FQFT and cohomology
The term topological conformal field theory (TCFT) is used for a linearization or stabilization of something that is like a conformal field theory (CFT) up to homotopy. It is a notion somewhere half-way between a (2-dimensional) TQFT and a CFT.
(Actually, the remnant of conformal structure here should be just an artefact of the way to parameterize the moduli space of surfaces. As the classification result by Lurie discussed below shows, TCFTs are really -TFTs.)
This formalizes the physics notion of “the topological string”, a topologically twisted superconformal field theory, such as, notably, the A-model and the B-model. TCFTs are therefore a tool for formalizing homological mirror symmetry.
Recall that an ordinary conformal field theory (CFT) is, in FQFT-language, a symmetric monoidal functor on a category whose objects are disjoint unions of intervals and circles, and whose morphisms are Riemann surfaces with these 1d manifolds as incoming and outgoing punctures.
Since Riemann surfaces form a well-understood moduli space, one can turn this also into a Top-enriched category, i.e. an (∞,1)-category, whose hom-spaces are these moduli spaces of Riemann surfaces with given 1d manifolds as incoming and outgoing punctures.
A “truly topological conformal field theory” would be an (∞,1)-functor of the form
or similar. But what is actually called a “topological conformal field theory” is the linearization or stabilization of this:
in a TCFT, this (∞,1)-category of conformal cobordisms is replaced by a stable (∞,1)-category whose hom-objects (when modeled by a dg-category) are just the homology chain complexs of the original hom-spaces.
to the symmetric monoidal dg-category of chain complexes.
This means in particular that when two Riemann surfaces and are homologous as chains in the moduli space of Riemann surfaces, then the TCFT will send them to two equivalent morphisms and of chain complexes between the in- and the output states. The equivalence between and , however, is not unique neither up to equivalence. Rather, it funtorially depends on the 1-chain realizing the homology equivalence between and as 0-chains in the moduli space. In particular, two non-homologous 1-chains between and will in general lead to non-equivalent equivalences between and .
According to ClassTFT the original definition of the domain for TCFTs can be formulated as follows (without reference to any conformal or Riemann structure).
Definition The -category of non-compact 2-dimensional cobordism is defined as follows:
The objects of are oriented 0-manifolds.
Given a pair of objects , a 1-morphism from to is an oriented bordism .
Given a pair of 1-morphsims in , a 2-morphism from to in is an oriented bordism (which is trivial along and ) with the following property: every connected component of has nonempty intersection with .
Higher morphisms in are given by (orientation preserving) diffeomorphisms, isotopies between diffeomorphisms, and so forth.
Then, the cobordism hypothesis-theorem for becomes
are equivalent to Calabi-Yau objects in : the functor sends the point to .
This is ClassTFT, theorem 4.2.11. One can “unfold” and the theorem above, obtaining a statement in terms of symmetric monoidal (∞,1)-categories. Actually it was the unfolded version to be proven first, by Kevin Costello in Cos04, in the particular case . We state it below in the general version given by Jacob Lurie in ClassTFT.
Let be the (infinity,1)-category of open-closed strings, described as follows:
objects are oriented 1-manifolds with boundary;
morphisms are oriented bordisms between 1-manifolds such that each connected component has non-vanishing intersection with the codomain 1-manifold;
the higher morphisms are given by orientation preserving diffeomorphisms, isotopies between these, and so forth.
Write for the full sub-(∞,1)-category on disjoint unions of intervals (open strings sector).
This is ClassTFT, above theorem 4.2.13.
The original statement of the classification result for TCFTs concerned symmetric homotopy-monoidal functors :
The category of open TCFTs with set of D-branes is equivalent to that of Calabi-Yau categories with set of objects.
In Cos04 this is proven using information about cell decompositions of the moduli space of punctured Riemann surfaces, thus effectively presenting by generators-and-relations, The then theorem amounts to noticing that representations of these generators and relations define the operations in an -category with pairing operation.
are equivalent to Calabi-Yau algebra object?s in : the functor sends the interval to .
This is a special case of the general cobordism hypothesis-theorem.
The idea of the proof is that a topological open string theory, i.e., a symmetric monoidal (∞,1)-functor has a Kan extension to an open-closed topological string theory, i.e., to a symmetric monoidal (∞,1)-functor , which is the unfolded version of a symmetric monoidal (∞,2)-functor? from to a symmetric monoidal -category .
One imagines generally that one obtains TCFTs, in their formal definition given above, from worldsheet action functionals as familiar from the physics literature (such as on the A-model and the B-model) by performing the path integral and finding from it a collection of differential forms on moduli space of bosonic field configurations.
It seems there is at this point no literature giving a direct construction along these lines, but there is the following:
(In the discussion leading up to Lemma 4.5.1 there. The proof that this yields a TCFT is theorem 4.5.4.)
the partition function of the string perturbation series for the above TCFT is
So this construxts a 2d TCFT and shows that its effective background quantum field theory is a Chern-Simons theory. While the action functionl on the worldsheet itself, whose path integral should give the differential forms on moduli space considered above, is not explicitly considered here, this does formalizes at least some aspects of an observation that was earlier made in (Witten 92) where it was observed that Chern-Simons theory is the effective background string theory of 2d TFTs obtained from action functionals of the A-model and the B-model.
So via the detour over the effective background field theory, this sort of shows that the physicist’s A-model and B-model are indeed captured by the abstract FQFT definition of TCFT as given above.
The definition was given independently by
The classification of TCFTs by Calabi-Yau categories was discussed in
following conjectures by Maxim Kontsevich, e.g.
This classification is a precursor of the full cobordism hypothesis-theorem. This, and the reformulation of the original TCFT constructions in full generality is in
Here are notes from a seminar on these definitions and results:
Discussion of the construction of TCFTs from differential forms on moduli space and the way this induces by “second quantization” effective background Chern-Simons theories is in
formalizing at least aspects of the observations in
Si Li, BCOV theory on the elliptic curve and higher genus mirror symmetry (arXiv:1112.4063)
Si Li, Variation of Hodge structures, Frobenius manifolds and Gauge theory (arXiv:1303.2782)