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These lecture notes are based on notes from the 18.999 geometry seminar class taught by Tomasz Mrowka and from the IAS/Park city Mathematic series book Gauge theory and the topology of four-manifolds 1. Moduli spaces A moduli space can be viewed as a geometric object which classifies the solutions of some problem. For example, there is the moduli space of $n$-tuples of points in $I=[0,1]$,that is, points in $I^{n}$ modulo symmetric transformations:$I^{n}/ Sym_{n}$. Given a complex vector space $V$ of dimension $n$, we can look at the space of endomorphisms of $V$ modulo isomorphisms:

$$\End(V)/Iso(V) \simeq Mat_{n \times n}( \mathbb{C})/Gl_{n}(\mathbb{C}) = \mathcal{M}_{n}$$ The Jordan canonical form gives a description of $\mathcal{M}_{n}$ as a set.
In this class, we will look at moduli spaces which arise from the study of connections on $G$-bundles such as the moduli space of instantons on a 4-dimension manifold (ADHM) or the moduli space of Yang-Mills-Higgs monopoles on $\R^{3}$.

In the next section, we list some basic notions of principal $G$-bundles which we will need for this class.

2. G-bundles

Given $g \in G, q \in P$, we set

$$\begin{eqnarray*} r_{g}: P &\rightarrow P \\ p &\rightarrow p\cdot g \end{eqnarray*}$$
and
$$\begin{eqnarray*} \iota_{q}: P &\rightarrow P \\ h &\rightarrow q\cdot h \end{eqnarray*}$$
The map $\iota_{q}$ induces an isomorphism
$$\begin{eqnarray*} (\iota_{q})_{\ast}:\mathfrak{g}\rightarrow T_{p}P_{x} \end{eqnarray*}$$ where $\mathfrak{g}$ is the Lie algebra of $G$, $x= \pi(p)$ and $P_{x}= \pi^{-1}(x)$ the fiber over $x$. We will also denote $T_{p}P_{x}$ by $VTP_p$ (the space of vertical tangent vectors at $p$).

2.1. Associated bundles Suppose $\pi: P \rightarrow X$ is a smooth principal $G$-bundle and $F$ a smooth manifold equipped with a smooth left $G$-action. Then,we can define a fiber bundle over $X$ with fiber $F$ as follows. We set

$$P \times_{G}F = ( P \times F) / \sim$$ where $(p,y)\sim (p.g,g^{-1} \cdot y)$ for all $p \in P, y \in F$ and $g \in G$. We define the map

$$\begin{eqnarray*} \tau : P \times_{G}F &\rightarrow X \\ \{p,y\} & \rightarrow \pi(p) \end{eqnarray*}$$
where $\{p,y\}$ is the equivalence class of $(p,y) \in P \times F$.Here are some important examples of this construction.

1. Let $F=G$ and let $G$ act on itself via conjugation :$g \cdot h = ghg^{-1}$.Then $P \times_{G}G \rightarrow X$ is a smooth fiber bundle with fiber $G$ which is usually denoted by $Ad(P)$.
2. Suppose that $F$ is a vector space $V$ and $\rho: G \rightarrow Gl(V)$ is a linear representation. Then $P \times_{G}V \rightarrow X$ is a smooth vector bundle.In particular, if we consider the adjoint representation $ad: G \rightarrow Gl(\mathfrak{g})$, then the corresponding smooth vector bundle is denoted by $ad(P)$.

2.2. Connections

For every $p \in P$,we will denote by $j^{A}_{p}$ the projection map
$$\begin{eqnarray*} TP_{p} \rightarrow VTP_{p} \end{eqnarray*}$$ with kernel $\mathcal{H}^{A}_{p}$.Then, given $g \in G$, we have $$\begin{eqnarray*} j^{A}_{pg} \circ (r_{g})_{\ast} = (r_{g})_{\ast} \circ j^{A}_{p} \end{eqnarray*}$$
Given a connection on a $G$-bundle $\pi: P \rightarrow X$, we can lift smooth paths on $X$ to smooth paths on $P$. Suppose $\gamma : [ 0,1] \rightarrow X$ , and $p \in P_{\gamma(0)}$.There is a unique smooth path $\tilde{\gamma} : [ 0,1] \rightarrow X$ such that $\tilde{\gamma}(0)=p$ and $\tilde{\gamma}^{\prime}(t) \in \mathcal{H}_{\tilde{ \gamma}(t)}$ for all $t \in [ 0,1]$.Hence,a connection gives us a notion of parallel transport.

