].
    ==quote Schmidt and Hui abstract==
    Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (even superhorizon), leaving as a signature a large-angle modulation of the observed cosmic microwave background (CMB) power spectrum.
    ==endquote==
    Ashtekar and Barrau are pointing out that the LQC bounce would leave an observable imprint in the CMB via the inflationary mechanism analyzed in the Schmidt Hui paper. So they are making essential use of recent non-LQC analysis
    http://arxiv.org/pdf/1210.2965v2.pdf
    http://arxiv.org/abs/1210.2965
    CMB Power Asymmetry from Non-Gaussian Modulation
    Fabian Schmidt, Lam Hui
    (Submitted on 10 Oct 2012 (v1), last revised 14 Dec 2012 (this version, v2))
    Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (even superhorizon), leaving as a signature a large-angle modulation of the observed cosmic microwave background (CMB) power spectrum. This provides an alternative origin for a power asymmetry which is otherwise often ascribed to a breaking of statistical isotropy. The non-Gaussian modulation effect can be significant even for typical ~10^{-5} perturbations, while respecting current constraints on non-Gaussianity, if the squeezed limit of the bispectrum is sufficiently infrared divergent. Just such a strongly infrared-divergent bispectrum has been claimed for inflation models with a non-Bunch-Davies initial state, for instance. Upper limits on the observed CMB power asymmetry place stringent constraints on the duration of inflation in such models.
    Comments: 4+epsilon pages, 1 figure; v2: minor edits and references added, reflects PRL version in press
    Phys. Rev. Lett. 110, 011301 (2013)
    This paper lists Fabian Schmidt as an Einstein Fellow with appointments at both Princeton and Caltech. I wanted to find out more so I looked up his Inspire profile
    http://inspirehep.net/author/profile/F.Schmidt.2
    and his arXiv list of 93 papers:
    http://arxiv.org/find/astro-ph/1/au:+Schmidt_F/0/1/0/all/0/1
    He has co-authored with some top people in cosmology such as Wayne Hu (Chicago) and Eiichiro Komatsu (lead author of the WMAP reports), Mark Kamionkowski, Dragan Huterer, Eric Linder.... Institutional associations include Chicago and the Kavli Institute for Cosmological Physics.
    He got his PhD at Chicago in 2009. And already has published over 70 papers with an average number of 66 citations each.
    It would seem the guy is a rising star in cosmology---and I didn't know about him until I looked up Ashtekar and Barrau's reference [2]
     
    Last edited: Jul 29, 2015
  4. Jul 29, 2015 #3

    marcus

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    I think the most challenging thing to understand is not the specifically LQC work here but instead the non-LQC work of Fabian Schmidt which in effect provides the avenue for pre-inflation effects to have a observable imprint on the (post-inflation) CMB power spectrum.
    As they say at the beginning, this non-LQC work is what Ashtekar and Barrau are using. I have highlighted the reference:
    ==quote Ashtekar Barrau http://arxiv.org/pdf/1504.07559.pdf ==
    Thanks to systematic investigations over the past decade, loop quantum cosmology (LQC) is now sufficiently developed to address these issues. As is common in physics, a more fundamental analysis introduces a new scale at which novel phenomena can occur. In LQC, as we discuss in section 2, because of the underlying quantum geometry, the big bang singularity is resolved and replaced by a quantum bounce. The curvature at the bounce is universal and introduces a new length scale lLQC. The key new phenomenon is the following: Pre-inflationary LQC dynamics modifies the standard inflationary predictions in a universal way for modes whose wave length at the bounce is larger than lLQC. Detailed analysis shows that these correspond either to the longest wave length modes observable today and/or modes whose wave length is larger than the radius of the observable universe but which can couple to the observable modes [2]. Therefore the pre-inflationary dynamics of LQC can have interesting ramifications for the ∼ 3σ anomalies in the Planck data associated with the largest angular scales.

    At first reading, this assertion may seem counter-intuitive on two accounts. First, one generally expects quantum gravity effects to modify only the short-distance behavior. How could they have any implications to predictions for the longest wave length modes? Second, it is often claimed that while quantum gravity effects may be conceptually interesting, they will not be relevant for cosmological observations because all they will all be diluted away during inflation. We will now discuss why these expectations are not borne out.

    The belief that the pre-inflationary dynamics does not matter stems from the following argument (left panel of FIG. 1). If one evolves the modes that are seen in the CMB back in time using GR, their physical wave lengths λphy continue to remain smaller than the curvature radius Rcurv all the way to the big bang. The equations governing the evolution of these modes then imply that they propagate as though they were in flat space-time and cannot get excited in the pre-inflationary stage. Therefore, the argument goes, they will be in the Bunch-Davies (BD) vacuum at the onset of inflation.

