Multiplicative Chow-Künneth decomposition and homology splitting of configuration spaces

Abstract

We construct a splitting of the cohomology of configuration spaces of points on a smooth proper variety with a multiplicative Chow--Künneth decomposition. Applied to hyperelliptic curves, this shows that the hyperelliptic Torelli group acts trivially on the rational cohomology of ordered configuration spaces of points. Moreover, if $H_{g,n}$ denotes the moduli space of $n$-pointed hyperelliptic curves, the Leray spectral sequence for the forgetful map $H_{g,n}\to H_g$ degenerates immediately, in sharp contrast to the forgetful map from $M_{g,n}$ to $M_g$. This allows for new detailed calculations of the cohomology of $M_{2,n}$ for $n\le 5$, and the stable cohomology of $H_{g,n}$ for $n\le 5$. We also give a detailed study of the cohomology of symplectic local systems on $M_2$.

Richard Hain has communicated to us that Corollary 1.10 contradicts known statements in the literature