DIFFERENTIABLE MANIFOLDS Section c course 2003 Nigel Hitchin [email protected] 1 1 Introduction This is an introductory course on differentiable manifolds. These are higher dimen- sional analogues of surfaces like this: This is the image to have, but we shouldn’t think of a manifold as always sitting inside a fixed Euclidean space like this one, but rather as an abstract object. One of the historical driving forces of the theory was General Relativity, where the manifold is four-dimensional spacetime, wormholes and all: Spacetime is not part of a bigger Euclidean space, it just exists, but we need to learn how to do analysis on it, which is what this course is about. Another input to the subject is from mechanics – the dynamics of complicated me- chanical systems involve spaces with many degrees of freedom. Just think of the different configurations that an Anglepoise lamp can be put into: 2 How many degrees of freedom are there? How do we describe the dynamics of this if we hit it? The first idea we shall meet is really the defining property of a manifold – to be able to describe points locally by n real numbers, local coordinates. Then we shall need to define analytical objects (vector fields, differential forms for example) which are independent of the choice of coordinates. This has a double advantage: on the one hand it enables us to discuss these objects on topologically non-trivial manifolds like spheres, and on the other it also provides the language for expressing the equations of mathematical physics in a coordinate-free form, one of the fundamental principles of relativity. The most basic example of analytical techniques on a manifold is the theory of dif- ferential forms and the exterior derivative. This generalizes the grad, div and curl of ordinary three-dimensional calculus. A large part of the course will be occupied with this. It provides a very natural generalization of the theorems of Green and Stokes in three dimensions and also gives rise to de Rham cohomology which is an analytical way of approaching the algebraic topology of the manifold. This has been important in an enormous range of areas from algebraic geometry to theoretical physics. More refined use of analysis requires extra data on the manifold and we shall simply define and describe some basic features of Riemannian metrics. These generalize the first fundamental form of a surface and, in their Lorentzian guise, provide the substance of general relativity. A more complete storydemands a muchlonger course, but here we shall consider just two aspects which draw on the theory of differential forms: the study of geodesics via a vector field, the geodesic flow, on the cotangent bundle, and some basic properties of harmonic forms. Certain standard technical results which we shall require are proved in the Appendix 3 so as not to interrupt the development of the theory. A good book to accompany the course is: An Introduction to Differential Manifolds by Dennis Barden and Charles Thomas (Imperial College Press £19 (paperback)). 2 Manifolds 2.1 Coordinate charts The concept of a manifold is a bit complicated, but it starts with defining the notion of a coordinate chart. Definition 1 A coordinate chart on a set X is a subset U ⊆ X together with a bijection ϕ : U → ϕ(U) ⊆ Rn onto an open set ϕ(U) in Rn. Thus we can parametrize points of U by n coordinates ϕ(x) = (x ,...,x ). 1 n We now want to consider the situation where X is covered by such charts and satisfies some consistency conditions. We have Definition 2 Ann-dimensionalatlasonX isacollectionofcoordinatecharts{U ,ϕ } α α α∈I such that • X is covered by the {U } α α∈I • for each α,β ∈ I, ϕ (U ∩U ) is open in Rn α α β • the map ϕ ϕ−1 : ϕ (U ∩U ) → ϕ (U ∩U ) β α α α β β α β is C∞ with C∞ inverse. Recall that F(x ,...,x ) ∈ Rn is C∞ if it has derivatives of all orders. We shall also 1 n say that F is smooth in this case. It is perfectly possible to develop the theory of manifolds with less differentiability than this, but this is the normal procedure. 