Let and be smooth oriented manifolds of dimension and , and let a proper smooth map. There is a theorem called the "Disintegration Theorem" which says roughly that a measure on induces a measure on the fibers of (except for a set of measure 0) such that integrating over fibers and then over is the same as integrating over .
Now, since is a smooth manifold, most measures that anyone would care about are given by -forms , so disintegration of such a measure would hopefully give us -forms on the regular fibers of .
In fact, I sort of see how one would define this: the pushforward of to is a top form there, and there is a dual -vector field on that pairs to 1 with this top form (maybe with some singularities at non-regular fibers). At any regular point in , we can choose a subspace in which maps isomorphically to , and transfer the -vector field to this subspace, and contract with and pull back to . This should be uniquely defined because the components where you've contracted with vectors parallel to the fiber drop out when you do the pull-back.
I don't especially like this phrasing, since it is quite non-canonical, but I'm confident it must work because the disintegration theorem is true.
I have to imagine that I am not the first person to have had this thought. Is there some better way of describing this process and making computations with it? There should be a text book or something that covers this, but I cannot find it. Anyone have suggestions?
