nLab geometry of physics -- principal connections

This entry contains one chapter of geometry of physics.

previous chapters: flat connections, de Rham coefficients

next chapters: prequantum geometry, WZW terms

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Principal connections

Model Layer

Motivation

• Dirac charge quantization implies that the electromagnetic field is not just given by a closed differential 2-form but by a refinement of that to a circle group principal connection

• analogous arguments show that more generally a configuration of the Yang-Mills field for some gauge group $G$ is a $G$-principal connection;

• analogous arguments say that the B-field and the C-field are given by circle n-bundles with connection for $n = 2$ and $n = 3$, respectively.

In this way all of gauge theory is much the study of principal connections and all of higher gauge theory is much the study of principal infinity-connections.

More is true: also a configuration of the field of gravity is represented by a connection, in this case a Cartan connection, which is a Poincare group-principal connection satisfying an extra condition (that “solders” it to the underlying manifold).

Deligne cohomology

It turns out that one of the many equivalent incarnations of circle n-bundles with connection are cocycles in Deligne cohomology. That this is so we discuss below in the Semantics layer. Here instead we discuss Deligne cohomology taken at face value.

Preliminaries on sheaf cohomology

In order to be somewhat self-contained, this section reviews some elements of abelian sheaf cohomology specified to the context that we need. It also sets up some notation. The definition of the Deligne complex itself is below in def. .

Recall the following definition, which is discussed in much detail in the chapter geometry of physics – smooth sets.

Definition

Write CartSp for the site whose

• objects are Cartesian space $\mathbb{R}^n$ for $n \in \mathbb{N}$,

• morphisms are smooth functions $\mathbb{R}^{n_1} \to \mathbb{R}^{n_2}$ between these;

• whose coverage is given by “differentially good open covers”, those open covers of $\mathbb{R}^n$s all whose finite non-empty intersections are diffeomorphic to an open ball, hence again to $\mathbb{R}^n$.

Write $PSh(CartSp) = Func(CartSp^{op},Set)$ for the category of presheaves over this site. Write

$$Smooth0Type \coloneqq Sh(CartSp)$$

for its category of sheaves, also called the cohesive topos of smooth spaces.

Remark

Instead of the site CartSp of def. one could use the site SmoothMfd of all smooth manifolds. All of the statements and constructions in the following go through in that case just as well. In fact CartSp is a dense subsite of SmoothMfd. On the one hand this implies that the abelian sheaf cohomology is the same for both sites, but on the other hand means that it is convenient to restrict to the much “smaller” site of Cartesian spaces. In fact since the stalks of sheaves over smooth manifolds are evaluations on small open balls and since every open ball is diffeomorphic to a Cartesian space, many statements that are true (only) stalkwise over SmoothMfd are actually true globally over $CartSp$. It is the “descent” or “infinity-stackification” which is implicit in abelian sheaf cohomology that takes care of these global statements over CartSp to translate into the same local statements as one gets over SmoothMfd.

Example

The assignment $C^\infty(-,\mathbb{R}) \colon \mathbb{R}^n \mapsto C^\infty(\mathbb{R}^n,\mathbb{R})$ of smooth functions with values in the real numbers is a sheaf. Since this is representable we are entitled to identify this with the smooth manifold $\mathbb{R}$ (the real line) itself, and just write $\mathbb{R} \in Sooth0Type$.

Similarly for $X$ any other smooth manifold, it represents a sheaf on CartSp and we just write $X \in Smooth0Type$ for this.

Of particular interest below is the case where $X = S^1 = \mathbb{R}/\mathbb{Z} = U(1)$ is the circle, to be regarded as the circle group.

Notice that traditionally the sheaf represented by $\mathbb{R}$ or $U(1)$ is indicated by an underline as in $\underline{\mathbb{R}}$ and $\underline{U}(1)$, but we do not follow this tradition here.

Instead, if we consider the other sheaf that might deserve to be denoted by $\mathbb{R}$, namely the constant sheaf on $\mathbb{R}$, which sends each $U \in CartSp$ to the set underlying $\mathbb{R}$, then we write $\flat \mathbb{R}$ for that. Similarly

$$\flat U(1) \in Smooth0Type$$

is the sheaf sending each test manifold to the set of points in the circle, and each smooth function between Cartesian spaces to the identity function on that set.

Example

For $k \in \mathbb{N}$ write

$$\mathbf{\Omega}^k \in Sh(CartSp)$$

for the sheaf $\mathbf{\Omega}^k \colon U \mapsto \Omega^k(U)$ of smooth differential k-forms on $X$. The de Rham differential extends to a morphism of sheaves

$$\mathbf{d} \colon \mathbf{\Omega}^k \to \mathbf{\Omega}^{k+1} \,.$$

For positive $k$ its kernel is the sub-sheaf

$$0 \to \mathbf{\Omega}^k_{cl} \hookrightarrow \mathbf{\Omega}^k \stackrel{\mathbf{d}}{\longrightarrow} \mathbf{\Omega}^{k+1}$$

of closed differential forms; and for $k = 0$ its kernel is the sub-sheaf of constant functions

$$0 \to \flat \mathbb{R} \hookrightarrow \mathbb{R} \stackrel{\mathbf{d}}{\longrightarrow} \mathbf{\Omega}^1 \,.$$

In the background, what plays a role for the following is the full cohesive homotopy theory of smooth ∞-groupoids. This receives a map from the following coarse homotopy theory of chain complexes of abelian sheaves, which is all that is necessary for the present purpose.

Definition

Write

$$Ch_+(Smooth0Type) = Ch_+(Sh(CartSp)) = Sh(CartSp,Ch_+)$$

for the category of chain complexes in the smooth sheaves of def. , hence for the 1-category whose objects are chain complexes of abelian sheaves on $CartSp$.

Regard this as equipped with the structure of a category of fibrant objects induced by the projective model structure on chain complexes, hence with classes of morphisms labeled as follows: a chain map $f_\bullet \colon A_\bullet \to B_\bullet$ is called

• a weak equivalence if it is a quasi-isomorphism, hence if it induces isomorphisms on (sheaves of) chain homology groups;

• a fibration if it is an epimorphism (of abelian sheaves) in positive degree.

Remark

For our purpose the main use of this structure is to compute homotopy fibers via the factorization lemma. Namely

1. every chain map may be replaced, up to weak equivalence of its domain, by a fibration;

2. the homotopy fiber of a chain map is the ordinary fiber of any of its fibration replacements.

Remark

That the properties in def. are interpreted in sheaves simply means that they apply stalk-wise. For instance a morphism of chain complexes of presheaves $f_\bullet \colon A_\bullet \to B_\bullet$ is a weak equivalence precisely if the underlying presheaf of chain complexes becomes a quasi-isomorphism for each point $x$ in each Cartesian space $\mathbb{R}^n$ after restricting (via the presheaf structure maps) to a small enough open neighbourhood of that point. Similarly for epimorphisms.

Remark

There is a canonical map of homotopy theories from $Ch_+(Smooth0Type)$ to the full (∞,1)-topos Smooth∞Grpd which is given by applying the Dold-Kan correspondence followed by ∞-stackification. The key point is that this map preserves homotopy fiber products, which is the universal construction that already captures most of the relevant properties of the Deligne complex. In this way it is sufficient to concentrate on $Ch_+(Smooth0Type)$ for much of the theory.

When writing out the components of chain complexes we will use square brackets always denote the group in degree-0 to the far right, and the group in degree $k$ being $k$ steps to the left from that.

Example

For $A \in Ab(Smooth0Type) = Sh(CartSp,Ab)$ any abelian sheaf and for $n \in \mathbb{N}$ we write

$$(\mathbf{B}^n A)_{\bullet} \coloneqq A[-n] = \left[ A \to 0 \to \cdots \to 0 \right]$$

for the chain complex of sheaves concentrated on $A$ in degree $n$.

Example

There is a weak equivalence, def. ,

$$\left[\mathbb{Z}\to \mathbb{R} \right] \stackrel{\simeq}{\longrightarrow} U(1)$$

given by the chain map

$$\array{ \mathbb{Z} &\hookrightarrow& \mathbb{R} \\ \downarrow && \downarrow^{\mathrlap{mod\,\mathbb{Z}}} \\ 0 &\to& U(1) }$$

(which is just the exponential sequence regarded as a chain map).

The de Rham differential extends through this equivalence to produce a morphism denoted $\mathbf{d} log$:

$$\array{ (\mathbb{Z} \to \mathbb{R}) &\stackrel{\mathbf{d}}{\longrightarrow}& \mathbf{\Omega}^1 \\ \downarrow^{\mathrlap{\simeq}} & \nearrow_{\mathrlap{\mathbf{d} log}} \\ U(1) \,. }$$

On a given $U(1)$-valued function this is given by representing the function by a smooth $\mathbb{R}$-valued function under mod-$\mathbb{Z}$-reduction (which is always possible over a Cartesian space) and applying the de Rham differential to that.

The kernel of that is the constant sheaf $\flat U(1)$ of example

$$0 \to \flat U(1) \hookrightarrow U(1) \stackrel{\mathbf{d} log}{\longrightarrow} \mathbf{\Omega}^1 \,.$$

Under addition of differential forms, the sheaves $\mathbf{\Omega}^k$ of example becomes abelian sheaves, and we will implicitly understand them this way now.

Definition

Write $\widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet \in Ch(Smooth0Type)$ for the complex of sheaves given by the truncated de Rham complex:

$$\widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet \coloneqq \left[ \mathbb{R} \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^1 \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^2 \stackrel{\mathbf{d}}{\to} \cdots \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^{n+1}_{cl} \right] \,.$$

Proposition

The morphism

$$(\flat \mathbf{B}^{n+1}\mathbb{R})_\bullet \longrightarrow \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet$$

given by the canonical chain map

$$\array{ \flat \mathbb{R} &\stackrel{}{\to}& 0 &\stackrel{}{\to}& 0 &\stackrel{}{\to}& \cdots &\stackrel{}{\to}& 0 \\ \downarrow && \downarrow && \downarrow && \cdots && \downarrow \\ \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^2 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n+1}_{cl} }$$

is a weak equivalence in the sense of def. .

Proof

By the Poincaré lemma. This is the Poincaré Lemma.

Every $A_\bullet \in Ch_+(Smooth0Type)$ serves as the coefficients for an abelian sheaf cohomology theory on smooth manifolds. Abelian sheaf cohomology has a general abstract characterization (see at cohomology) in terms of derived hom-spaces. For definiteness, we recall the model for this construction given by Cech cohomology .

Definition

(Čech complex)

Let $X$ be a smooth manifold and let $A_\bullet \in Ch_+(Smooth0Type)$ be a sheaf of chain complexes. Let $\{U_i \to X\}$ be a good open cover of $X$, i.e. an open cover such that each finite non-empty intersection $U_{i_0, \cdots, i_k}$ is diffeomorphic to an open ball/Cartesian space.

The Čech cochain complex $C^\bullet((X,\{U_i\}),A_\bullet)$ of $X$ with respect to the cover $\{U_i \to X\}$ and with coefficients in $A_\bullet$ is in degree $k \in \mathbb{N}$ given by the abelian group

$$C^((X,\{U_i\}),A_\bullet) \coloneqq \oplus_{{l,n} \atop {k = n-l}} \oplus_{i_0, i_1, \cdots, i_n} A_l(U_{i_0, \cdots, i_n})$$

which is the direct sum of the values of $A_\bullet$ on the given intersections as indicated; and whose differential

$$d \colon C^{k}((X,\{U_i\}),A_\bullet) \longrightarrow C^{k+1}((X,\{U_i\}),A_\bullet)$$

is defined componentwise (see at matrix calculus for conventions on maps between direct sums) by

$$\begin{aligned} (d a)_{i_0, \cdots, i_{k+1}} & \coloneqq (\partial_A + (-1)^k \delta a)_{i_0, \cdots, i_{k+1}} \\ & \coloneqq \partial_A a_{i_0, \cdots, i_{k+1}} + (-1)^k \sum_{0 \leq j \leq k+1} (-1)^{j} a_{i_0, \cdots, i_{j-1}, i_{j+1}, \cdots, i_{k+1}} |_{U_{i_0, \cdots, i_{k+1}}} \end{aligned}$$

where on the right the sum is over all components of $a$ obtained via the canonical restrictions obtained by discarding one of the original $(k+1)$ subscripts.

The Cech cohomology groups of $X$ with coefficients in $A_\bullet$ relative to the given cover are the chain homology groups of the Cech complex

$$H_{Cech}^k((X,\{U_i\}), A_\bullet) \coloneqq H^k(C^\bullet((X,\{U_i\}),A_\bullet)) \,.$$

The Cech cohomology groups as such are the colimit (“directed limit”) of these groups over refinements of covers

$$H^k_{Cech}(X, A_\bullet) \coloneqq \underset{\longrightarrow}{\lim}_{\{U_i \to X\}} H_{Cech}^k((X,\{U_i\}), A_\bullet) \,.$$

Remark

Often Cech cohomology is considered for the case that $A_\bullet$ is concentrated in a single degree, in which case the first term in the sum defining the differential in def. disappears. When $A_\bullet$ is not concentrated in a single degree, then for emphasis one sometimes speaks of hypercohomology. This is the case of relevance for Deligne cohomology.

Remark

The Cech chain complex in def. is the total complex of the double complex whose vertical differential is that of $A_\bullet$ and whose horizontal differential is the Cech differential $\delta$ given by alternating sums over restrictions along patch inclusions

$$\array{ \vdots && \vdots \\ \downarrow^{\mathrlap{\partial_A}} && \downarrow^{\mathrlap{\partial_A}} \\ \oplus_i A_1(U_i) &\stackrel{\delta}{\longrightarrow}& \oplus_{i_1, i_2} A_1(U_{i_1, i_2}) &\stackrel{\delta}{\longrightarrow}& \cdots \\ \downarrow^{\mathrlap{\partial_A}} && \downarrow^{\mathrlap{\partial_A}} \\ \oplus_i A_0(U_i) &\stackrel{\delta}{\longrightarrow}& \oplus_{i_1, i_2} A_0(U_{i_1, i_2}) &\stackrel{\delta}{\longrightarrow}& \cdots }$$

Remark

For analyzing the properties of Deligne cohomology below, all one needs is the following fact about Cech cohomology, which is discussed for instance at infinity-cohesive site:

For $X$ a smooth manifold (in particular paracompact),

1. $X$ admits a good open cover $\{U_i \to X\}$ (by charts $U_i$ all whose finite non-empty intersections are diffeomorphic to an open ball/Cartesian space $\mathbb{R}^n$);

2. for any such good open cover the Cech complex $C^\bullet((X,\{U_i\}),A_\bullet)$ already computes Cech cohomology (i.e. there is no further need to form the colimit of Cech complexes over refinements of covers);

3. the functor $C^\bullet((X,\{U_i\}),-) \colon Ch_+(Smooth0Type) \to Ch_+$ preserves weak equivalences and fibrations.

This means in particular that if $X_\bullet \to Y_\bullet \to Z_\bullet$ is a homotopy fiber sequence in $Ch_+(Smooth0Type)$, then also

$$C^\bullet((X,\{U_i\}), X_\bullet) \to C^\bullet((X,\{U_i\}), Y_\bullet) \to C^\bullet((X,\{U_i\}), Z_\bullet)$$

is a homotopy fiber sequence of chain complexes, and therefore the cohomology groups sit in the long exact sequence in homology of this sequence of chain complexes.

Example

For $A_\bullet = (\mathbf{B}^{n+1}\mathbb{Z})_\bullet$ as in example , then for $X$ a smooth manifold

$$H^0_{Cech}(X, (\mathbf{B}^{n+1}\mathbb{Z})_\bullet) \simeq H^{n+1}(X,\mathbb{Z})$$

is the ordinary cohomology of $X$ with integer coefficients, the cohomology which is also computed as the singular cohomology of the underlying topological space of $X$.

Similarly for $A_\bullet = (\flat \mathbf{B}^n U(1))_{\bullet}$ then

$$H^0_{Cech}(X, (\flat\mathbf{B}^{n}U(1))_\bullet) \simeq H^{n}(X,U(1))$$

is the ordinary cohomology of $X$ with circle group coefficients, the cohomology which is also computed as the singular cohomology of the underlying topological space of $X$ with $U(1)$-coefficients.

Example

Passing to abelian sheaf cohomology (e.g. via def. ), then prop. is the de Rham theorem.

We will have need to give names to truncations of the de Rham complex. One is this:

Definition

For $n \in \mathbb{N}$ write

$$\mathbf{\Omega}^{\bullet \leq n} \in Ch_+(Smooth0Type)$$

for the chain complex of the form

$$\mathbf{\Omega}^{\bullet \leq n} \coloneqq \left[ \mathbb{R} \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^1 \stackrel{\mathbf{d}}{\to} \cdots \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^{n-1} \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^{n} \right]$$

with all $n$-forms, not just the closed ones, in degree 0.

Example

The abelian sheaf cohomology of the truncated de Rham complex in def. is $\Omega^n(X)/im(\mathbf{d})$.

The Deligne complex

Definition

For $n \in \mathbb{N}$ the smooth Deligne complex of degree $n$

$$(\mathbf{B}^n U(1)_{conn})_\bullet \in Ch_+(Smooth0Type)$$

is the chain complex of abelian sheaves given by

$$(\mathbf{B}^n U(1)_{conn})_\bullet \; \coloneqq \; \left[ U(1) \stackrel{\mathbf{d} log}{\to} \mathbf{\Omega}^1 \stackrel{\mathbf{d}}{\to} \cdots \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^n \right]$$

with $U(1)$ in degree $n$ and with the differentials as in def. and example .

We write

$$H^{n+1}_{conn}(X,\mathbb{Z}) \coloneqq H^0_{Cech}(X,(\mathbf{B}^n U(1)_{conn})_\bullet)$$

for its abelian sheaf cohomology.

Remark

By example the obvious chain map

$$\array{ \mathbb{Z} &\hookrightarrow& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \downarrow && \downarrow^{\mathrlap{(-)/\mathbb{Z}}} && \downarrow^{id} && && \downarrow^{id} \\ 0 &\to& U(1) &\stackrel{\mathbf{d} log}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n }$$

is a weak equivalence, def. , and one could define the top chain complex here as “the” Deligne complex, just as well. In the context of homotopy theory/homological algebra, all that matters is the complex up to zig-zags of weak equivalences.

Remark

In def. the de Rham complex is truncated to the right by discarding what would be the next differentials, without passing to their kernel, i.e. in degree 0 the Deligne complex has all differential $n$-forms, not just the closed $n$-fomrs. This simple point is the key aspect of the Deligne complex. If one instead truncates while preserving the chain homology in the lowest degree, then one obtains the following complex with the sheaf $\mathbf{\Omega}^{n}_{cl}$ of closed forms in lowest degree, which gives ordinary cohomology.

Definition

For $n \in \mathbb{N}$ the flat smooth Deligne complex of degree $n$

$$(\mathbf{B}^n U(1)_{flat})_\bullet \in Ch_+(Smooth0Type)$$

is the chain complex of abelian sheaves given by

$$(\mathbf{B}^n U(1)_{flat})_\bullet \; \coloneqq \; \left[ U(1) \stackrel{\mathbf{d} log}{\to} \mathbf{\Omega}^1 \stackrel{\mathbf{d}}{\to} \cdots \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^n_{cl} \right]$$

with $U(1)$ in degree $n$ and with the differentials as in def. and example , and with the closed $n$-forms on the right.

