In mathematics, the classifying space for the unitary group U(n) is a space BU(n) together with a universal bundle EU(n) such that any hermitian bundle on a paracompact space X is the pull-back of EU(n) by a map X → BU(n) unique up to homotopy.
This space with its universal fibration may be constructed as either
the Grassmannian of n-planes in an infinite-dimensional complex Hilbert space; or, the direct limit, with the induced topology, of Grassmannians of n planes. Both constructions are detailed here.
Construction as an infinite Grassmannian
The total space EU(n) of the universal bundle is given by
Here, H denotes an infinite-dimensional complex Hilbert space, the ei are vectors in H, and is the Kronecker delta. The symbol is the inner product on H. Thus, we have that EU(n) is the space of orthonormal n-frames in H.
The group action of U(n) on this space is the natural one. The base space is then
and is the set of Grassmannian n-dimensional subspaces (or n-planes) in H. That is,
so that V is an n-dimensional vector space.
Case of line bundles
For n = 1, one has EU(1) = S∞, which is known to be a contractible space. The base space is then BU(1) = CP∞, the infinite-dimensional complex projective space. Thus, the set of isomorphism classes of circle bundles over a manifold M are in one-to-one correspondence with the homotopy classes of maps from M to CP∞.
One also has the relation that
that is, BU(1) is the infinite-dimensional projective unitary group. See that article for additional discussion and properties.
For a torus T, which is abstractly isomorphic to U(1) × ... × U(1), but need not have a chosen identification, one writes BT.
The topological K-theory K0(BT) is given by numerical polynomials; more details below.
Construction as an inductive limit
Let Fn(Ck) be the space of orthonormal families of n vectors in Ck and let Gn(Ck) be the Grassmannian of n-dimensional subvector spaces of Ck. The total space of the universal bundle can be taken to be the direct limit of the Fn(Ck) as k → ∞, while the base space is the direct limit of the Gn(Ck) as k → ∞.
Validity of the construction
In this section, we will define the topology on EU(n) and prove that EU(n) is indeed contractible.
The group U(n) acts freely on Fn(Ck) and the quotient is the Grassmannian Gn(Ck). The map
is a fibre bundle of fibre Fn−1(Ck−1). Thus because is trivial and because of the long exact sequence of the fibration, we have
whenever . By taking k big enough, precisely for , we can repeat the process and get
This last group is trivial for k > n + p. Let
be the direct limit of all the Fn(Ck) (with the induced topology). Let
be the direct limit of all the Gn(Ck) (with the induced topology).
Lemma: The group is trivial for all p ≥ 1.
Proof: Let γ : Sp → EU(n), since Sp is compact, there exists k such that γ(Sp) is included in Fn(Ck). By taking k big enough, we see that γ is homotopic, with respect to the base point, to the constant map.
In addition, U(n) acts freely on EU(n). The spaces Fn(Ck) and Gn(Ck) are CW-complexes. One can find a decomposition of these spaces into CW-complexes such that the decomposition of Fn(Ck), resp. Gn(Ck), is induced by restriction of the one for Fn(Ck+1), resp. Gn(Ck+1). Thus EU(n) (and also Gn(C∞)) is a CW-complex. By Whitehead Theorem and the above Lemma, EU(n) is contractible.
Cohomology of BU(n) Proposition: The cohomology of the classifying space H*(BU(n)) is a ring of polynomials in n variables c1, ..., cn where cp is of degree 2p.
Proof: Let us first consider the case n = 1. In this case, U(1) is the circle S1 and the universal bundle is S∞ → CP∞. It is well known that the cohomology of CPk is isomorphic to , where c1 is the Euler class of the U(1)-bundle S2k+1 → CPk, and that the injections CPk → CPk+1, for k ∈ N*, are compatible with these presentations of the cohomology of the projective spaces. This proves the Proposition for n = 1.
There are homotopy fiber sequences
Concretely, a point of the total space is given by a point of the base space classifying a complex vector space , together with a unit vector in ; together they classify while the splitting , trivialized by , realizes the map representing direct sum with