Fiber bundle

A cylindrical hairbrush showing the intuition behind the term "fiber bundle". This hairbrush is like a fiber bundle in which the base space is a cylinder and the fibers (bristles) are line segments. The mapping π:E→B would take a point on any bristle and map it to the point on the cylinder where the bristle attaches.

In mathematics, in particular in topology, a fiber bundle (or fibre bundle) is a space which looks locally like a product space. It may have a different global topological structure in that the space as a whole may not be homeomorphic to a product space. Every fiber bundle consists of a continuous surjective map

\pi�: E \to B

where small regions in the total space E look like small regions in the product space B × F. Here B is the base space while F is the fiber space. For example, setting E as the product space B × F, equipped with π = pr1 (projection onto the first coordinate), is a fiber bundle. This is called the trivial bundle. One goal of the theory of bundles is to quantify, via algebraic invariants, what it means for a bundle to be non-trivial, or in other words twisted in the large.

Fiber bundles generalize vector bundles, where the main example is the tangent bundle of a manifold, as well as principal bundles. They play an important role in the fields of differential topology and differential geometry. They are also a fundamental concept in the mathematical formulation of gauge theory. Fiber bundles specialize the more general bundle.

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Formal definition

A fiber bundle consists of the data (E, B, π, F), where E, B, and F are topological spaces and π : E → B is a continuous surjection satisfying a local triviality condition outlined below. B is called the base space of the bundle, E the total space, and F the fiber. The map π is called the projection map (or bundle projection). We shall assume in what follows that the base space B is connected.

We require that for any x in B, there is an open neighborhood U of x (which will be called a trivializing neighborhood) such that π−1(U) is homeomorphic to the product space U × F, in such a way that π carries over to the projection onto the first factor. That is, the following diagram should commute:

Local triviality condition

where proj1 : U × F → U is the natural projection and φ : π−1(U) → U × F is a homeomorphism. The set of all {(Ui, φi)} is called a local trivialization of the bundle.

For any x in B, the preimage π−1({x}) is homeomorphic to F and is called the fiber over x. A fiber bundle (E, B, π, F) is often denoted

FiberBundle-02.png

to indicate a short exact sequence of spaces. Note that every fiber bundle π : E → B is an open map, since projections of products are open maps. Therefore B carries the quotient topology determined by the map π.

A smooth fiber bundle is a fiber bundle in the category of smooth manifolds. That is, E, B, and F are required to be smooth manifolds and all the functions above are required to be smooth maps.

Examples

Trivial bundle

Let E = B × F and let π : E → B be the projection onto the first factor. Then E is a fiber bundle (of F) over B. Here E is not just locally a product but globally one. Any such fiber bundle is called a trivial bundle.

Möbius strip

The Möbius strip is a nontrivial bundle over the circle.

Perhaps the simplest example of a nontrivial bundle E is the Möbius strip. It has the circle that runs lengthwise along the center of the strip as a base B and a line segment for the fiber F, so the Möbius strip is a bundle of the line segment over the circle. A neighborhood U of a point x ∈ B is an arc; in the picture, this is the length of one of the squares. The preimage \pi^{-1}(U) in the picture is a (somewhat twisted) slice of the strip four squares wide and one long. The homeomorphism φ maps the preimage of U to a slice of a cylinder: curved, but not twisted.

The corresponding trivial bundle B × F would be a cylinder, but the Möbius strip has an overall "twist". Note that this twist is visible only globally; locally the Möbius strip and the cylinder are identical (making a single vertical cut in either gives the same space).

Klein bottle

A similar nontrivial bundle is the Klein bottle which can be viewed as a "twisted" circle bundle over another circle. The corresponding trivial bundle would be a torus, S1 × S1.

The Klein bottle immersed in three-dimensional space.
A torus.

Covering map

A covering space is a fiber bundle such that the bundle projection is a local homeomorphism. It follows in particular, that the fiber is a discrete space.

Vector and principal bundles

A special class of fiber bundles, called vector bundles, are those whose fibers are vector spaces (to qualify as a vector bundle the structure group of the bundle — see below — must be a linear group). Important examples of vector bundles include the tangent bundle and cotangent bundle of a smooth manifold. From any vector bundle, one can construct the frame bundle of bases which is a principal bundle (see below).

Another special class of fiber bundles, called principal bundles, are bundles on whose fibers a free and transitive action by a group G is given, so that each fiber is a principal homogeneous space. The bundle is often specified along with the group by referring to it as a principal G-bundle. The group G is also the structure group of the bundle. Given a representation ρ of G on a vector space V, a vector bundle with ρ(G)⊆Aut(V) as a structure group may be constructed, known as the associated bundle.

Sphere bundles

A sphere bundle is a fiber bundle whose fiber is an n-sphere. Given a vector bundle E with a metric (such as the tangent bundle to a Riemannian manifold) one can construct the associated unit sphere bundle, for which the fiber over a point x is the set of all unit vectors in Ex. When the vector bundle in question is the tangent bundle T(M), the unit sphere bundle is known as the unit tangent bundle, and is denoted UT(M).

A sphere bundle is partially characterized by its Euler class, which is a degree n+1 cohomology class in the total space of the bundle. In the case n=1 the sphere bundle is called a circle bundle and the Euler class is equal to the first Chern class, which characterizes the topology of the bundle completely. For any n, given the Euler class of a bundle, one can calculate its cohomology using a long exact sequence called the Gysin sequence.

Mapping tori

If X is a topological space and f:X → X is a homeomorphism then the mapping torus Mf has a natural structure of a fiber bundle over the circle with fiber X. Mapping tori of homeomorphisms of surfaces are of particular importance in 3-manifold topology.