Definition

Write

$$(\flat\mathbf{B}^n U(1))_\bullet = (\mathbf{B}^n \flat U(1))_\bullet \in Ch_+(Smooth0Type)$$

for the chain complex of abelian sheaves given by

$$(\flat \mathbf{B}^n U(1))_\bullet \; \coloneqq \; \left[ \flat U(1) \stackrel{}{\to} 0 \stackrel{}{\to} \cdots \stackrel{}{\to} 0 \right]$$

with the constant sheaf $\flat U(1)$ of example in degree $n$.

Proposition

For $(\flat \mathbf{B}^n U(1))_\bullet$ as in def. , then the morphism

$$(\flat \mathbf{B}^n U(1))_\bullet \stackrel{\simeq}{\longrightarrow} (\mathbf{B}^n U(1)_{flat})_\bullet$$

given by the chain map

$$\array{ \flat U(1) &\to& 0 &\to& \cdots &\to& 0 \\ \downarrow && \downarrow && \cdots && \downarrow \\ U(1) &\stackrel{\mathbf{d}log}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n_{cl} }$$

(with the vertical morphism on the left being the inclusion of example ) is a weak equivalence, def. .

Proof

By the Poincaré lemma, this is just an immediate variant of prop. .

The Cech-Deligne complex

The Cech complex, def. , for Deligne cohomology of degree $(p+2)$ is the total complex of a double complex of the following form

$$\array{ 0 &\longrightarrow& 0 &\longrightarrow& 0 &\longrightarrow & \cdots \\ \uparrow && \uparrow && \uparrow \\ \Omega^{p+1}(\coprod_i U_i) &\stackrel{\delta}{\longrightarrow}& \Omega^{p+1}(\coprod_{i,j} U_{ i j}) &\stackrel{\delta}{\longrightarrow}& \Omega^{p+1}(\coprod_{i,j, k} U_{i j k}) &\stackrel{\delta}{\longrightarrow}& \cdots \\ \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \\ \vdots && \vdots && \vdots \\ \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \\ \Omega^2(\coprod_i U_i) &\stackrel{\delta}{\longrightarrow}& \Omega^2(\coprod_{i,j} U_{ i j}) &\stackrel{\delta}{\longrightarrow}& \Omega^2(\coprod_{i,j, k} U_{i j k}) &\stackrel{\delta}{\longrightarrow}& \cdots \\ \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \uparrow^{\mathrlap{d}} && \\ \Omega^1(\coprod_i U_i) &\stackrel{\delta}{\longrightarrow}& \Omega^1(\coprod_{i,j} U_{ i j}) &\stackrel{\delta}{\longrightarrow}& \Omega^1(\coprod_{i,j, k} U_{i j k}) &\stackrel{\delta}{\longrightarrow}& \cdots \\ \uparrow^{\mathrlap{d log}} && \uparrow^{\mathrlap{d log}} && \uparrow^{\mathrlap{d log}} && \\ C^\infty(\coprod_i U_i, U(1)) &\stackrel{\delta}{\longrightarrow}& C^\infty(\coprod_{i,j} U_{i j}, U(1)) &\stackrel{\delta}{\longrightarrow}& C^\infty(\coprod_{i,j,k} U_{i j k}, U(1)) &\stackrel{\delta}{\longrightarrow}& \cdots }$$

where vertically we have the de Rham differential and horizontally the Cech differential given by alternating sums of pullback of differential forms.

The corresponding total complex has in degree $n$ the direct sum of the entries in this double complex which are on the $n$th nw-se off-diagonal and has the total differential

$$D = d + (-1)^{deg} \delta$$

with $deg$ denoting form degree.

Example

A Cech-Deligne cocycle in degree $3$ (“bundle gerbe with connection”) is data $(\{B_{i}\}, \{A_{i j}\}, \{g_{i j k}\})$ such that

$$\array{ \{B_i\} &\stackrel{\delta}{\longrightarrow}& {{\{B_j - B_i\}} = {d A_{i j}}} && && \\ && \uparrow^{\mathrlap{d}} && && \\ && \{A_{i j}\} &\stackrel{\delta}{\longrightarrow}& \{-A_{ j k} + A_{i k} - A_{i j}\} = \{d log g_{i j k}\} && \\ && && \uparrow^{\mathrlap{d log}} && \\ && && \{g_{i j k}\} &\stackrel{\delta}{\longrightarrow}& \{g_{j k l} g_{i k l}^{-1} g_{i j l} g_{i j k}^{-1} \} = 1 }$$

Curvature and characteristic classes

We discuss the construction of two canonical morphisms out of Deligne cohomology, and two canonical morphisms into it. Below these are shown to form two interlocking exact sequences and in fact an exact differential cohomology hexagon which accurately characterizes Deligne cohomology as the differential cohomology extension of integral ordinary cohomology by differential forms.

Throughout, for ease of notation, we assume $n \in \mathbb{N}$ to be positive,

$$n \geq 1 \,.$$

The remaining case $n = 0$ describes “circle 0-bundles with connection”, which are just $U(1)$-valued functions, and is hence essentially trivial in itself.

In the following $X$ is any smooth manifold.

Definition

Let $(\mathbf{B}^{n+1}\mathbb{Z})_\bullet \in Ch_+(Smooth0Type)$ be as in example . Write

$$(\mathbf{B}^n U(1)_{conn})_{\bullet} \stackrel{\simeq}{\longleftarrow} \stackrel{DD_\bullet}{\longrightarrow} (\mathbf{B}^{n+1} \mathbb{Z})_{\bullet}$$

for the zig-zag of chain complexes where the left weak equivalence is that of remark , i.e. for the chain maps given by

$$\array{ \mathbb{Z} &\to& 0 &\to& 0 &\to& \cdots &\to& 0 \\ \uparrow^{\mathrlap{id}} && \uparrow^{\mathrlap{}} && \uparrow^{\mathrlap{}} && \cdots && \uparrow^{\mathrlap{}} \\ \mathbb{Z} &\hookrightarrow& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \downarrow^{\mathrlap{0}} && \downarrow^{\mathrlap{mod\,\mathbb{Z}}} && \downarrow^{\mathrlap{id}} && \cdots && \downarrow^{\mathrlap{id}} \\ 0 &\to& U(1) &\stackrel{\mathbf{d} log}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n }$$

Passing to abelian sheaf cohomology this gives, by def. and example , a morphism

$$\array{ H^{n+1}_{conn}(X,\mathbb{Z}) \\ & \searrow^{\mathrlap{DD}} \\ && H^{n+1}(X,\mathbb{Z}) }$$

from Deligne cohomology to ordinary cohomology with integer coefficients in degree $n+1$.

For $[\nabla] \in H^{n+1}_{conn}(X,\mathbb{Z})$ we call $DD(\nabla) \in H^{n+1}(X,\mathbb{Z})$
the Dixmier-Douady class of the underlying circle n-bundle.

