Universal bundles – or mapping cocylinders – are intermediate steps in the computation of homotopy fibers, dual to the that way mapping cone are intermediate steps in the computation of homotopy cofibers.
It is familiar from topology that one can form the path fibration of a topological space. This can be understood as an example of a general construction where one computes homotopy pullbacks of the point – or, if things are not groupoidal, comma objects.
Since universal bundles are examples of this construction, we here speak of generalized universal bundles. Another appropriate term might be generalized path fibrations.
One generalizaton of “generalized universal bundles” is that the objects in question need not be groupoidal, i.e. they behave like directed spaces. In this case the homotopy pullbacks familiar from topology are replaced by comma object constructions. This is useful in various applications. For instance the constructions category of elements and Grothendieck construction can be understood as such directed homotopy pullbacks of the point.
and in particular
which is the total composite vertical morphism of the pullback diagram
Then a (generalized) “-bundle” on some object is a morphism which is the pullback of the generalized universal -bundle along a “classifying morphism”
This can be understood as a “(directed) homotopy pullback” of the point:
The fiber of the generalized universal bundle is the loop monoid :
is exact in that is the kernel of in the sense of kernels of morphisms of pointed objects (see there).
In (higher) categorical contexts, take the interval object to the the interval category . Then
This is the universal -bundle in its groupoid incarnation. It is a theorem by Segal from the 1960s that indeed this maps, under geometric realization to the familiar universal -bundle in . Moreover, it can be seen that every -principal bundle in the ordinary sense is the pullback of in the following sense:
the -bundle is classified by a nonabelian -valued 1-cocycle (the transition function of any of its local trivializations), which is an anafunctor
(For instance could be the Čech groupoid of a cover of .)
The universal groupoid bundle may now be pulled back along this anafunctor to yield the groupoid bundle given by the total left vertical morphism in
This bundle of groupoids is weakly equivalent to the -principal bundle we started with in that there is a morphism of bundles of groupoids (with regarded as a bundle of discrete groupoids)
This is recalled in the following reference.
For , strict 2-categories , a strict one-object 2-groupoid corresponding to a strict 2-group with the unique point, was described under the name in
This was shown to be action bigroupoid of acting on itself in
One can show that every -principal 2-bundle as described in
Toby Bartels, 2-Bundles (arXiv)
Christoph Wockel, …
Igor Bakovič, Bigroupoid 2-torsors PhD thesis, Munich (2008) (pdf).
indeed is recovered as the pullback of along the corresponding cocycle, along the lines described above.
The way this works is indicated briefly in the last section of Roberts-Schreiber above. A more detailed description for the moment is in the notes
is . Pullback of this is the Grothendieck construction.
It was David Roberts in the blog comment
who first pointed out that these (higher) subobject classifiers are just generalized universal bundles in the above sense.
These cases for and have been considered in the context of universal category bundles in
The discussion there becomes more manifestly one of bundles if one regards all morphisms appearing there as being the right legs of anafunctors.
A morphism to a pointed object (needs not be a basepoint preserving morphism!) can be regarded as a representation of on the point of . The pullback of the universal -bundle along this morphism
can be addressed as the -bundle -associated to the universal -bundle .
If is a groupoid, then is the action groupoid of acting on the point of .
Further pulling this back along a cocycle of a -principal bundle yields the -accociated bundle of that.
For instance for and with a representation of the group on a vector space , the -associated -bundle on is
Pulling that further back along the cocycle classifying a -principal bundle , one obtains the familiar vector bundle which is -associated to , along the lines described above: