Cartan connection
In the mathematical field of differential geometry, a Cartan connection is a flexible generalization of the notion of an affine connection. It may also be regarded as a specialization of the general concept of a principal connection, in which the geometry of the principal bundle is tied to the geometry of the base manifold using a solder form. Cartan connections describe the geometry of manifolds modelled on homogeneous spaces.
The theory of Cartan connections was developed by Élie Cartan, as part of (and a way of formulating) his method of moving frames (repère mobile).[1] The main idea is to develop a suitable notion of the connection forms and curvature using moving frames adapted to the particular geometrical problem at hand. In relativity or Riemannian geometry, orthonormal frames are used to obtain a description of the Levi-Civita connection as a Cartan connection. For Lie groups, Maurer–Cartan frames are used to view the Maurer–Cartan form of the group as a Cartan connection.
Cartan reformulated the differential geometry of (pseudo) Riemannian geometry, as well as the differential geometry of manifolds equipped with some non-metric structure, including Lie groups and homogeneous spaces. The term 'Cartan connection' most often refers to Cartan's formulation of a (pseudo-)Riemannian, affine, projective, or conformal connection. Although these are the most commonly used Cartan connections, they are special cases of a more general concept.
Cartan's approach seems at first to be coordinate dependent because of the choice of frames it involves. However, it is not, and the notion can be described precisely using the language of principal bundles. Cartan connections induce covariant derivatives and other differential operators on certain associated bundles, hence a notion of parallel transport. They have many applications in geometry and physics: see the method of moving frames, Cartan formalism and Einstein–Cartan theory for some examples.
Introduction[edit]
At its roots, geometry consists of a notion of congruence between different objects in a space. In the late 19th century, notions of congruence were typically supplied by the action of a Lie group on space. Lie groups generally act quite rigidly, and so a Cartan geometry is a generalization of this notion of congruence to allow for curvature to be present. The flat Cartan geometries—those with zero curvature—are locally equivalent to homogeneous spaces, hence geometries in the sense of Klein.
A Klein geometry consists of a Lie group G together with a Lie subgroup H of G. Together G and H determine a homogeneous space G/H, on which the group G acts by left-translation. Klein's aim was then to study objects living on the homogeneous space which were congruent by the action of G. A Cartan geometry extends the notion of a Klein geometry by attaching to each point of a manifold a copy of a Klein geometry, and to regard this copy as tangent to the manifold. Thus the geometry of the manifold is infinitesimally identical to that of the Klein geometry, but globally can be quite different. In particular, Cartan geometries no longer have a well-defined action of G on them. However, a Cartan connection supplies a way of connecting the infinitesimal model spaces within the manifold by means of parallel transport.
Motivation[edit]
Consider a smooth surface S in 3-dimensional Euclidean space R3. Near to any point, S can be approximated by its tangent plane at that point, which is an affine subspace of Euclidean space. The affine subspaces are model surfaces—they are the simplest surfaces in R3, and are homogeneous under the Euclidean group of the plane, hence they are Klein geometries in the sense of Felix Klein's Erlangen programme. Every smooth surface S has a unique affine plane tangent to it at each point. The family of all such planes in R3, one attached to each point of S, is called the congruence of tangent planes. A tangent plane can be "rolled" along S, and as it does so the point of contact traces out a curve on S. Conversely, given a curve on S, the tangent plane can be rolled along that curve. This provides a way to identify the tangent planes at different points along the curve by affine (in fact Euclidean) transformations, and is an example of a Cartan connection called an affine connection.
Another example is obtained by replacing the planes, as model surfaces, by spheres, which are homogeneous under the Möbius group of conformal transformations. There is no longer a unique sphere tangent to a smooth surface S at each point, since the radius of the sphere is undetermined. This can be fixed by supposing that the sphere has the same mean curvature as S at the point of contact. Such spheres can again be rolled along curves on S, and this equips S with another type of Cartan connection called a conformal connection.
