Wednesday, October 09, 2013

Quantum Gravity in the Cosmic Microwave Background?

Gravity waves. They are pretty but have
nothing to do with gravitational waves.
Image Source: UWO.
Krauss and Wilczek recently posted a brief note on the arXiv. They present a dimensional argument that claims signatures of relic gravitational waves in the cosmic microwave background (CMB) would be evidence for quantum gravity.

Relic gravitational waves are perturbations of space-time created at the Big Bang. They cannot presently be directly detected, but if they exist they would affect the polarization of CMB photons. The Planck satellite mission is about to deliver data on CMB polarization, so Krauss and Wilczek’s is a very timely contribution.

While their dimensional argument is original and compelling in its simplicity, what they say is not particularly surprising and known to researchers familiar with the subject. The argument means essentially if there are no suitable matter sources that could cause space-time perturbations, then the only way relic gravitational waves can have been created is through quantum effects. That’s because it needs a mass scale to get the dimensions right and Newton’s constant will only give a mass-scale when suitably combined with Planck’s constant, thus indicating a quantum effect.

The argument however only works without matter that brings in anisotropic stress. It would still work if the matter was solely scalar fields because these don’t contribute to the anisotropic stress, but electromagnetic radiation could deliver such a contribution. Be that as it may, this means by a purely dimensional argument alone it is hard if not impossible to reverse the logical arrow, that being the question whether relic gravitational waves could have been created in a non-quantum fashion.

Few few people doubt that relic gravitational waves exist and are quantized. It would certainly be exciting to have evidence that this treatment of the early universe is correct, but it must be said that this is not evidence for what the community commonly refers to as quantum gravity.

“Quantum gravity” is normally meant to be the fundamental theory for the quantum nature of space and time. The quantization that is being used for gravity in the early universe is normally explicitly referred to as “perturbatively quantized gravity”. It is expected by all but a few dissidents that perturbatively quantized gravity is the correct effective limit of any theory of quantum gravity. The mere existence of such quantized perturbations thus tells us little. More telling is the spectrum of the perturbations which depends on what happened in the early universe, for example on whether there was a Big Bang or a Big Bounce, and that does indeed depend on the full theory of quantum gravity.

Evidence for relic gravitational waves would give strong support to the validity of perturbatively quantized gravitational waves (essentially quantum field theory in curved background), but it takes more than a dimensional argument to show that other models cannot produce the same observation. And even if that could be shown, the mere existence of the gravitational wave background does not teach us much about the non-perturbative theory of quantum gravity. Thus, Krauss and Wilczek’s argument makes a good point but its relevance for research in quantum gravity is limited.

Kudos to Jakub Mielczarek for helpful communication.

Bonus: Krauss at a recent discussion following his public lecture in Stockholm. Spot the American among the Swedes :p

Lawrence Krauss in Stockholm. Still from this YouTube Video.


  1. Yep, sounds like thin gruel Sabine, clutching at straws again. I was reading this piece fairly happily until I came to Big Bounce and then curved background. It strikes me that people in quantum gravity would benefit from having another look at classical electromagnetism and relativity, and the wave nature of matter. To understand what I mean by this, try explaining why an electron and a positron move together, and then why an electron falls down. IMHO this is where it starts.

  2. "Spot the American among the Swedes".

    Somehow, that reminded me of this comparison between Sweden and the UK. Sorry, couldn't resist. :-))

  3. The subject of quantum gravity represents all phenomena between general relativity and quantum mechanic scales, i.e. beetles, plants, etc.. - i.e. not just CMBR background. In AWT the CMBR noise, gravitons and gravitational waves are equivalent concepts. The CMBR exhibits a quantization of the red shift.

  4. Zephir so true but can we not imagine a higher concept beyond these hard fought for models of physics to which looking back it can seem thin and irrelevant gruel indeed. Some hold we are still very much children in what can be known in coming science and philosophy. The poetic idea of seeing the universe in the palm of your hand. Very young I touched my small globe and imagined whole new universes in the electrons. At first I removed my finger to consider if I would be smashing anything. Sometimes the child in our genius have such moments (awe struck when I understood Newton's laws so late on the way to middle)
    school. Even Carl Sagan mentioned this idea in a video to which same say if we could see into the wobble of an electron we could see evidence of whole other parallel universes. Timely, indeed as we make clearer our eyes.

    Sabine, in the sensible search for the how of it all... thank you for the reality check.

  5. Gravity waves are a subset of Kelvin–Helmholtz instability from shear of two fluids (re Big Bang photons and matter). Curl is in there, too (arxiv:1112.0567). If the Big Bang had (local, then cosmic inflation to our bubble) non-zero curl, it need not have been achiral isotropic, ever and ever after.
    Instabilty evolution/time

  6. Hi Bee,

    A central theme to a lot of your posts.

    Such clarifications do separate what you hold with regard to phenomenological approach and what cannot be measured. I get that.

    Yet, you are lead to questions around experimental design in order to show whether something can be scientifically proven?

    It seems to lay waste to any effort to logically ask questions, with regard to using theoretical models that would include calculations mathematically deduced?


