Dark matter is a quantum liquid? (buzz over McElrath's paper)

[marcus]

Rightly or wrongly, there's considerable excitement about a paper which a postdoc theorist at CERN, Bob McElrath, recently posted. One physics blogger headlined his comment "Dark matter is a quantum liquid."
The headline may itself be a misinterpretation of what McElrath is saying but I couldn't resist quoting it. Proceed with caution.

Here's the paper:
http://arxiv.org/abs/0812.2696
Emergent Electroweak Gravity
Bob McElrath
4 pages
(Submitted on 15 Dec 2008)
"We show that any massive cosmological relic particle with small self-interactions is a super-fluid today, due to the broadening of its wave packet, and lack of any elastic scattering. The WIMP dark matter picture is only consistent its mass M >> M_Pl in order to maintain classicality. The dynamics of a super-fluid are given by the excitation spectrum of bound state quasi-particles, rather than the center of mass motion of constituent particles. If this relic is a fermion with a repulsive interaction mediated by a heavy boson, such as neutrinos interacting via the Z⁰, the condensate has the same quantum numbers as the vierbein of General Relativity. Because there exists an enhanced global symmetry SO(3,1)_space x SO(3,1)_spin among the fermion's self-interactions broken only by its kinetic term, the long wavelength fluctuation around this condensate is a Goldstone graviton. A gravitational theory exists in the low energy limit of the Standard Model's Electroweak sector below the weak scale, with a strength that is parametrically similar to G_N."

John Conway, a professor at UC Davis (http://www.physics.ucdavis.edu/~conway/) had this interpretive comment:
http://blogs.discovermagazine.com/cosmicvariance/2008/12/27/gravity-emergesfrom-neutrinos/
Conway paraphrased McElrath's message this way:
" In a single sentence, it goes like this: What we know as gravity is actually the result of interactions with relic neutrinos, which satisfy all the conditions necessary to form a superfluid once the universe has expanded sufficiently."

He also quotes this from McElrath's concluding paragraph:
“…WIMP dark matter scenarios are inconsistent: WIMPs cannot both be decoupled and localized for the age of the universe...”"

Michael Schmidt, the blogger whose response was "dark matter is a quantum liquid!" is tenured faculty at Northwestern University. Here's his post:
http://muon.wordpress.com/2008/12/27/dark-matter-is-a-quantum-liquid/
Schmidt quotes this sentence from McElrath's paper:

"…today, WIMPs and at least two neutrino mass eigenstates are definitely quantum liquids."

And goes on to comment as follows:
"In the last third of this short paper, Bob makes some more exciting claims. He points out that a condensate will break Lorentz invariance. More specifically, Poincare invariance will be broken spontaneously and dynamically, and relic neutrino density will vary in space. This links standard model neutrino interactions with the space-time Lorentz group, and Bob identifies a generator for the broken symmetry and the associated Goldstone bosons. The field operator for the neutrino condensate and the propagating Goldstone bosons together form a 4-tensor that would be identified with the spin-2 graviton. Hence, Bob speculates, gravity arises from the standard model interactions of relic neutrinos which necessarily form a quantum super-fluid!"

 

[marcus]

Another physics blogger, Stephen "Cooley" Sekula, comments here:
http://steve.cooleysekula.net/blog/2008/12/16/gravity-in-daylight/

Here's a sample of what Sekula has to say:

"What if gravity is a consequence...of a known phenomenon: superfluidity?

In the paper, Bob suggests something which has apparently not been suggested before: neutrinos, or any other purely weakly interacting particle left over from the big bang (so-called “relics” of the big bang), interact SO rarely and SO weakly that by now, their wave functions are bigger than the space between them - they are “non-localized”, in the language of quantum mechanics. Just like electrons in a superconductor, this means that these relics are in a super-state - a super-fluid, in this case. Like electrons in a very cold conductor, below the superconducting critical temperature, if there is any force that is even slightly attractive between them they will form bound states - pairs, in the simplest case. This is analogous to the electron “Cooper pairs” of superconductors which make them superconducting.

