This series consists of talks in the area of Quantum Gravity.
Effective field
theory techniques allow reliable quantum calculations in general relativity at
low energy. After a review of these techniques, I will discuss the attempts to
define the gravitational corrections to running gauge couplings and to the
couplings of gravity itself. I will also describe an attempt to understand the
relation between the effective field theory and Asymptotic Safety in the region
where they overlap.
I will describe recent work in collaboration with Adam
Henderson, Alok Laddha, and Madhavan Varadarajan on the loop quantization of a
certain $G_{\mathrm{N}}\rightarrow 0$ limit of Euclidean gravity, introduced by
Smolin. The model allows one to test various quantization choices one is faced
with in loop quantum gravity, but in a simplified setting. The main results are the construction of
finite-triangulation Hamiltonian and diffeomorphism constraint operators whose
continuum limits can be evaluated in a precise sense, such that the quantum
I present a candidate for a new derivation of black hole
entropy. The key observation is that the action of General Relativity in
bounded regions has an imaginary part, arising from the boundary term. The
formula for this imaginary part is closely related to the Bekenstein-Hawking
entropy formula, and coincides with it for certain classes of regions. This
remains true in the presence of matter, and generalizes appropriately to
Lovelock gravity. The imaginary part of the action is a versatile notion,
I
will describe a discrete model of spacetime which is quantum-mechanical,
causal, and background free. The kinematics is described by networks whose
vertices are labelled with arrows. These networks can be evolved forwards (or
backwards) in time by using unitary replacement rules. The arrow structure
permits one to define dynamics without using an absolute time parameter.
Based on arXiv:1201.2489.
Causal
Dynamical Triangulations” (CDT) is a lattice theory where aspects of quantum
gravity can be studied. Two-dimensional CDT can be solved analytically and the
continuum (quantum) Hamiltonian obtained.
In this talk I will show that this continuum Hamiltonian is the one obtained by
quantizing two-dimensional projectable Horava-Lifshitz gravity.
We construct a
self-consistent model which describes a black hole from formation to
evaporation including the back reaction from the Hawking radiation. In the case
where a null shell collapses, at the beginning the evaporation occurs, but it
stops eventually, and a horizon and singularity appear. On the other hand, in
the generic collapse process of a continuously distributed null matter, the
black hole evaporates completely without forming a macroscopically large
Both AdS/CFT duality and more general reasoning from quantum gravity point to a rich collection of boundary observables that always evolve unitarily. The physical quantum gravity states described by these observables must be solutions of the spatial diffeomorphism and Wheeler-deWitt constraints, which implies that the state space does not factorize into a tensor product of localized degrees of freedom.
I will review some problems of the black hole paradigm and explore other
possibilities for the final state of stellar collapse other than an evaporating
black hole. In particular I will use the so-called transplanckian problem as a
guide in this search for a compelling scenario for the evaporation of
ultracompact objects.
I will recall the
main motivations for considering spin foam models in their Group Field Theory
(GFT) versions, which are quantum field theories defined on group manifolds. As
for any other quantum field theory, a fully consistent definition of the latter
must involve renormalization. I will briefly review a specific class of GFTs,
called tensorial, for which progress in this direction has recently been possible.
A new just-renormalizable model, in three dimensions and on the SU(2) group,