Date	Speaker	Title
January 22	Mikhail Skopenkov Higher School of Economics, Moscow and King Abdullah University, Saudi Arabia	TBA
November 13	Irving Day UT Austin	TBA
November 6	Ivan Dynnikov Moscow State University and Steklov Mathematical Institute, Moscow	TBA
September 25	Eric Ling University of Copenhagen	Singularities, topology, and rigidity in cosmological spacetimes satisfying the null energy conditions

January 22, 2026: Mikhail Skopenkov "TBA"

Abstract:

November 13, 2025: Irving Dai "TBA"

Abstract:

November 6, 2025: Ivan Dynnikov "TBA"

Abstract:

September 25, 2025: Eric Ling "Singularities, topology, and rigidity in cosmological spacetimes satisfying the null energy condition"

Abstract: We consider globally hyperbolic spacetimes with compact Cauchy surfaces (i.e., Riemannian manifolds which appear as time slices within the spacetime) in a setting compatible with the presence of a positive cosmological constant. More specifically, for 3+1 dimensional spacetimes which satisfy the null energy condition and contain a future expanding compact Cauchy surface, we establish a precise connection between the topology of the Cauchy surfaces and the occurrence of past singularities. Our proof makes use of the Penrose singularity theorem along with the fact that every aspherical closed 3-manifold admits a finite covering with positive first Betti number, which is a consequence of the positive resolution of the virtual Haken conjecture. We also obtain several rigidity results when the spacetime does not contain past singularities and the Cauchy surface is only everywhere noncontracting. This is joint work with Greg Galloway and with Carl Rossdeutscher, Walter Simon, and Roland Steinbauer.