Neutrinos - fermions, with half-integer spin - will then form bosonic bound states with integer spin. Bob identifies one of these as having the same properties as the hypothetical graviton, the particle that is alleged to carry the gravitational force. Gravity emerges as a consequence of the electroweak force - thus the title of the paper.

He might be wrong. In fact, in seminars he’s given recently he says as much. He notes that the theory is testable and falsifiable, although I would bet that will take some work even with our available technology. I find the whole notion intriguing. Rather than appealing to some far off energy scale, or vibrating 11-dimensional objects, what if gravity is as simple as neutrinos in a superfluid state?..."

Incidental information: Stephen Sekula and Edward Witten are two of the people whom McElrath thanks, for their useful comments, in the Acknowledgements section.

 

[Vanadium 50]

Marcus, Bob's idea is clearly an interesting new direction. It does have observational problems if you try and use it to explain Dark Matter. In particular, it explains Dark Matter by having a larger effective G in regions of space due to differing relic neutrino density. The problem is that doing this messes up stellar evolution and smears out the H-R diagram.

 

[marcus]

McElrath argues persuasively that weakly interacting relic particles such as neutrinos would have gelled into a superfluid over cosmological time
(the fact that collisions are so rare would mean disruptive noise is at a low level that could prevent their condensing)

OK. and it seems reasonable that this superfluid would collect in structures of varying density, forming strands and voids and haloes around clusters of galaxies---behaving as dark matter seems to behave.

And it seems to me that collections of such superfluid could either amplify gravity (vaguely analogous to how an iron core contributes to the field of a coil.) Or could---but this requires a stretch of the imagination and perhaps further assumptions---simply be gravity.

McElrath seems to be conjecturing that the imagined condensate is all there is to gravity and that has a definite appeal. But even if the condensate is not the whole story, if it merely enhances gravity, that would already be very interesting. It could help explain dark matter effects without explaining the whole caboodle.

So far I can't tell how opinion is going in the blog responses---can't exactly tell in which light commenters see the idea. Besides the obvious caveat that 99 percent of theoretical physics ideas turn out wrong and besides the fact that a fair number of people seem excited, what's to say?

 

[marcus]

Marcus, Bob's idea is clearly an interesting new direction. It does have observational problems if you try and use it to explain Dark Matter. In particular, it explains Dark Matter by having a larger effective G in regions of space due to differing relic neutrino density. The problem is that doing this messes up stellar evolution and smears out the H-R diagram.

Vanadium, just saw your post. Thanks for the comment! Have to go to supper, hoping to see more responses later.

 

[Vanadium 50]

McElrath seems to be conjecturing that the imagined condensate is all there is to gravity and that has a definite appeal.

That's definitely the direction Bob is going.

But even if the condensate is not the whole story, if it merely enhances gravity, that would already be very interesting. It could help explain dark matter effects without explaining the whole caboodle.

Probably not Dark Matter. The size of the effect is too large: you would need "effective G" varying by at least a factor of 5 and probably more like 10 to explain observations. That would cause a number of other easily visible effects: I mentioned stellar evolution before. Another problem is with Type 1a supernovae, which would have differing brightnesses depending on the local neutrino density.

Dark Energy on the other hand, is a whole 'nother kettle of fish. This theory has a time-varying gravitational constant, so you have a non-constant expansion rate out of the box. However, you also have a time-varying SN1a brightness, so the interpretation is non-trivial. I have suggested this to Bob.

By the way, Bob would argue that this is in the wrong section, as there's really nothing in this theory that's not in the SM.

 

[atyy]

Seems to be descended from Sakharov? That's my personal favourite line of thought. The main comments on the blogs seem to be how the coupling will be universal, and the Weinberg-Witten theorem. Usually to get round the WW theorem, people have emergent Lorentz invariance or spacetime, but how can that happen in the 4D Lorentz invariant SM of particle physics? But technicalities aside which I (a biologist) certainly can't judge, doesn't string theory itself point towards this direction (but beyond the SM, unless McElrath's idea works!) via AdS/CFT?

 

[atyy]

The Weinberg-Witten paper is not free online, but an essay by Lobbert is http://edoc.mpg.de/377397.