Quotient spaces

If G is a topological group and H is a closed subgroup, then under some circumstances, the quotient space G/H together with the quotient map π : G → G/H is a fiber bundle, whose fiber is the topological space H. A necessary and sufficient condition for (G,G/H,π,H) to form a fiber bundle is that the mapping π admit local cross-sections (Steenrod & 1951 §7).

The most general conditions under which the quotient map will admit local cross-sections are not known, although if G is a Lie group and H a closed subgroup (and thus a Lie subgroup), then the quotient map is a fiber bundle. One example of this is the Hopf fibration, S3 → S2 which is a fiber bundle over the sphere S2 whose total space is S3. From the perspective of Lie groups, S3 can be identified with the special unitary group SU(2). The abelian subgroup of diagonal matrices is isomorphic to the circle group U(1), and the quotient SU(2)/U(1) is diffeomorphic to the sphere.

More generally, if G is any topological group and H a closed subgroup which also happens to be a Lie group, then G G/H is a fiber bundle.

Sections

A section (or cross section) of a fiber bundle is a continuous map f : B → E such that π(f(x))=x for all x in B. Since bundles do not in general have globally-defined sections, one of the purposes of the theory is to account for their existence. The obstruction to the existence of a section can often be measured by a cohomology class, which leads to the theory of characteristic classes in algebraic topology.

The most well-known example is the hairy ball theorem, where the Euler class is the obstruction to the tangent bundle of the 2-sphere having a nowhere vanishing section.

Often one would like to define sections only locally (especially when global sections do not exist). A local section of a fiber bundle is a continuous map f : U → E where U is an open set in B and π(f(x))=x for all x in U. If (U, φ) is a local trivialization chart then local sections always exist over U. Such sections are in 1-1 correspondence with continuous maps U → F. Sections form a sheaf.

Structure groups and transition functions

Fiber bundles often come with a group of symmetries which describe the matching conditions between overlapping local trivialization charts. Specifically, let G be a topological group which acts continuously on the fiber space F on the left. We lose nothing if we require G to act effectively on F so that it may be thought of as a group of homeomorphisms of F. A G-atlas for the bundle (E, B, π, F) is a local trivialization such that for any two overlapping charts (Ui, φi) and (Uj, φj) the function

\phi_i\phi_j^{-1}�: (U_i \cap U_j) \times F \to (U_i \cap U_j) \times F

is given by

\phi_i\phi_j^{-1}(x, \xi) = (x, t_{ij}(x)\xi)

where tij : Ui ∩ Uj → G is a continuous map called a transition function. Two G-atlases are equivalent if their union is also a G-atlas. A G-bundle is a fiber bundle with an equivalence class of G-atlases. The group G is called the structure group of the bundle; the analogous term in physics is gauge group.

In the smooth category, a G-bundle is a smooth fiber bundle where G is a Lie group and the corresponding action on F is smooth and the transition functions are all smooth maps.

The transition functions tij satisfy the following conditions

  1. t_{ii}(x) = 1
  2. t_{ij}(x) = t_{ji}(x)^{-1}
  3. t_{ik}(x) = t_{ij}(x)t_{jk}(x)

The third condition applies on triple overlaps Ui ∩ Uj ∩ Uk and is called the cocycle condition (see Čech cohomology). The importance of this is that the transition functions determine the fibre bundle (if one assumes the Čech cocycle condition).

A principal G-bundle is a G-bundle where the fiber F is a principal homogeneous space for the left action of G itself (equivalently, one can specify that the action of G on the fibre F is free and transitive). In this case, it is often a matter of convenience to identify F with G and so obtain a (right) action of G on the principal bundle.

Bundle maps

It is useful to have notions of mapping between bundles on B for the same type of fiber F. These are called bundle maps. The most common are the mappings between vector bundles that are linear on each fiber; these occur also for different bases, for example from one tangent bundle to another. Another kind is the bundle map of principal bundles, which is G-equivariant fiber by fiber.

If p_i�: E_i \to B_i are fiber bundles for i=1,2 a bundle map from p_1 to p_2 consists of a pair of functions f: E_1 \to E_2, g: B_1 \to B_2 such that p_2 \circ f = g \circ p_1. If f and g are homeomorphisms (resp. diffeomorphisms), they are called an isomorphism of bundles.

Differentiable fiber bundles

In the category of differentiable manifolds, fiber bundles arise naturally as submersions of one manifold to another. Not every (differentiable) submersion ƒ : M → N from a differentiable manifold M to another differentiable manifold N gives rise to a differentiable fiber bundle. For one thing, the map must be surjective. However, this necessary condition is not quite sufficient, and there are a variety of sufficient conditions in common use.

If M and N are compact and connected, then any submersion f : M → N gives rise to a fiber bundle in the sense that there is a fiber space F diffeomorphic to each of the fibers such that (E,B,π,F) = (M,N,ƒ,F) is a fiber bundle. (Surjectivity of ƒ follows by the assumptions already given in this case.) More generally, the assumption of compactness can be relaxed if the submersion ƒ : :M → N is assumed to be a surjective proper map, meaning that ƒ-1(K) is compact for every compact subset K of N. Another sufficient condition, due to Ehresmann (1951), is that if ƒ : M → N is a surjective submersion with M and N differentiable manifolds such that the preimage ƒ-1{x} is compact and connected for all x ∈ N, then ƒ admits a compatible fiber bundle structure (Michor 2008, §17).

See also

References

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