Schreiber
differential cohomology in an (∞,1)-topos

Differential cohomology in an (,1)(\infty,1)-topos

  1. survey

  2. general structures

  3. paths

  4. Lie theory

  5. differential cohomology

  6. examples

  7. references


Every (∞,1)-topos comes with its intrinsic cohomology. For the special case of an cohesive (∞,1)-topos we define an intrinsic differential cohomology.

Contents

Idea

A cocycle g:XAg : X \to A in the intrinsic cohomology of an (∞,1)-topos H\mathbf{H} classifies a principal ∞-bundle PXP \to X in H\mathbf{H}. A refinement of this cocycle to a cocycle in intrinsic differential cohomology corresponds to equipping this principal \infty-bundle with a connection.

The notion of differential cohomology makes sense when H\mathbf{H} is a cohesive (∞,1)-topos or at least an ∞-connected (∞,1)-topos: in this case it comes with an intrinsic fundamental path ∞-groupoid functor Π:HGrpd\Pi : \mathbf{H} \to \infty Grpd whose internal reflection (Π):HH(\mathbf{\Pi} \dashv \flat) : \mathbf{H} \stackrel{\to}{\leftarrow} \mathbf{H} allows to encode the behaviour of a bare cocycle XAX \to A under parallel transport along the geometric paths defined by Π\Pi.

The intrinsic differential cohomology in H\mathbf{H} is the twisted cohomology induced by the obstruction problem of finding extensions flat\nabla_{flat} in

X g A flat Π(X) \array{ X &\stackrel{g}{\to}& A \\ \downarrow &\nearrow_{\nabla_{flat}} \\ \mathbf{\Pi}(X) }

If such an extension exists, we say that flat\nabla_{flat} is a cocycle in intrinsic flat differential cohomology in H\mathbf{H}, also known as a flat connection or a local system.

In general a given cocycle does not admit a flat differential refinement. But the obstructions to its existence arrange themselves neatly into curvature characteristic classes . The curvature-twisted cohomology induced by these classes we identify with intrinsic differential cohomology.

Assumptions

Throughout, we take HSh (,1)(C)\mathbf{H} \simeq Sh_{(\infty,1)}(C) to be a ∞-connected (∞,1)-topos of (∞,1)-sheaves on some (∞,1)-site CC, with global section essential geometric terminal morphism

HΓLConstΠGrpd \mathbf{H} \stackrel{\overset{\Pi}{\to}}{\stackrel{\overset{LConst}{\leftarrow}}{\underset{\Gamma}{\to}}} \infty Grpd

and induced internal path ∞-groupoid adjunction

(Π):=(LConstΠLConstΓ). (\mathbf{\Pi} \dashv \mathbf{\flat}) := (LConst \circ \Pi \dashv LConst \circ \Gamma) \,.
Example

A list of examples is at Ambient (∞,1)-toposes.

For discussion of concrete representative of differential cocycles we make use of models for ∞-stack (∞,1)-toposes in terms of the model structure on simplicial presheaves [C op,sSet] proj,cov[C^{op}, sSet]_{proj,cov} obtained from the global model structure on simplicial presheaves on the sSet-site CC by left Bousfield localization? at Cech nerve projections.

Moreover, when dealing with fibrant objects in this model structure we make use of standard constructions in categories of fibrant objects, notably the factorization lemma .

In particular for BA\mathbf{B} A a fibrant object with essentially unique point *BA* \to \mathbf{B}A – modelling the delooping of an ∞-group object AA – we write EABA\mathbf{E}A \to \mathbf{B}A for the fibration that is the GG-universal principal ∞-bundle.

Flat differential cohomology

We conceive all of differential cohomology in H\mathbf{H} as the theory of extensions along the natural unit morphism XΠ(X)X \to \mathbf{\Pi}(X) that includes any object XX into its path ∞-groupoid. In the simplest case the extension simply exists and we are left with cocycles on Π(X)\mathbf{\Pi}(X). These we identify as cocycles in flat differential cohomology, which one may also think of as generalizations of the notion of local systems.