Definition

Write

$$(F_{(-)})_\bullet \colon (\mathbf{B}^n U(1)_{conn})_\bullet \stackrel{}{\longrightarrow} \mathbf{\Omega}^{n+1}_{cl}$$

for the morphism given by the chain map which is just the de Rham differential in degree 0

$$\array{ 0 &\to& U(1) &\stackrel{\mathbf{d} log}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \downarrow && \downarrow && \downarrow && \cdots && \downarrow^{\mathrlap{}} && \downarrow^{\mathrlap{\mathbf{d}}} \\ 0 &\to& 0 &\to& 0 &\to& \cdots &\stackrel{}{\to}& 0 &\to& \mathbf{\Omega}^{n+1}_{cl} }$$

Passing to abelian sheaf cohomology this gives a morphism of the form

$$\array{ && \Omega^{n+1}_{cl}(X) \\ & {}^{\mathllap{F_{(-)}}}\nearrow \\ H^{n+1}_{conn}(X,\mathbb{Z}) }$$

We call this the curvature map, i.e. for $[\nabla] \in H^{n+1}_{conn}(X,\mathbb{Z})$ the class of a Deligne cocycle, we call

$$F_\nabla \in \Omega^{n+1}_{cl}(X)$$

its curvature form.

Definition

Consider the zig-zag

$$\mathbf{\Omega}^{\bullet\leq n} \stackrel{}{\longrightarrow} (\mathbf{B}^n U(1)_{conn})_\bullet$$

out of the complex of def. , given by the chain maps

$$\array{ 0 &\to& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \downarrow && \downarrow && \downarrow^{\mathrm{id}} && \cdots && \downarrow^{\mathrm{id}} && \downarrow^{\mathrm{id}} \\ \mathbb{Z} &\hookrightarrow& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \downarrow && \downarrow && \downarrow^{\mathrm{id}} && \cdots && \downarrow^{\mathrm{id}} && \downarrow^{\mathrm{id}} \\ 0 &\to& U(1) &\stackrel{\mathbf{d}log}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n }$$

where the bottom quasi-isomorphism is from remark .

On passing to abelian sheaf cohomology this gives, by example , a morphism

$$\array{ \Omega^n(X)/im(\mathbf{d}) \\ & \searrow \\ && H^{n+1}_{conn}(X,\mathbb{Z}) }$$

Definition

Consider the canonical morphism

$$(\flat \mathbf{B}^n U(1))_\bullet \stackrel{\simeq}{\longrightarrow} (\mathbf{B}^n U(1)_{flat})_\bullet \longrightarrow (\mathbf{B}^n U(1)_{conn})_\bullet$$

via def. , prop. .

Passing to abelian sheaf cohomology this induces a morphism

$$\array{ && H^{n+1}_{conn}(X,\mathbb{Z}) \\ & \nearrow \\ H^n(X,U(1)) }$$

We call this map the inclusion of the flat infinity-connections into all circle n-connections.

Combining what we have so far:

Proposition

The composite of the morphisms of def. and of the curvature morphism of def.

$$\array{ \Omega^n(X)/im(\mathbf{d}) && \stackrel{\mathbf{d}}{\longrightarrow} && \Omega^{n+1}_{cl}(X) \\ & \searrow && \nearrow_{\mathrlap{F_{(-)}}} \\ && H^{n+1}_{conn}(X,\mathbb{Z}) }$$

is given by the de Rham differential $\mathbf{d}$ on differential forms.

The composite of the morphisms of def. and def. is the Bockstein homomorphism:

$$\array{ && H^{n+1}_{conn}(X,\mathbb{Z}) \\ & \nearrow && \searrow^{\mathrlap{DD}} \\ H^n(X,U(1)) && \underset{\beta}{\longrightarrow} && H^{n+1}(X,\mathbb{Z}) }$$

Proof

By composing the defining zig-zags of chain maps the statement is immediate.

The Chern character

While the explicit definition of the Deligne complex in def. is easy enough, all its good abstract properties are best understood by realizing that it is the homotopy fiber product of a kind of higher abelian Chern character map with the closed differential forms $\mathbf{\Omega}^{n+1}_{cl}$. This is the content of prop. below.

Definition

Write

$$ch_\bullet \colon (\mathbf{B}^{n+1}\mathbb{Z})_\bullet \longrightarrow (\flat \mathbf{B}^{n+1}\mathbb{R})_\bullet$$

for the morphism given as the composite

$$(\mathbf{B}^{n+1}\mathbb{Z})_\bullet = (\flat \mathbf{B}^{n+1}\mathbb{Z})_\bullet \longrightarrow (\flat \mathbf{B}^{n+1}\mathbb{R})_\bullet$$

where the second morphism is induced by the canonical inclusion $\mathbb{Z} \hookrightarrow \mathbb{R}$.

Passing to abelian sheaf cohomology this induces a morphism

$$\array{ && H^{n+1}(X,\mathbb{R}) \\ & \nearrow_{\mathrlap{ch}} \\ H^{n+1}(X,\mathbb{Z}) }$$

Remark

The morphism in def. is just the traditional map from ordinary cohomology with integer coefficients to that with real numbers coefficients given for instance via singular cohomology simply by forming the tensor product of abelian groups with $\mathbb{R}$. In the broader context of differential cohomology however it is useful to think of this map as the Chern character, whence the notation.

While this is easy enough to construct in itself, the underlying chain map here is not a fibration in the sense of def. (having as non-trivial component the inclusion $\mathbb{Z} \hookrightarrow \mathbb{R}$, which is evidently not an epimorphism). But the homotopy fiber of this map plays a crucial role in the theory, and so in view of remark we consider now a fibration resolution of this map

Definition

Consider the chain complex

$$\widehat {(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet \coloneqq \left[ \array{ \mathbb{Z} &\stackrel{}{\hookrightarrow}& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \oplus &\nearrow_{-\mathrlap{id}}& \oplus &\nearrow_{+\mathrlap{id}}& \oplus &\nearrow_{-\mathrlap{id}}& \cdots &\nearrow_{\pm\mathrlap{id}}& \oplus &\nearrow_{\mp\mathrlap{id}}& \\ \mathbb{R} &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^2 &\underset{\mathbf{d}}{\to}& \cdots &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^{n} && } \right]$$

where we use matrix calculus-notation as for mapping cones (see at mapping cone – Examples – In chain complexes).