An interesting footnote in Weinberg and Witten's 1980 paper is a list of theories of emergent gravity not excluded by the theorem(s): A Sakharov Sov Phys Dokl 1968, O Klein Phy Scr 1974, P Minkowski Phys Lett 1977, A Zee PRL 1979, L Smolin Nucl Phys B 1979, Ikama et al Prog Theor Phys 1978, S Adler PRL 1980, Hasslacher and Mottola Phys Lett 1980.

 

[Hyperreality]

atyy, if you look on SPIRES, you may find a scanned version of the Weinberg-Witten paper via the KEK library,

http://ccdb4fs.kek.jp/cgi-bin/img_index?8101322

 

[dx]

L. Motl's opinion: http://motls.blogspot.com/2008/12/bob-mcelrath-gravitons-are-not-made-out.html

 

[MTd2]

Lubos is angry. That's all.

BTW, speaking of universal superconductors, I am halfway on Wilzec book. I was a bit lazy to read it, since all its beginning was aimed at a general public, but I noticed he has a really nice conceptual understanding of things, starting at ch.7

One can save money, by reading these short articles:

http://arxiv.org/abs/physics/0212025
http://arxiv.org/abs/physics/0511067v2
http://arxiv.org/abs/hep-ph/0502113
http://arxiv.org/abs/hep-ph/0605188
http://arxiv.org/abs/0708.4236v3

The essence is still in those articles.

 

[dx]

Lubos is angry. That's all.

Yes, but the question is, is he angry for the right reasons?

 

[MTd2]

Yes, but the question is, is he angry for the right reasons?

I really don't care about his private life...

 

[Phrak]

It seems, in the early universe, there would be a transition to superfluidity--and not necessarily a smooth one. Is this compatable with observation?

 

[atyy]

Quantum gravity as a Fermi liquid
Stephon H.S. Alexander, Gianluca Calcagni
http://arxiv.org/abs/0807.0225

Is this related?
Do LQG people accept this reformulation?

 

[Xezlec]

My understanding of this subject is microscopic, but the idea of knocking out one of the forces, explaining what the remaining 96% of the universe is, giving a theory of quantum gravity, giving all those kajillions of snooty neutrinos out there some actual work to do, AND finding a really sweet application of the theory of superfluidity all in one shot just sounds too damn good to be true. As a pessimist, I vote wrong, just because God would never be that nice to us.

(EDIT: changed "superconductivity" to "superfluidity". D'oh! Perils of being an EE.)

 

[atyy]

Quantum gravity as a Fermi liquid
Stephon H.S. Alexander, Gianluca Calcagni
http://arxiv.org/abs/0807.0225

Is this related?
Do LQG people accept this reformulation?

Brian Eno is a LQG fan?!
http://www.interactions.org/quantumdiaries/bios/stephon_alexander.html

 

[marcus]

Atyy, thanks for reminding me of that Stephon Alexander paper. It's an unusual and remarkably interesting paper. I added it to the biblio thread when it came out in July https://www.physicsforums.com/showpost.php?p=1788251&postcount=728
but was uncertain what to make of it. Until you called attention to it, I didn't notice that they had revised it and posted a new version 21 November, now accepted for publication.

Maybe we should include it as a write-in candidate in the poll

http://arxiv.org/abs/0807.0225
Quantum gravity as a Fermi liquid
Stephon H.S. Alexander, Gianluca Calcagni
43 pages, 1 figure
(Submitted on 1 Jul 2008)

 

[Haelfix]

Theres a lot of ways this could go really wrong as it gets fleshed out, in much the same way as many of the spinor gravity models ate dust long before Weinberg Witten. The typical problem that usually kills these types of ideas are potential violations of the equivalence principle b/c that's been tested to very high accuracy and very constraining.

Bob desperatedly needs to show universality here, but that's going to be extremely hard. Its not hard to imagine any number of quantum effects that could spoil things, and trying to hide them or finetune them away will be quite a chore, particularly if you accept the premise that there exists very high energy processes that are unmasked in the universe (eg black hole collisions and so forth)