Rigidity of ancient ovals in higher dimensional mean curvature flow

Abstract: In this paper, we consider the classification of compact ancient noncollapsed mean curvature flows of hypersurfaces in arbitrary dimensions. More precisely, we study $k$-ovals in $\mathbb{R}^{n+1}$, defined as ancient noncollapsed solutions whose tangent flow at $-\infty$ is given by $\mathbb{R}^k \times S^{n-k}((2(n-k)|t|)^{\frac{1}{2}})$ for some $k \in {1,\dots,n-1}$, and whose fine cylindrical matrix has full rank. A significant advance achieved recently by Choi and Haslhofer suggests that the shrinking $n$-sphere and $k$-ovals together account for all compact ancient noncollapsed solutions in $\mathbb{R}^{n+1}$. We prove that $k$-ovals are $\mathbb{Z}^{k}_2 \times \mathrm{O}(n+1-k)$-symmetric and are uniquely determined by $(k-1)$-dimensional spectral ratio parameters. This result is sharp in view of the $(k-1)$-parameter family of $\mathbb{Z}^{k}_2 \times \mathrm{O}(n+1-k)$-symmetric ancient ovals constructed by Du and Haslhofer, as well as the conjecture of Angenent, Daskalopoulos and Sesum concerning the moduli space of ancient solutions. We also establish a new spectral stability theorem, which suggests the local $(k-1)$-rectifiability of the moduli space of $k$-ovals modulo space-time rigid motion and parabolic rescaling. In contrast to the case of $2$-ovals in $\mathbb{R}^4$, resolved by Choi, Daskalopoulos, Du, Haslhofer and Sesum, the general case for arbitrary $k$ and $n$ presents new challenges beyond increased algebraic complexity. In particular, the quadratic concavity estimates in the collar region and the absence of a global parametrization with regularity information pose major obstacles. To address these difficulties, we introduce a novel test tensor that produces essential gradient terms for the tensor maximum principle, and we derive a local Lipschitz continuity result by parameterizing $k$-ovals with nearly matching spectral ratio parameters.