Of particular relevance is the case where the underlying cocycle of a flat differential cocycle is trivial. This we identify below with a cocycle in (nonabelian) de Rham cohomology intrinsic to H\mathbf{H}.

Intrinsic de Rham cohomology serves as the home of curvature characteristic forms discussed further below in terms of which non-flat differential cohomology is defined. Passing to de Rham coefficients is also related to \infty-Lie differentiation in H\mathbf{H}.

\infty-Local systems

Definition

Given objects XX and AA in H\mathbf{H}, a flat differential cocycle or \infty-local system on XX with coefficients in AA is a morphism Π(X)A\mathbf{\Pi}(X) \to A in H\mathbf{H}, equivalently a morphism X(A)X \to \mathbf{\flat}(A).

We write

H flat(X,A):=H(Π(X),A)H(X,(A)) \mathbf{H}_{flat}(X,A) := \mathbf{H}(\mathbf{\Pi}(X),A) \simeq \mathbf{H}(X, \mathbf{\flat}(A))

for the ∞-groupoid of flat differential AA-cocycles on XX and accordingly

H flat(X,A):=π 0H flat(X,A) H_{flat}(X,A) := \pi_0 \mathbf{H}_{flat}(X,A)

for the corresponding cohomology set. The constant path inclusion XΠ(X)X \to \mathbf{\Pi}(X) induces the projection

H flat(X,A)H(X,A). \mathbf{H}_{flat}(X,A) \to \mathbf{H}(X,A) \,.

The image of a cocycle under this morphism we call the underlying cocycle or bare cocycle of a given flat differential cocycle.

We therefore have an obstruction problem: given a bare underlying cocycle in H(X,A)\mathbf{H}(X,A), what is the obstruction to equipping it with a flat connection and thus lifting it to H flat(X,A)\mathbf{H}_{flat}(X,A)? The next sections provide answers to this question.

Atiyah \infty-groupoids

Let g:XBGg : X \to \mathbf{B}G be a cocycle classifying a GG-principal ∞-bundle PXP \to X. Above we defined a flat differential cocycle that refines gg as an extension P:Π(X)A\nabla_P : \mathbf{\Pi}(X) \to A of gg along the constant path inclusion

X g BG P Π(X). \array{ X &\stackrel{g}{\to}& \mathbf{B}G \\ \downarrow & \nearrow_{\mathrlap{\nabla_P}} \\ \mathbf{\Pi}(X) } \,.

It is sometimes useful to reformulate this extension problem as a lifting problem. We would like to have an object At(P)Π(X)At(P) \to \mathbf{\Pi}(X) in H\mathbf{H} such that a section ^ g\hat \nabla_g of this projection

At(P) ^ g Π(X) = Π(X) \array{ && At(P) \\ &{}^{\mathllap{\hat \nabla_g}}\nearrow& \downarrow \\ \mathbf{\Pi}(X) &\stackrel{=}{\to}& \mathbf{\Pi}(X) }

corresponds to a choice of flat differential cocycle g\nabla_g refining gg.

For GG an ordinary Lie group and PP an ordinary GG-principal bundle, the nature of the object At(P)At(P) is well known: it is the Atiyah Lie groupoid of the principal bundle PP, a Lie groupoid whose Lie algebroid is the Atiyah Lie algebroid of PP.

The following definition is supposed to generalize this notion to the general context of a locally \infty-connected (,1)(\infty,1)-topos H\mathbf{H}

Definition

For a cocycle g:XAg : X \to A, a morphism in H\mathbf{H}, we say its Atiyah \infty-groupoid is the object

At(g):=Π(X× A(A)). At(g) := \mathbf{\Pi}(X \times_A \mathbf{\flat}(A)) \,.
Remark

The universal property of the fiber product X× A(A)X \times_A \mathbf{\flat}(A) says that sections σ:XX× A(A)\sigma : X \to X \times_A \mathbf{\flat}(A)

X× A(A) (A) σ X Id X g A \array{ && X \times_A \mathbf{\flat}(A) &\to& \mathbf{\flat}(A) \\ & {}^{\mathllap{\sigma}}\nearrow & \downarrow && \downarrow \\ X &\stackrel{Id}{\to}& X &\stackrel{g}{\to}& A }

correspond precisely to lifts

(A) σ X g A. \array{ && \mathbf{\flat}(A) \\ & {}^{\mathllap{\sigma}}\nearrow & \downarrow \\ X &\stackrel{g}{\to}& A } \,.

