## Further research projects of Research Area B members

•

**Comparison and rigidity for scalar curvature ** Questions involving the scalar curvature bridge many areas inside mathematics including geometric analysis, differential geometry and algebraic topology, and they are naturally related to the mathematical description of general relativity.

There are two main flavours of methods to probe the geometry of scalar curvature: One goes back to Lichnerowicz and uses various versions of index theory for the Dirac equation on spinors. The other is broadly based on minimal hypersurfaces and was initiated by Schoen and Yau. On both types of methods there has been tremendous progress over recent years sparked by novel quantitative comparison and rigidity questions due to Gromov and by on-going attempts to arrive at a deeper geometric understanding of lower scalar curvature bounds.

In this proposal we view established landmark results, such as the non-existence of positive scalar curvature on the torus, together with the more recent quantitative problems from a conceptually unified standpoint, where a comparison principle for scalar and mean curvature along maps between Riemannian manifolds plays the central role.

Guided by this point of view, we aim to develop fundamentally new tools to study scalar curvature that bridge long-standing gaps in between the existing techniques. This includes a far-reaching generalization of the Dirac operator approach expanding upon techniques pioneered by the PI, and novel applications of Bochner-type methods. We will also study analogous comparison problems on domains with singular boundary motivated by a first synthetic characterization of lower scalar curvature bounds in terms of polyhedral domains, and by the general quest for extending the study of scalar curvature beyond smooth manifolds. At the same time, we will treat subtle almost rigidity questions corresponding to the rigidity aspect of our comparison principle.

onlineProject members:

**Rudolf Zeidler**• **Global Estimates for non-linear stochastic PDEs **

Semi-linear stochastic partial differential equations: global solutions’ behaviours

Partial differential equations are fundamental to describing processes in which one variable is dependent on two or more others – most situations in real life. Stochastic partial differential equations (SPDEs) describe physical systems subject to random effects. In the description of scaling limits of interacting particle systems and in quantum field theories analysis, the randomness is due to fluctuations related to noise terms on all length scales. The presence of a non-linear term can lead to divergencies. Funded by the European Research Council, the GE4SPDE project will describe the global behaviour of solutions of some of the most prominent examples of semi-linear SPDEs, building on the systematic treatment of the renormalisation procedure used to deal with these divergencies.

onlineProject members:

**Hendrik Weber**• **Quantization, Singularities and Holomorphic Dynamics **

The goal of the team brought together in this project is to bundle our
expertise with the aim of making important contributions to a number of
fundamental problems and conjectures in Quantization, Holomorphic
Dynamics and Foliation Theory. We will exhibit and exploit the deep ties
between these areas and bring them to bear on diverse open questions.
Our goal is to provide fresh perspectives and novel problem-solving
strategies to encompass these fields and in the long-term to foster in
the wider research community a stronger unification of these parts of
mathematics.Using as a cornerstone the development of the theory of
currents in the complex setting and of the Bergman/Szegö kernel
(including L^2 methods) and their systematic exploitation in the study
of a number of topics, we address the following interrelated
questions: commutation of quantization and reduction on Kähler spaces
and Cauchy-Riemann manifolds; hamiltonian actions; quantization of the
space of Kähler potentials and of adapted complex structures; Bergman
kernel asymptotics, analytic torsion, Newlander-Nirenberg theorem on
complex spaces; singularities and accumulation points of a leaf of a
holomorphic foliation, especially with non-hyperbolic singularities;
unique ergodicity for singular holomorphic foliations; quantitative
counting of dynamical phenomena for holomorphic dynamical systems, both
in the phase and parameter spaces; equidistribution of zeros of random
holomorphic sections.

onlineProject members:

**Ursula Ludwig**• **Dynamical systems and irregular gradient flows ** The central goal of this project is to study asymptotic properties for gradient flows (GFs) and related dynamical systems. In particular, we intend to establish a priori bounds and related regularity properties for solutions of GFs, we intend to study the behaviour of GFs near unstable critical regions, we intend to derive lower and upper bounds for attracting regions, and we intend to establish convergence speeds towards global attrators. Special attention will be given to GFs with irregularities (discontinuities) in the gradient and for such GFs we also intend to reveal sufficient conditions for existence, uniqueness, and flow properties in dependence of the given potential. We intend to accomplish the above goals by extending techniques and concepts from differential geometry to describe and study attracting and critical regions, by using tools from convex analysis such as subdifferentials and other generalized derivatives, as well as by employing concepts from real algebraic geometry to describe domains of attraction. In particular, we intend to generalize the center-stable manifold theorem from the theory of dynamical systems to the considered non-smooth setting. Beside finite dimensional GFs, we also study GFs in their associated infinite dimensional limits. The considered irregular GFs and related dynamical systems naturally arise, for example, in the context of molecular dynamics (to model the configuration of atoms along temporal evoluation) and machine learning (to model the training process of artificial neural networks).