  7. Better yet, at 40:32 how may of it sparked this post?

    Just wondering.

  8. Regarding “seeing quantum gravity in Big Bang traces”, it sounds a bit like reading fortunes in tea leaves. There is a strong inclination to find what you are looking for whether it is really there or not.

  9. Robert,

    It isn't quite as bad, but it is true that the more people analyze the CMB data by different methods, over and over again, the more likely it becomes that somebody will eventually find something just by chance. Unfortunately, the 'many people trying different things' factor isn't taken into account in the statistical significance. Be that as it may, the actual problem for the polarization is that there are many models that make similar predictions and without other data it'll probably be impossible to tell them apart. Best,


  10. Agreed Robert. But I don't think that's really the problem here. See this in the paper:

    "Indeed, recently Freeman Dyson and colleagues [2] have cogently estimated that it may in fact be infinitely more daunting, namely that it is likely to be impossible to physically realize a detector sensitive to individual gravitons without having the detector collapse into a black hole in the process".

    That's totally barking up the wrong tree. So's this:

    "Since no field other than gravity is involved, we infer that quantization of the gravitational field is an essential ingredient".

  11. I will sum it up as on my post this morning on my creative science and philosophy blog:

    Physicality of Intrinsic Linearity in
    Superconductive Stereometries

    L. Edgar Otto 10 October, 2013

    In the maximal bonding of carbon, linearly, between atoms with 2+2 or 3+1 bonds, we should pose the question: In what respects can it relate to superconductivity?

    This I imagine intimately relates to the issues of artificial magnetism induced in graphene that considers the Bose condensates by virtue of holes in broken congruence field overlapping flat orthogonal structures that depends on the local (quasic) proximity of isotopes in the Bell assumptions of non-locality, thus thermodynamic symmetries.

    In the principles of symmetry breaking, or conservation in the widest field of identities at a given dimension, our models should be compared as to the grounding of matter with these more general principles of stereometry in mind.

    In disconnects between with bridges between the generations and fundamental forces, we may have even higher exclusions such as magnetism where the combinations of generational hierarchies inter-relate across the given physical dimensions of standard forces. But in matters of gravity or monopoles as ultimate these higher force considerations my analogously be ultimate exclusions to which we cannot explain by magnetism the evaporation of black holes even if that the last mechanism left to model.

    This so mirrored implies at an entity of singularity or a sea of singularities, that which is superdetermined so looped and measured intelligibly as part of the teleology, dark matter and energy ideas, if substance in theory as well as physicality, as an analogy of the Fermi and Bose distinction at a higher level, are reasonably explained.

    If this is not close or the Omnium as a theory of everything in the spirit of partial models half seen or half-hearted in our quest... what more can it be?

    * * * * *

  12. @ L. Edgar Otto "In the maximal bonding of carbon, linearly, between atoms with 2+2 or 3+1 bonds..." Tell us the molecular orbital difference between (-C#C-)_n and (=C=C=)_n. Polycarbyne synthesis via acetylene oxidative coupling is no biggie on paper. What surprises await you when doing it?

    Your et sequens word salad is a physics cold compost heap. Benzyne plus anthracene Diels-Alder into triptycene. 9,10-Dibromoanthracene, benzyne, then Wurtz couple into polymer (MW control with 9-bromoanthracene chain terminator), then anhydrous FeCl_3 in nitromethane to close rings. (Re hexaphenylbenzene to a lovely yellow product with melting point higher than its glass mp capillary.) P- or n-dope to a supercon. For polymer solubility, anthracene monomer should have a tail or two of -O-(CH_2)_nCH3, or poly(ethylene oxide), or poly(propylene oxide) oligomers. Spontaneous lyotropic liquid crystal ordering for Green chemistry filament spinning suggests Br_2An-O-(PPO-PEO block oligomer) starting material.

    As Nunavut First Americans say," Iqualuit."

  13. I've noticed that when famous physicists enter their decline with advancing age, one common symptom of their problems is a belief that extremely difficult problems can be solved with elementary techniques. The notion that one can say anything useful about this subject using nothing more than dimensional analysis just makes me want to say, "You're not serious, are you?" [Something I once heard Wilczek say.]

  14. Two fermions walk into a bar. The first says “I’d like a vodka martini with a twist.” The second says “Dammit, that’s what I wanted!”

    Well forgive me, not too many places to use that joke.

  15. And the bartender (who is also a fermion)says "Well, since its quitting time I think I'll join you. I feel like getting a bit twisted myself."

  16. Go on then, I'll have a go at that:

    Biaxial wave motion in a Dirac's-belt closed loop. See magnetic dipole moment and the Einstein-de Haas effect then look at the ring torus here. Then note that the electron's "electric field" is spherically symmetric so move on to the spindle-sphere torus. Then lose the surface and think frame-dragging instead.

    A cyclone has intrinsic spin. It makes it what it is. Take that away using an anticyclone and all you've got is wind. An electron has intrinsic spin. Take that away using a positron, and all you've got is light.


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