But by the adjunction (ΠLConstΓ)(\Pi \dashv LConst \dashv \Gamma) we have (see essential geometric morphism) that this corresponds to extensions

X g A g Π(X). \array{ X &\stackrel{g}{\to}& A \\ \downarrow & \nearrow_{\mathrlap{\nabla_g}} \\ \mathbf{\Pi}(X) } \,.

Similarly by the adjunction property, we have that every such section also gives a lift

At(P) Π(X) = Π(X). \array{ && At(P) \\ & {}^{\mathllap{\nabla}}\nearrow & \downarrow \\ \mathbf{\Pi}(X) &\stackrel{=}{\to}& \mathbf{\Pi}(X) } \,.

Of particular interest is the special case of Atiyah \infty-groupoids over the point . These we identify in the next section with coefficient objects for closed differential forms.

Intrinsic de Rham cohomology

We describe now an abstract intrinsic notion of (abelian and nonabelian) de Rham cohomology in any locally ∞-connected (∞,1)-topos H\mathbf{H} as a constrained form of intrinsic flat differential cohomology in H\mathbf{H} as described above. We give intrinsic definitions for fundamental de Rham cohomological concepts such as the the Maurer-Cartan form-datum on an ∞-group object in H\mathbf{H} and show that in the case H=\mathbf{H} = ∞LieGrpd these definitions reproduce ordinary smooth ∞-Lie algebroid valued differential forms.

Below in Differential cohomology with groupal coefficients we show that the obstructions to lifts from bare intrinsic cohomology of H\mathbf{H} to cocycles in intrinsic flat differential cohomology live precisely in this intrinsic de Rham cohomology: these are the intrinsic curvature characteristic forms.

Intrinsic flat differential forms

Definition

(intrinsic de Rham coefficients)

Let *A* \to A be a pointed object in H\mathbf{H}. We write A dRA_{dR} or dRA\mathbf{\flat}_{dR} A for the Atiyah ∞-groupoid of the unique AA-principal ∞-bundle over the point, i.e. for the (∞,1)-pullback

dRA (A) * A \array{ \mathbf{\flat}_{dR} A &\to& \mathbf{\flat}(A) \\ \downarrow && \downarrow \\ * &\to& A }

in H\mathbf{H}.

Similarly for XHX \in \mathbf{H} any object, we write Π(X)/X\mathbf{\Pi}(X)/X or Π dR(X)\mathbf{\Pi}_{dR}(X) for the pushout

X * Π(X) Π dR(X). \array{ X &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}(X) &\to& \mathbf{\Pi}_{dR}(X) } \,.
Remark

In the model [C op,sSet] proj,cov[C^{op}, sSet]_{proj,cov} of H\mathbf{H} let (Π R R)(\mathbf{\Pi}_R \dashv \mathbf{\flat}_R) be a Quillen adjunction that model the adjoint (∞,1)-functors (Π)(\mathbf{\Pi} \dashv \mathbf{\flat}) such that the counit components R(A)A\mathbf{\flat}_R(A) \to A are fibration (see path ∞-groupoid for onstructions of such models).

Then for XX given by a cofibrant object in [C op,sSet] proj[C^{op},sSet]_{proj}, the object Π dR(A)H\mathbf{\Pi}_{dR}(A) \in \mathbf{H} is represented by the ordinary pushout

X * Π R(X) Π R,dR(A) \array{ X &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(X) &\to& \mathbf{\Pi}_{R,dR}(A) }

and is hence in particular itself cofibrant.