Differential geometers in the late 19th and early 20th centuries were very interested in using model families such as planes or spheres to describe the geometry of surfaces. A family of model spaces attached to each point of a surface S is called a congruence: in the previous examples there is a canonical choice of such a congruence. A Cartan connection provides an identification between the model spaces in the congruence along any curve in S. An important feature of these identifications is that the point of contact of the model space with S always moves with the curve. This generic condition is characteristic of Cartan connections.
In the modern treatment of affine connections, the point of contact is viewed as the origin in the tangent plane (which is then a vector space), and the movement of the origin is corrected by a translation, and so Cartan connections are not needed. However, there is no canonical way to do this in general: in particular for the conformal connection of a sphere congruence, it is not possible to separate the motion of the point of contact from the rest of the motion in a natural way.
In both of these examples the model space is a homogeneous space G/H.
- In the first case, G/H is the affine plane, with G = Aff(R2) the affine group of the plane, and H = GL(2) the corresponding general linear group.
- In the second case, G/H is the conformal (or celestial) sphere, with G = O+(3,1) the (orthochronous) Lorentz group, and H the stabilizer of a null line in R3,1.
The Cartan geometry of S consists of a copy of the model space G/H at each point of S (with a marked point of contact) together with a notion of "parallel transport" along curves which identifies these copies using elements of G. This notion of parallel transport is generic in the intuitive sense that the point of contact always moves along the curve.
In general, let G be a group with a subgroup H, and M a manifold of the same dimension as G/H. Then, roughly speaking, a Cartan connection on M is a G-connection which is generic with respect to a reduction to H.
Affine connections[edit]
An affine connection on a manifold M is a connection on the frame bundle (principal bundle) of M (or equivalently, a connection on the tangent bundle (vector bundle) of M). A key aspect of the Cartan connection point of view is to elaborate this notion in the context of principal bundles (which could be called the "general or abstract theory of frames").
Let H be a Lie group, its Lie algebra. Then a principal H-bundle is a fiber bundle P over M with a smooth action of H on P which is free and transitive on the fibers. Thus P is a smooth manifold with a smooth map
Let Rh denote the (right) action of h ∈ H on P. The derivative of this action defines a vertical vector field on P for each element
A principal H-connection on P is a 1-form on P, with values in the Lie algebra of H, such that
- for any ,
ω (Xξ ) =ξ (identically on P).
The intuitive idea is that
Frame bundles have additional structure called the solder form, which can be used to extend a principal connection on P to a trivialization of the tangent bundle of P called an absolute parallelism.
In general, suppose that M has dimension n and H acts on Rn (this could be any n-dimensional real vector space). A solder form on a principal H-bundle P over M is an Rn-valued 1-form
The pair (
Cartan connections generalize affine connections in two ways.
- The action of H on Rn need not be effective. This allows, for example, the theory to include spin connections, in which H is the spin group Spin(n) rather than the orthogonal group O(n).
- The group G need not be a semidirect product of H with Rn.
Klein geometries as model spaces[edit]
Klein's Erlangen programme suggested that geometry could be regarded as a study of homogeneous spaces: in particular, it is the study of the many geometries of interest to geometers of 19th century (and earlier). A Klein geometry consisted of a space, along with a law for motion within the space (analogous to the Euclidean transformations of classical Euclidean geometry) expressed as a Lie group of transformations. These generalized spaces turn out to be homogeneous smooth manifolds diffeomorphic to the quotient space of a Lie group by a Lie subgroup. The extra differential structure that these homogeneous spaces possess allows one to study and generalize their geometry using calculus.
The general approach of Cartan is to begin with such a smooth Klein geometry, given by a Lie group G and a Lie subgroup H, with associated Lie algebras and , respectively. Let P be the underlying principal homogeneous space of G. A Klein geometry is the homogeneous space given by the quotient P/H of P by the right action of H. There is a right H-action on the fibres of the canonical projection
π : P → P/H
given by Rhg = gh. Moreover, each fibre of
A vector field X on P is vertical if d
- Ad(h) Rh*
η =η for all h in H η (Xξ ) =ξ for allξ in- for all g∈P,
η restricts a linear isomorphism of TgP with (η is an absolute parallelism on P).
