FQFT and cohomology
Types of quantum field thories
For semisimple Lie algebra targets
For discrete group targets
For discrete 2-group targets
For Lie 2-algebra targets
For targets extending the super Poincare Lie algebra
for higher abelian targets
for symplectic Lie n-algebroid targets
This is a sub-entry of sigma-model. See there for background and context.
Above we have discussed some standard classical sigma-models and higher gauge theories as sigma-models, also mostly classically. Here we talk about the quantization of these models (or some of them) to QFTs: quantum -models .
for the moment see the discussion at
that generalizes the Goldman bracket on . Since this operation is induced from concatenating loops, they called it the string product . Its study has come to be known as string topology, now a branch of differential topology.
for a 2-dimensional surface with incoming and outgoing boundary components , the “space of states” of the theory ought to be given by the homology groups and , and the path integral as a pull-push transform along ought to be given by push-forward and dual fiber integration
For the 3-holed sphere with two incoming and one outgoing circle, this would describe an operation on string states induced by the merging of two closed strings to a single one
and this ought to be Chas-Sullivan string product operation.
That this is indeed the case was finally demonstrated by Ralph Cohen and Veronique Godin. (See string topology for all references.) While the idea is rather simple, the concrete realization, especially when taking open strings into account, is fairly technical (see for instance this MO discussion).
But there should be more to it: one expects that these operations on homology groups are just a shadow of a refined construction on chain complexes (for instance singular chains): while Godin’s construction gives an HQFT – a quantum field theory that depends only on the homology of the moduli spaces of the relevant cobordisms – one expect that this is the homology of a genuine extended TQFT (which in this dimension is widely but somewhat unfortunately known under the term “TCFT”). Remarks on how that might be obtained have been made in print by Costello and Lurie.
In the context of our discussion of -models, we would want to refine this even one further step and ask: is the string-topology TCFT of a manifold (given that it exists) formally a -model with target space that manifold, and using some suitable background gauge field?
Given that the string topology TCFT itself has not been fully identified yet, we cannot expect a complete answer to this at the moment, but we will discuss some crucial ingredients that are available.
Notably we can first ignore the dynamics of the system, just consider the kinematics and ask the simple question: which quantum -models on have (∞,n)-vector spaces of states whose decategorification are graded homology groups of mapping spaces of ?
The answer to this question is more more transparent after we formulate the question in more generality: as observed by Cohen and Godin in their A Polarized View of String Topology , we may assume without restriction that the homology groups here are with respect to the generalized homology with coefficients in any commutative ∞-ring (as long as this is an “-field” and as long as is -oriented):
Most every statement about ordinary commutative rings has its analog for commutative ∞-rings, and so we can just follow our nose:
An (∞,1)-vector space over is an -module spectrum and we have an (∞,1)-category Mod of such -vector spaces. Notice that these are a categorification of the ordinary notion of vector space only in the “”-direction of the lattice of (r,n)-categories. A genuine (∞,n)-vector space over – as appears in the description of general -dimensional -models – is instead an object of . In particular the -category of -vector spaces over is something like .
out of the fundamental ∞-groupoid of . This assigns to each point of a -module – the fiber of the (∞,1)-vector bundle thus encoded – to each path in an equivalence between the fibers over its endpoints, and so on: this is the higher parallel transport of a flat -connection. We can also think of this as an ∞-representation of on -modules, also called a representation up to homotopy . For instance if is the classifying space of a discrete ∞-group, then flat (∞,1)-vector bundles on are precisely ∞-representations of .
There is an evident full sub-(∞,1)-category
of 1-dimensional -vector spaces: -lines – -modules that are equivalent to itself regarded as a -module.
An -vector bundle that factors through this inclusion is a -line -bundle .
One finds, as for the case of ordinary 1-vector spaces, that
as the corresponding associated ∞-bundle.
This was amplified by Ando, Blumberg, Gepner, Hopkins, and Rezk (see the reference at (∞,1)-vector bundle), who notice much of the theory of K-(co)homology – including notably its Thom spectrum theory and its twisted cohomology – is neatly captured by simple statements about such -line -bundles. For instance the notion of orientation in generalized cohomology simply boils down to the notion of trivialization of such -line -bundles:
For our discussion here this means that Cohen-Godin’s finding that the string topology HQFT exists for such that is -orientable meaks that the -line -bundle background field that we are to consider in this context are to be trivializable.
Recall that if we interpret an -vector bundle as a background gauge field for a -model, then for any 2-dimensional cobordism the corresponding (∞,1)-vector space of states assigned to, say, the incoming boundary is defined to be the -vector space of sections of the transgression of this -vector bundle to an (∞,1)-vector bundle the mapping space . The transgression of a trivial bundle is again the trivial bundle. And the -vector space of (co)sections is, in the discrete case, as we had discussed above, the (∞,1)-colimit
This one can compute. By triviality of the bundle, Ando-Blumberg-Gepner-Hopkins-Rezk observe that this is the -homology spectrum of
(Here the main point is that for the bundle not being trivial the result encodes the corresponding twisted cohomology , but for our purposes at the moment we want the oriented/trivializable case.)
and in fact
betwee the (∞,1)-category of -module spectra/-algebra spectra (see there for details on this equivalence) and the -category presented by the model structure on chain complexes/on dg-algebras. Under this equivalence the above module spectrum-space of states over the circle ought to be identified with the ordinary integral homology chain complex
The “decategorification” of this -vector space of states is precisely the tower of homology groups of :
of . For the moment see there for more details.
For the moment see