Write

$$\widehat {(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet \stackrel{\widehat{ch}_\bullet}{\longrightarrow} \widehat {(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet$$

for the morphism to the chain complex of def. which is given by the chain map that in positive degree projects onto the lower row in the above direct sum expression and in degree 0 is given by the de Rham differential:

$$\array{ \mathbb{Z} &\stackrel{}{\hookrightarrow}& \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n-1} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^n \\ \oplus &\nearrow_{-\mathrlap{id}}& \oplus &\nearrow_{+\mathrlap{id}}& \oplus &\nearrow_{-\mathrlap{id}}& \cdots &\nearrow_{\pm\mathrlap{id}}& \oplus &\nearrow_{\mp\mathrlap{id}}& \\ \mathbb{R} &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^2 &\underset{\mathbf{d}}{\to}& \cdots &\underset{\mathbf{d}}{\to}& \mathbf{\Omega}^{n} && \\ \downarrow && \downarrow && \downarrow && \cdots && \downarrow && \downarrow^{\mathrlap{\mathbf{d}}} \\ \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^2 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n+1} }$$

Finally write

$$(\mathbf{B}^{n+1}\mathbb{Z})_\bullet \longrightarrow \widehat {(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet$$

for the morphism given by the chain map which in degree $n+1$ is given by

$$\mathbb{Z} \to \mathbb{Z} \oplus \mathbb{R}$$

$$n \mapsto (n,n) \,.$$

Lemma

The construction in def. gives a fibration resolution of the Chern character morphism of def. in that it gives a commuting diagram of chain maps

$$\array{ (\mathbf{B}^{n+1}\mathbb{Z})_\bullet &\stackrel{ch_\bullet}{\longrightarrow}& (\flat\mathbf{B}^{n+1} \mathbb{R})_\bullet \\ \downarrow^{\mathrlap{\simeq}} && \downarrow^{\mathrlap{\simeq}} \\ \widehat {(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet &\stackrel{\widehat {ch}_\bullet}{\longrightarrow}& \widehat {(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet }$$

with, on the right, the weak equivalence of prop. , where

1. the left vertical morphism is a weak equivalence;

2. the bottom horizontal morphism $\widehat{ch}_\bullet$ is a fibration.

Proof

That the diagram commutes is a straightforward inspection, unwinding the definitions. That $\widehat{ch}_\bullet$ is a fibration according to def. is by its very construction, being a projection in positive degree. That the left morphism is a weak equivalence comes down to the Poincaré lemma, in a slight variant of the simple argument that proves prop. .

Definition

Write

$$\mathbf{\Omega}^{n+1}_{cl} \stackrel{\iota_\bullet}{\longrightarrow} \widehat{\flat \mathbf{B}^{n+1}\mathbb{R}}_\bullet \stackrel{\simeq}{\longleftarrow} (\flat \mathbf{B}^{n+1}\mathbb{R})_\bullet$$

for the zig-zag whose right morphism is the weak equivalence of prop. and whose left morphism is given by the chain map

$$\array{ 0 &\to& 0 &\to& \cdots &\to& 0 &\to& \mathbf{\Omega}^{n+1}_{cl} \\ \downarrow && \downarrow && \vdots && \downarrow && \downarrow^{\mathrlap{id}} \\ \mathbb{R} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^1 &\stackrel{\mathbf{d}}{\to}& \cdots &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n} &\stackrel{\mathbf{d}}{\to}& \mathbf{\Omega}^{n+1}_{cl} } \,.$$

Proposition

The chain maps

• $(F_{(-)})_\bullet$, def. ;

• $DD_\bullet$, def. ;

• $\iota_\bullet$, def.

• $\widehat{ch}_\bullet$, def.

fit into a commuting diagram

$$\array{ && \mathbf{\Omega}^{n+1}_{cl} \\ & {}^{\mathllap{(F_{(-)})_\bullet }}\nearrow && \searrow^{\mathrlap{\iota_\bullet}} \\ (\mathbf{B}^n U(1)_{conn})_\bullet && && \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet \\ & {}_{\mathllap{DD_\bullet}}\searrow && \nearrow_{\mathrlap{\widehat{ch}_\bullet}} \\ && \widehat{(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet }$$

which is a pullback diagram in $Ch_+(Smooth0Type)$. This exhibits the Deligne complex $(\mathbf{B}^n U(1)_{conn})_\bullet$ as the homotopy pullback of the inclusion of $\mathbf{\Omega}^{n+1}_{cl}$ along the Chern character map $ch_\bullet$.

Proof

The first statement follows straightforwardly by inspection, using that pullbacks of chain complexes are computed componentwise. From this the second statement follows then since by $\widehat{ch}_\bullet$ is a fibration resolution of $ch_\bullet$.

The exact sequences for curvature and characteristic classes

Definition

Write

$$\Omega^{n+1}_{int}(X) \hookrightarrow \Omega^{n+1}_{cl}(X)$$

for the inclusion of those closed differential forms whose periods (integration over $(n+1)$-cycles) takes values in the integers.

Proposition

The image of the curvature map $F_{(-)} \colon H^{n+1}_{conn}(X,\mathbb{Z}) \longrightarrow \Omega^{n+1}_{cl}(X)$ of def. are the integral forms of def. .

Proof

The homotopy pullback characterization of of prop. implies that the image consists of precisely those closed differential forms which under the de Rham theorem, remark , represent real cohomology classes that are in the image of integral cohomology classes. These are the differential forms with integral periods.

Proposition

(curvature exact sequence)

The Deligne cohomology group fits into a short exact sequence (of abelian groups) of the form

$$0 \to H^n(X,U(1)) \longrightarrow H^{n+1}_{conn}(X,\mathbb{Z}) \stackrel{F_{(-)}}{\longrightarrow} \Omega^{n+1}_{int}(X) \to 0$$

where $F_{(-)}$ is the curvature map of def. .

Proof

By prop. the morphism on the right is indeed an epimorphism. It remains to determine its kernel.

To that end, consider the pasting diagram of homotopy pullbacks obtained form the homotopy pullback in prop. . Using the pasting law and the fact that the loop space object of a 0-truncated object such as $\mathbf{\Omega}^{n+1}_{cl}$ is trivial, this is of the form

$$\array{ 0 &\longrightarrow& \widehat{(\flat \mathbf{B}^{n}(\mathbb{R}/\mathbb{Z}))}_\bullet &\longrightarrow& 0 \\ \downarrow && \downarrow && \downarrow^{} \\ 0 &\longrightarrow& (\mathbf{B}^n U(1)_{conn})_\bullet &\stackrel{(F_{(-)}_\bullet)}{\longrightarrow}& \mathbf{\Omega}^{n+1}_{cl} \\ && \downarrow^{\mathrlap{DD_\bullet}} && \downarrow^{\mathrlap{\iota_\bullet}} \\ && \widehat{(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet &\stackrel{\widehat{ch}_\bullet}{\longrightarrow}& \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet } \,.$$

Passing to abelian sheaf cohomology and applying the induced long exact sequence in homology, in view of remark , implies the claim.

