This page contains my research papers. Frequently the version on this page is more up-to-date than the version on the arXiv.

**Realization problems for diffeomorphism groups (with Kathryn Mann)**

This survey article discusses recent results and open questions on the broad theme of Nielsen realization problems with an emphasis on infinite subgroups of the mapping class group.

A basic problem in the study of fiber bundles is to compute the ring H*(BDiff(M)) of characteristic classes of bundles with fiber a smooth manifold M. When M is a surface, this problem has ties to algebraic topology, geometric group theory, and algebraic geometry. We have a good understanding of the cohomology in the *stable range, *but this accounts for a small percentage of the total cohomology, and very little is known beyond that. In this paper we produce new classes (independent from the stable classes) for some special M that come from the unstable cohomology of arithmetic groups. Dually these nontrivial characteristic classes imply the existence of new topologically interesting manifold bundles. We also give an application to bundles with fiber a K3 surface.

**Groups with 2-sphere Bowditch boundary(with Genevieve Walsh)**

Bestvina-Mess showed that the duality properties of a group are encoded in any boundary that gives a Z-set compactification; in particular, a hyperbolic group with Gromov boundary S^n is a PD(n+1) group. For relatively hyperbolic pairs (*G*,*P*), the natural boundary -- the Bowditch boundary -- does not give a Z-set compactification of *G*. Nevertheless we show that if the Bowditch boundary of (*G,P*) is S^2, then (*G*,*P*) is a PD(3) pair.

## Characteristic classes of fiberwise branched surface bundles via arithmetic groups

Mich. Math. J. to appear

Let *S* be a closed surface of genus *g*>1. The cohomology of its mapping class group Mod(*S*) and its finite-index subgroups plays an important role in many areas of mathematics. One source of cohomology classes is the symplectic representation*R*: Mod(*S*) --> Sp(2*g*,Z). A well-known computation shows that the image of

R*: H*( Sp(2g,Z) ;Q) --> H*( Mod(S) ;Q) is the algebra generated by the odd Miller-Morita-Mumford (MMM) classes in the stable range. In this paper we extend this example. Let *G* < Diff(*S*) be a finite group, and consider the subgroup Mod^*G*(*S*) of Mod(*S*) of mapping classes that can be realized by diffeomorphisms that commute with *G*. The image of Mod^*G*(*S*) under *R* lands in the centralizer Sp^*G*(2*g*,Z). The main result of this paper is a computation of *R**: H^2( Sp^*G*(2*g*,Z) ;Q) --> H^2( Mod^*G*(*S*) ;Q)

when *G* is a finite cyclic group. We apply this computation to compute Toledo invariants of surface group representations to SU(*p*,*q*) arising from the Atiyah-Kodaira construction, and we show that classes in the image of *R** gives equivariant cobordism invariants for surface bundles with fiberwise *G* action, following Church-Farb-Thibault.

## On the non-realizability of braid groups by diffeomorphisms

(with Nick Salter)

Bull. Lond. Math. Soc. 2016

For every compact surface we show that when *n* is sufficiently large there is no lift of the surface braid group Br(*n*,*S*) to Diff(*S*,*n*), the group of diffeomorphisms preserving *n* marked points and restricting to the identity on the boundary. This generalizes work of Bestvina-Church-Souto, but our main tool (the Thurston stability theorem) is different. Our methods are applied to give a new proof of Morita's non-lifting theorem in the best possible range. These techniques extend to the more general setting of spaces of codimension-2 embeddings, and we obtain corresponding results for spherical motion groups, including the string motion group.

## Hyperbolic groups with boundary an n-dimensional Sierpinski space

(with Jean Lafont)

J. Topol. Anal. to appear

The fundamental group of a closed aspherical manifold *M* is an example of a Poincare duality group. Whether or not all finitely presented Poincare duality groups arise in this fashion is an open problem that goes back to Wall. This paper addresses a relative version of this problem for a special class of groups. Fixing *n*>6, we show that if *G* is a torsion-free hyperbolic group whose visual boundary is an (*n*-2)-dimensional Sierpinski space, then *G* = pi_1(*W*) for some aspherical n-manifold *W* with boundary. The proof involves the total surgery obstruction and is based on work of Bartels-Lueck-Weinberger. Concerning the converse, we construct examples of aspherical manifolds with boundary whose fundamental group is hyperbolic but with visual boundary not homeomorphic to a Sierpinski space.

In this note we compute low degree rational Pontryagin classes for every closed locally symmetric manifold of noncompact type. In particular, we answer the question: Which locally symmetric M have at least one nonzero Pontryagin class? This computation was motivated by a Nielsen realization problem. Specifically, in my paper "Cohomological obstructions to Nielsen realization", I show that Pontryagin classes are obstructions to realizing the point-pushing subgroup for nonpositively curved manifolds (and in particular locally symmetric manifolds of noncompact type).

For a manifold *M* and a subgroup *G* < Mod(*M*) of the mapping class group, the question of whether *G* can be lifted to the diffeomorphism group Diff(*M*) is an example of a Nielsen realization problem. This question is related to an existence question for flat connections on fiber bundles with with monodromy *G*. In this paper we consider the case *M* is a locally symmetric manifold with a basepoint and *G* is the *point-pushing subgroup*. The simplest instance -- when *M* is a closed orientable surface of genus at least 2 -- was worked out by Bestvina-Church-Souto, who showed a lift does not exist. We give new cohomological techniques to generalize their result to higher dimensional locally symmetric manifolds. The main tools include Chern-Weil theory, Milnor-Wood inequalities, and Margulis superrigidity.