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CMB Anisotropies Reveal Quantized Gravity

  1. Jan 9, 2004 #1
    Dated as of today:
    http://arxiv.org/abs/astro-ph/0401070

    What do you think guys? :)
     
  2. jcsd
  3. Jan 11, 2004 #2
    From a brane world perspective some of this is to be expected. As brane tension has local effects on gravitation and also the local vacuum state. Following a normalized Schrodinger equation the quantum spectrem should display aspects of whatever eventual model is found to be correct. A good place to do local experimentation would be the study of quantum scale black holes for their Hawking radiation signature since the differences would be more pronounced at that scale when and if some are eventually created in accelerator experiments.
     
  4. Jan 11, 2004 #3

    marcus

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    I think first off it makes a difference who is Ken-ji Hamada (the principal author of the paper you mentioned: "CMB Anisotropies Reveal Quantized Gravity")
    and what earlier papers has he published about the same thing.
    Before 1998 he was doing string theory but around 1998 his interest seems to have shifted. Now most of his work explicitly concerns his own 4D quantization of GR which he calls "Quantum Gravity": a 4D theory (no extra dimensions) that has Background Independence and Diffeo-Invariance. Here are titles

    http://arxiv.org/abs/hep-th/9810095
    Background-metric Independent Formulation of 4D Quantum Gravity

    http://arxiv.org/abs/hep-th/0005063
    On The BRST Formulation of Diffeomorphism Invariant 4D Quantum Gravity

    http://arxiv.org/abs/hep-th/0105069
    Note on Quantum Diffeomorphism Invariance, Physical States and Unitarity

    http://arxiv.org/abs/hep-th/0203250
    Resummation and Higher Order Renormalization in 4D Quantum Gravity

    Hamada has tended to publish his work in "Progress in Theoretical Physics" and in "Modern Physics Letters".
    Before 1997 he had a long run of string/brane papers. Also published in various established journals.

    I have to give him my respectful attention when he says something even tho it sounds strange and unexpected. In the recent (7 January 2004) paper you cited he is saying surprising stuff about seeing quantum gravity effects present in the early universe that have been "magnified" by the lens of inflation----seeing "evidence" of his
    (background indep, diffeo-invariant) model of 4D "quantum geometry".

    In several ways it is very much like Bojowald's loop quantum cosmology which also looks at early universe (big bang, inflation,etc.) with a BI-DI 4-D quantum geometric model, but of a
    different type, and gets results. But Bojowald did not say anything yet, that I know, about seeing ripples in the CMBR spectrum that tell him quantum things about the early universe. I have to say Hamada seems to me rather boldly going out on a limb, but good for him, more power to him.

    Well alexok, thanks for the link making us notice Hamada. And you asked "What do you think?" so I am telling you my immediate reaction. I hope you have or somebody has more thoughtful comments than mine about this!
     
    Last edited: Jan 11, 2004
  5. Jan 11, 2004 #4

    hellfire

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    Despite from the fact that I did not understand anything regarding quantum gravity, I am interested in CMB physics and I found the following paragraph confusing:

    So, why? What is the reason to look for the cause of the fall off in dynamics? Are there explicite reasons, or is this a paper which shows that QG could explain the fall off in case that it would not be produced by statistics?
     
  6. Jan 11, 2004 #5

    marcus

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    hello hellfire, since no one else has stepped in I will try a brief intro.

    the crown jewells data from COBE and WMAP satellite obs of the CMB is the "angular power spectrum", more has been told abt the early universe and overall shape of universe than anything else I can think of. so better get a rough idea of a CONVENTIONAL interpretation of the spectrum (before trying to fathom Hadama's new spin on it)

    better look at Figure 10 of the great and indispensible Lineweaver
    http://arxiv.org/astro-ph/0305179
    this is on page 25 of Lineweaver
    there is a CalTech online version of Lineweaver with higher resolution Figures but the ordinary arxiv.org PDF download is good enough.

    the CMB ripples!!!!! and there are different relative amounts of rippliness at different scales and the important numbers are
    C1
    C2
    C3
    ....
    Cl
    ....

    This parameter l called "ell" is the multiplicity, how many peaks and troughs in the whole 4 pi steradians, the whole sphere.
    ell = 1 is the dipole anisotropy, just one hotspot and one coldspot.

    ell = 10 corresonds to ripple peaks about 10 degrees apart
    ell = 100 is ripples with peaks about 1 degree apart
    ell = 1000 is very fine ripples with peaks about 1/10 degree apart

    You can see this from the Lineweaver Figure 10.

    now Lineweaver describes in a kind of comic strip cartoon way (Figure 11) how conditions in very early universe can show up in the rippling of CMB and in particular in the acoustic power SPECTRUM which says comparatively how much rippliness there is at varying closenesses

    like how much power is in ripples with peaks 10 degrees apart versus in ripples with peaks 1 degree apart.

    I have to go help with supper but will get back later
     
  7. Jan 11, 2004 #6

    Nereid

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    Interesting.

    How well does the data match his theory? Figure 2 looks good, but it's only eye candy; there's no formal analysis of goodness of fit in the paper.

    Some questions:
    - why aren't error bars displayed for the WMAP data points?
    - why is [tex]l = 6[/tex] chosen for normalisation?

    [Edit: I had intended this post to refer to alexsok's thread-starter; due to an unconscionable delay in submitting my post, it ended up looking like it referred to marcus' comment. Sorry if this mislead any readers or caused confusion.]
     
    Last edited: Jan 11, 2004
  8. Jan 11, 2004 #7

    marcus

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    Nereid you are moving into discussing Hadama when we didnt yet really talk about the CONVENTIONAL discussion of the acoustic power spectrum.
    I just had supper and I'm back. I would soooooo love it if you Nereid would explain Lineweaver Figures 10 and 11 and give the usual spiel about how the acoustic power spectrum of the CMB agrees with the U being spatially flat.
    The reading of the CMB temperature ripples is so recent and so important and so nifty, such a neat trick.

    How about it Nereid. Could you do the tutorial?
    Notice Figure 11 gives you 4 levels of comic strip (time moving to the left) to explain the humps in the spectrum---the spectrum is the same as in Figure 10 but plastered on the lefthand margin so the comic strips can explain its humps.

    I think its good pedagogical graphics.

    And then follow up with INTEGRATED SACHS WOLFE to see how light that passes thru a deeper gravity well gets hotter
    so then the hotspots in the CMB (if you take away the dipole due to our motion relative to the expansion of the universe) correspond to overdensity spots in the early universe that existed at z = 1100
    the decoupling redshift.

    Of course Hadama talks about Integrated Sachs Wolfe ISW (something any skateboarder would understand) but who cares about Hadama if we arent even at first base with this yet. ISW is also a bit like the way you "pump" a swing. You coast down into the well and then the universe expands so it takes less energy to get out of the well and you have gained some.

    I'm actually leery of Hadama because he is so much on the edge and who knows if entirely kosher.

    That deviation arould ell=1 is something you can see in Lineweaver's
    Figure 10 and it looks reasonably much like random scatter of which maybe not so much should be made as Hadama makes of it
     
  9. Jan 13, 2004 #8

    hellfire

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    My understanding is that the Sachs-Wolfe effect is the dominating one and therefore cold spots correspond to overdensities, since photons are redshifted when escaping the potentials just after recombination. You can read here:
    http://www.astro.ubc.ca/people/scott/faq_intermediate.html about that.

    This is an interesting figure. I do not understand why the doppler power is out of phase with the acoustic power. Any help?

    Regards.
     
  10. Jan 13, 2004 #9

    marcus

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    Glad to meet you, hellfire. Thanks for the link to that British Columbia FAQ about the Cosmic Microwave Background, which I had not seen. (Douglas Scott, 1999)

    this is just the right kind of question to be asking, its not overwhelming but has a little grit to it. If you keep researching and probing I (for one) am going to learn from the discussion, maybe you too.

    I probably got something backwards. Doug Scott (at least in intermediate FAQ) says hotspot corresponds to UNDERDENSITY!!!
    Whereas I had hastily supposed (not being particularly knowledgeable about this) that a hotspot would correspond to overdensity.

    In case you are curious how I got that notion, here's how I thought about it: the photon we see coming from an overdensity FELL INTO that overdense patch and gained energy and then (while it was down in the well) space expanded and the well got shallower. So climbing out used up less energy than what it gained by falling in. I believe that is the mechanism for a type of blueshifting called "integrated" S-W. But I think I misapplied it.

    At "recombination" when space becomes transparent, or at the "last scattering" time, however you call it, it is probably just as Scott says and hotspots are underdensities-----later there may be a slight amount of blueshifting as the photon travels thru exapanding overdensities along the way, but that doesnt alter the basic correspondence between hotspot and underdensity.
    Does this agree with how you understand it?

    See if this makes sense. The dotted line tracks the doppler power. The power spectrum does not distinguish between hotspots and coldspots but measures the amount of rippliness at a certain "ell" (a kind of wavenumber). So ANY motion towards or away, in the scattering medium, contributes power.

    So the dotted line has peaks or humps both when the cloud is collapsing and when it is expanding, due to the falling in and rebounding motion. The dotted line basically tracks the speed of
    motion in the medium. The speed is going to be 90 degrees out of phase
    with the density (the usual 90 degrees in all kinds of oscillators).

    The figure shows both this doppler effect and the gravity-well effect. Since the light solid line tracks the gravitational effect, it has its humps where the blob is either very dense or very sparse, and the gravity-well is either very deep or the opposite. So these two have to be 90 degrees out of phase. D'accord?

    It does seem like the doppler contribution is less important, as I think you indicated. So the dark solid line which is the sum of the two mainly follows the humps of the gravitational effect (light solid) curve.
     
  11. Jan 13, 2004 #10

    hellfire

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    Marcus, I agree with you in the distinction you made between SW and ISW effects. But I think ISW does not necessarily lead to the contrary effect than the SW (which leads unambiguously to a redshift). It depends whether the potentials are increasing or decreasing over time. If they decrease, then I agree completely with your exposition. But if they increase, the ISW will lead to an additional redshift. As you probably know, the behaviour of potentials does not depend merely on the expansion of the universe, but on which 'kind' of expansion is taking place: radiation dominated (potentials will decrease), matter dominated (potentials will increase), or cosmological constant dominated (potentials will decrease also).

    Regarding your explanation of the figure: I think it will take me more time to understand. I'll read this again carefully. Thanks.
     
  12. Jan 13, 2004 #11

    marcus

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    Likewise! (maybe Nereid or one of the others will step in and clarify too, but anyway if there are any stray thanks buzzing around, they go to you)
    BTW do you happen to have any links or numerical estimates about the cosmic neutrino background you think worth passing on.
    Lineweaver has something on page 24---the expected temperature is
    1.95 kelvin for the neutrinos, a bit less than the measured
    2.725 for the cosmic photon background (CMB)

    I just happened across an estimate of the number density of
    the neutrino background---100 particles per cubic centimeter.
    would really like to have some better handles on what the neutrino background is expected to look like (when eventually seen)
    the things are so old! (making CMB photons look like John-Lately)
     
  13. Jan 14, 2004 #12

    hellfire

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    My understanding is that the light solid line in Figure 11 of Lineweaver corresponds to the gravitational and temperature effects and has its first maximum (at low l ~ 2) produced by the Sachs-Wolfe effect.

    It is not clear for me which are the different contributions of SW and ISW to the low multipoles.

    On the other hand I’ve also read somewhere, that it is precisely the ISW effect the one which is confined to low multipoles and suffers of cosmic variance (statistical uncertainty for the generation of the correlation function).

    This is also unclear for me and this may be one of the points to understand the reasons which lead to the paper of Hamada.

    The second maximum of the light solid line (l ~ 200, as can be seen in Figure 10) is produced by collapsing density perturbations which reach the sound horizon scale during recombination. This brings these perturbations to start oscillating (Jeans condition) and is reflected in the CMB with a higher temperature of photons in the corresponding overdensity regions.

    Perturbations which are smaler did already start oscillating, but do not necessarily freeze in a maximum and perturbations which are bigger did not start oscillating (and obviously will not start, since the fluid stops oscillating at recombination).

    I do not understand how this is depicted in the figure 11.

    I understand now why doppler peaks are 90° out of phase with acoustic oscillations: the acoustic velocity in the line of sight vanishes where the acoustic density oscillation reaches its maximum.

    It would be nice if someone could confirm these points or ellaborate on my questions. Thanks.
     
    Last edited: Jan 14, 2004
  14. Jan 16, 2004 #13

    Nereid

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    I was coming at the paper from the perspective of the CMB being an observational test of the theory. This is my primary interest - how well does a theory/idea/hypothesis match the observational data? In this case, they claimed their theory matches the WMAP data; IMHO, without a 'goodness of fit' analysis, the claim is weak, being based simply on 'looks good to the eye'. Also, they did their normalisation at l = 6, but didn't say why.

    It could be that their theory is strongly inconsistent with the WMAP data; for example, it doesn't account for the strong l = 4 peak, nor the peaks at 10, 27, and 31; nor the trough at l = 21. Perhaps Hamada and Yukawa might say these discrepencies are due to other effects? Perhaps; their paper is silent on this.
     
    Last edited: Jan 16, 2004
  15. Jan 16, 2004 #14

    marcus

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    that is, a test of Hadama's ideas about quantizing Relativity. They dont seem to be specifically String Theory ideas (although he did string research earlier), or Loop ideas either.

    My reaction was, I guess, similar to yours----skeptical that there was any real fit to the data (beyond random fluctuation at one or two datapoints giving a seeming fit) or much substance to the "Hadama theory".

    What I wish is we could consign "Hadama theory" to the back shelf (since I'm skeptical of its importance) and take this as an opportunity to learn a little about the basics of the CMB Acoustic Power Spectrum.

    Hellfire has asked some pointed questions (not about Hadama but) about the wrinkles in the Background. How we interpret them as information about very early fluctuations in density.

    Do we have someone who knows the basics of "CMB Wrinkles" well enough to reply to Hellfire?
     
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