Similarly, for AA a fibrant representative the object dRA\mathbf{\flat}_{dR} A is constructed by the ordinary pullback

dRA R(A) * A \array{ \mathbf{\flat}_{dR} A &\to& \mathbf{\flat}_{R}(A) \\ \downarrow && \downarrow \\ * &\to& A }

in [C op,sSet][C^{op}, sSet], and is in particular itself fibrant.

An equivalent way to compute this is using the global model structure on functors [I,sPSh(C) proj loc] proj[I,sPSh(C)_{proj}^{loc}]_{proj}, where I=Δ[1]={01}I = \Delta[1] = \{0 \to 1\} is the interval. Notice that a cofibration between cofibrant objects in sPSh(C) proj,covsPSh(C)_{proj,cov} is a cofibrant object in [I,sPSh(C) proj,cov] proj[I,sPSh(C)_{proj,cov}]_{proj}.

Since all representables UU are cofibrant in sPSh(C) proj,covsPSh(C)_{proj,cov} and since UΠ R(U)U \to \mathbf{\Pi}_R(U) is a cofibration in sPSh(C) proj,covsPSh(C)_{proj,cov} it is a cofibrant object in the functor category. Therefore for AA fibrant in H\mathbf{H} we have that dRA\mathbf{\flat}_{dR}A is presented in [I,sPSh(c) proj,loc] proj[I,sPSh(c)_{proj,loc}]_{proj} by

dRA:U[I,sPSh(C)]([U Π R(U)],[* A]). \mathbf{\flat}_{dR} A \;\; : \;\; U \mapsto [I,sPSh(C)] \left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right] \,, \left[ \array{ * \\ \downarrow \\ A } \right] \right) \,.
Proof

By the standard lore about computing homotopy pullbacks and homotopy pushouts.

Observation

Formation of loop space objects commutes with passing to de Rham coefficients, in that

Ω( dRA) dR(ΩA). \Omega (\mathbf{\flat}_{dR} A) \simeq \mathbf{\flat}_{dR}(\Omega A) \,.
Observation

For *A* \to A pointed, we have a natural equivalence

H(A, dRA)H(Π dR(X),A). \mathbf{H}(A,\mathbf{\flat}_{dR} A) \simeq \mathbf{H}(\mathbf{\Pi}_{dR}(X), A) \,.

… details …

Definition

(intrinsic de Rham cohomology)

We call

H dR(X,A):=H(X, dRA)H(Π dR(X),A) \mathbf{H}_{dR}(X,A) := \mathbf{H}(X, \mathbf{\flat}_{dR} A) \simeq \mathbf{H}(\mathbf{\Pi}_{dR}(X), A)

the intrinsic de Rham cohomology of XX with coefficients in AA.

Remark

A cocycle in H dR(X,A)\mathbf{H}_{dR}(X,A) is given by a diagram

X * Π(X) A \array{ X &\to& * \\ \downarrow &\swArrow& \downarrow \\ \mathbf{\Pi}(X) &\to& A }

in H\mathbf{H}. However in terms of the model [I,sPSh(C)][I,sPSh(C)] every such cocycle is represented, up to equivalence, by a strictly commuting diagram

Y * Π R(Y) A \array{ Y &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{}{\to}& A }

for AA fibrant and YY a cofibrant representative of YY. In practice one is often faced with flat differential cocycles modeled by morphisms :Π R(Y)A\nabla : \mathbf{\Pi}_R(Y) \to A whose underlying bare cocycle g:YΠ R(Y)Ag : Y \to \mathbf{\Pi}_R(Y) \to A is not yet trivialized, but only trivializable. Given any such trivialization η:g*\eta : g \to * one would like to lift it to a coboundary η^:\hat \eta : \nabla \to \nabla' that identifies an equivalent flat differential cocycle \nabla' whose underlying bare cocycle is exactly trivial.

