Integral geometry and curvature integrals in hyperbolic space. Gil Solanes April 29, 2003 Contents 1 Mean curvature integrals 13 1.1 Hyperbolic space . 13 1.1.1 De¯nition and basic facts . 13 1.1.2 Moving frames . 15 1.1.3 Models . 19 1.2 Mean curvature integrals . 21 2 Spaces of planes 27 2.1 Spaces of planes . 27 2.1.1 Invariant metrics and measures . 29 2.2 Kinematic formulas . 33 2.3 The de Sitter sphere . 37 2.3.1 Spaces of planes in the de Sitter sphere . 40 2.3.2 The Gauss map . 42 2.4 Contact measures. 46 2.5 Cauchy-Crofton formula in the de Sitter sphere. 48 3 Total Curvature 53 3.1 The Gauss-Bonnet Theorem in Euclidean Space . 53 3.2 The Gauss-Bonnet Theorem in the sphere . 55 3.3 Variation Formulas. 60 3.4 The Gauss-Bonnet Theorem in Hyperbolic space . 66 3.5 Total Absolute Curvature in Hyperbolic Space . 70 3.5.1 Total Absolute Curvature in Euclidean Space . 70 3.5.2 Surfaces in Hyperbolic 3-Space. 72 3.5.3 Tight Immersions in Hyperbolic Space . 75 4 Inequalities between mean curvature integrals 85 4.1 Introduction . 85 4.2 Inequalities between Quermassintegrale . 87 4.3 Slice expectation for random geodesic planes . 91 4.4 Inequalities between the mean curvature integrals . 92 4.5 Examples . 96 3 Contents 5 Integral geometry of horospheres and equidistants 101 5.1 Introduction . 101 5.2 De¯nitions and invariant measures . 102 5.3 Volume of intersections with ¸-geodesic planes . 104 5.4 Mean curvature integrals of intersections with ¸-geodesic planes . 106 5.5 Measure of ¸-geodesic planes meeting a ¸-convex domain. 111 5.6 h-convex domains . 112 5.7 Expected slice with ¸-geodesic planes . 115 4 Introduction The sum of the inner of a hyperbolic triangle is always below ¼. The di®erence with that value is the so-called defect of the triangle, and coincides with its area. This result belongs to the beginnings of non-euclidean geometry but it is also a consequence of the well-known Gauss-Bonnet theorem. By virtue of this theorem, the integral of the geodesic curvature along a smooth simple closed curve in hyperbolic plane is known to be the area of the bounded domain plus the Euler characteristic of this domain multiplied by 2¼. The defect of a curve in hyperbolic plane could be de¯ned to be the di®erence between its geodesic curvature integral and that of a topologically equivalent curve in euclidean plane. Then one would state that the defect of a hyperbolic plane curve is the area of the domain it bounds. Such a defect would measure the extra e®ort that a sailor needs to make at the rudder in order to make the ship give a complete loop when sailing in a hyperbolic sea. When going to dimension 3, the Gauss-Bonnet theorem states that the Gauss cur- vature integral of a closed surface is 2¼ times its Euler characteristic plus its area. Then one can say that the defect of such a surface equals its area. In particular it is clear that surfaces in hyperbolic space must provide more curvature than in euclidean space in order to get closed. In hyperbolic space of general dimension one ¯nds a quite disappointing version of the Gauss-Bonnet theorem: for even n and a domain Q Hn of volume V , ½ M (@Q) + c M (@Q) + + c M (@Q) + ( 1)n=2V = O Â(Q) n¡1 n¡3 n¡3 ¢ ¢ ¢ 1 1 ¡ n¡1 while for odd n, O M (@Q) + c M (@Q) + + c M (@Q) M (@Q) = n¡1 Â(@Q) n¡1 n¡3 n¡3 ¢ ¢ ¢ 2 2 ¡ 0 2 where Oi is the volume of the i-dimensional unit sphere, the constants ci depend only on the dimension, and Mi(@Q) is the integral over @Q of the i-th mean curvature function. In particular Mn¡1(@Q) is the Gauss curvature integral or total curvature of @Q. This Gauss-Bonnet formula is deduced from the general intrinsic version proved by Chern for abstract riemannian manifolds. However it is too bad to loose the simplicity we had in dimensions 2 and 3. For instance, its is not so clear, even for a convex Q, whether if the total curvature is greater in hyperbolic or in euclidean space. Nevertheless, integral geometry will allow us to give a much smarter version of the Gauss-Bonnet theorem in 5 Introduction hyperbolic space. Indeed, it will be proved M (@Q) = O Â(Q) + c Â(L Q)dL (1) n¡1 n¡1 n¡2 \ n¡2 ZLn¡2 n where