Wednesday, June 8, 2016

Strings meet Loops via AdS/CFT (Helsinki Workshop on Quantum Gravity 2016)

This rerecorded talk was originally given at the Helsinki Workshop on Quantum Gravity on June 2, 2016. I was invited by the organisers to talk about a recent paper, which was intended as an invitation for people to become interested in the subject, as opposed to giving concrete and detailed results.

In particular, I am very much looking forward to discussions about this topic and criticism of the ideas, in particular from experts in string theory. In the long run, much can be gained in my point of view from intensifying the exchange between researchers in loop quantum gravity and string theory.

In this context, it is certainly worth pointing out a recent article by Sabine Hossenfelder for Quanta Magazine, as well as a blogpost of hers on the paper I wrote.

Wednesday, June 1, 2016

Graph superspositions and improved regularisations

Sum of graphs in loop quantum gravity
In loop quantum gravity, the elements of a certain basis in the Hilbert space
can be (roughly) interpreted as lattices. Generic quantum states can be constructed
as superpositions thereof and a priori have different properties which cannot
be realised at the level of individual lattices. 

Let us pick up the topic of extracting effective cosmological dynamics from loop quantum gravity again. Today’s post is about a recent paper by Emanuele Alesci and Francesco Cianfrani within their framework of “Quantum reduced loop gravity”. This version of loop quantum gravity is arrived at by a gauge fixing to the diagonal metric gauge at the quantum level (as opposed to the classical level as e.g. here), along with a truncation of the Hamiltonian constraint consistent with a spatially homogeneous setting (in particular, the non-trivial shift resulting from the gauge fixing is dropped). The simplified quantum dynamics resulting form this gauge fixing have allowed to compute the expectation value of the Hamiltonian constraint in suitable coherent states, leading to the effective Hamiltonian that one finds in loop quantum cosmology.

An open issue in this context has been to properly derive the so called “improved “ dynamics of loop quantum cosmology, which are consistent with observation and do not feature some unphysical properties of the original formulation. This is somewhat tricky if one uses standard connection variables along with a gauge group like SU(2) or U(1) for the following reason: