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Deligne cohomology

Idea

Deligne cohomology is an abelian sheaf cohomology that models ordinary differential cohomology.

The standard Deligne complex (of abelian sheaves) is under the Dold-Kan correspondence the sheaf of n-groupoids of smooth n-functors from the path n-groupoid to the $n$-fold delooping $B^{n}U(1)$:

$$\mathbb{Z}(n+1{)}_D^{\mathrm{\infty }}\simeq {\overline{B}}^{n}U(1)\stackrel{N}{{\to }^{\simeq }}[P_n(-),B^{n}U(1)].$$\mathbb{Z}(n+1)_D^\infty \simeq \bar \mathbf{B}^n U(1) \stackrel{N}{\to^\simeq} [P_n(-), \mathbf{B}^n U(1)] \,.

Smooth Deligne cohomology in degree $n$, of a smooth space $X$ is cohomology with coefficients in ${\overline{B}}^{n}U(1)$.

$$\mathrm{Deligne}\mathrm{cohomology}=H(X,{\overline{B}}^{n}U(1)).$$Deligne cohomology = H(X, \bar \mathbf{B}^n U(1)) \,.

Here the notation on the right is as at the end of motivation for sheaves, cohomology and higher stacks.

This is a realization of the differential refinement (or smooth extension) ${\overline{H}}^n(X,\mathbb{Z})$ of the integral cohomology $H^n(X,\mathbb{Z})$ of $X$ in terms of abelian sheaf cohomology.

Recall that analgous to how $H^n(X,\mathbb{Z})$ classifies line $(n-1)$-bundles and equivalently line $(n-2)$-gerbes on $X$, ${\overline{H}}^n(X,\mathbb{Z})$ classifies line $(n-2)$-gerbes with connection.

Accordingly, the Deligne complex of sheaves $\mathbb{Z}(n{)}_D^{\mathrm{\infty }}$ is a complex of sheaves of differential forms.

Definition

Definition

For $k\in \mathbb{N}$ write ${\Omega }^k(-):U\mapsto {\Omega }^k(U)$ for the sheaf of smooth differential $k$-forms on $X$ and $C^{\mathrm{\infty }}(-,V)$ for the sheaf of smooth $V$-valued functions on $X$.

The degree $(n+1)$ Deligne complex is the complex of sheaves

$$\mathbb{Z}(n+1{)}_D^{\mathrm{\infty }}:=(\cdots \to 0\to C^{\mathrm{\infty }}(-,\mathbb{Z})\hookrightarrow C^{\mathrm{\infty }}(-,ℝ)\stackrel{d}{\to }{\Omega }^1(-)\stackrel{d}{\to }\cdots \stackrel{d}{\to }{\Omega }^n(-)).$$\mathbb{Z}(n+1)_D^\infty \; := \; \left( \cdots \to 0 \to C^\infty(-,\mathbb{Z}) \hookrightarrow C^\infty(-,\mathbb{R}) \stackrel{d }{\to} \Omega^1(-) \stackrel{d}{\to} \cdots \stackrel{d}{\to} \Omega^n(-) \right) \,.

Often it is useful to consider the quasi-isomorphic complex

$${\overline{B}}^{n}U(1):=(\cdots 0\to C^{\mathrm{\infty }}(-,U(1))\stackrel{d\log }{\to }{\Omega }^1(-)\stackrel{d}{\to }\cdots \stackrel{d}{\to }{\Omega }^n(-))$$\bar \mathbf{B}^n U(1) \;\; := \;\; \left( \cdots 0 \to C^\infty(-,U(1)) \stackrel{d log}{\to} \Omega^1(-) \stackrel{d}{\to} \cdots \stackrel{d}{\to} \Omega^n(-) \right)

Here $C^{\mathrm{\infty }}(-,U(1))\stackrel{d\log }{\to }{\Omega }^1(-)$ is the morphism of sheaves induced by regarding a $U(1)\simeq ℝ/\mathbb{Z}$-valued function locally as a $ℝ$-valued function and applying the deRham differential $d$ to that.

The obvious morphism of complexes

$$\begin{array}{ccccccccc}{C}^{\mathrm{\infty }}(-,\mathbb{Z})& \hookrightarrow & C^{\mathrm{\infty }}(-,ℝ)& \stackrel{d\log }{\to }& {\Omega }^1(-)& \stackrel{d}{\to }& \cdots & \stackrel{d}{\to }& {\Omega }^n(-)\\ \downarrow & & {\downarrow }^{(-)/\mathbb{Z}}& & {\downarrow }^{\mathrm{Id}}& & & & {\downarrow }^{\mathrm{Id}}\\ 0& \to & C^{\mathrm{\infty }}(-,U(1))& \stackrel{d\log }{\to }& {\Omega }^1(-)& \stackrel{d}{\to }& \cdots & \stackrel{d}{\to }& {\Omega }^n(-)\end{array}$$\array{ C^\infty(-,\mathbb{Z}) &\hookrightarrow& C^\infty(-,\mathbb{R}) &\stackrel{d log}{\to}& \Omega^1(-) &\stackrel{d}{\to}& \cdots &\stackrel{d}{\to}& \Omega^n(-) \\ \downarrow && \downarrow^{(-)/\mathbb{Z}} && \downarrow^{Id} && && \downarrow^{Id} \\ 0 &\to& C^\infty(-,U(1)) &\stackrel{d log}{\to}& \Omega^1(-) &\stackrel{d}{\to}& \cdots &\stackrel{d}{\to}& \Omega^n(-) }

clearly induces isomorphism on homology groups: the homology in degree $n$ is locally constant $ℝ$-valued functions modulo locally constant $\mathbb{Z}$-valued functions in the first case and constant $U(1)$-valued functions in the second case, which is the same.

Definition

Deligne cohomology in degree $n+1$ of $X$ is the cohomology (which is abelian sheaf cohomology in this case) with coefficients in ${\overline{B}}^{n}U(1)$.

$$H(X,\mathbb{Z}(n+1{)}_D^{\mathrm{\infty }})\simeq H(X,{\overline{B}}^{n}U(1)).$$H(X, \mathbb{Z}(n+1)_D^\infty) \simeq H(X, \bar \mathbf{B}^n U(1)) \,.

Here the notation on the right is motivated from the discussion at the end of motivation for sheaves, cohomology and higher stacks.

Characteristic classes of Deligne cocycles

There are two natural morphisms of abelian cohomology groups out of Deligne cohomology:

• the map to the underlying non-differential cocycle, the class of the underlying principal infinity-bundle:

$$\mathrm{cl}:H(X,{\overline{B}}^{n}U(1))\to H(X,B^{n}U(1))\simeq H(X,B^{n+1}\mathbb{Z})\simeq H^{n+1}(X,\mathbb{Z})$$cl : H(X,\bar \mathbf{B}^n U(1)) \to H(X,\mathbf{B}^n U(1)) \simeq H(X, \mathbf{B}^{n+1} \mathbb{Z}) \simeq H^{n+1}(X,\mathbb{Z})

• the map to the curvature characteristic class

$$[F]:H(X,{\overline{B}}^{n}U(1))\to H_{\mathrm{dR}}^{n+1}(X).$$[F] : H(X,\bar \mathbf{B}^n U(1)) \to H_{dR}^{n+1}(X) \,.

These are induced from the canonical morphisms of coefficient objects

$${\overline{B}}^{n}U(1)\simeq \mathbb{Z}(n+1{)}_D^{\mathrm{\infty }}\to B^{n+1}\mathbb{Z}$$\bar \mathbf{B}^n U(1) \simeq \mathbb{Z}(n+1)_D^\infty \to \mathbf{B}^{n+1} \mathbb{Z}

given by

$$\begin{array}{ccccccccc}{C}^{\mathrm{\infty }}(-,\mathbb{Z})& \hookrightarrow & C^{\mathrm{\infty }}(-,ℝ)& \stackrel{d}{\to }& {\Omega }^1(-)& \stackrel{d}{\to }& \cdots & \stackrel{d}{\to }& {\Omega }^n(-)\\ {\downarrow }^{\mathrm{Id}}& & {\downarrow }^0& & {\downarrow }^0& & & & {\downarrow }^0\\ C^{\mathrm{\infty }}(-,\mathbb{Z})& \to & 0& \to & 0& \to & \cdots & \to & 0\end{array}$$\array{ C^\infty(-,\mathbb{Z}) &\hookrightarrow& C^\infty(-,\mathbb{R}) &\stackrel{d }{\to}& \Omega^1(-) &\stackrel{d}{\to}& \cdots &\stackrel{d}{\to}& \Omega^n(-) \\ \downarrow^{Id} && \downarrow^{0} && \downarrow^{0} && && \downarrow^{0} \\ C^\infty(-, \mathbb{Z}) &\to& 0 &\to& 0 &\to& \cdots &\to& 0 }

and

$${\overline{B}}^{n}U(1)\simeq \mathbb{Z}(n+1{)}_D^{\mathrm{\infty }}\to ({\Omega }^0(-)\stackrel{d}{\to }{\Omega }^1(-)\stackrel{d}{\to }\cdots \stackrel{d}{\to }{\Omega }^{n+1}(-))$$\bar \mathbf{B}^n U(1) \simeq \mathbb{Z}(n+1)_D^\infty \to (\Omega^0(-) \stackrel{d}{\to} \Omega^1(-) \stackrel{d}{\to} \cdots \stackrel{d}{\to} \Omega^{n+1}(-) )

given by

$$\begin{array}{ccccccccc}{C}^{\mathrm{\infty }}(-,\mathbb{Z})& \hookrightarrow & C^{\mathrm{\infty }}(-,ℝ)& \stackrel{d}{\to }& {\Omega }^1(-)& \stackrel{d}{\to }& \cdots & \stackrel{d}{\to }& {\Omega }^n(-)\\ {\downarrow }^0& & {\downarrow }^d& & {\downarrow }^d& & & & {\downarrow }^d\\ C^{\mathrm{\infty }}(-,ℝ)& \stackrel{d}{\to }& {\Omega }^1(-)& \stackrel{d}{\to }& {\Omega }^2(-)& \stackrel{d}{\to }& \cdots & \stackrel{d}{\to }& {\Omega }^{n+1}(-)\end{array}$$\array{ C^\infty(-,\mathbb{Z}) &\hookrightarrow& C^\infty(-,\mathbb{R}) &\stackrel{d }{\to}& \Omega^1(-) &\stackrel{d}{\to}& \cdots &\stackrel{d}{\to}& \Omega^n(-) \\ \downarrow^{0} && \downarrow^d && \downarrow^d && && \downarrow^d \\ C^\infty(-,\mathbb{R}) &\stackrel{d}{\to}& \Omega^1(-) &\stackrel{d}{\to}& \Omega^2(-) &\stackrel{d}{\to}& \cdots &\stackrel{d}{\to}& \Omega^{n+1}(-) }

Theorem

These two morphisms exhibit Deligne cohomology as a refinement in differential cohomology of ordinary (i.e. integral Eilenberg-MacLane) cohomology, in that the diagram

$$\begin{array}{ccc}H(X,{\overline{B}}^{•}U(1))& \stackrel{[F]}{\to }& H_{\mathrm{dR}}^{•+1}(X)\\ {\downarrow }^{\mathrm{cl}}& & \downarrow \\ H^{•+1}(X,\mathbb{Z})& \to & H^{•+1}(X,ℝ)\end{array}$$\array{ H(X,\bar \mathbf{B}^\bullet U(1)) &\stackrel{[F]}{\to}& H^{\bullet+1}_{dR}(X) \\ \downarrow^{cl} && \downarrow \\ H^{\bullet+1}(X,\mathbb{Z}) &\to& H^{\bullet + 1}(X,\mathbb{R}) }

is the cohomology of a homotopy pullback diagram, i.e. satisfies the axioms described at differential cohomology.

Interpretation in terms of parallel transport

There is a natural way to understand the Deligne complex of sheaves as a sheaf which assigns to each patch the Lie $n$-groupoid of smooth parallel transport $n$-functors. This perspective is helpful for understanding how Deligne cohomology relates to the bigger picture of differential cohomology.

We start by discussing this in low degree.

There is path groupoid $P_1(X)$ whose smooth space of objects is $X$ and whose smooth space of morphisms is a space of classes of smooth paths in $X$. Every smooth 1-form $A\in {\Omega }^1(X)$ induces a smooth functor ${\mathrm{tra}}_A:P_1(X)\to BU(1)$ from $P_1(X)$ to to the smooth groupoid $BU(1)$ with one object and $U(1)$ as its smooth space of morphisms by sending each path $\gamma :[\mathrm{0,1}]\to X$ to $\exp (2\pi i{\int }_0^1{\gamma }^{*}A)$. This map from 1-forms to smooth functors turns out to be bijective: every smooth functor of this form uniquely arises this way. Similarly, one finds that smooth natural transformation ${\eta }_f:{\mathrm{tra}}_A\to {\mathrm{tra}}_{A\prime }$ between two such functors is in components precisely a smooth function $f:X\to U(1)$ such that $A\prime =A+d\log f$.

Since the analogous statements are true for every open subset $U\subset X$ this defines a sheaf of Lie groupoids

$${\mathrm{Funct}}^{\mathrm{\infty }}(P_1(-),BU(1)):\mathrm{Op}(X{)}^{\mathrm{op}}\to \mathrm{LieGrpd}.$$Funct^\infty(P_1(-), \mathbf{B}U(1)) : Op(X)^{op} \to LieGrpd \,.

By the Dold-Kan correspondence this sheaf of groupoids corresponds to a sheaf of complexes of groups. This complex of sheaves is nothing but the degree 2 Deligne complex

$${\mathrm{Funct}}^{\mathrm{\infty }}({\Pi }_1(-),BU(1))\simeq \mathbb{Z}(2{)}_D^{\mathrm{\infty }}.$$Funct^\infty(\Pi_1(-), \mathbf{B}U(1)) \simeq \mathbb{Z}(2)^\infty_D \,.

This way Deligne cohomology is realized as computing the stackification of the pre-stack ${\mathrm{Funct}}^{\mathrm{\infty }}(P_1(-),B(1))$ of smooth $U(1)$-valued parallel transport functors.

The identification generalizes: for all $n$ there is a path n-groupoid $P_n(X)$ whose $k$-morphisms are $k$-dimensional smooth paths in $X$. Smooth $n$-functors ${\mathrm{tra}}_C{:}_n(X)\to B^{n}U(1)$ are canonically identified with smooth $n$-forms $C\in {\Omega }^n(X)$ and under the Dold-Kan correspondence the Deligne-complex in degree $n+1$ is identified with the sheaf of $n$-groupoids of such smooth $n$-functors

$$n{\mathrm{Funct}}^{\mathrm{\infty }}(P_n(-),B^n)\simeq \mathbb{Z}(n+1{)}_D^{\mathrm{\infty }}.$$n Funct^\infty(P_n(-), \mathbf{B}^n) \simeq \mathbb{Z}(n+1)^\infty_D \,.

See

• John Baez, Urs Schreiber, Higher Gauge Theory (arXiv)

The full proof for $n=1$ this is in

• Urs Schreiber, Konrad Waldorf, Parallel transport and functors (arXiv);

for $n=2$ in

• Urs Schreiber, Konrad Waldorf, Smooth functors versus differential forms (arXiv)

For more on this see differential nonabelian cohomology.

For higher $n$ there is as yet no detailed proof in the literature, but the low dimensional proofs have obvious generalizations.

Examples

As described in smoe detail at electromagnetic field in abelian higher gauge theories the background field naturally arises as a Čech–Deligne cocycle, i.e. a Čech cocycle representative with values in the Deligne complex.

• Degree 2 Deligne cohomology classifies $U(1)$-principal bundles with connection. The Deligne complex $\overline{B}U(1)$ in this case coincides with the groupoid of Lie-algebra valued forms for the Lie algebra of $U(1)$.

  • In physics the electromagnetic field is modeled by a degree 2 Deligne cocycle. See there for a derivation of Čech–Deligne cohomology from physical input.

• Degree 3 Deligne cohomology classifies bundle gerbes with connection.

  • In electromagnetism Degree 3 Deligne cocycles model magnetic charge. In formal high energy physics the Kalb-Ramond field is modeled by a Deligne 3-cocycle.

• Degree 4 Deligne cohomology classifies bundle 2-gerbe?s with connection. In particular Chern-Simons bundle 2-gerbes whose degree 4 curvature characteristic class is a multiple of the Pontryagin 4-form on some $\mathrm{SO}(n)$-principal bundle.

  • In formal high energy physics degree 4 Deligne classes model the supergravity C-field.

References

A standard textbook reference is section 5 of

• J.-L. Brylinski, Loop Spaces, Characteristic Classes and geometric Quantization, Birkhaeuser

A concise review is for instance section 2 of

• K. Gomi, Projective unitary representations of smooth Deligne cohomology groups (arXiv)