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Occam's lost his razor on the seesaw


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#1    Startraveler

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Posted 01 August 2007 - 07:27 PM

Dark energy's a very mysterious something that's pushing the universe apart faster and faster. This is an interesting idea as to where it comes from but it doesn't seem particularly parsimonious as Occam's razor ("entities should not be multiplied beyond necessity") would like. Perhaps we're bumping against the edge of necessity here. Anyway, have a read.

PhysicsWeb:

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'Seesaw' explains light dark-energy particles

Dark energy - the mysterious stuff thought to be causing the expansion of the universe to accelerate - could be the result of eight fields interacting via a "seesaw mechanism". This is the claim of physicists in Finland and Denmark, who say that the fields would be incarnated by a single, very light particle (Phys. Rev. Lett. 99 031301).

The top-contender for dark energy is the cosmological constant, which predicts that quantum fluctuations existing in the vacuum could create a constant energy density throughout space. But another dark-energy contender is quintessence, a scalar field with a negative pressure that, on large scales, would overcome the attractive force of gravity.

Unlike the cosmological constant, quintessence would be able to vary with time, thus explaining why the acceleration has only become significant in the past five billion years or so. Unfortunately, the particle-like mass associated with quintessence’s scalar field would have to be more than 38 orders of magnitude smaller than an electron, and so would easily be wiped-out by the vacuum fluctuations.

Kari Enqvist from the University of Helsinki and colleagues from the University of Aarhus, however, say they can solve this problem by borrowing an idea from particle physics that explains why neutrinos in the Standard Model are observed to have a very small, but not zero, mass. Called the seesaw mechanism, it suggests that the observed left-handed neutrinos - that is, those spinning anticlockwise in the direction they are travelling - can get their small mass if the equations are “balanced” by much heavier, right-handed neutrinos.

But to explain quintessence with a seesaw mechanism, say Enqvist and colleagues, you would not need just two types of particles, but eight different scalar fields. Each of these fields would be associated with a large particle of mass 1 TeV or more, but would interact to produce a single “physical” quintessence particle with a very small mass. Crucially, this particle would be protected from vacuum fluctuations, because they would only act on all eight fields as a whole.

Although eight fields may sound a little far-fetched, they would also have just the right properties to explain the early “inflation” era of the universe when the expansion was exponential. According to Martin Sloth, one of the Aarhus researchers, this would mean future observations of the cosmic microwave background - radiation left from an era after inflation - could shed light on the new theory’s credibility.

“Since explaining dark energy is a very challenging problem…it is certainly worth looking into such ideas,” Josh Frieman, the physicist who originally highlighted the problem of a small quintessence-particle mass, told physicsworld.com. Speaking about the possibility of eight dimensions, he added: “That could be the right explanation, but one might still hope that in the end nature is more parsimonious.”

About the author

Jon Cartwright is a reporter for physicsworld.com



#2    camlax

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Posted 01 August 2007 - 11:45 PM

Very interesting, where did they publish I dont have time to look it up.

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#3    Startraveler

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Posted 01 August 2007 - 11:47 PM

It's in PRL.


#4    camlax

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Posted 03 August 2007 - 12:08 PM

Quote

It's in PRL.



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indeed, the present scenario yields the possibility of a radiatively stable particle physics motivated scenario for both quintessence and inflation


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As an interesting curiosity, one may note that an eightby-eight matrix is the smallest which can provide the particular features that we require of the quintessence seesaw mass matrix, yielding small enough eigenvalues and mixings. However, to realize such a matrix with the type of brane configurations discussed in the previous section, we must require exactly six extra dimensions, which is also the generic prediction of string theory.


Very interesting indeed. That it works with current models is very compelling. It seems they may be on to something here. Will be something to keep on eye on over the next year.

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#5    Harte

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Posted 03 August 2007 - 02:49 PM

The agreement with the extra-dimensional requirements of the current incarnation of String Theory makes this idea very interesting.  Now I'm wondering if the authors' started out with the idea of six extra dimensions because of their knowledge of string theory (which would be disappointing,) or if the six extra dimensions simply "resolved out" of their theory for quintessence (which would be somewhat exciting.)

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#6    camlax

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Posted 03 August 2007 - 03:17 PM

Quote

The agreement with the extra-dimensional requirements of the current incarnation of String Theory makes this idea very interesting.  Now I'm wondering if the authors' started out with the idea of six extra dimensions because of their knowledge of string theory (which would be disappointing,) or if the six extra dimensions simply "resolved out" of their theory for quintessence (which would be somewhat exciting.)

Harte



It appears from reading it, the extra dimensions were emergent from the matrix they used to extrapolate their branes. To predict the brane configuration exactly 6 dimensions (no fracturals it appears as well) are needed.

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In the directions where there are zeros in the off diagonal, the branes are separated by a distance r  262=M. If we assume that there are three brane fixed points, A, B, and C, in each dimension, with A and B separated by r  60=M and C further away separated at a distance r  262=M, this leads to the brane configuration for eight branes in six extra dimensions shown in Table I. For instance, in dimension one we will have Brane 1 and Brane 4 localized on top of each other at fixed point A while Brane 2, 7, and 8 are localized on top of each other at the fixed point B. Brane 3 is localized at the fixed  point C in this direction. The remaining branes are not localized in this direction. In order to stabilize the setup under perturbations, we can assume for simplicity that the branes are sitting at the fixed points of some orbifold symmetries.


Edit:Bah sorry I have recently wiped my work laptop and I dont have my symbol font back on it. (Also not sure if the board would support it anyway). The squares should be approximation symbols.

Edited by camlax, 03 August 2007 - 03:19 PM.

"Sorry, but my inner voice tells me to tell your inner voice the following:
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