    But in the pre-inflationary calculations, dynamical equations of GR cannot be trusted in the Planck regime; we must use instead a candidate quantum gravity theory. In LQC, if a mode has λphy > lLQC at the bounce, it does experience curvature during pre-inflationary dynamics and can get excited (right panel of FIG. 1). For suitable choices of initial conditions at the bounce, these modes correspond to the largest angular scales seen in the CMB, roughly to l ≤ 30 in the spherical harmonics decomposition of correlation functions. Thus, the ultraviolet modifications of the background dynamics that cure the big bang singularity can directly influence the infrared behavior of perturbations. These longest wave-length modes, then, will not be in the BD vacuum at the onset of inflation [3, 4]. But why will this fact alter the observable predictions of inflation? Will not these excitations just get washed away during inflation? The answer is in the negative because of the accompanying stimulated emission. Agullo, Navarro-Salas and Parker have shown that if one were to start with a candidate non-BD vacuum at the onset of inflation, the stimulated particle creation would result in certain departures from the standard predictions based on the BD vacuum [5]. The pre-inflationary dynamics of LQC provides specific non-BD initial states at the onset of inflation, thereby streamlining the possibilities, leading to an interplay between the Planck scale physics and observations.
     
    Last edited: Jul 29, 2015
  5. Jul 29, 2015 #4

    Chalnoth

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    It looks like they're predicting some observational differences between LQC and inflation at large angular scales, claiming that their model provides a better fit to the data.

    Sadly, the statistics are a bit sparse at large angular scales, such that it's really hard to say that there's anything going on from that data alone. We'd need another, independent check to confirm. Not that we shouldn't go for such a check in any case, but the statistics can't ever be anything but mildly suggestive with this particular observational signal.
     
  6. Jul 29, 2015 #5
    Tahnsk guys, So what would it take to find the effect they are looking for?
     
  7. Jul 29, 2015 #6

    Chalnoth

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    It's not really possible to improve the statistics of the CMB temperature at large angular scales to any significant degree, unfortunately. So we really would need to add an entirely different observational signature to this one to get an answer.

    The problem is that the errors at large angular scales are today dominated by the fact that we can only observe one observable universe. At large angular scales, there are only so many degrees of freedom, and today our measurements have gathered at least 60% of the possible data that is available in this regime. If we could push that 60% to 100%, it would only drop our error bars by about 30%, which would hardly be enough to make a not-very-significant statistical result into a significant one. It doesn't help that chasing that last bit of the data is fraught with concerns over systematic errors (that's why we exclude so much of the sky in the first place: the stuff in our own galaxy gets in the way, making the measurements less certain, with the final result dependent upon precisely how you remove the foregrounds).

    One potential way to get at an answer to this issue would be to detect primordial B-mode polarization. The problem with that is that the LQC models don't predict any, so all that you could do is falsify the LQC model: you could never verify the model using B-mode polarization. That is to say, there are a large class of inflation models that predict B-mode polarization that is too small to detect, so if we don't detect any B-mode polarization, then we can't say which model is preferred. But if we do detect this polarization, then we know LQC is out.
     
  8. Jul 30, 2015 #7
    Really, I have to say that comes as a surprise to me. I understood that LQC generally includes inflation and inflation predicts B modes. When people thought BICEP 2 had found the B mode i didnt see anyone saying it was the death of LQC if true. I am not technically competent enough to understand all the technical papers but I try and read the conclusions as they are often written in plain English and I haven't seen this claim. Can you provide a reference?
     
  9. Jul 30, 2015 #8

    Chalnoth

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    My information may be out of date. It looks like they do predict some B-mode signal with LQC, and one that is distinct from at least some inflation models:
    http://arxiv.org/abs/1111.4661

    But it is true that the LQC bounce is a different sort of thing than more standard models of inflation, which (usually) use a scalar field to drive inflation, and assume that General Relativity remains accurate.
     
  10. Aug 1, 2015 #9
    On page 13 Ashtekar mentions suppression of low l for the T-E and EE correlations, if you could give a laymans guide to what this means and what sort of mission it would take to measure that would be very much appreciated.
     
  11. Aug 1, 2015 #10

    Chalnoth

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    The low-##\ell## modes have also been measured quite well for the TE and EE spectra. A little less well than the TT (temperature) spectrum because the foreground signal is much greater compared to the signal in TE and EE, but there still isn't all that much room for improvement.

    The best way to improve on current measurements is a new satellite mission with better control of systematic errors for polarization and more frequency coverage.

    The systematic error issue is a significant one for Planck, because it wasn't really designed as a polarization mission from the start. A number of balloon and ground-based designs have dramatically improved upon the systematic errors when measuring CMB polarization.

    A satellite is needed for low-##\ell## measurements largely because you really need to view the entire sky to accurately determine these modes, and you just can't do that from the ground or a balloon. These experiments are mostly good for small angular scales, or for B-mode polarization which has yet to be detected from satellite observations.

    Finally, because the foreground signal is so strong compared to the signal for E-mode polarization, you need to subtract the foreground signal. And the way to distinguish between what photons are coming from the CMB and what photons are coming from objects between us and the CMB is to look at the sky in a lot of different frequency bands. The more bands, the better we'll be able to determine how much signal is coming from the CMB.
     
  12. Aug 1, 2015 #11

    Chalnoth

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    As for *what* the polarization is, this is a good description:
    http://cosmology.berkeley.edu/~yuki/CMBpol/

    Regarding TE and EE specifically, the power spectrum in general is the variance of the fluctuations. When measuring polarization, though, what is measured is the covariance in a 3x3 matrix, with the dimensions being temperature, E-mode, and B-mode. TT, then, is the variance of the temperature fluctuations. EE is the variance of the E-mode polarization fluctuations. And TE is the covariance between the two types of fluctuations.
     
  13. Aug 2, 2015 #12
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