Pushforward dynamics on Wasserstein spaces and measure rigidity
For an endomorphism $\phi$ of a closed Riemannian manifold $M$, we study the pushforward action $\phi_\ast$ on the Wasserstein space $\mathcal{P}(M)$ at a measure $\mu_0$ preserved by $\phi$. We show that if $\phi$ is a $C^2$ covering map and $\mu_0$ has positive $C^1$ density, then $\phi_\ast$ is G\^ateaux differentiable at $\mu_0$ along tangent directions, with derivative given by the transfer operator of $\phi$ acting on vector fields, followed by orthogonal projection onto the tangent space. The derivative is the adjoint of the Koopman operator restricted to the tangent space, and its fixed space consists of the directions in which $\mu_0$ can be deformed while preserving invariance to first order. For appropriate pairs of endomorphisms of $\mathbb{T}^d$, we compute the intersection of their fixed spaces, show that it contains an infinite family of linearly independent continuous vector fields, and construct, for every $n$, an embedded $n$-dimensional family of measures that are nearly invariant under both endomorphisms. First-order rigidity therefore fails in every dimension. In particular, rigidity phenomena such as higher-dimensional analogues of the Furstenberg conjecture, if true, are genuinely nonlinear.