online

Project members: **Christoph Böhm**, **Arnulf Jentzen**

• **CRC 1442: Geometry: Deformation and Rigidity - B05: Scalar curvature in Kähler geometry ** In this project we propose to study the degeneration of Kähler manifolds with constant or bounded scalar curvature under a non-collapsing assumption. For Riemannian manifolds of bounded sectional curvature, this is the content of the classical Cheeger-Gromov convergence theory from the 1970s. For Riemannian manifolds of bounded Ricci curvature, definitive results were obtained by Cheeger-Colding-Naber in the past 10-20 years, with spectacular applications to the Kähler-Einstein problem on Fano manifolds. Very little is currently known under only a scalar curvature bound even in the Kähler case. We propose to make progress in two different directions: (I) Gather examples of weak convergence phenomena related to the stability of the Positive Mass Theorem for Kähler metrics and to Taubes' virtually infinite connected sum construction for ASD 4-manifolds. (II) Study uniqueness and existence of constant scalar curvature Kähler metrics on non-compact or singular spaces by using direct PDE methods. online

Project members: **Bianca Santoro**, **Hans-Joachim Hein**, **Johan Klemmensen**

• **The Black Hole Stability Problem and the Analysis of asymptotically anti-de Sitter spacetimes ** The present proposal is concerned with the analysis of the Einstein equations of general relativity, a non-linear system of geometric partial differential equations describing phenomena from the bending of light to the dynamics of black holes. The theory has recently been confirmed in a spectacular fashion with the detection of gravitational waves.The main objective of the proposal is to consolidate my research group based at Imperial College by developing novel mathematical techniques that will fundamentally advance our understanding of the Einstein equations. Here the proposal builds on mathematical progress in the last decade resulting from achievements in the fields of partial differential equations, differential geometry, microlocal analysis and theoretical physics.The Black Hole Stability ProblemA major open problem in general relativity is to prove the non-linear stability of the Kerr family of black hole solutions. Recent advances in the problem of linear stability made by the PI and collaborators open the door to finally address a complete resolution of the stability problem. In this proposal we will describe what non-linear techniques will need to be developed in addition to achieve this goal. A successful resolution of this program would conclude an almost 50-year-old problem.The Analysis of asymptotically anti-de Sitter (aAdS) spacetimesWe propose to prove the stability of pure AdS if so-called dissipative boundary conditions are imposed at the boundary. This result would align with the well-known stability results for the other maximally-symmetric solutions of the Einstein equations, Minkowski space and de Sitter space.As a second -- related -- theme we propose to formulate and prove a unique continuation principle for the full non-linear Einstein equations on aAdS spacetimes. This goal will be achieved by advancing techniques that have recently been developed by the PI and collaborators for non-linear wave equations on aAdS spacetimes. online

Project members: **Gustav Holzegel**

• **CRC 1442: Geometry: Deformation and Rigidity - D02: Exotic crossed products and the Baum–Connes conjecture ** The Baum–Connes conjecture on the K-theory of crossed products by group actions on C*-algebras is one of the central problems in noncommutative geometry. The conjecture holds for large classes of groups and has important applications in other areas of mathematics. However, there are groups for which the conjecture fails to be true and in this project we study a new formulation of the conjecture due to Baum, Guentner and Willett which avoids the known counterexamples for the classical one. This involves new exotic crossed product functors which differ from the classical maximal or reduced crossed products. online

Project members: **Siegfried Echterhoff**

• **CRC 1442: Geometry: Deformation and Rigidity - B01: Curvature and Symmetry ** The question of how far geometric properties of a manifold determine its global topology is a classical problem in global differential geometry. In a first subproject we study the topology of positively curved manifolds with torus symmetry. We think that the methods used in this subproject can also be used to attack the Salamon conjecture for positive quaternionic Kähler manifolds. In a third subproject we study fundamental groups of non-negatively curved manifolds. Two other subprojects are concerned with the classification of manifolds all of whose geodesics are closed and the existence of closed geodesics on Riemannian orbifolds. online

Project members: **Burkhard Wilking**, **Michael Wiemeler**

• **CRC 1442: Geometry: Deformation and Rigidity - Geometric evolution equations ** Hamilton’s Ricci flow is a geometric evolution equation on the space of Riemannian metrics of a smooth manifold. In a first subproject we would like to show a differentiable stability result for noncollapsed converging sequences of Riemannian manifolds with nonnegative sectional curvature, generalising Perelman’s topological stability. In a second subproject, next to classifying homogeneous Ricci solitons on non-compact homogeneous spaces, we would like to prove the dynamical Alekseevskii conjecture. Finally, in a third subproject we would like to find new Ricci flow invariant curvature conditions, a starting point for introducing a Ricci flow with surgery in higher dimensions. online

Project members: **Burkhard Wilking**, **Christoph Böhm**

• **CRC 1442: Geometry: Deformation and Rigidity - C03: K-theory of group algebras ** The Farrell–Jones conjecture gives a homological formula for the K-theory of group rings. On the one hand this conjecture has concrete applications, e.g. to the rigidity of aspherical manifolds. On the other hand its underlying principle is very general, and applies for example also to Hecke algebras of p-adic Lie groups. We will extend the scope of positive results for this conjecture, develop new tools to study the conjecture and extend the framework where is can be formulated and applied. online

Project members: **Arthur Bartels**

• **CRC 1442: Geometry: Deformation and Rigidity - Z01: Central Task of the Collaborative Research Centre ** online

Project members: **Arthur Bartels**

• **CRC 1442: Geometry: Deformation and Rigidity ** From its historic roots, geometry has evolved into a cornerstone in modern mathematics, both as a tool and as a subject in its own right. On the one hand many of the most important open questions in mathematics are of geometric origin, asking for example to what extent an object is determined by geometric properties. On the other hand, abstract mathematical problems can often be solved by associating them to more geometric objects that can then be investigated using geometric tools. A geometric point of view on an abstract mathematical problem quite often opens a path to its solution.

Deformations and rigidity are two antagonistic geometric concepts which can be applied in many abstract situations making transfer of methods particularly fruitful. Deformations of mathematical objects can be viewed as continuous families of such objects, like for instance evolutions of a shape or a system with time. The collection of all possible deformations of a mathematical object can often be considered as a deformation space (or moduli space), thus becoming a geometric object in its own right. The geometric properties of this space in turn shed light on the deeper structure of the given mathematical objects. We think of properties or of quantities associated with mathematical objects as rigid if they are preserved under all (reasonable) deformations. A rigidity phenomenon refers to a situation where essentially no deformations are possible. Rigidity then implies that objects which are approximately the same must in fact be equal, making such results important for classifications.

The overall objective of our research programme can be summarised as follows:

Develop geometry as a subject and as a powerful tool in theoretical mathematics focusing on the dichotomy of deformations versus rigidity. Use this unifying perspective to transfer deep methods and insights between different mathematical subjects to obtain scientific breakthroughs, for example concerning the Langlands programme, positive curvature manifolds, K-theory, group theory, and C*-algebras. online

Project members: **Arthur Bartels**

• **CRC 1442: Geometry: Deformation and Rigidity - D03: Integrability ** The project investigates a novel integrable system which arises from a quantum field theory on noncommutative geometry. It is characterised by a recursive system of equations with conjecturally rational solutions. The goal is to deduce their generating function and to relate the rational coefficients in the generating function to intersection numbers of tautological characteristic classes on some moduli space. online

Project members: **Raimar Wulkenhaar**

• **CRC 1442: Geometry: Deformation and Rigidity - D01: Amenable dynamics via C*-algebras ** Three regularity properties and their interplay have been at the heart of exciting recent developments in the structure and classification theory of nuclear C*-algebras: finite nuclear dimension, tensorial absorption of the Jiang–Su algebra, and strict comparison of positive elements. There are corresponding properties for group actions; we will study these dynamic regularity properties in order to gain new insights into amenable groups and their actions, and on rigidity properties of their associated C*-algebras. Taking a dual viewpoint, we will study - and to some extent classify - Cartan subalgebras of C*-algebras. These are maximal abelian subalgebras, the position of which encapsulates crucial information about the underlying dynamics of a C*-algebra. online

