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."(adsbygoogle = window.adsbygoogle || []).push({});

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 Z^{0}, 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!"

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

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