Blowdowns along Ricci flow in dimension four

Abstract

We show that the blowdown shrinking Ricci soliton of Feldman, Ilmanen and Knopf is strictly dynamically stable in the following strong local sense: (1) The Ricci flow from every metric on a closed 4-manifold containing a region close to a rescaling of a sufficiently large compact subset of the soliton in either orientation and with controlled geometry elsewhere develops a singularity with unique tangent flow modeled on the soliton. (2) The singular time slice has an isolated conical singularity, and by Gianniotis-Schulze the flow can be continued by blowing down a sphere of self-intersection $\pm1$. (3) On a closed 4-manifold, the set of metrics whose Ricci flows have their first singularity modeled on the soliton is $C^{14}$-open. It is nonempty if and only if the manifold contains a sphere of self-intersection $\pm1$. (4) The soliton is strongly rigid among shrinking Ricci solitons. As applications, we construct $C^{14}$-open sets of initial data whose Ricci flows perform any prescribed ordered sequence of blowdowns. (1) Every closed simply connected smooth 4-manifold, including spin and exotic examples, can be reached by Ricci flow from a standard $k\mathbb{CP}^2\#\ell\overline{\mathbb{CP}}{}^2$, through finitely many blowdown singularities. (2) On the standard $2\mathbb{CP}^{2}\#9\overline{\mathbb{CP}}{}^{2}$, there are infinitely many open sets of initial metrics whose Ricci flows, after a single blowdown, reach pairwise nondiffeomorphic manifolds. The proof uses the spectral analysis of Naff and the second author and deals with the gauge-fixing problem along Ricci flow through a Ważewski argument inspired by Stolarski. We also use Bamler-Lai's PDE-ODI principle. Except for continuation (requiring nonnegative curvature of the asymptotic cone), all results apply to linearly stable nonflat asymptotically conical shrinkers in all dimensions.