4 Examples: 1. Let X = Rn and take U = X with ϕ = id. We could also take X to be any open set in Rn. 2. Let X be the set of straight lines in the plane: Each such line has an equation Ax+By +C = 0 where if we multiply A,B,C by a non-zero real number we get the same line. Let U be the set of non-vertical lines. 0 For each line ‘ ∈ U we have the equation 0 y = mx+c where m,c are uniquely determined. So ϕ (‘) = (m,c) defines a coordinate chart 0 ϕ : U → R2. Similarly if U consists of the non-horizontal lines with equation 0 0 1 x = m˜y +c˜ we have another chart ϕ : U → R2. 1 1 Now U ∩U is the set of lines y = mx+c which are not horizontal, so m 6= 0. Thus 0 1 ϕ (U ∩U ) = {(m,c) ∈ R2 : m 6= 0} 0 0 1 which is open. Moreover, y = mx+c implies x = m−1y −cm−1 and so ϕ ϕ−1(m,c) = (m−1,−cm−1) 1 0 which is smooth with smooth inverse. Thus we have an atlas on the space of lines. 3. Consider R as an additive group, and the subgroup of integers Z ⊂ R. Let X be the quotient group R/Z and p : R → R/Z the quotient homomorphism. Set U = p(0,1) and U = p(−1/2,1/2). Since any two elements in the subset p−1(a) 0 1 differ by an integer, p restricted to (0,1) or (−1/2,1/2) is injective and so we have coordinate charts ϕ = p−1 : U → (0,1), ϕ = p−1 : U → (−1/2,1/2). 0 0 1 1 5 Clearly U and U cover R/Z since the integer 0 ∈ U . 0 1 1 We check: ϕ (U ∩U ) = (0,1/2)∪(1/2,1), ϕ (U ∩U ) = (−1/2,0)∪(0,1/2) 0 0 1 1 0 1 which are open sets. Finally, if x ∈ (0,1/2), ϕ ϕ−1(x) = x and if x ∈ (1/2,1), 1 0 ϕ ϕ−1(x) = x−1. These maps are certainly smooth with smooth inverse so we have 1 0 an atlas on X = R/Z. 4. Let X be the extended complex plane X = C∪{∞}. Let U = C with ϕ (z) = 0 0 z ∈ C ∼= R2. Now take U = C\{0}∪{∞} 1 and define ϕ (z˜) = z˜−1 ∈ C if z˜6= ∞ and ϕ (∞) = 0. Then 1 1 ϕ (U ∩U ) = C\{0} 0 0 1 which is open, and x y ϕ ϕ−1(z) = z−1 = −i . 1 0 x2 +y2 x2 +y2 This is a smooth and invertible function of (x,y). We now have a 2-dimensional atlas for X, the extended complex plane. 5. Let X be n-dimensional real projective space, the set of 1-dimensional vector subspaces of Rn+1. Each subspace is spanned by a non-zero vector v, and we define U ⊂ RPn to be the subset for which the i-th component of v ∈ Rn+1 is non-zero. i Clearly X is covered by U ,...,U . In U we can uniquely choose v such that the 1 n+1 i ith component is 1, and then U is in one-to-one correspondence with the hyperplane i x = 1 in Rn+1, which is a copy of Rn. This is therefore a coordinate chart i ϕ : U → Rn. i i The set ϕ (U ∩U ) is the subset for which x 6= 0 and is therefore open. Furthermore i i j j ϕ ϕ−1 : {x ∈ Rn+1 : x = 1,x 6= 0} → {x ∈ Rn+1 : x = 1,x 6= 0} i j j i i j is 1 v 7→ v x i which is smooth with smooth inverse. We therefore have an atlas for RPn. 6 2.2 The definition of a manifold All the examples above are actually manifolds, and the existence of an atlas is suf- ficient to establish that, but there is a minor subtlety in the actual definition of a manifold due to the fact that there are lots of choices of atlases. If we had used a different basis for R2, our charts on the space X of straight lines would be different, butwewouldliketothinkofX asanobjectindependentofthechoiceofatlas. That’s why we make the following definitions: Definition 3 Two atlases {(U ,ϕ )}, {(V ,ψ )} are compatible if their union is an α α i i atlas. What this definition means is that all the extra maps ψ ϕ−1 must be smooth. Com- i α patibility is clearly an equivalence relation, and we then say that: Definition 4 A differentiable structure on X is an equivalence class of atlases. Finally we come to the definition of a manifold: Definition 5 An n-dimensional differentiable manifold is a space X with a differen- tiable structure. The upshot is this: to prove something is a manifold, all you need is to find one atlas. The definition of a manifold takes into account the existence of many more atlases. Many books give a slightly different definition – they start with a topological space, and insist that the coordinate charts are homeomorphisms. This is fine if you see the world as a hierarchy of more and more sophisticated structures but it suggests that in order to prove something is a manifold you first have to define a topology. As we’ll see now, the atlas does that for us. First