Week 09.11.2014 – 15.11.2014

I discuss the quantization of a perfect fluid. This differs from textbook QFT, because of the presence of vortex modes, which map to an infinite collection of quantum mechanical free particles rather than harmonic oscillators. As a result, there is no Fock space and no S-matrix. I argue that there exists, nevertheless, a consistent effective field theory description, valid at large distances and times.

Galaxy clusters are the most efficient convertors of axion-like particles to photons in the universe. I discuss the physics and phenomenology of ALPs, and describe their astrophysical implications, with particular reference to the recently observed 3.5 keV X-ray line that is a candidate for a dark matter decay line.

In many instances of holographic correspondences between a d dimensional boundary theory and a d+1 dimensional bulk, a simple argument in the boundary theory implies that there must exist a direct relation between the on-shell Euclidean gravitational bulk action and the on-shell Euclidean action of a (d-1)-brane probing the bulk geometry. This relation is crucial for the consistency of holography but puzzling from the bulk perspective. We provide a full bulk derivation in the case of pure gravity. A central role is played by a non-trivial isoperimetric inequality that must be satisfied in a large class of Poincaré-Einstein spaces. Remarkably, this inequality follows from a theorem by John Lee.