Ricci flow of warped Berger metrics on ${\mathbb{R}}^4$

Abstract

We study the Ricci flow on ${\mathbb{R}}^4$ starting at an SU(2)-cohomogeneity 1 metric $g_0$ whose restriction to any hypersphere is a Berger metric. We prove that if $g_0$ has no necks and is bounded by a cylinder, then the solution develops a global Type-II singularity and converges to the Bryant soliton when suitably dilated at the origin. This is the first example in dimension $n>3$ of a non-rotationally symmetric Type-II flow converging to a rotationally symmetric singularity model. Next, we show that if instead $g_0$ has no necks, its curvature decays and the Hopf fibers are not collapsed, then the solution is immortal. Finally, we prove that if the flow is Type-I, then there exist minimal 3-spheres for times close to the maximal time.

40 pages, final version. Accepted in Calc. Var