**Seminars for the year:**
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**Time: ** 3. November 2017, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Maja Buric

**Title: ** Geometry of noncommutative spaces

**Abstract:**

We discuss geometry of fuzzy sphere and Moyal plane in terms of coherent states and symmetries of these spaces.

**Time: ** 27. October 2017, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Ilija Ivanisevic

**Title: ** Quantization of the scalar field on the fuzzy sphere

**Abstract:**

In the process of the quantization of the scalar field on the fuzzy sphere, in addition to ordinary (planar) diagrams one encounters also nonplanar diagrams. They imply the existence of a finite noncommutative anomaly, i.e. the lack of commutativity between the quantization and the commutative limit. Nonplanar diagrams give rise to the so-called UV/IR mixing, i.e. the emergence of IR divergences during the removal of UV divergences using usual renormalization techniques. UV/IR mixing is explained as one form of a noncommutative anomaly.

**Time: ** 13. October 2017, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Tijana Radenkovic

**Title: ** Quantum gravity on piecewise-flat manifolds

**Abstract:**

First, we introduce the Ponzano-Regge model and it is shown that pure three-dimensional gravity is just the BF theory. Four-dimensional General Relativity can be formulated as BF theory with an additional constraint term added to the Lagrangian, so we present here the quantization procedure of the topological sector for both 3D and 4D models. We see how General Relativity can be formulated as a gauge theory for the Poincare 2-group like a constrained topological theory based on BFCG action. Covariant quantisation of the topological sector of the spin cube model - BFCG action is implemented the same way as in BF case. We see how the issues of spinfoam models are successfully resolved by spin cube model - the edge lengths of a triangulation become the basic dynamical variables and matter can be consistently coupled to gravity in this formulation. Additionally, a brief introduction to the concepts of Regge calculus and 2-groups in higher category theory is presented.

**Time: ** 6. October 2017, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Maja Buric

**Title: ** Fuzzy de Sitter space

**Abstract:**

We discuss properties of fuzzy de Sitter space defined as an algebra of de Sitter group SO(1,4) in a unitary irreducible representation. It was shown before that it is possible, on this noncommutative space, to choose a local frame with metric which reduces to the de Sitter metric in the commutative limit. Here we show that the embedding coordinates have discrete spectra for (\rho,s=1/2) unitary irreducible representations of the principal continuous series of the SO(1,4). The result is obtained in the Hilbert space representation, and it is very likely that it can be extended to the other irreps of the principal continuous series.

**Time: ** 21. August 2017, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Vitaly Vanchurin

**Title: ** Covariant information theory and emergent gravity

**Abstract:**

Informational dependence between statistical or quantum subsystems can be described with Fisher matrix or Fubini-Study metric obtained from variations of the sample/configuration space coordinates. Using these non-covariant objects as macroscopic constraints we consider statistical ensembles over the space of classical probability distributions or quantum wave-functions. The ensembles are covariantized using dual field theories with either complex or real scalar fields identified with complex wave-functions or square root of probabilities. We argue that a full space-time covariance on a field theory side is dual to local computations (defined in terms of parallel computing) on the information theory side. We define a fully covariant information-computation tensor and show that it must satisfy conservation equations. Then we switch to a thermodynamic description and argue that the (inverse of) space-time metric tensor is a conjugate thermodynamic variable to the ensemble-averaged information-computation tensor. In the equilibrium the entropy production vanishes and the metric is not dynamical, but away from equilibrium the entropy production gives rise to an emergent dynamics of the metric. This dynamics can be described by expanding the entropy production into products of generalized forces (derivatives of metric) and conjugate fluxes. Near equilibrium these fluxes are given by an Onsager tensor contracted with generalized forces and on the grounds of time-reversal symmetry the Onsager tensor is expected to be symmetric. We show that a particularly simple and highly symmetric form of the Onsager tensor gives rise to the Einstein-Hilbert term. This proves that general relativity is equivalent to a theory of non-equilibrium (thermo)dynamics of the metric which is expected to break down far away from equilibrium where the symmetries of the Onsager tensor are to be broken.

**Time: ** 17. March 2017, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Marko Vojinovic

**Title: ** Quantum entanglement between gravity and matter

**Abstract:**

We show that gravitational and matter fields are always entangled, as a consequence of local Poincare symmetry. This entanglement is not of dynamical origin, as is common for interacting quantum systems, but rather of kinematical origin, enforced by gauge symmetry at the quantum level. We will give a general argument why one should expect the symmetry-protected entanglement in quantum gravity theories, and then support it by demonstrating explicitly that the Hartle-Hawking state is entangled, in the framework of the Regge quantum gravity model. These results may have various interesting consequences, including the universal gravitational decoherence of matter and a possible small violation of the weak equivalence principle.

**Time: ** 10. February 2017, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Rodrigo Olea

**Title: ** From conformal to Einstein gravity

**Abstract:**

We provide a simple derivation of the equivalence between Einstein and Conformal Gravity (CG) with Neumann boundary conditions given by Maldacena. As Einstein spacetimes are Bach flat, a generic solution to CG would contain both Einstein and non-Einstein part. Using this decomposition of the spacetime curvature in the Weyl tensors, makes manifest the equivalence between the two theories, both at the level of action and variation of it. As a consequence, we show that the on-shell action for Critical Gravity in four dimensions is given uniquely in terms of the Bach tensor. The formulation presented here is expected to provide a shortcut in the computation of holographic correlation functions in Critical Gravity.

**Time: ** 11. January 2017, 11:00h

**Place: ** Faculty of Physics, room 661

**Speaker: ** Igor Prlina

**Title: ** Finding the Landau singularities using the amplituhedron

**Abstract:**

We propose a simple geometric algorithm for determining the complete set of branch points of amplitudes in planar N=4 super-Yang-Mills theory directly from the amplituhedron, without resorting to any particular representation in terms of local Feynman integrals. This represents a step towards translating integrands directly into integrals. In particular, the algorithm provides information about the symbol alphabets of general amplitudes. We illustrate the algorithm applied to the one- and two-loop MHV amplitudes.

**Seminars for the year:**
2017
2016
2015
2014
2013
2012
2011
2010
2009
2008
2007