**SMS scnews item created by Boris Lishak at Fri 1 Feb 2019 1613**

Type: Seminar

Modified: Tue 12 Feb 2019 1356

Distribution: World

**Calendar1: 12 Feb 2019 1100-1200**

**CalLoc1: Carslaw 159**

CalTitle1: Galatius -- Tropical curves, graph homology, and top weight cohomology of \(M_g\)

Auth: borisl@dora.maths.usyd.edu.au

### Geometry and Topology Seminar

# Tropical curves, graph homology, and top weight cohomology of \(M_g\)

### Soren Galatius (Copenhagen)

Please join us for lunch after the talk.
**Abstract:**

We study the topology of a space parametrizing stable tropical curves of genus \(g\) with volume \(1\), showing that its reduced rational homology is canonically identified with both the top weight cohomology of \(M_g\) and also with the genus g part of the homology of Kontsevich's graph complex. Using a theorem of Willwacher relating this graph complex to the Grothendieck-Teichmueller Lie algebra, we deduce that \(H^{4g-6}(M_g;Q)\) is nonzero for \(g=3\), \(g=5\), and \(g\) at least \(7\). This disproves a recent conjecture of Church, Farb, and Putman as well as an older, more general conjecture of Kontsevich. We also give an independent proof of another theorem of Willwacher, that homology of the graph complex vanishes in negative degrees.