Flows of geometric structures

Abstract

Abstract We develop an abstract theory of flows of geometric 𝐻-structures, i.e., flows of tensor fields defining 𝐻-reductions of the frame bundle, for a closed and connected subgroup H ⊂ SO ⁢ ( n ) H\subset\mathrm{SO}(n) , on any connected and oriented 𝑛-manifold with sufficient topology to admit such structures. The first part of the article sets up a unifying theoretical framework for deformations of 𝐻-structures, by way of the natural infinitesimal action of GL ⁢ ( n , R ) \mathrm{GL}(n,\mathbb{R}) on tensors combined with various bundle decompositions induced by 𝐻-structures. We compute evolution equations for the intrinsic torsion under general flows of 𝐻-structures and, as applications, we obtain general Bianchi-type identities for 𝐻-structures, and, for closed manifolds, a general first variation formula for the L 2 L^{2} -Dirichlet energy functional ℰ on the space of 𝐻-structures. We then specialise the theory to the negative gradient flow of ℰ over isometric 𝐻-structures, i.e., their harmonic flow. The core result is an almost-monotonocity formula along the flow for a scale-invariant localised energy, similar to the classical formulas by Chen–Struwe [M. Struwe, On the evolution of harmonic maps in higher dimensions, J. Differential Geom. 28 (1988), 3, 485–502; Y. M. Chen and M. Struwe, Existence and partial regularity results for the heat flow for harmonic maps, Math. Z. 201 (1989), 1, 83–103] for the harmonic map heat flow. This yields an 𝜀-regularity theorem and an energy gap result for harmonic structures, as well as long-time existence for the flow under small initial energy, relative to the L ∞ L^{\infty} -norm of initial torsion, in the spirit of Chen–Ding [Y. M. Chen and W. Y. Ding, Blow-up and global existence for heat flows of harmonic maps, Invent. Math. 99 (1990), 3, 567–578]. Moreover, below a certain energy level, the absence of a torsion-free isometric 𝐻-structure in the initial homotopy class imposes the formation of finite-time singularities. These seemingly contrasting statements are illustrated by examples on flat 𝑛-tori, so long as the set [ S n , SO ⁢ ( n ) / H ] [\mathbb{S}^{n},\mathrm{SO}(n)/H] of homotopy classes of maps S n → SO ⁢ ( n ) / H \mathbb{S}^{n}\to\mathrm{SO}(n)/H contains more than one element and the universal cover of SO ⁢ ( n ) / H \mathrm{SO}(n)/H is a sphere, e.g. when n = 7 n=7 and H = G 2 H=\mathrm{G}_{2} , or n = 8 n=8 and H = Spin ⁢ ( 7 ) H=\mathrm{Spin}(7) .