universal bundle
Let be a topological group. A universal bundle for is a principal bundle
such that for any principal bundle , with a CW-complex
, there is a map , unique up to homotopy
, such that the pullback bundle is equivalent
to , that is such that there is a bundle map
.
with , such that any bundle map of any bundle over extending factors uniquely through .
As is obvious from the universal property, the universal bundle for a group is unique up to unique homotopy equivalence
.
The base space is often called a classifying space of , since homotopy classes of maps to it from a given space classify -bundles over that space.
There is a useful criterion for universality: a bundle is universal if and only if all the homotopy groups of , its total space, are trivial. This allows us to construct the universal bundle any subgroup from that of a larger group. Assume and that is a universal bundle for . Then also acts freely on which is contractable so must be a universal bundle for .
In 1956, John Milnor gave a general construction of the universal bundle for any topological group (see Annals of Mathematics, Second Series, Volume 63 Issue 2 and Issue 3 for details). His construction uses the infinite join of the group with itself to define the total space of the universal bundle.