Motivic Steenrod algebra and Thom obstructions without desingularization

Abstract

We give a desingularization-free proof of Voevodsky's identification of bistable mod-$\ell$ motivic cohomology operations with the motivic Steenrod algebra over fields of characteristic zero. This supplies the direct argument anticipated by Voevodsky in his study of motivic Eilenberg--MacLane spaces. As an application, we study Thom's topological obstructions to desingularization, which arose in his work on the Steenrod realization problem. Without invoking desingularization, we prove that the primary and all higher Thom obstructions vanish on complex algebraic cycles. The proof uses the motivic bidegrees of the successive $k$-invariants in the Brown--Peterson tower to show that every lifting obstruction vanishes.