Proposition

(characteristic class exact sequence)

The Deligne cohomology group fits into a short exact sequence (of abelian groups) of the form

$$0 \to \Omega^{n}(X)/\Omega^n_{int}(X) \stackrel{}{\longrightarrow} H^{n+1}_{conn}(X,\mathbb{Z}) \stackrel{DD}{\longrightarrow} H^{n+1}(X,\mathbb{Z}) \to 0$$

where $DD$ is the characteristic class map of def. .

Proof

The chain map that represents the Dixmier-Douady class by def. is manifestly a fibration in the sense of def. . Therefore its ordinary fiber is already its homotopy fiber. That ordinary fiber is evidently the domain of the morphism constructed in def. , in its second weakly equivalent incarnation as displayed there.

Therefore the long exact sequence in homology, induced by the chain map $DD_\bullet$ under passage to abelian sheaf cohomology (in view of remark ) goes as

$$\cdots \longrightarrow H^n(X,\mathbb{Z}) \longrightarrow \Omega^n(X)/im(\mathbf{d}) \stackrel{}{\longrightarrow} H^{n+1}_{conn}(X,\mathbb{Z}) \stackrel{DD}{\longrightarrow} H^{n+1}(X,\mathbb{Z})$$

As in the proof of prop. it follows that the rightmost morphism is an epimorphism. Hence we get a short exact sequence by dividing out the image of $H^n(X,\mathbb{Z})$ in $\Omega^n/im(\mathbf{d})$. That image is $\Omega^n_{int}(X)$. Since this image contains $im(\mathbf{d})$ (as the closed differential forms all whose periods are $0 \in \mathbb{N}$ ) the resulting quotient is $\Omega^n(X)/Omega^n_{int}(X)$ and the claim follows.

Remark

In words the statement of prop. and prop. is that Deligne cohomology groups $H^{n+1}_{conn}(X,\mathbb{Z})$ constitute a group extension

1. of integral ordinary cohomology by differential $n$-forms modulo integral forms;

2. of closed $(n+1)$-forms by $U(1)$-valued ordinary cohomology.

The first statement is what gives the name to “differential cohomology” as it makes precise how $H^{n+1}_{conn}(X)$ is a combination of ordinary integral cohomology with differential form-data.

The second statement is secretly of the same flavor, if maybe not as manifestly so: the $U(1)$-valued ordinary cohomology is really what classifies flat circle n-connections on circle n-group principal infinity-bundles (either, depending on perspective, by def. or else, more intrinsically, by the very statement of prop. ) and hence again describes a combination of underlying bundles with differential form data.

The exact differential cohomology hexagon

Summing up, the homotopy pullback square of prop. together with the maps of prop. form a commuting diagram in $Ch_+(Smooth0Type)$ of the form.

$$\array{ \mathbf{\Omega}^{\bullet \leq n} && \stackrel{\mathbf{d}}{\longrightarrow} && \mathbf{\Omega}^{n+1}_{cl} \\ &\searrow& & {}^{\mathllap{(F_{(-)})_\bullet }}\nearrow && \searrow^{\mathrlap{\iota_\bullet}} \\ && (\mathbf{B}^n U(1)_{conn})_\bullet && && \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet \\ &\nearrow& & {}_{\mathllap{DD_\bullet}}\searrow && \nearrow_{\mathrlap{\widehat{ch}_\bullet}} \\ \widehat{(\flat \mathbf{B}^n U(1))}_\bullet && \underset{\beta}{\longrightarrow} && \widehat{(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet }$$

Proposition

This extends to a diagram in $Ch_+(Smooth0Type)$ of the form

$$\array{ && \mathbf{\Omega}^{\bullet \leq n} && \stackrel{\mathbf{d}}{\longrightarrow} && \mathbf{\Omega}^{n+1}_{cl} \\ & \nearrow& &\searrow& & {}^{\mathllap{(F_{(-)})_\bullet }}\nearrow && \searrow^{\mathrlap{\iota_\bullet}} \\ \widehat{(\flat \mathbf{B}^n \mathbb{R})}_{\bullet} && && (\mathbf{B}^n U(1)_{conn})_\bullet && && \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet \\ &\searrow & &\nearrow& & {}_{\mathllap{DD_\bullet}}\searrow && \nearrow_{\mathrlap{\widehat{ch}_\bullet}} \\ && \widehat{(\flat \mathbf{B}^n U(1))}_\bullet && \underset{\beta}{\longrightarrow} && \widehat{(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet }$$

such that

1. both square are homotopy pullback squares;

2. both diagonals are homotopy fiber sequences;

3. the two outer sequences are long homotopy fiber sequences.

Proof

For the first statement consider the pasting of homotopy pullback diagrams as in the proof of prop. , now extended to the left, via the pasting law, as

$$\array{ \widehat{(\flat \mathbf{B}^n \mathbb{R})}_\bullet &\longrightarrow& \widehat{(\flat \mathbf{B}^{n}(\mathbb{R}/\mathbb{Z}))}_\bullet &\longrightarrow& 0 \\ \downarrow && \downarrow && \downarrow^{} \\ \mathbf{\Omega}^{\bullet \leq n} &\longrightarrow& (\mathbf{B}^n U(1)_{conn})_\bullet &\stackrel{(F_{(-)}_\bullet)}{\longrightarrow}& \mathbf{\Omega}^{n+1}_{cl} \\ && \downarrow^{\mathrlap{DD_\bullet}} && \downarrow^{\mathrlap{\iota_\bullet}} \\ && \widehat{(\mathbf{B}^{n+1}\mathbb{Z})}_\bullet &\stackrel{\widehat{ch}_\bullet}{\longrightarrow}& \widehat{(\flat \mathbf{B}^{n+1}\mathbb{R})}_\bullet } \,.$$

That the NE-diagonal is a homotopy fiber sequence is the statement in the proof of prop. . That the SE-diagonal is a homotopy fiber sequence follows by inspection as remarked in the proof of prop. .

From this the last statement now is implied by using the pasting law yet once more, as show in the proof here.

Remark

The form of the exact hexagon characterizing the Deligne complex via prop. is in fact a general abstract consequence of the fact that all universal constructions in $Ch_+(Smooth0Type)$ considered here indeed may be understood as taking place in the cohesive homotopy theory of smooth ∞-groupoids, via remark . This is discussed in detail at differential cohomology hexagon.

Semantic Layer

We give now a general abstract formulation of principal connections in any cohesive (∞,1)-topos $\mathbf{H}$. The structures discussed above turn out to be models for these axioms.

Throughout, let $\mathbb{G}$ be a braided ∞-group in the given cohesive (∞,1)-topos, write $\mathbf{B}\mathbb{G}$ for its delooping and assume that this is precisely n-truncated. Write $\mathbf{B}^2 \mathbb{G}$ for the second delooping, given by the braidedness.