In addition to these properties,
Conversely, one can show that given a manifold M and a principal H-bundle P over M, and a 1-form
A Cartan geometry is a generalization of a smooth Klein geometry, in which the structure equation is not assumed, but is instead used to define a notion of curvature. Thus the Klein geometries are said to be the flat models for Cartan geometries.[3]
Pseudogroups[edit]
Cartan connections are closely related to pseudogroup structures on a manifold. Each is thought of as modelled on a Klein geometry G/H, in a manner similar to the way in which Riemannian geometry is modelled on Euclidean space. On a manifold M, one imagines attaching to each point of M a copy of the model space G/H. The symmetry of the model space is then built into the Cartan geometry or pseudogroup structure by positing that the model spaces of nearby points are related by a transformation in G. The fundamental difference between a Cartan geometry and pseudogroup geometry is that the symmetry for a Cartan geometry relates infinitesimally close points by an infinitesimal transformation in G (i.e., an element of the Lie algebra of G) and the analogous notion of symmetry for a pseudogroup structure applies for points that are physically separated within the manifold.
The process of attaching spaces to points, and the attendant symmetries, can be concretely realized by using special coordinate systems.[4] To each point p ∈ M, a neighborhood Up of p is given along with a mapping
φ ′p = hpφ p.[5]
This freedom corresponds roughly to the physicists' notion of a gauge.
Nearby points are related by joining them with a curve. Suppose that p and p′ are two points in M joined by a curve pt. Then pt supplies a notion of transport of the model space along the curve.[6] Let
τ t =φ pt oφ p0−1.
Intuitively,
Typical of Cartan, one motivation for introducing the notion of a Cartan connection was to study the properties of pseudogroups from an infinitesimal point of view. A Cartan connection defines a pseudogroup precisely when the derivative of the transport map
In this case,
So
This form, however, is dependent on the choice of parametrized coordinate system. If h : U → H is an H-relation between two parametrized coordinate systems
where
Formal definition[edit]
A Cartan geometry modelled on a homogeneous space G/H can be viewed as a deformation of this geometry which allows for the presence of curvature. For example:
- a Riemannian manifold can be seen as a deformation of Euclidean space;
- a Lorentzian manifold can be seen as a deformation of Minkowski space;
- a conformal manifold can be seen as a deformation of the conformal sphere;
- a manifold equipped with an affine connection can be seen as a deformation of an affine space.
There are two main approaches to the definition. In both approaches, M is a smooth manifold of dimension n, H is a Lie group of dimension m, with Lie algebra , and G is a Lie group of dimension n+m, with Lie algebra , containing H as a subgroup.
Definition via gauge transitions[edit]
A Cartan connection consists[7][8] of a coordinate atlas of open sets U in M, along with a -valued 1-form
θ U : TU → .θ U mod : TuU → is a linear isomorphism for every u ∈ U.- For any pair of charts U and V in the atlas, there is a smooth mapping h : U ∩ V → H such that
- where
ω H is the Maurer-Cartan form of H.
By analogy with the case when the
The curvature of a Cartan connection consists of a system of 2-forms defined on the charts, given by
- If the forms
θ U andθ V are related by a function h : U ∩ V → H, as above, thenΩ V = Ad(h−1)Ω U
The definition can be made independent of the coordinate systems by forming the quotient space
of the disjoint union over all U in the atlas. The equivalence relation ~ is defined on pairs (x,h1) ∈ U1 × H and (x, h2) ∈ U2 × H, by
- (x,h1) ~ (x, h2) if and only if x ∈ U1 ∩ U2,
θ U1 is related toθ U2 by h, and h2 = h(x)−1 h1.
Then P is a principal H-bundle on M, and the compatibility condition on the connection forms
Definition via absolute parallelism[edit]
Let P be a principal H bundle over M. Then a Cartan connection[9] is a -valued 1-form
- for all h in H, Ad(h)Rh*
η =η - for all
ξ in ,η (Xξ ) =ξ - for all p in P, the restriction of
η defines a linear isomorphism from the tangent space TpP to .
