Some problems in hyperbolic 3-manifolds

Yair Minsky
June 9, 2004

Geometry from topology

The geometrization conjecture supplies necessary and sufficient topological conditions for a 3-manifold M to admit a hyperbolic structure. When M is closed this structure is unique by Mostow rigidity. There is currently only incomplete understanding of how to predict, from topological data, the geometric properties of the hyperbolic metric. For example, how may we predict the volume, the injectivity radius, the number of closed geodesics with a given length bound, etc. etc.? (Culler-Shalen et al have given some lower volume bounds from topological data, e.g. first betti numbers. Weeks' "Snappea" program gives precise numerical estimates from triangulation data).
  1. A "gluing problem": Let M1,M2 be compact 3-manifolds and f:M1 ® M2 be a homeomorphism. Give a uniform bilipschitz model for M = M1 Èf M2, with estimates depending only on the topology of Mi and the map f.
  2. Incompressible case: If Mi are incompressible, then the gluing problem reduces to (a) the model manifold constructed by Brock-Canary-Minsky for the solution of the Ending Lamination Conjecture, and (b) Thurston's "bounded image theorem" which says when Mi is acylindrical that the image of the "skinning map" in Teich(Mi) is compact. Given a topological description of Mi, can one give a quantitative bound on the diameter of the skinning map image?

Structure of deformation spaces

If M is a compact hyperbolizable 3-manifold with boundary we may consider the space of marked hyperbolic structures on int(M), which lies in the representation space p1(M) ® PSL(2,\mathbb C) modulo conjugation. (More generally we may look at all discrete faithful representations in this space, which gives manifolds homotopy-equivalent to M).
  1. Give a complete topological description of this space. (This is perhaps impossible. Although the space is enumerated by end-invariants, its topological structure is quite complicated. The end-invariants vary discontinuously, and Bromberg has shown the deformation space is not locally connected).
  2. Bers slice: If M=S×[-1,1] then quasifuchsian structures are parametrized by Teich(S)×Teich(S). A Bers slice BX is the set parametrized by {X}×Teich(S). It is homeomorphic to \mathbb R6g(S)-6. Is the closure of BX a ball?
  3. What is the Hausdorff dimension of the boundary of the deformation space?
  4. Let MCG(M) denote the group of homeomorphisms of M modulo homotopy. Describe the dynamics of MCG(M) on the representation space. Do the convex-cocompact representations comprise the largest open set on which the action is properly discontinuous?

Representations of surface groups

Consider representations r:p1(S) ® PSL(2,\mathbb C) where S is a compact surface and r is required to be parabolic on the boundary of S.
  1. Cannon-Thurston maps: If r is discrete and faithful, show that there is a continuous equivariant map from S1 to S2 (where p1(S) acts on S1 via any Fuchsian representation and PSL(2,\mathbb C) acts on S2 by Möbius transformations). This is known when r has "bounded geometry" (lower bound on injectivity radius), and there have been a number of extensions, notably a complete solution by McMullen when S is a one-holed torus. There is a general form of this conjecture for any finitely-generated group replacing S1 by the Gromov boundary. A positive solution would complete our picture of the action of a Kleinian group on the 2-sphere.
  2. Bowditch's conjecture: Consider the quantity
    rr =
    inf
    g 
    lr(g)

    l0(g)
    where g varies over simple (non-self-intersecting) closed curves in S, lr denotes translation distance of the image in \mathbb H3, and l0 denotes translation length in some Fuchsian structure on S. Show that rr > 0 if and only if r is quasi-fuchsian. This is known if r is discrete and faithful, but not for general representations. I.e. show that if r is indiscrete or nonfaithful then rr=0. This would be trivial if g were not required to be simple.
  3. Simple loop conjecture for representations: philosophically related to Bowditch's conjecture. If the kernel of r is nonempty, does it contain a simple element?
  4. An interesting special case of (*): Consider the action of MCG(one-holed torus)=PSL(2,\mathbb Z) on the representation space for the one-holed torus. This space may be parametrized (through traces of generators) as \mathbb C3, and the group action is generated by (x,y,z) ® (x,y,xy-z) and (x,y,z) ® (y,z,x).



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On 14 Jan 2005, 13:09.