Monday, 26 May 2014

Planck 2014 Liveblog: Day One Session 3

The first post-lunch session looks like a string theory session.

2:30 pm: Pre-inflationary Clues from String Theory? Augusto Sagnotti

First, let us consider the problem of SUSY breaking in string theory.  Can be broken in the bulk of an extra dimension through the Scherk-Schwarz mechanism, or on branes using ?fluxes?

All approaches require redefinitions of vacua, with all the problems that implies for the very definitions of operators.

Okay, I'm lost already.

It would also help if I could actually download the slides, I might be able to look at the early ones and figure out where it lost me.

3:00 pm: Cosmology with Massive Gravity and Beyond, Gregory Gabadadze

Okay, not a string theory talk which makes me wonder why it has been sandwiched between two of them.

Simple extension of GR: replace DE term Λ gμν with Λ Xμν, for some X.  Useful as alternative to GR, but aim to address CC problem without a landscape.  Essentially, Λ is scale where GR is modified; will correspond to graviton mass.

This can be expressed as: what is the (allowed) potential of gravity?  Theory requires "vector" of scalars; this reminds me of something I once worked on, but I can see the error I made.  The scalars are not a true 4-vector, and their index needs contraction with an internal metric.  This lets us write a true gravity potential up to quartic terms.  Though the definition of the matrix K in terms of the square root of the scalar kinetic terms makes me worry about things.  This worry is unfounded: theory has natural expression in terms of vierbeins that are linear in the scalar fields.

Theory gives five unitary degrees of freedom on (nearly) Minkowski backgrounds.  However, there is no guarantee that the theory remains unitary on arbitrary backgrounds, and indeed counter-examples exist.

Theory also exhibits superluminal velocities, though this may not lead to acausal behaviour due to running into strong coupling regimes.

Problems may be due to lack of Higgs analogue in massive gravity.

Theory generically has self-accelerating solutions, but also generically has ghosts/strong coupling and also anisotropies, so not yet able to really address the CC problem.

3:30 pm: Gobal F-theory models with U(1) symmetries, Sakura Schafer-Nameki

F-theory allows for a systematic approach to model-building (which is "bottom up" by string theory standards).  Generically must include symmetries to forbid various dangerous operators.

F-theory based on "minimal" theory: N=1 SUSY SU(5) GUT.  Symmetries to forbid various nasty proton-decay operators etc.

The bottom-up approach includes constraints systematically.

I'm lost again, but this time it's a limit of my knowledge rather than a quality of the talk.

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