Gaboriau’s criterion and fixed price one for locally compact groups

Let G 1 G_1 be a semisimple real Lie group and G 2 G_2 another locally compact second countable unimodular group. We prove that G 1 × G 2 G_1 × G_2 has fixed price one if G 1 G_1 has higher rank, or if G 1 G_1 has rank one and G 2 G_2 is a p p -adic split reductive group of rank at least one. As an application we resolve a question of Gaboriau showing S L ( 2 , Q ) SL(2,Q) has fixed price one. Inspired by the very recent work of Mikolaj Fraczyk, Sam Mellick, and Amanda Wilkens [ Poisson-voronoi tessellations and fixed price in higher ranks , Ann. of Math., to appear], we employ the method developed by the author and Miklós Abért to show that all essentially free probability measure preserving actions of groups weakly factor onto the Cox process driven by their amenable subgroups. We then show that if an amenable subgroup can be found satisfying a double recurrence property then the Cox process driven by it has cost one.