A connection can also be defined via a 1-form. The Maurer-Cartan form $\omega_{mc}$ is a 1-form on $G$ with values in $\mathfrak{g}$ defined by :

$$\omega_{mc}(v)= (L_{g^{-1}})_{\ast}(v)$$ where $v \in T_{g}G$ and $L_{g^{-1}}$ is left translation by $g^{-1}$.

Proof:Suppose we are given a connection $A$.For every $p \in P$ , we set

$$\omega_p=(\iota_{p})^{-1}_{\ast}\circ j^{A}_{p}: TP_{p} \rightarrow \mathfrak{g}$$ $\omega$ is clearly smooth.Given $g \in G$ and $v \in T_{p}P$, we have \begin{eqnarray*}

\omega_{pg}((r_{g})_{\ast}( v ))&=&(\iota_{pg})^{-1}_{\ast}\circ j^{A}_{pg}((r_{g})_{\ast}( v))\\
&=&( L_{g^-1})_{\ast}\circ (\iota_{p})^{-1} \circ (r_{g})_{\ast} \circ j^{A}_{p}(v)\\
&=&( ( L_{g^-1})_{\ast}\circ (\iota_{p})^{-1} \circ (r_{g})_{\ast})\circ j^{A}_{p}(v)\\
&=& (ad_{g^{-1}}\circ (\iota_{p})^{-1})\circ j^{A}_{p}(v)\\
&=& ad_{g^{-1}}( \omega_{p}(v))
\end{eqnarray*}
Hence, the first condition is satisfied and ,by doing another simple computation, we can show that $\omega$ also has the second property.

Conversely, suppose $\omega \in \Omega^{1}(P;\mathfrak{g})$ has the properties mentioned above. Define a connection $A$ by setting $\mathcal{H}^{A}_{p}$ to be the kernel of the map $\omega_p: TP_{p} \rightarrow \mathfrak{g}$ for every $p \in P$.The second property implies that $\omega_{p}$ induces an isomorphism $VTP_{p} \simeq \mathfrak{g}$.Hence,$\mathcal{H}^{A}$ is indeed a smooth distribution such that $\pi_{\ast}: \mathcal{H}^{A}_{p} \rightarrow T_{x}X$ is an isomorphism for every $p$.The condition$(r_{g})_{\ast}(\mathcal{H}^{A}_{p})= \mathcal{H}^{A}_{p.g}$ follows form the first property of $\omega$.

Covariant derivatives.Finally, a connection $A$ on $\pi: P \rightarrow X$ induces a connection on the associated vector bundle $ad(P)$.

$$\nabla^{A} :C^{\infty}(X,ad(P)) \rightarrow C^{\infty}(X,ad(P) \otimes T^{\ast}X )$$ It is defined as follows: Suppose $v$ is a vector field on $X$ and $s: X \rightarrow ad(P)$ is a section of $ad(P)$.Let $\tilde{v}$ be the vector field on $P$ defined by $\pi_{\ast}(\tilde{v})= v$ and $\tilde{v}_{p} \in \mathcal{H}^{A}_{p}$ for all $p \in P$. We also define the vector field $\tilde{s}$ on $P$ by $\tilde{s}_{p}=( \iota_{p})_{\ast}(\varepsilon)$ where $s(\pi(p))= [p,\varepsilon]$.Hence $\tilde{s}_{p} \in VTP_P$ for evey $p \in P$. For $x \in X$, we set
$$\nabla^{A}_{v}(s)_{(x)}=\{p,(\iota_{p})^{-1}_{\ast} ( [\tilde{v},\tilde{s}]_{p})\}$$ where $p \in P_{x}$.Note that

