This series consists of talks in the area of Quantum Gravity.
Spin foams provide models for quantum gravity and hence quantum space time. One of the key outstanding questions is to show that they reproduce smooth space time manifolds in a continuum limit.I will start with a very short introduction to spin foams and the structure of quantum space time they encode.
The most relevant evidences in favour of the Lorentzian EPRL-FK spinfoam model come from its capibility of reproducing the expected semiclassical limit in the large spin regime. The main examples of this are the large spin limit of the vertex amplitude, later extended to arbitrary triangulations, and that of the spinfoam graviton propagator, which was calculated on the simplest possible two complex. These results are very promising. Nonetheless, their relevance may be endangered by the effects associated to radiative corrections.
In several approaches
to quantum-gravity, the spectral dimension of spacetime runs from the standard
value of 4 in the infrared (IR) to a smaller value in the ultraviolet (UV).
Describing this running in terms of deformed dispersion relations, I show that
a striking cosmological implication is that that UV behavior leading to 2
spectral dimensions results in an exactly scale-invariant spectrum of vacuum scalar
and tensor fluctuations. I discuss scenarios that break exact scale-invariance
An exciting frontier in physics is to understand the quantum nature of gravitation in finite regions of spacetime. Study of these regions from ``below'', that is, by studying the quantum geometry of finite regions emerging from loop gravity and spin networks has recently resulted in a new road to the quantization of volume and to evidence that there is a robust gap in the volume spectrum. In this talk I will complement these results with recent work on conformal field theories in a particular finite region, a spherical ball of space.
The Functional
Renormalisation Group technique has received great attention in recent times
proving itself as a powerful tool to describe the high energy behaviour of
gravitational interactions.
Its key ingredient is a nontrivial fixed point of the theory renormalization
group flow which controls the behavior of the coupling constants in the
ultraviolet regime and ensures that physical quantities are safe from
divergences. I will briefly review the main ingredients of the gravitational asymptotic
Causal set theory is discrete, fully covariant theory of quantum
gravity. The discrete framework makes it necessary to reformulate
continuum concepts. One of these concepts is that of a derivative operator. It is
possible to define a derivative operator in causal sets that in
the continuum limit agrees with the d'Alembertian for a scalar
field. This operator can be used to define a causal set action, which
enables Monte-Carlo simulations. In this seminar I will present this operator and action and then
The functional renormalization group
is a tool in the systematic search for Euclidean QFTs that works with
very little input: All one needs to specify is a field content,
symmetries and a notion of locality. The functional renormalization
group then allows one to scan this theory space for bare actions for
which the path integral can be performed nonperturbatively. These
actions appear as fixed points (and relevant deformations) of the
renormalization group flow (so-called asymptotic safety). Such a
The
late physicist John Wheeler, was renowned for his Socratic method of conducting
physics discussions. "Why is general relativity the way it is? What makes
it special?" were reportedly questions one should expect in his
presence. There are different answers to these questions, each requiring a set
of assumptions - which Wheeler would likely question again - and each bringing
with it new insights into physics as a whole. This talk will put forward
new principles for deriving general relativity. Perhaps more than is the case
AdS/CFT is a duality
relating the degrees of freedom in a D dimensional bulk gravity theory to a (D-1) dimensional theory living on the boundary. I will argue that in fact the boundary theory contains only a subset of the bulk observables. For each state of the boundary theory, there are multiple bulk states dual to it, which can be operationally distinguished by observers who fall across event horizons. Based on arXiv:1210.3590.
We argue that the
scale-free spectrum that is observed in the cosmic microwave background is the
result of a phase transition in the early universe. The observed tilt of
the spectrum, which has been measured to be 0.04, is shown to be equal to the
anomalous scaling dimension of the correlation function. The phase
transition replaces inflation as the mechanism that produces this spectrum. The tilt further indicates that there is a fundamental small length scale in
nature that we have not yet observed in any other way.