Project members: **Wilhelm Winter**

• **CRC 1442: Geometry: Deformation and Rigidity - C01: Automorphisms and embeddings of manifolds ** This project concerns the homotopical properties of spaces of smooth and topological automorphisms of manifolds, their classifying spaces and spaces of smooth and topological embeddings of manifolds. Known characteristic classes for manifold bundles will play an important role. It is conceivable that new ones will be constructed. The action of automorphisms and embeddings on finite subsets of manifolds, more precisely on the configuration category of a manifold, will be exploited. online

Project members: **Michael Weiss**, **Johannes Ebert**

• **CRC 1442: Geometry: Deformation and Rigidity - B03: Moduli spaces of metrics of positive curvature ** In this project, the space of Riemannian metrics of positive scalar curvature on closed manifolds will be studied. Central research questions concern the nontriviality of secondary index invariants, rigidity theorems for the homotopy type of those spaces and the action of the diffeomorphism group, and the comparison of two iterated loop space structures. We will use techniques from differential geometry, higher index theory, metric geometry, differential topology and homotopy theory. online

Project members: **Johannes Ebert**

• **RTG 2149: Strong and Weak Interactions - from Hadrons to Dark Matter ** The Research Training Group (Graduiertenkolleg) 2149 "Strong and Weak Interactions - from Hadrons to Dark Matter" funded by the Deutsche Forschungsgemeinschaft focuses on the close collaboration of theoretical and experimental nuclear, particle and astroparticle physicists further supported by a mathematician and a computer scientist. This explicit cooperation is of essence for the PhD topics of our Research Training Group.Scientifically this Research Training Group addresses questions at the forefront of our present knowledge of particle physics. In strong interactions we investigate questions of high complexity, such as the parton distributions in nuclear matter, the transition of the hot quark-gluon plasma into hadrons, or features of meson decays and spectroscopy. In weak interactions we pursue questions, which are by definition more speculative and which go beyond the Standard Model of particle physics, particularly with regard to the nature of dark matter. We will confront theoretical predictions with direct searches for cold dark matter particles or for heavy neutrinos as well as with new particle searches at the LHC.The pillars of our qualification programme are individual supervision and mentoring by one senior experimentalist and one senior theorist, topical lectures in physics and related fields (e.g. advanced computation), peer-to-peer training through active participation in two research groups, dedicated training in soft skills, and the promotion of research experience in the international community. We envisage early career steps through a transfer of responsibilities and international visibility with stays at external partner institutions. An important goal of this Research Training Group is to train a new generation of scientists, who are not only successful specialists in their fields, but who have a broader training both in theoretical and experimental nuclear, particle and astroparticle physics. online

Project members: **Raimar Wulkenhaar**

• **Amenability, Approximation and Reconstruction ** Algebras of operators on Hilbert spaces were originally introduced as the right framework for the mathematical description of quantum mechanics. In modern mathematics the scope has much broadened due to the highly versatile nature of operator algebras. They are particularly useful in the analysis of groups and their actions. Amenability is a finiteness property which occurs in many different contexts and which can be characterised in many different ways. We will analyse amenability in terms of approximation properties, in the frameworks of abstract C*-algebras, of topological dynamical systems, and of discrete groups. Such approximation properties will serve as bridging devices between these setups, and they will be used to systematically recover geometric information about the underlying structures. When passing from groups, and more generally from dynamical systems, to operator algebras, one loses information, but one gains new tools to isolate and analyse pertinent properties of the underlying structure. We will mostly be interested in the topological setting, and in the associated C*-algebras. Amenability of groups or of dynamical systems then translates into the completely positive approximation property. Systems of completely positive approximations store all the essential data about a C*-algebra, and sometimes one can arrange the systems so that one can directly read of such information. For transformation group C*-algebras, one can achieve this by using approximation properties of the underlying dynamics. To some extent one can even go back, and extract dynamical approximation properties from completely positive approximations of the C*-algebra. This interplay between approximation properties in topological dynamics and in noncommutative topology carries a surprisingly rich structure. It connects directly to the heart of the classification problem for nuclear C*-algebras on the one hand, and to central open questions on amenable dynamics on the other. online

Project members: **Wilhelm Winter**