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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{Axiomatic field theories and their motivation from topology} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{quantum_field_theory}{}\paragraph*{{Quantum field theory}}\label{quantum_field_theory} [[!include functorial quantum field theory - contents]] This is a sub-entry of [[geometric models for elliptic cohomology]] and [[A Survey of Elliptic Cohomology]] See there for background and context. This entry here indicates, generally, how [[FQFT]]s may be related to [[cohomology theory|cohomology theories]]. \begin{quote}% \textbf{raw material}: this are notes taken more or less verbatim in a seminar -- needs polishing \end{quote} next: \begin{itemize}% \item [[(1,1)-dimensional Euclidean field theories and K-theory]] \item [[(2,1)-dimensional Euclidean field theories and tmf]] \end{itemize} \vspace{.5em} \hrule \vspace{.5em} All of the following assumes that the reader is well familiar with the basic ideas indicated at [[FQFT]]. In the following $d-RB$ or $R Bord_d$ and the like denotes a category of $d$-dimensional [[Riemannian cobordism]]s that are equipped with \emph{Riemannian structure}, i.e. with [[Riemannian metric]]. Similarly $d-E B$ or $E Bord_d$ or the like denotes a category of cobordisms with \emph{Euclidean structure}, by which is meant a \emph{flat} Riemannian metric. The definition of this is discussed in detail in \begin{itemize}% \item [[bordism categories following Stolz-Teichner]] \end{itemize} \textbf{definition} $d$-dimensional Riemannian field theories are symmetric monoidal functors $d-RB \to TV$ from $d$-dimensional Riemannian bordisms to [[topological vector space]]s. A field theory is very similar to a [[representation]] of a group. Only where a representation of a [[group]] $G$ is a functor from the [[delooping]] $\mathbf{B}G = {*}//G$ of $G$ to [[Vect]], an [[FQFT]] is a representation of a more complicated domain category. \textbf{how does [[topology]] enter?} for $X$ some [[topological space]] there is also a [[symmetric monoidal category]] \begin{displaymath} d-RB(X) \end{displaymath} of Riemannian bordisms equipped with a continuous map to $X$. Notice that $d RB(X)$ does depend covariantly on $X$. This means that $Fun^\otimes(d RB(X), TV)$ is contravariant in $X$. When special structure is around, however, we also have a push-forward of such functors along morphisms. \textbf{Example: push-forward to the point}: for $X$ as above and $X \to {*}$ the unique map to the [[point]] heuristically we want a map \begin{displaymath} d RFT(X) \stackrel{p_*}{\to} d RFT(pt) \end{displaymath} notice that this push-forward is not an [[adjoint functor]]. Instead, it is a map that comes from integration over fibers. In particular it will change the degree of [[cohomology theory|cohomology theories]]. heuristically the pushforward \begin{displaymath} d RFT(X) \stackrel{p_*}{\to} d RFT(pt) \end{displaymath} acts on field theories $E_X$ over $X$ \begin{displaymath} E_X \mapsto E \end{displaymath} by the assignment \begin{displaymath} E(Y^{d-1}) \mapsto \Gamma\left( \itexarray{ E_X(Y) \\ \downarrow \\ Maps(Y,X) } regarded as a vector bundle \right) \end{displaymath} for instance when $E_X(Y) = \mathbb{C}$ then $E(Y) = \Gamma(Maps(Y,X))$. For instance take $\Sigma$ to be the pair of pants with input $Y_0$ and output $Y_1$ and let $F : \Sigma \to X$ be a map. Then $E(\Sigma)$ will take some section $\Psi \in E(Y_0)$ to \begin{displaymath} E(\Sigma)(\Psi) : (f_1 : Y_1 \to X) \mapsto \int_{\{F : \Sigma \to X\} | F/Y_1 = f_1} E_X(F)(\Psi(f_0)) \frac{1}{Z}\exp(-S(F) dvol) \,. \end{displaymath} Here the expression $\frac{1}{Z}\exp(-S(F) dvol)$ denotes a would-be measure which is still to be defined. Here $f_0 = F/Y_0$ is the restriction of $F$ to $Y_0$. Contravariant functors with push-forward also arise as part of a [[cohomology theory]] \begin{displaymath} h^n : Diff^{op} \to Ab \end{displaymath} from the category [[Diff]] of smooth [[manifold]]s to the category [[Ab]] of abelian groups that satisfies the axioms of [[generalized (Eilenberg-Steenrod) cohomology]] theory. These Eilenberg-Steenrod axioms are \begin{enumerate}% \item \textbf{homotopy axiom} for $h^n$ \item \textbf{Mayer-Vietoris axiom} for $h^n$ \item \textbf{suspension isomorphism} $h^n(X) \simeq h^{n+1}_{cvs}(X \times \mathbb{R})$ \end{enumerate} here in the last axiom for [[topological space]]s we'd simply have the [[suspension]] $h^{n+1}(\Sigma X)$. Here in order to stay within [[manifold]]s we instead do as indicated, where ${}_{cvs}$ means ``compact vertical support''. Concerning the \textbf{homotopy axiom}: given any [[functor]] that sends manifolds to a category of [[FQFT]]s over $X$ \begin{displaymath} d-FTs := h : Diff^{op} \to C \end{displaymath} in $dRFT$s we can make it a homotopy functor by defining $\omega_0, \omega_1 \in h(X)$ to be [[concordance|concordant]] if $\exists \omega \in h(X \times \mathbb{R})$ such that $\omega/(X\times \{i\}) \simeq \omega_i$ in $h(X)$, $i = 0,1$ then under this relation \begin{displaymath} X \mapsto d-RFT(X)/\simeq \end{displaymath} is a homotopy invariant functor \textbf{Homework}: for $h(X) = \Omega^n_{closed}(X)$ we have that $\omega_0$ concordant to $\omega_1$ exactly when $\omega_0 = \omega_1 + d \alpha$ for some $\alpha \in \Omega^{n-1}(X)$ Concerning the \textbf{Mayer-Vietoris axiom}: if $X = U \cup V$ then the functor should respect this gluing. suppose $E \in 2-RFT(X)$ is a 2d Riemmanian field theory. let $\gamma : S^1 \to X$ be a loop in $X$. then $E(\gamma)$ is a [[vector space]]. If $\gamma$ sits neither entirely in $U$ or in $V$, then there is no way that the vector space $E(\gamma)$ can be reconstructed by knowing just the restriction of $R$ to $U$ and $V$. So this is a \textbf{problem} for the definition of field theories so far. The \textbf{proposed solution} (from [[What is an elliptic object?]]) is to use \emph{extended} [[FQFT]]s instead. This introduces \textbf{locality} into [[FQFT]]s, at the expense of working with [[n-category|n-categories]]. This will however not be studied here for the moment. Concerning the \textbf{suspension isomorphism}: for that first we need for $n \in \mathbb{Z}$ the notion of a field theory over $X$ of \textbf{degree $n$}, i.e. \begin{displaymath} X \mapsto d-RFTs^n(X) \end{displaymath} such that for $n=0$ this is an ordinary Riemannian field theory, in $d-RFT(X)$. This requires to replace [[manifold]]s by [[supermanifold]]s. \textbf{Example} let $d = 0$ and consider 0-dimensional TFTs over $X$. so consider \begin{displaymath} Fun^\otimes(0 Bord(X), TV) \end{displaymath} in $0 Bord(X)$ there is only a single [[object]]: the [[empty set]] $\emptyset$ which has a unique map $\emptyset \to X$ the collecton of morphisms is $\{finite set \to X\}$ in there is $Hom_{Diff}(pt,X)$. here composition of morphisms is the same as tensor product of objects: both comes from the disjoint union of these finite set domains. \begin{displaymath} E : 0 Bord(X) \to TV_{\mathbb{R}} \end{displaymath} \begin{displaymath} \emptyset \mapsto \mathbb{R} \end{displaymath} \begin{displaymath} x \in X \mapsto E(X) \in \mathb{R} \end{displaymath} so a priori we have \begin{displaymath} Fun^\otimes(0 Bord(X), TV) \simeq Maps(X, \mathbb{R}) \,, \end{displaymath} where on the right we have \emph{all} maps. This is not quite what is intended. We want to see \emph{smooth} maps on the right. To get that, we need to talk about \emph{smooth functors} on the left. This is the topic of later discussion, which will yield \begin{displaymath} SmoothFun^\otimes(0 Bord(X), TV) \simeq C^\infty(X, \mathbb{R}) \,. \end{displaymath} But one other ting goes wrong the: the corresponding homotopy functor is $C^\infty(X)/\simeq = \{0\}$. So turning this into an Eilenberg-Steenrod theory yields the trivial theory. The way out to that will be to go to [[supermanifold]]s. \textbf{Punchline} of this session here: \begin{displaymath} (0|1)-TFTs(X) \simeq SmoothFun^\otimes((0|1)Bord(X), TV) \end{displaymath} and then it is a $lemma^G$ that \begin{displaymath} C^\infty(SuperDiff(\mathbb{R}^{0|1}, X))^G \simeq \Omega^\bullet(X)^G = (\oplus_{n = 0}^{\infty} \Omega^n(X))^G \end{displaymath} where $G$ is the [[supergroup]] $G = Diff(\mathbb{R}^{0|1}) \wim42 \mathbb{R}^{0|1} \rtimes \mathbb{R}^\times$. The degree decomposition on forms then turns out to be the eigenvalue decompositon of the $\mathbb{R}^\times$-part, while the deRham differential is the $\mathbb{R}^{0|1}$-action (cite Kontsevich, Severa here \ldots{}) so then from that one finds that the $G$-invariant bit is the \emph{closed} 0-forms \begin{displaymath} \Omega^\bullet(X)^G \simeq \Omega^0_{closed}(X) \end{displaymath} So we get $n$-\textbf{Lemma} : \begin{displaymath} (0|1)-TFT^n(X) \simeq \Omega^n_{cl}(X) \end{displaymath} and \begin{displaymath} (0|1)-TFT^n(X)_{concord} \simeq H^n_{dR}(X) \end{displaymath} \textbf{push-forward} Now with the super-directions included, there is a notion of push-forward of the TFTs that does shift the degree, and we get the following \textbf{Theorem (Stolz-Teichner-Kreck-Hohnhold)} when $X$ is oriented \begin{displaymath} (0|1)TFT^n(X) \stackrel{push}{\to} (0|1)TFT^0 \simeq \mathbb{R} \end{displaymath} \begin{displaymath} \Omega^n_{cl}(X) \stackrel{\int_{X_n}}{\to} \Omega^0_{cl}(pt) \simeq \mathbb{R} \end{displaymath} and similarly after dividing out concordance, the push-forward becomes the push-forward in [[deRham cohomology]]. \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Stefan Stolz]] (notes by Arlo Caine), \emph{Supersymmetric Euclidean field theories and generalized cohomology} Lecture notes (2009) (\href{http://www.nd.edu/~jcaine1/pdf/Lectures_complete.pdf}{pdf}) \end{itemize} \end{document}