The last condition is sometimes called the Cartan condition: it means that
The curvature of a Cartan connection is the -valued 2-form
Note that this definition of a Cartan connection looks very similar to that of a principal connection. There are several important differences, however. First, the 1-form
An intuitive interpretation of the Cartan connection in this form is that it determines a fracturing of the tautological principal bundle associated to a Klein geometry. Thus Cartan geometries are deformed analogues of Klein geometries. This deformation is roughly a prescription for attaching a copy of the model space G/H to each point of M and thinking of that model space as being tangent to (and infinitesimally identical with) the manifold at a point of contact. The fibre of the tautological bundle G → G/H of the Klein geometry at the point of contact is then identified with the fibre of the bundle P. Each such fibre (in G) carries a Maurer-Cartan form for G, and the Cartan connection is a way of assembling these Maurer-Cartan forms gathered from the points of contact into a coherent 1-form
From the Cartan connection, defined in these terms, one can recover a Cartan connection as a system of 1-forms on the manifold (as in the gauge definition) by taking a collection of local trivializations of P given as sections sU : U → P and letting
As principal connections[edit]
Another way in which to define a Cartan connection is as a principal connection on a certain principal G-bundle. From this perspective, a Cartan connection consists of
- a principal G-bundle Q over M
- a principal G-connection
α on Q (the Cartan connection) - a principal H-subbundle P of Q (i.e., a reduction of structure group)
such that the pullback
The principal connection
Since
Definition by an Ehresmann connection[edit]
Yet another way to define a Cartan connection is with an Ehresmann connection on the bundle E = Q ×G G/H of the preceding section.[10] A Cartan connection then consists of
- A fibre bundle
π : E → M with fibre G/H and vertical space VE ⊂ TE. - A section s : M → E.
- A G-connection
θ : TE → VE such that
- s*
θ x : TxM → Vs(x)E is a linear isomorphism of vector spaces for all x ∈ M.
- s*
This definition makes rigorous the intuitive ideas presented in the introduction. First, the preferred section s can be thought of as identifying a point of contact between the manifold and the tangent space. The last condition, in particular, means that the tangent space of M at x is isomorphic to the tangent space of the model space at the point of contact. So the model spaces are, in this way, tangent to the manifold.
This definition also brings prominently into focus the idea of development. If xt is a curve in M, then the Ehresmann connection on E supplies an associated parallel transport map
To show that this definition is equivalent to the others above, one must introduce a suitable notion of a moving frame for the bundle E. In general, this is possible for any G-connection on a fibre bundle with structure group G. See Ehresmann connection#Associated bundles for more details.
Special Cartan connections[edit]
Reductive Cartan connections[edit]
Let P be a principal H-bundle on M, equipped with a Cartan connection
η =η +η .
Note that the 1-form
η (X) = 0 for every vertical vector X ∈ TP. (η is horizontal.)- Rh*
η = Ad(h−1)η for every h ∈ H. (η is equivariant under the right H-action.)
In other words,
Hence, P equipped with the form
Parabolic Cartan connections[edit]
If is a semisimple Lie algebra with parabolic subalgebra (i.e., contains a maximal solvable subalgebra of ) and G and P are associated Lie groups, then a Cartan connection modelled on (G,P,,) is called a parabolic Cartan geometry, or simply a parabolic geometry. A distinguishing feature of parabolic geometries is a Lie algebra structure on its cotangent spaces: this arises because the perpendicular subspace ⊥ of in with respect to the Killing form of is a subalgebra of , and the Killing form induces a natural duality between ⊥ and . Thus the bundle associated to ⊥ is isomorphic to the cotangent bundle.
Parabolic geometries include many of those of interest in research and applications of Cartan connections, such as the following examples:
- Conformal connections: Here G = SO(p+1,q+1), and P is the stabilizer of a null ray in Rn+2.
- Projective connections: Here G = PGL(n+1) and P is the stabilizer of a point in RPn.
- CR structures and Cartan-Chern-Tanaka connections: G = PSU(p+1,q+1), P = stabilizer of a point on the projective null hyperquadric.