n¡2 is the space of codimension 2 totally geodesic planes of H , and dLn¡2 is the L invariant measure in this space. This way, the defect of a hyperbolic closed hypersurface is the measure with some multiplicity of the set of codimension 2 planes intersecting it. Integral geometry allows not only to state such a pretty formula but it is also the key tool for the proof. Since one can easily deduce the Gauss-Bonnet theorem in Hn from this formula, we will get a new proof of this theorem using the methods of integral geometry. This remarkable fact has an equivalent in spherical geometry that was already dis- covered by E. Teufel in [Teu80]. In this case, the integral over the space on (n 2)-planes ¡ in formula (1) appears with a minus sign. Let us take a look to the proof in this spher- ical case, and we will have an idea of the di±culties hidden in the hyperbolic case. To simplify take a strictly convex domain Q Sn. Then one can de¯ne a Gauss map γ by ½ taking the inner unit normal vector at the points of S = @Q. The image γ(S) of such a Gauss map is a hypersurface in Sn with volume equal to the total curvature of S. This is due to the fact that the Gauss curvature at a point is the in¯nitesimal volume defor- mation of γ. On the other hand, the Cauchy-Crofton formula in Sn allows to compute this volume by just integrating the number of points where γ(S) is cut by all the great n circles of S . Up to some constant, one has that Mn¡1(S) is the integral of the number of intersection points #(l γ(S)) over the space of great circles l of Sn. Consider the \ bundle of hyperplanes orthogonal to a great circle l. They all contain a common (n 2)- ¡ plane L which we call the pole of l. Moreover, every point of l γ(S) corresponds to n¡2 \ a hyperplane of the bundle that is tangent to S. But, depending on whether the pole Ln¡2 meets S or not, the hyperplane bundle has none or two hyperplanes tangent to S. Therefore, the total curvature of S is, up to some constant, the di®erence of the total measure of (n 2)-planes with the measure of those planes meeting the convex Q. ¡ The general plan for hyperbolic space is similar. Consider a strictly convex domain Q Hn in the hyperboloid model ½ n n+1 H = x R L(x; x) = 1 x0 > 0 f 2 j ¡ g with the Lorentz metric L(x; y) = x y + x y + + x y . Take, at every point in ¡ 0 0 1 1 ¢ ¢ ¢ n n S = @Q, the inner unit normal vector. This is a space-like vector of Rn+1; so one has a Gauss map γ from S to the following quadric, called de Sitter sphere, n n+1 ¤ = x R L(x; x) = 1 : f 2 j g The image γ(S) is a space-like hypersurface and, as in Sn, the Gauss curvature integral over S coincides with the volume of this image. The next step would be to use a Cauchy- Crofton formula in ¤n to ¯nd the volume of γ(S). Unfortunately, such a formula does not exist. In fact, the set of lines in ¤n intersecting any small piece of hypersurface has 6 Introduction in¯nite measure. This was the hidden di±culty of hyperbolic space: one needs to do integral geometry in a Lorentz space, and this gives some di±culties. That the target of the Gauss map of Hn is a Lorentz manifold is not accidental, nor it is due to the fact that we have used a model contained in Minkowski space. As we will see, the reasons for this fact are closely related to the existence of ultraparallel hyperplanes. Therefore, it is a fact deeply related to the geometrical properties of hyperbolic space, and not to those of any particular model. However, we will here get over this di±culty while getting a Cauchy-Crofton-like formula for hypersurfaces of ¤n. To be precise we will prove O (2 #(l R))dl = n¡2 (vol(R) O ) ¡ \ n 1 ¡ n¡1 ZL ¡ where R is any space-like hypersurface embedded in ¤n, and stands for the set of L space-like geodesics l. Then one can apply this formula to the hypersurface R = γ(S). For every vector of some space-like geodesic in ¤n, take the orthogonal subspace with respect to the Lorentz metric. We get a bundle of hyperplanes in Hn containing all of n them an (n 2)-dimensional geodesic plane Ln¡2 of H . Now, the intersections of l and ¡ γ(S) correspond to hyperplanes of the bundle tangent to S.