recall what a topological space is: a set X with a distinguished collection of subsets V called open sets such that 1. ∅ and X are open 2. an arbitrary union of open sets is open 3. a finite intersection of open sets is open 7 Now suppose M is a manifold. We shall say that a subset V ⊆ M is open if, for each α, ϕ (V ∩U ) is an open set in Rn. One thing which is immediate is that V = U is α α β open, from Definition 2. We need to check that this gives a topology. Condition 1 holds because ϕ (∅) = ∅ α and ϕ (M ∩U ) = ϕ (U ) which is open by Definition 1. For the other two, if V is α α α α i a collection of open sets then because ϕ is bijective α ϕ ((∪V )∩U ) = ∪ϕ (V ∩U ) α i α α i α ϕ ((∩V )∩U ) = ∩ϕ (V ∩U ) α i α α i α and then the right hand side is a union or intersection of open sets. Slightly less obvious is the following: Proposition 2.1 With the topology above ϕ : U → ϕ (U ) is a homeomorphism. α α α α Proof: If V ⊆ U is open then ϕ (V) = ϕ (V ∩U ) is open by the definition of the α α α α topology, so ϕ−1 is certainly continuous. α Now let W ⊂ ϕ (U ) be open, then ϕ−1(W) ⊆ U and U is open in M so we need α α α α α to prove that ϕ−1(W) is open in M. But α ϕ (ϕ−1(W)∩U ) = ϕ ϕ−1(W ∩ϕ (U ∩U )) (1) β α β β α α α β From Definition 2 the set ϕ (U ∩ U ) is open and hence its intersection with the α α β open set W is open. Now ϕ ϕ−1 is C∞ with C∞ inverse and so certainly a homeo- β α morphism, and it follows that the right hand side of (1) is open. Thus the left hand side ϕ (ϕ−1W ∩ U ) is open and by the definition of the topology this means that β α β ϕ−1(W) is open, i.e. ϕ is continuous. 2 α α To make any reasonable further progress, we have to make two assumptions about this topology which will hold for the rest of these notes: • the manifold topology is Hausdorff • in this topology we have a countable basis of open sets Without these assumptions, manifolds are not even metric spaces, and there is not much analysis that can reasonably be done on them. 8 2.3 Further examples of manifolds We need better ways of recognizing manifolds than struggling to find explicit coordi- nate charts. For example, the sphere is a manifold and although we can use stereographic projection to get an atlas: there are other ways. Here is one. Theorem 2.2 Let F : U → Rm be a C∞ function on an open set U ⊆ Rn+m and take c ∈ Rm. Assume that for each a ∈ F−1(c), the derivative DF : Rn+m → Rm a is surjective. Then F−1(c) has the structure of an n-dimensional manifold which is Hausdorff and has a countable basis of open sets. Proof: Recall that the derivative of F at a is the linear map DF : Rn+m → Rm a such that F(a+h) = F(a)+DF (h)+R(a,h) a 9 where R(a,h)/khk → 0 as h → 0. If we write F(x ,...,x ) = (F ,...,F ) the derivative is the Jacobian matrix 1 n+m 1 m ∂F i (a) 1 ≤ i ≤ m,1 ≤ j ≤ n+m ∂x j Now we are given that this is surjective, so the matrix has rank m. Therefore by reordering the coordinates x ,...,x we may assume that the square matrix 1 n+m ∂F i (a) 1 ≤ i ≤ m,1 ≤ j ≤ m ∂x j is invertible. Now define G : U ×Rm → Rn+m by G(x ,...,x ) = (F ,...,F ,x ,...,x ). (2) 1 n+m 1 m m+1 n+m Then DG is invertible. a We now apply the inverse function theorem to G, a proof of which is given in the Appendix. It tells us that there is a neighbourhood V of x, and W of G(x) such that G : V → W is invertible with smooth inverse. Moreover, the formula (2) shows that G maps V ∩ F−1(c) to the intersection of W with the copy of Rn given by {x ∈ Rn+m : x = c ,1 ≤ i ≤ m}. This is therefore a coordinate chart ϕ. i i If we take two such charts ϕ ,ϕ , then ϕ ϕ−1 is a map from an open set in {x ∈ α β α β Rn+m : x = c ,1 ≤ i ≤ m} to another one which is the restriction of the map G G−1 i 1 α β of (an open set in) Rn+m to itself. But this is an invertible C∞ map and so we have the requisite conditions for an atlas. Finally, in the induced topology from Rn+m, G is a homeomorphism, so open sets α in the manifold topology are the same as open sets in the induced topology. Since Rn+m is Hausdorff with a countable basis of open sets, so is F−1(c). 2 We can now give further examples of manifolds: Examples: 1. Let n+1 X Sn = {x ∈ Rn+1 : x2 = 1} i 1 10