$$\pi_{\ast} [\tilde{v},\tilde{s}]= [\pi_{\ast}(\tilde{v}),\pi_{\ast}(\tilde{s})]=[v,0]=0$$
Hence $[\tilde{v},\tilde{s}] \in VTP \simeq \mathfrak{g}$. Also $[\tilde{v},\tilde{s}]_{pg}=ad_{g^{-1}}( [\tilde{v},\tilde{s}]_{p})$.So $\nabla^{A}_{v}(x)$ is well-defined. Finally, if $h,f \in C^{\infty}(X)$, we have :
$$[\widetilde{fv},\tilde{s}]=\pi^{\ast}f [\tilde{v},\tilde{s}]-\tilde{s}(\pi^{\ast}f)\tilde{v}=f[\tilde{v},\tilde{s}]-\pi_{\ast}(\tilde{s})(f)\tilde{v}=f[\tilde{v},\tilde{s}]$$
and
$$[\tilde{v},\tilde{hs}]=h[\tilde{v},\tilde{s}]+\tilde{v}(h)\tilde{s}$$
This implies that $\nabla^{A}_{v}$ is indeed a connection.( It is $C^{\infty}$- linear in $v$ and satisfies the Leibniz rule).

We will denote by $\mathcal{A}_P$ the space of connections on the bundle $P \rightarrow X$.Any smooth principal $G$ bundle has a connection.So $\mathcal{A}_P$ is non-empty. Furthermore, one can show that $\mathcal{A}_P$ is an affine space for the vector space $C^{\infty}(X,ad(P)\otimes T^{\ast}X )= \Omega^{1}(X,ad(P))$.

2.3. The Gauge group

Consider the fiber bundle $Ad(P) \rightarrow X$ with fiber $G$. The group structure on $G$ induces a group structure on $C^{\infty}(X,Ad(P))$,the space of smooth sections of $Ad(P)$, via fiber-wise multiplication.Let $\mathcal{M}(P,G)$ be the space of smooth maps $\psi:P \rightarrow G$ such that $\psi(p \cdot g)= g^{-1}\psi(p)g$ for all $p\in P$ and $g \in G$.The group structure on $G$ alos induces a group sturucture on $\mathcal{M}(P,G)$. Furthermore,

$$\mathcal{G}_{P} \simeq \mathcal{M}(X,G) \simeq C^{\infty}(X,Ad(P))$$
Indeed, suppose $u:P \rightarrow P$ is an automorphism. Since u is a fiber-preserving map,there exists a unique smooth map $\psi:P \rightarrow G$ such that $u(p)=p\cdot \psi(p)$ for all $p \in P$.The condition $u(p \cdot g)=u(p)\cdot g$ implies that $\psi(p \cdot g)= g^{-1}\psi(p)g$.

Conversely, given a map $\psi:P \rightarrow G$ with the above property, we define $u:P \rightarrow P$ by $u(p)=p\cdot \psi(p)$.Hence, $\mathcal{G}_{P} \simeq \mathcal{M}(X,G)$ as claimed. Finally, the group $\mathcal{M}(X,G)$ can be natuarlly identified with $C^{\infty}(X,Ad(P))$.

Induced action on the space of connections.The group $\mathcal{G}_{P}$ acts on $\mathcal{A}_P$

$$\begin{eqnarray*} \mathcal{A}_P \times \mathcal{G}_{P} \rightarrow \mathcal{A}_P\\ (A,u) \rightarrow u\cdot A \end{eqnarray*}$$
If we view the connection $A$ as a distribution, then
$$\mathcal{H}^{u\cdot A}_{p}=(u_{\ast})^{-1}(\mathcal{H}^{ A}_{u(p)})$$
for every $p \in P$. If we represent $A$ by a one-form $\omega \in \Omega^{1}(P;\mathfrak{g})$, then $u \cdot A$ is represented by $u^{\ast}\omega$. Finally, in terms of the covariant derivative induced on ad(P), we have:
$$\nabla^{A}(s)= \tilde{u}^{-1}(\nabla^{A}(\tilde{u}(s))$$
where $\tilde{u}: ad(P) \rightarrow ad(P)$ is the automorphism induced by u.

We will be studying the quotient space

$$\mathcal{A}_{P}/ \mathcal{G}_{P}= \mathcal{B}_P$$
We will show that $\mathcal{B}_{P}$ is a Hausdorff space. In fact, certain completions of this space are smooth Banach or Hilbert orbifolds.