Post a Comment On: Backreaction

Apparently, the world did not end yesterday. Too bad, I was really looking forward to it. Now I will have to pay all the past due bills that accumulated in my mailbox while I was away.

As I mentioned earlier, I was in Paris last week, on the Planck 2006. The conference was fairly well organized, everything went without major disasters. The only annoyance were the security scans at the entrance every morning, where some uniformed guys repeatedly looked through the trash in my backpack.

Here are some comments about my personal favourite talks:



Most interesting theory talkMost interesting phenomenology talkMost entertaining talkMost deafening talk




Most interesting theory talk:

Keith Dienes
Temperature, Duality, And the Hagedorn Transition

Is based on the papers hep-th/0312216, hep-th/0312217, and hep-th/0507201 with Mike Lennek from the University of Arizona. The idea is roughly to find a formulation of thermodynamics that is compatible with string-theoretical expectations by construction. Remember how a finite temperature description is achieved in QFT. One goes from zero, T=0, temperature to a finite temperature, T>0, by compactifying a timelike coordinate on radius 1/T.
Now, to get stringy thermodynamics, assume that T-duality (symmetry under replacing the radius R with a dual radius ~ 1/R) holds for the compactified time-like coordinate. This consequently results in a duality under the replacement T -> Td2/T, where Td is some dual temperature.

However, standard thermodynamics unfortunately does not respect this duality. Instead, it is necessary to introduce a 'covariant' exterior derivative to generate thermodynamical quantities that respect the symmetry. Here, the covariant derivative just defined such that it respects the symmetry.

I will write more about this topic at some point because I really like it. Once the string-compatible thermodynamics is formulated, it provides a useful effective description of stringy effects at high temperatures. This in turn can be quite useful for cosmological implications.

I still wonder why this nice work has gotten so little attention.





Most interesting phenomenology talk:

Ann. E. Nelson
Mass Varying Neutrinos and Neutrino Oscillation Tests

Is based on the very readable paper Dark Energy from Mass Varying Neutrinos (astro-ph/0309800). The possibility that neutrino masses vary with the medium they propagate in has recently received increased attention. Though I don't particularly like this model, it is an interesting proposal.

The concept of the mass-varying neutrinos assumes a relation between neutrinos and the dark energy of the universe through a scalar field, the so-called acceleron. This implies then that the neutrino oscillation parameters in vacuum and a medium could be very different (this is not the standard MSW-effect).

Nelson discussed the implications of these models for neutrino phenomenology, for example in the sun, for cosmology, and astrophysics. Maybe most importantly, it could also explain the LSND data. However, she also pointed out that tests of mass-varying effects are very difficult, since the different experiments obtain data under different conditions, and little or no direct information is available on oscillation parameters in vacuum or air.

I should add that I personally don't like to fix problems by introducing additional, fairly unmotivated, scalar fields. Also, the precise predictions of the mass-varying neutrinos seems to be rather model dependent. For my taste, there are too many ambiguities in the
model.

However, in principle, the idea to connect the dark energy to the smallness of neutrino masses is nice, and it is a reasonable phenomenological model which is experimentally testable.






Most entertaining talk:

John March-Russell

Spoke about the always fascinating topic of TBA. Based on the paper Signals of Inflation in a Friendly String Landscape (astro-ph/0604254), it was a talk delivered with humor and an non-negligible amount of self-confidence.

Nevertheless, it left me with the impression that the consequence of the anthropic principle is that we now look for probability distributions of parameters that favor correlations we observe. Should it happen that a distribution has a high probability for correlations we don't observe, well, then we better look for another distribution. But in any case, there is no need to worry, coz the universe can always be just unlikely.

It seems to me that we have replaced looking for theories with looking for probability distributions. Given some distribution, hopefully motivated in one way or the other, a vacuum energy of value of so-and-so is with probability x correlated to a curvature of this-and-that value. But what do we learn from that?




Most deafening talk:

H. Nielsen
Fine Structure Constant relation and multi Point principle

Despite the enormous volume the talk was delivered in, I have not the slightest idea what it actually was about. I only vaguely recall that the speaker repeatedly mentioned some cheat, where he put factors 3 or 1/2 when necessary.

The lesson I learned from this talk is that with some accent, cheat sounds suspiciously like shit.