This can always be achieved by solving a lifting problem. The given data specifies the following diagram, except for the diagonal morphism η^\hat \eta:

A I× A* * Y η A I d 1 A η^ d 0 Π R(Y) A \array{ && A^I \times_A * &\to& * \\ & \nearrow & \downarrow && \downarrow \\ Y &\stackrel{\eta}{\to}& A^I &\stackrel{d_1}{\to}& A \\ \downarrow &{}^{\hat \eta}\nearrow& \downarrow^{\mathrlap{d_0}} \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla}{\to}& A }

Here A IA^I is the path object of AA used to present this right homotopy, and therefore d 0:A IAd_0 : A^I \to A is an acyclic fibration. Since moreover the constant path inclusion YΠ R(Y)Y \hookrightarrow \mathbf{\Pi}_R(Y) is a cofibration, a lift η^\hat \eta exists as indicated.

The resulting diagram

Y * Π R(Y) :=d 1η^ A \array{ Y &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla' := d_1 \hat \eta}{\to}& A }

is a cocycle representative in the model [I,sPSh(C)][I,sPSh(C)] of the cocycle \nabla in H dR(Y,A)\mathbf{H}_{dR}(Y,A) that we started with.

Remark

A k-morphism in A dRA_{dR}(is modeled in [I,sPSh(C) proj,loc] proj[I,sPSh(C)_{proj,loc}]_{proj} by a diagram

U×Δ[k] * Π R(U)×Δ[k] A. \array{ U \times \Delta[k] &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(U)\times \Delta[k] &\to& A } \,.

In particular this means that coboundaries given by A 1A_1-valued functions on UU may appear in A flat(U)A_{flat}(U) but not in dRA(U)\mathbf{\flat}_{dR}A(U). For AA 1-truncated but not 1-connected all coboundaries in H flat(X,A)\mathbf{H}_{flat}(X,A) disappear in H dR(X,A)\mathbf{H}_{dR}(X,A), such that every cocycle in H dR(X,A)\mathbf{H}_{dR}(X,A) represents its own cohomology class.

As we shall see, this maybe somewhat curious phenomenon is precisely correct for dRA\mathbf{\flat}_{dR} A to be the right coefficient object for general \infty-categorical curvatures: it implies that, in contrast to all higher degree form curvature?s, 1-form curvatures – those that arise in lowest degree for Lie algebroid-valued differential cocycles – are not to be taken up to de Rham coboundaries (doing which would make them all trivial). More on this in the section Example: Lie algebroid-valued connections.

Flat \infty-Lie algebra valued differential forms

The intrinsic flat differential forms that we described are in general cocycles in nonabelian de Rham cohomology: a generalization of Lie-algeba valued 1-forms? for general Lie algebras. We now describe in more detail how the intrinsic flat differential forms may be thought of as being ∞-Lie algebroid valued differential forms.

Intrinsic \infty-Lie algebroids

Above we defined a cocycle in the intrinsic de Rham cohomology in H\mathbf{H} of XHX \in \mathbf{H} with coefficients in AHA \in \mathbf{H} as a morphism

(Π dR(X)A). (\mathbf{\Pi}_{dR}(X) \to A) \,.

Being a morphism out of a left adjoint this factors through a universal morphism Π dRflat dRAA\mathbf{\Pi}_{dR}\mathbf{flat}_{dR}A \to A as

Π dR dRA Π dR(X) A. \array{ && \mathbf{\Pi}_{dR} \mathbf{\flat}_{dR} A \\ & \nearrow & \downarrow \\ \mathbf{\Pi}_{dR}(X) &\to& A } \,.

We will now identify exp(𝔞):=exp(Lie(A)):=Π dR dRA\exp(\mathfrak{a}) := \exp(Lie(A)) := \Pi_{dR}\mathbf{\flat}_{dR} A with the intrinsic incarnation of the exponentiated ∞-Lie algebroid underlying AHA \in \mathbf{H} and morphisms Π dR(X)A\mathbf{\Pi}_{dR}(X) \to A accordingly as generalizations of ordinary flat Lie-algebra valued 1-forms: ∞-Lie algebroid valued differential forms.

