Interesting Paper on the Immirzi Parameter, and more…

In Loop Quantum Gravity, the Immirzi parameter is somewhat troubling…largely because it’s such an odd value.

It was once calculated to be some ugly value like:

Or something like that. Bizarre!

Well, a paper came out that argued that:

Microscopic state counting for a black hole in Loop Quantum Gravity yields a result proportional to horizon area, and inversely proportional to Newton’s constant and the Immirzi parameter. It is argued here that before this result can be compared to the Bekenstein-Hawking entropy of a macroscopic black hole, the scale dependence of both Newton’s constant and the area must be accounted for. The two entropies could then agree for any value of the Immirzi parameter, if a certain renormalization property holds.

(Emphasis added). Fascinating!

The paper is called “Renormalization and black hole entropy in Loop Quantum Gravity” by Ted Jacobson.

There was a second paper that crossed my eye because of something that stuck out from Carlip’s Class. This student with a British accent asked why not place the universe within a box? We do it for the Hydrogen atom, among other things, so why not do it for quantum gravity?

Well, the obvious answer is the philosophical problems with this in the context of classical general relativity. However, a paper has come out about this very subject! It’s very exciting purely from the nostalgic feeling the abstract conjures:

The curvature perturbation in a box by David H. Lyth

The stochastic properties of cosmological perturbations are best defined through the Fourier expansion in a finite box. I discuss the reasons for that with reference the curvature perturbation, and explore some issues arising from it.

Next on the reading List is a lengthy piece dealing with particle propagators in arbitrary backgrounds.

This piece fascinates me partially because it comforts my inner general relativist in dealing with background independency (or more spacifically, a sort of pseudo-background independency) for quantum theory.

Particle propagation in non-trivial backgrounds: a quantum field theory approach by Daniel Arteaga

The basic aim of the thesis is the study of the propagation of particles and quasiparticles in non-trivial backgrounds from the quantum field theory point of view. By “non-trivial background” we mean either a non-vacuum state in Minkowski spacetime or an arbitrary state in a curved spacetime. Starting with the case of a flat spacetime, the basic properties of the particle and quasiparticle propagation are analyzed using two different methods other than the conventional mean-field-based techniques: on the one hand, the quantum state corresponding to the quasiparticle excitation is explicitly constructed; on the other hand, the spectral representation of the two-point propagators is analyzed. Both methods lead to the same results: the energy and decay rate of the quasiparticles are determined by the real and imaginary parts of the retarded self-energy respectively. These general results are applied to two particular quantum systems: first, a scalar particle immersed in a thermal graviton bath; second, a simplified atomic model, seizing the opportunity to connect with other statistical and first-quantized approaches. In the second part of the thesis the results are extended to curved spacetime. Working with a quasilocal quasiparticle concept the flat-spacetime results are recovered. In cosmology, within the adiabatic approximation, it is possible to go beyond the flat spacetime results and find additional effects due to the universe expansion. The cosmologically-induced effects are analyzed, obtaining that there might be an additional contribution to the particle decay due to the universe expansion. In the de Sitter case, this additional contribution coincides with the decay rate in a thermal bath in a flat spacetime at the corresponding de Sitter temperature.

Fascinating papers, all of them well worth reading.