Organisation des séminaires et calendrier

Not all effective field theories are created equal. Some appear perfectly healthy at low energies and yet may contain the seeds of their own inconsistency. Dispersion relations and positivity provide one diagnostic of such inconsistencies. Are there other criteria capable of identifying theories that cannot admit a consistent ultraviolet completion? It has been proposed that a suitable notion of black holes’ stability could provide such a criterion. More specifically, quasinormal-modes in EFT extensions of gravity have been argued to decay faster than their GR counterpart, and if not the deviation should remain unresolvable.
I will show that, besides lacking a compelling physical motivation, this proposal contains no information beyond that already encoded in existing positivity constraints, with one curious exception. I will support these claims in the context of GREFT and scalar-tensor theories, using Einstein–dilaton–Gauss–Bonnet and Einstein–Maxwell gravity as explicit examples.

I will describe an exact relation between 1D sigma models on homogeneous spaces and Gaudin spin models, which in the physics setup originates from Haldane’s mapping between spin chains and sigma models. As the main example, I will consider the sigma model with target space U(3)/U(1)^3 (the flag manifold) and the three-site Gaudin model, respectively. The goal of the talk is to describe an explicit solution to both the classical and quantum problems for the sigma model: namely, finding the geodesics and calculating the spectrum of the Laplace operator on the flag manifold equipped with an arbitrary invariant metric. This method is also applicable to homogeneous spaces of higher-rank groups, where the integrable case corresponds to the metrics of Manakov–Mishchenko–Fomenko discovered in the 1970s in the study of higher-dimensional integrable generalizations of the Euler top.

In principle my affiliations are Steklov Math. Inst. and ITMP (MSU). However, if it can cause trouble for you to host people with Russian affiliations, please feel free to put instead BIMSA or IHES as my affiliation (it’s not that important for me which affiliation you put).

Abstract: I will present a new geometric formulation of two-dimensional conformal field theory. To every unitary 2d CFT with c>1, it canonically associates a von Neumann algebra: the algebra of functions on a noncommutative space. The existence of this algebra is equivalent to the Virasoro conformal bootstrap, and crossing symmetry becomes associativity of multiplication. Virasoro symmetry is replaced by the action of a quantum group on the noncommutative space. Unlike in algebraic QFT, boundedness and symmetry coexist: every Virasoro family is realized by bounded operators transforming irreducibly under the quantum group. Liouville theory arises as the most basic example: the quantum group acting on itself by left and right translations. Finally, I will discuss how this framework opens a path to constructing chaotic 2d CFTs, as quantum analogues of compact hyperbolic surfaces.

Based on https://arxiv.org/pdf/2611.06907

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