This series consists of talks in the area of Quantum Gravity.
A number of recent proposals for a quantum theory of gravity are based on the idea that spacetime geometry and gravity are derivative concepts and only apply at an approximate level. Two fundamental challenges to any such approach are, at the conceptual level, the role of time in the emergent context and, technically, the fact that the lack of a fundamental spacetime makes difficult the straightforward application of well-known methods of statistical physics and quantum field theory to the problem.
A serious shortcoming of spinfoam loop gravity is the absence of matter.
I present a minimal and surprisingly simple coupling of a chiral fermion field in the framework of spinfoam quantum gravity.
This result resonates with similar ones in early canonical loop theory: the naive fermion hamiltonian was found to be just the extension of the simple
Emergent gravity scenarios have become increasingly popular in recent times. In this talk I will review some evidence in this sense and discuss some lessons from toy models based on condensed matter analogues of gravity. These lessons suggest some (possibly) general features of the emergent gravity framework which not only can be tested with current astrophysical observations but can also improve our understanding of cosmological puzzles such as the dark energy one.
Usually in quantum field theory one considers two different interpretations:
1: The field is an infinite number of quantum oscillators, giving rise to a wave functional \Psi(\phi).
2: The positive frequency component of a field, \phi_+(x), is a wave function analogous to standard quantum mechanics.
While interpretation 2 is often only mentioned implicitly it is crucial to standard computations of measurable scattering probabilities.
We extend the formalism of embedded spin networks and spin foams to include topological data that encode the underlying three-manifold or
four-manifold as a branched cover. These data are expressed as
monodromies, in a way similar to the encoding of the gravitational field
via holonomies. We then describe convolution algebras of spin networks and
spin foams, based on the different ways in which the same topology can be
realized as a branched covering via covering moves, and on possible
The functional Renormalization Group is a continuum method to study quantum field theories in the non-perturbative regime. In Yang-Mills theory, it can be used to relate fully nonperturbative low-order correlation functions in particular gauges to observables such as confinement order parameters. As a special application, we determine the order of the phase transition and the critical temperature for various gauge groups (SU(N), N=3,.,12, Sp(2) and E(7)). This also allows to investigate what determines the order of the deconfinement phase transition.
Why is a vertical column of gas at thermal equilibrium slighly hotter at the bottom than a the top? My answer in this talk will be that time runs slower in a deeper gravitational potential, and temperature is nothing but the (inverse) speed of time. Specifically, I will (i) introduce Rovelli's notion of thermal time, (ii) use it to provide a "principle" characterization of thermal equilibrium in stationary spacetimes, and (iii) effortlessly derive the Tolman-Ehrenfest relation.
Interwiners describe quanta of space in loop quantum gravity. In this talk I show that the Hilbert space of SU(2) intertwiners has as semiclassical limit the phase space of a classical system originally considered by Minkowski: convex polyhedra with N facets of given areas and normals. This result sharpens Penrose spin-geometry theorem. The knowledge of the classical system associated to intertwiner space can be fruitfully used: I show that many properties of the spectrum of the volume operator can be derived via Bohr-Sommerfeld quantization of the volume of a classical polyhedron.
In the context of loop quantum gravity and spin foam models, the simplicity constraints are essential in that they allow to write general relativity as a constrained topological theory.
I will first recall the spin foam quantization procedure and focus more particularly on the step consisting in implementing the simplicity constraints.