Welcome to the homepage of the mathematical physics seminar at Radboud University Nijmegen, which features talks in the area of mathematical physics (in a broad sense). This seminar is intended to take place in person, but might occasionally take place online via Zoom. If you wish to join the online talks, please send an email here with a request for the Zoom connection details.

Organizers: Annegret Burtscher, Dejan Gajic, Teri Soultanis and Walter van Suijlekom

◆ Thursday 19 May, 15:00–16:00 CEST in HG02.802

Athanasios Chatzikaleas (WWU Münster)

Non-linear periodic waves on the Einstein cylinder

◆ Thursday 23 June, 15:00–16:00 CEST in HG03.085

Eleni-Alexandra Kontou (Universiteit van Amsterdam)

TBA

Past talks

- ◆ Thursday 21 April, 15:00–16:00 CEST in HG02.802

Leonardo García-Heveling (Radboud University)

When is topology change physically reasonable?

- ◆ Thursday 31 March, 15:00–16:00 CEST in HG00.086

Koen van den Dungen (University of Bonn)

Homotopy equivalence in unbounded KK-theory

- ◆ Thursday 9 December, 11:00–12:00 CET

Claude Warnick (University of Cambridge)

Quasinormal modes of black holes

Date and time: Thursday 23 June, 15:00–16:00 CEST

Location: HG03.085

Speaker: Eleni-Alexandra Kontou (Universiteit van Amsterdam)

Title: TBA

Abstract: TBA

Date and time: Thursday 19 May, 15:00–16:00 CEST

Location: HG02.802

Speaker: Athanasios Chatzikaleas (WWU Münster)

Title: Non-linear periodic waves on the Einstein cylinder

Abstract: Motivated by the study of small amplitudes non-linear waves in the AdS spacetime and in particular the conjectured existence of periodic in time solutions to the Einstein equations, we construct families of arbitrary small time-periodic solutions to various toy models that mimic certain properties of nonlinear waves in the AdS spacetime. These include the conformal cubic wave equation and the spherically-symmetric Yang–Mills equations on the Einstein cylinder. Our proof relies on modifications of a theorem of Bambusi–Paleari for which the main assumption is the existence of a seed solution, given by a non-degenerate zero of a non-linear operator associated with the resonant system. This is a joint work with Jacques Smulevici.

Date and time: Thursday 21 April, 15:00–16:00 CEST

Location: HG02.802 (2nd floor of the Huygens building)

Speaker: Leonardo García-Heveling (Radboud University)

Title: When is topology change physically reasonable?

Abstract: General Relativity is famous for merging the concepts of space and time into a single entity, spacetime, represented by a 4-dimensional manifold. In practice, however, most interesting spacetime manifolds can still be equipped with a function that plays the role of time. Topology change refers to the situation when, say, the {time=1} set has a different topology than the {time=0} set. There is an ongoing debate about whether the laws of physics should allow topology change or not. This is partly because some examples are known where the change in topology leads to unphysical behaviour, such as infinite energy bursts of a quantum field propagating on spacetime. In this talk I will review previous ideas and work related to topology change and present recent progress on a conjecture of Borde and Sorkin which aims to distinguish between "good" and "bad" topology change.

Date and time: Thursday 31 March, 15:00–16:00 CEST

Location: HG00.086 (ground floor of the Huygens building)

Speaker: Koen van den Dungen (University of Bonn)

Title: Homotopy equivalence in unbounded KK-theory

Abstract: Kasparov's KK-theory assigns an abelian group KK(A,B) to any pair of C*-algebras A and B. Elements of this group are homotopy equivalence classes of so-called (bounded) Kasparov modules. Every such element can also be represented by an unbounded Kasparov module. In this talk I will describe the notion of homotopy equivalence for such unbounded modules. This leads to an alternative definition of KK-theory in terms of unbounded representatives, and I will show that the resulting group of unbounded homotopy equivalence classes is indeed isomorphic to Kasparov's KK-group.

Date and time: Thursday 9 December, 11:00–12:00 CET

Location: online via Zoom (email here for connection details)

Speaker: Claude Warnick (University of Cambridge)

Title: Quasinormal modes of black holes

Abstract: It has been known since numerical studies in the early '70s that a perturbed black hole will typically return to its ground state by emitting radiation at certain characteristic (complex) frequencies. These "quasinormal" frequencies carry information about the geometry of the black hole. The last few years have seen new mathematical approaches to understand the spectral problem underlying the determination of the quasinormal frequencies for a particular spacetime. In this talk I will review these new approaches and their connection to the underlying physical problem.