- Contact projective connections:[12] Here G = SP(2n+2) and P is the stabilizer of the ray generated by the first standard basis vector in Rn+2.
- Generic rank 2 distributions on 5-manifolds: Here G = Aut(Os) is the automorphism group of the algebra Os of split octonions, a closed subgroup of SO(3,4), and P is the intersection of G with the stabilizer of the isotropic line spanned by the first standard basis vector in R7 viewed as the purely imaginary split octonions (orthogonal complement of the unit element in Os).[13]
Associated differential operators[edit]
Covariant differentiation[edit]
Suppose that M is a Cartan geometry modelled on G/H, and let (Q,
where denotes the space of k-forms on M with values in V so that is the space of sections of V and is the space of sections of Hom(TM,V). For any section v of V, the contraction of the covariant derivative ∇v with a vector field X on M is denoted ∇Xv and satisfies the following Leibniz rule:
for any smooth function f on M.
The covariant derivative can also be constructed from the Cartan connection
- .
In order to show that ∇v is well defined, it must:
- be independent of the chosen lift
- be equivariant, so that it descends to a section of the bundle V.
For (1), the ambiguity in selecting a right-invariant lift of X is a transformation of the form where is the right-invariant vertical vector field induced from . So, calculating the covariant derivative in terms of the new lift , one has
since by taking the differential of the equivariance property at h equal to the identity element.
For (2), observe that since v is equivariant and is right-invariant, is equivariant. On the other hand, since
The fundamental or universal derivative[edit]
Suppose that V is only a representation of the subgroup H and not necessarily the larger group G. Let be the space of V-valued differential k-forms on P. In the presence of a Cartan connection, there is a canonical isomorphism
given by where and .
For each k, the exterior derivative is a first order operator differential operator
and so, for k=0, it defines a differential operator
Because
Notes[edit]
- ^ Although Cartan only began formalizing this theory in particular cases in the 1920s (Cartan 1926), he made much use of the general idea much earlier. The high point of his remarkable 1910 paper on Pfaffian systems in five variables is the construction of a Cartan connection modelled on a 5-dimensional homogeneous space for the exceptional Lie group G2, which he and Engels had discovered independently in 1894.
- ^ Chevalley 1946, p. 110.
- ^ See R. Hermann (1983), Appendix 1–3 to Cartan (1951).
- ^ This appears to be Cartan's way of viewing the connection. Cf. Cartan 1923, p. 362; Cartan 1924, p. 208 especially ..un repère définissant un système de coordonnées projectives...; Cartan 1951, p. 34. Modern readers can arrive at various interpretations of these statements, cf. Hermann's 1983 notes in Cartan 1951, pp. 384–385, 477.
- ^ More precisely, hp is required to be in the isotropy group of
φ p(p), which is a group in G isomorphic to H. - ^ In general, this is not the rolling map described in the motivation, although it is related.
- ^ Sharpe 1997.
- ^ Lumiste 2001a.
- ^ This is the standard definition. Cf. Hermann (1983), Appendix 2 to Cartan 1951; Kobayashi 1970, p. 127; Sharpe 1997; Slovák 1997.
- ^ Ehresmann 1950, Kobayashi 1957, Lumiste 2001b.
- ^ For a treatment of affine connections from this point of view, see Kobayashi & Nomizu (1996, Volume 1).
- ^ See, for example, Fox (2005).
- ^ Sagerschnig 2006; Čap & Sagerschnig 2009.
- ^ See, for instance, Čap & Gover (2002, Definition 2.4).
References[edit]
- Čap, Andreas; Gover, A. Rod (2002), "Tractor calculi for parabolic geometries]", Transactions of the American Mathematical Society, 354 (4): 1511–1548, doi:10.1090/S0002-9947-01-02909-9.
- Čap, A.; Sagerschnig, K. (2009), "On Nurowski's Conformal Structure Associated to a Generic Rank Two Distribution in Dimension Five", Journal of Geometry and Physics, 59 (7): 901–912, arXiv:0710.2208, Bibcode:2007arXiv0710.2208C, doi:10.1016/j.geomphys.2009.04.001, S2CID 12850650.
