The Action homomorphism, quasimorphisms and moment maps on the space of compatible almost complex structures

TL;DR

This paper extends Weinstein's Action homomorphism to infinite-dimensional Lie group actions on symplectic manifolds, constructing quasimorphisms that relate to various geometric invariants and demonstrating their properties and applications.

Contribution

It introduces a generalized action homomorphism for Hamiltonian group actions, linking it to quasimorphisms and moment maps, with applications to symplectic geometry and complex structures.

Findings

Constructed a quasimorphism on the universal cover of Hamiltonian diffeomorphisms.

Connected the quasimorphism to known invariants like Futaki and Calabi.

Proved the L^2_2-distance on the universal cover is unbounded.

Abstract

We extend the definition of Weinstein's Action homomorphism to Hamiltonian actions with equivariant moment maps of (possibly infinite-dimensional) Lie groups on symplectic manifolds, and show that under conditions including a uniform bound on the symplectic areas of geodesic triangles the resulting homomorphism extends to a quasimorphism on the universal cover of the group. We apply these principles to finite dimensional Hermitian Lie groups like Sp(2n,R), reinterpreting the Guichardet-Wigner quasimorphisms, and to the infinite dimensional groups of Hamiltonian diffeomorphisms Ham(M,\om) of closed symplectic manifolds (M,\om), that act on the space of compatible almost complex structures with an equivariant moment map given by the theory of Donaldson and Fujiki. We show that the quasimorphism on \widetilde{Ham}(M,\om) obtained in the second case is Symp(M,\om)-congjugation-invariant and compute its restrictions to \pi_1(Ham(M,\om)) via a homomorphism introduced by Lalonde-McDuff-Polterovich, answering a question of Polterovich; to the subgroup Hamiltonian biholomorphisms via the Futaki invariant; and to subgroups of diffeomorphisms supported in an embedded ball via the Barge-Ghys average Maslov quasimorphism, the Calabi homomorphism and the average Hermitian scalar curvature. We show that when c_1(TM)=0 this quasimorphism is proportional to a quasimorphism of Entov and when [\om] is a non-zero multiple of c_1(TM), it is proportional to a quasimorphism due to Py. As an application we show that the L^2_2-distance on \widetilde{Ham}(M,\om) is unbounded, similarly to the results of Eliashberg-Ratiu for the L^2_1-distance.