Write $\flat$ for the flat modality and recall the homotopy fiber sequence of its counit

$$\mathbf{B}\mathbb{G} \stackrel{\theta_{\mathbf{B}\mathbb{G}}}{\longrightarrow} \flat_{dR} \mathbf{B}^2 \mathbb{G} \stackrel{}{\longrightarrow} \flat \mathbf{B}^2\mathbb{G} \longrightarrow \mathbf{B}^2 \mathbb{G}$$

where $\theta_{\mathbf{B}\mathbb{G}}$ is the Maurer-Cartan form of the group $\mathbf{B}\mathbb{G}$. This will appear as the curvature characteristic form for $\mathbb{G}$-principal ∞-bundles now.

Definition

A $\mathbb{G}$-Hodge filtration is a choice of filtration of $\flat \mathbf{B}^2 \mathbb{G}$ (using the flat modality) such that the first stage $F^1 \flat \mathbf{B}^2 \mathbb{G}\longrightarrow F^0 \flat \mathbf{B}^2\mathbb{G}$ is equivalent to the canonical morphism $\flat_{dR} \mathbf{B}\mathbb{G} \longrightarrow \flat \mathbf{B}^2 \mathbb{G}$.

For the $n+1$st stage of the filtration we write

$$\Omega^2_{cl}(-,\mathbb{G}) \coloneqq F^{n+1}\flat\mathbf{B}^2 \mathbb{G} \,.$$

Example

Consider $\mathbf{H} =$ Smooth∞Grpd and for $n \gt 1$ consider $\mathbb{G} \coloneqq \mathbf{B}^{n-1} U(1)$ the circle n-group. Then (as in prop. )

$$\flat \mathbf{B}^2\mathbb{G} = \flat \mathbf{B}{n+1} U(1) \simeq L DK \left[ U(1) \stackrel{\mathbf{d}log}{\to} \mathbf{\Omega}^1 \stackrel{\mathbf{d}}{\to} \cdots \stackrel{\mathbf{d}}{\to} \mathbf{\Omega}^{n+1}_{cl} \right] \,,$$

where on the right we have the image of the truncated de Rham complex/flat smooth Deligne complex, def. , under the Dold-Kan correspondence $DK$ followed by ∞-stackification $L$.

The traditional Hodge filtration provides a filtration on this, in the general abstract sense of def. , whence the terminology.

Definition

Given a $\mathbb{G}$-Hodge filtration, def. , write $\mathbf{B}\mathbb{G}_{conn^\bullet}$ for the induced cofiltration given by homotopy pullback of the Maurer-Cartan form $\theta_{\mathbf{B}\mathbb{G}} \longrightarrow \flat_{dR}\mathbf{B}^2 \mathbb{G}$ along the Hodge filtration, i.e. the left sequence in the following pasting diagram of homotopy pullbacks

$$\array{ \flat \mathbf{B}\mathbb{G} &\longrightarrow& \ast & = & \{0\} \\ \downarrow && \downarrow \\ \mathbf{B}\mathbb{G}_{conn} &\longrightarrow& F^{n+1}\flat \mathbf{B}^2\mathbb{G} & \simeq & \Omega^2_{cl}(-,\mathbb{G}) \\ \downarrow && \downarrow \\ \vdots && \vdots \\ \downarrow && \downarrow \\ \mathbf{B}\mathbb{G}_{conn^2} &\longrightarrow& F^3 \flat \mathbf{B}^2 \mathbb{G} \\ \downarrow && \downarrow \\ \mathbf{B}\mathbb{G}_{conn^1} &\longrightarrow& F^2 \flat \mathbf{B}^2 \mathbb{G} \\ \downarrow && \downarrow \\ \mathbf{B}\mathbb{G} &\stackrel{\theta_{\mathbf{B}\mathbb{G}}}{\longrightarrow}& F^1 \flat \mathbf{B}^2 \mathbb{G} &\simeq& \flat_{dR} \mathbf{B}^2 \mathbb{G} }$$

We say that $\mathbf{B}\mathbb{G}_{conn}$ here is the coefficient object for $\mathbb{G}$-differential cohomology (with the given choice of Hodge filtration understood).

Proposition

With the choices as in example , then the general abstract def. of $\mathbb{G}$-differential coefficient object is represented by the smooth Deligne complex, def. :

$$\mathbf{B}^n U(1)_{conn} \simeq L DK (\mathbf{B}^n U(1)_{conn})_\bullet$$

where $L DK$ denotes the Dold-Kan correspondence followed by ∞-stackification.

Syntactic Layer

under construction

The dependent curvature type

The universal curvature characteristic, def. , has the syntax

$$\vdash curv_{G} \colon \mathbf{B}G \to \flat_{dR} \mathbf{B}^2 G \,.$$

Regarded as a dependent type in the de Rham coefficient context this is

$$\omega \colon \flat_{dR}\mathbf{B}^2 G \; \vdash \; \underset{\mathbf{c} \colon \mathbf{B}G}{\sum} \left( curv_G\left(\mathbf{c}\right) \simeq \omega \right) \colon Type$$

Therefore the syntax for a domain object $F \colon X \to \flat_{dR} \mathbf{B}^2 G$ in this context is

$$\omega \colon \flat_{dR} \mathbf{B}^2 G \;\vdash\; \underset{x \colon X}{\sum} \left( F_x \simeq \omega \right) \colon Type$$

and the syntax for a cocycle

$$\array{ X &&\stackrel{\bar P}{\to}&& \mathbf{B}G \\ & {}_{\mathllap{F}}\searrow &\swArrow_{\nabla}& \swarrow_{\mathrlap{curv_G}} \\ && \flat_{dR} \mathbf{B}^2 G }$$

in differential cohomology, def. , on $(X,F)$ is hence

$$\vdash \; (\bar P,\nabla) \colon \underset{\omega \colon \flat_{dR} \mathbf{B}^2 G}{\prod} \left( \left( \underset{x \colon X}{\sum} \left( F_x \simeq \omega \right) \right) \to \left( \underset{\mathbf{c} \colon \mathbf{B}G}{\sum} \left( curv_G(\mathbf{c}) \simeq \omega \right) \right) \right)$$

Fixed curvature twists

$$\begin{aligned} (\mathbf{B}^n \mathbb{G} \colon Type)_{conn} \colon & Type \\ \coloneqq & \sum_{\mathbf{c} \colon \mathbf{B}\mathbb{G}} \sum_{\omega \colon \Omega^{n+1}_{cl}} \left( curv(\mathbf{c}) = \omega \right) \end{aligned}$$