Registration - Welcome Room: 132:028

Asymptotic Safety in the bi-metric era

• Martin Reuter

Room: FB 54 After a brief review of the main structural properties of the bi-metric implementation of Background Independence, some recent results obtained by means of bi-metric truncations of the effective average action will be discussed. The focus of the talk is on the possible existence of a "C-theorem" in 4 dimensional QEG, some of the pressing positivity issues in QEG, and, more technically, on new reliability tests for truncations which can be applied to any theory.

Background independent exact renormalization group for conformally reduced gravity

• Tim Morris

Room: FB 54 Within the conformally reduced gravity model, where the metric is parametrised by a function of the conformal factor field, we keep dependence on both the background and fluctuation fields, to local potential approximation and order derivative-squared respectively, making no other approximation. The functional dependence on the background field is then dictated by realising a remnant diffeomorphism invariance. The standard non-perturbative Renormalization Group (RG) scale is inherently background dependent, which we show in general forbids the existence of RG fixed points. By combining the flow equations with the modified split Ward identity, we uncover a unique background independent notion of RG scale. The corresponding RG flow equations are then not only explicitly background independent along the entire RG flow but also explicitly independent of the way the conformal factor is parametrised.

Fluctuations, locality and the phase structure of quantum gravity

• Jan Pawlowski

Room: 132:028 In this talk I present results for UV-IR flows for fully momentum-dependent propagators, vertices in quantum gravity.  The momentum locality of the underlying block-spinning procedure is related to diffeomorphism invariance. The setting is used to compute  the scale-dependent Newton coupling and the cosmological constant. Physics implications as well as the extension to gauge-matter-gravity systems are discussed.

One-loop beta-function for an infinite-parameter family of gauge theories

• Kirill Krasnov

Room: 132:028 Any field theory is renormalisable once all terms compatible with the symmetries are added to the Lagrangian. One can then in principle attempt to compute the arising renormalisation group flow, and search for fixed points realising the asymptotic safety scenario. However, in practice the terms one needs to add to the Lagrangian contain higher derivatives, and the necessary computation, even at one loop, becomes unrealistic. I report on a one-loop computation where the salting point is taken to be the family of Lagrangians given by an arbitrary function of the self-dual part of the Yang-Mills curvature tensor. All such Lagrangians lead to just second order in derivatives field equations, and so seem to be clearly insufficient to renormalise the arising divergences. However, as I will show, the magic of self-duality makes this family of theories one-loop renormalisable. I will explicitly describe the arising renormalisation group flow in the infinite-dimensional space of coupling constants.

Discussion Room: 132:028

Wilson Loops in Causal Dynamical Triangulations

• Renate Loll

Room: 132:028

What is the large-scale effective action of (2+1)-dimensional causal dynamical triangulations?

• Joshua Cooperman

Room: 132:028 The literature contains several analyses suggesting that the large-scale effective action of (2+1)-dimensional causal dynamical triangulations has the form of a minisuperspace model of projectable Horava-Lifshitz gravity. I weigh in on these suggestions by reporting the results of a comprehensive statistical analysis comparing several candidate effective actions.  Controversial Statement: The large-scale effective action of (3+1)-dimensional causal dynamical triangulations has the form of a minisuperspace model of Einstein gravity.

RG flow in Causal Dynamical Triangulations

• Andrzej Görlich

Room: 132:028 Causal Dynamical Triangulations (CDT) is a background independent approach to quantum gravity. In this talk I will present recent studies of the effective transfer matrix. The introduced method allows observing a possibly new 'bifurcation' phase. I will also define a notion of 'lines of constant physics’ in the coupling constant space and investigate the renormalization group flow in CDT.

Real time renormalization

• Sandor Nagy

Room: 132:028 We present the real time formalism of the functional renormalization group method for the 3-dimensional $\phi^4$ theory. The original, single time formalism of the renormalization is based on the invariance of the transition amplitudes. By using the so-called closed time path formalism we will deduce the real time RG equations based on the invariance of the physically more relevant expectation values. In the case of zero temperature the phase space does not change qualitatively when we compare it by the ones obtained by the original RG treatment. The real time formalism enables us to take into account the finite temperature version of the $\phi^4$ theory in a simple manner. The evolution of the mixing couplings can account for the decoherence in the model.

Asymptotic safety and scalar-tensor theories

• Gian Paolo Vacca

Room: 132:028 I will focus on a class of scalar O(N) models non minimally coupled to gravity and discuss some results obtained within the functional renormalization group framework in the single metric truncation, using the exponential parametrization of the metric and a specific gauge fixing.

Scaling and Renormalization in Quantum Gravity

• Giulio D'Odorico

Room: 132:028 I will discuss scaling arguments in quantum gravity, in a covariant framework. After showing how these can be used to derive known results, such as the KPZ relations in the two dimensional case, I will describe how the renormalization group can be used together with scaling arguments to gain a deeper understanding of the theory at criticality.

