Given a completely arbitrary surface, whether or not it has bounded curvature, or even whether or not it is complete, there exists an instantaneously complete Ricci flow evolution of that surface that exists for a specific amount of time [GT11]. In the case that the underlying Riemann surface supports a hyperbolic metric, this Ricci flow always exists for all time and converges (after scaling by a factor 1 2t ) to this hyperbolic metric [GT11], i.e. our Ricci flow geometrises the surface. In this paper we show that there exist complete, bounded curvature initial metrics, including those conformal to a hyperbolic metric, which have subsequent Ricci flows developing unbounded curvature at certain intermediate times. In particular, when coupled with the uniqueness from [Top13], we find that any complete Ricci flow starting with such initial metrics must develop unbounded curvature over some intermediate time interval, but that nevertheless, the curvature must later become bounded and the flow must achieve geometrisation as t → ∞, even though there are other conformal deformations to hyperbolic metrics that do not involve unbounded curvature.Another consequence of our constructions is that while our Ricci flow from [GT11] must agree initially with the classical flow of Hamilton and Shi in the special case that the initial surface is complete and of bounded curvature, by uniqueness, it is now clear that our flow lasts for a longer time interval in general, with Shi's flow stopping when the curvature blows up, but our flow continuing strictly beyond in these situations.All our constructions of unbounded curvature developing and then disappearing are in two dimensions. Generalisations to higher dimensions are then immediate.
IntroductionHamilton [Ham82] and Shi [Shi89] proved that given a complete Riemannian manifold (M, g 0 ) with bounded curvature, there exists a complete Ricci flow g(t) on M for a short time, with g(0) = g 0 (see [Top06] for an introduction to this topic). The curvature of this Ricci flow is initially bounded, and the flow can be extended until such time that the curvature becomes unbounded.Ricci flows with possibly unbounded curvature in their initial condition and/or during the flow itself, were studied by the second author in [Top10] in the special case of surfaces, and in [GT11] we proved that one can always find an instantaneously complete Ricci flow starting at a completely arbitrary initial surface, whether of unbounded curvature or not, or indeed whether complete or not, which exists for a specific amount of time, and in [Top13] this solution was shown to be unique. More precisely, we proved: (1.1)Then there exists a smooth Ricci flow g(t) t∈[0,T ) such thatis complete for all t ∈ (0, T ));and this flow is unique in the sense that if g 2 (t) t∈[0,T2) is any other Ricci flow on M satisfying (i) and (ii), then T 2 ≤ T and g 2 (t) = g(t) for all t ∈ [0, T 2 ).
If T < ∞, then we haveand in particular, T is the maximal existence time.It has been understood since the work of Hamilton and C...