- Cartan, Élie (1910), "Les systèmes de Pfaff à cinq variables et les équations aux dérivées partielles du second ordre", Annales Scientifiques de l'École Normale Supérieure, 27: 109–192, doi:10.24033/asens.618.
- Cartan, Élie (1923), "Sur les variétés à connexion affine et la théorie de la relativité généralisée (première partie)", Annales Scientifiques de l'École Normale Supérieure, 40: 325–412, doi:10.24033/asens.751.
- Cartan, Élie (1924), "Sur les variétés à connexion projective", Bulletin de la Société Mathématique de France, 52: 205–241, doi:10.24033/bsmf.1053.
- Cartan, Élie (1926), "Les groupes d'holonomie des espaces généralisés", Acta Mathematica, 48 (1–2): 1–42, doi:10.1007/BF02629755.
- Cartan, Élie (1951), with appendices by Robert Hermann (ed.), Geometry of Riemannian Spaces (translation by James Glazebrook of Leçons sur la géométrie des espaces de Riemann, 2nd ed.), Math Sci Press, Massachusetts (published 1983), ISBN 978-0-915692-34-7.
- Chevalley, C. (1946), The Theory of Lie Groups, Princeton University Press, ISBN 0-691-08052-6.
- Ehresmann, C. (1950), "Les connexions infinitésimales dans un espace fibré différentiel", Colloque de Topologie, Bruxelles: 29–55, MR 0042768.
- Fox, D.J.F. (2005), "Contact projective structures", Indiana University Mathematics Journal, 54 (6): 1547–1598, arXiv:math/0402332, doi:10.1512/iumj.2005.54.2603, S2CID 17061926.
- Griffiths, Phillip (1974), "On Cartan's method of Lie groups and moving frames as applied to uniqueness and existence questions in differential geometry", Duke Mathematical Journal, 41 (4): 775–814, doi:10.1215/S0012-7094-74-04180-5, S2CID 12966544.
- Kobayashi, Shoshichi; Nomizu, Katsumi (1996), Foundations of Differential Geometry, Vol. 1 & 2 (New ed.), Wiley-Interscience, ISBN 0-471-15733-3.
- Kobayashi, Shoshichi (1970), Transformation Groups in Differential Geometry (1st ed.), Springer, ISBN 3-540-05848-6.
- Kobayashi, Shoshichi (1957), "Theory of Connections", Annali di Matematica Pura ed Applicata, Series 4, 43: 119–194, doi:10.1007/BF02411907, S2CID 120972987.
- Lumiste, Ü. (2001a) [1994], "Conformal connection", in Hazewinkel, Michiel (ed.), Encyclopedia of Mathematics, Kluwer Academic Publishers, ISBN 978-1-55608-010-4.
- Lumiste, Ü. (2001b) [1994], "Connections on a manifold", in Hazewinkel, Michiel (ed.), Encyclopedia of Mathematics, Kluwer Academic Publishers, ISBN 978-1-55608-010-4.
- Sagerschnig, K. (2006), "Split octonions and generic rank two distributions in dimension five", Archivum Mathematicum, 42 (Suppl): 329–339.
- Sharpe, R.W. (1997), Differential Geometry: Cartan's Generalization of Klein's Erlangen Program, Springer-Verlag, New York, ISBN 0-387-94732-9.
- Slovák, Jan (1997), Parabolic Geometries (PDF), Research Lecture Notes, Part of DrSc-dissertation, Masaryk University, archived from the original (PDF) on March 30, 2022.
Books[edit]
- Kobayashi, Shoshichi (1972), Transformations Groups in Differential Geometry (Classics in Mathematics 1995 ed.), Springer-Verlag, Berlin, ISBN 978-3-540-58659-3.
- The section 3. Cartan Connections [pages 127–130] treats conformal and projective connections in a unified manner.
External links[edit]
- Ü. Lumiste (2001) [1994], "Affine connection", Encyclopedia of Mathematics, EMS Press