Diffeomorphism invariance and critical scaling in quantum gravity

• Kevin Falls

Room: 132:028 The problem of obtaining a gauge independent beta function for Newton's constant is addressed. By a specific parameterisation of metric fluctuations a gauge independent functional integral is constructed for the semiclassical theory around an arbitrary Einstein space. The effective action then has the property that only physical polarisations of the graviton contribute, while all other modes cancel with the functional measure. We are then able to compute a gauge independent beta function for Newton's constant in d-dimensions to one-loop order. Going beyond the semiclassical theory, by means of the exact renormalisation group, the beta function for Newton's constant is found within a simple approximation exploiting the gauge independent parameterisation. We then assess the scaling at an asymptotically safe fixed point. Close to two dimensions the critical exponent for Newton’s constant is found to be regulator independent. In four dimensions we apply Litim’s optimisation criteria on the space of regulators. The critical exponents is then found to agree with the value found in numerical lattice simulations by Hamber.

Discussion Room: 132:028

Asymptotic safety in perturbation theory and beyond

• Daniel Litim

Room: 132:028

Rethinking Naturalness:  The Interacting Ultraviolet Safe Behaviour of Non-Abelian Gauge-Yukawa Theories

• Francesco Sannino

Room: 132:028 I will critically introduce, classify and discuss the fundamental open issues related to either composite or elementary extensions of the standard model.  As for the progress I will exhibit, among other things, the first proof of existence of nonsupersymmetric and non-asymptotically free 4D Gauge-Yukawa theories (structurally similar to the standard model) which are UV finite thanks to the existence of an exact interacting quantum UV fixed point in the gauge, Yukawa and scalar self-couplings. The quantum moduli space of the theory will also be precisely determined. Theories with this behaviour have been searched for on the lattice for the past several decades. Our results show the critical ingredients that are needed to construct these new classes of theories and offer a strategy for future lattice studies. I will then comment on the wide phenomenological impact of this discovery.

Search of scaling solutions in scalar-tensor gravity

• Roberto Percacci

Room: 132:028 We write new functional renormalization group equations for a scalar nonminimally coupled to gravity. Thanks to the choice of the parametrization and of the gauge fixing they are simpler than older equations and avoid some of the difficulties that were previously present. In three dimensions these equations admit, at least for sufficiently small fields, a solution that may be interpreted as a gravitationally dressed Wilson-Fisher fixed point. We also find for any dimension d>2 two analytic scaling solutions which we study for d=3 and d=4. One of them corresponds to the fixed point of the Einstein-Hilbert truncation, the others involve a nonvanishing minimal coupling.

Constraining Asymptotically Safe Inflation with cosmological observations

• Alfio Bonanno

Room: 132:028

Effective Field Theory of Gravity and Cosmology

• Alessandro Codello

Room: 132:028

Ghost free and singularity free construction of gravity : classical and quantum aspects

• Anupam Mazumdar

Room: 132:028 I will discuss generalisation of Einstein's gravity in the ultraviolet by extending it to infinite derivative theory of gravity which can ameliorate cosmological and black hole singularities. Furthermore at quantum level such class of action can also be made finite up to 2 loops. Beyond 2 loops there are some conjectures.

Probing the fundamental nature of fluctuating membranes with the Renormalization Group

• Omar Zanusso

Room: 132:028 Membranes provide exciting physical systems with a strong interplay between local order and geometric fluctuations. They are also a non-trivial arena, both theoretical and experimental, in which the findings of the non-perturbative renormalization group can be tested, thus enhancing the confidence of the same applications to quantum gravity. We will review the distinct universality classes of membranes, the corresponding order parameters and the effective long-range interactions, that determine the rich membranes' phase diagram. Guided by these results, we will attempt to draw some lesson that can be applied in the context of quantum gravity.

Phase transitions in Tensor Models

• Razvan Gurau

Room: 132:028 In tensor and matrix models, phase transitions are understood as a tuning to criticality of some parameters. But in field theory phase transitions are usually understood in terms of symmetry breaking and translations of the fields to non trivial vacua. In this talk I will discuss the relation between the two points of view.

Tensor Models, from branched polymers to Brownian spheres

• Vincent Rivasseau

Room: 132:028 Ordinary tensor models of rank D ≥ 3 are dominated at large N by tree-like graphs, known as melonic triangulations. We shall show that non-melonic contributions can be enhanced consistently, leading to different types of large N limits. For instance the most generic quartic tensor model at rank 4, with maximally enhanced non-melonic interactions, displays a branched polymer phase and a 2D quantum gravity phase, and a transition between them whose entropy exponent is positive. This work is in collaboration with V. Bonzom and T. Delepouve.

What defines and how to construct a theory of quantum gravity?

• Bianca Dittrich

Room: 132:028 Motivated by the loop quantum gravity framework I discuss what requirements a theory of quantum gravity needs to satisfy and how one could possible construct such a theory. I will then consider  how these ideas apply to other frameworks.

Fermions, gravity and chiral symmetry

• Holger Gies

Room: 132:028 Any quantum degree of freedom that has a simple overlap with an elementary degree of freedom has the chance to probe the nature of spacetime at the fundamental level - simply by means of its inherent quantum fluctuations. Chiral fermions are a good candidate for such degrees of freedom. Chiral symmetry is in fact a sensitive probe for interactions on all scales: chiral symmetry breaking can be triggered by comparatively weak interactions as well as by specific large-scale structures. The fact that chiral fermions can be observed in nature hence also represents a probe of spacetime properties. In this talk, the various connections between fermions, gravity and chiral symmetry will be discussed, highlighting potential implications for theories of quantum gravity.

Discussion Room: 132:028