Definition

(Lie differentiation to \infty-Lie algebroids)

We write

exp(Lie()):=Π dR dR \exp(Lie(-)) := \mathbf{\Pi}_{dR} \circ \mathbf{\flat}_{dR}

and for AHA \in \mathbf{H} call exp(𝔞):=exp(Lie(A))\exp(\mathfrak{a}) := \exp(Lie(A)) the intrinsic exponentiated ∞-Lie algebroid of AA.

Example/Proposition

For GG an ordinary Lie group regarded naturally as an ∞-group inside H=\mathbf{H} = ∞LieGrpd we have that dRBG\mathbf{\flat}_{dR} \mathbf{B}G is represented in the model by simplicial presheaves [CartSp op,sSet] proj,cov[CartSp^{op}, sSet]_{proj,cov} by the sheaf of flat Lie-algebra valued 1-forms

dRBG:UΩ flat 1(U,𝔤). \mathbf{\flat}_{dR}\mathbf{B}G : U \mapsto \Omega^1_{flat}(U,\mathfrak{g}) \,.
Proof

This and further examples are discussed at ∞-Lie groupoid. See the section Lie group – differential coefficients.

If H\mathbf{H} is given as an infinitesimal thickening of an underlying (∞,1)-topos H red\mathbf{H}_{red}

HΓ infLConst infΠ infH red \mathbf{H} \stackrel{\stackrel{\Pi_{inf}}{\to}}{\stackrel{\overset{LConst_{inf}}{\leftarrow}}{\underset{\Gamma_{inf}}{\to}}} \mathbf{H}_{red}

then we may extract from such an intrinsic definition of \infty-Lie algebroids a more explicit (but still intrinsic) definition of differential forms along the lines of synthetic differential geometry. Moreover, such synthetic differential forms with values in AA will factor through the ∞-Lie algebroid 𝔞A\mathfrak{a} \hookrightarrow A of AA and thus provide a generalization of ordinary Lie algebra valued differential forms to ∞-Lie algebroid valued differential forms.

Recall from the discussion at path ∞-groupoid that from the above infinitesimal thickening is induced the Infinitesimal path ∞-groupoid adjunction

(Π inf inf):HH. (\mathbf{\Pi}_{inf} \dashv \mathbf{\flat}_{inf}) : \mathbf{H} \to \mathbf{H} \,.

Accordingly we get analogs of the above definitions of de Rham objects.

Definition

Write

Π inf,dR(X):=Π inf(X) X* \mathbf{\Pi}_{inf,dR}(X) := \mathbf{\Pi}_{inf}(X) \coprod_{X} *

and for pointed *A* \to A in H\mathbf{H}

inf,dR(A):= inf(A)× A*. \mathbf{\flat}_{inf,dR}(A) := \mathbf{\flat}_{inf}(A) \times_{A} * \,.

The canonical natural inclusion

Π inf(X)Π(X) \mathbf{\Pi}_{inf}(X) \to \mathbf{\Pi}(X)

of infinitesimal paths into all paths accordingly induces natural inclusions

Π inf,dR(X)Π dR(X). \mathbf{\Pi}_{inf,dR}(X) \to \mathbf{\Pi}_{dR}(X) \,.
Proposition

As before we have on pointed objects adjoint (∞,1)-functors

Π inf,dR inf,dR. \mathbf{\Pi}_{inf,dR} \dashv \mathbf{\flat}_{inf,dR} \,.

The study of this adjunction is effectively the study of Lie theory inside H\mathbf{H}.

Definition

(Lie differentiation and \infty-Lie algebroids)

For AHA \in \mathbf{H} we say that

𝔞:=Π inf,dR inf,dRA \mathfrak{a} := \mathbf{\Pi}_{inf,dR} \mathbf{\flat}_{inf,dR}A

is the ∞-Lie algebroid of AA.

Notice that the counit of the adjunction gives a natural inclusion

𝔞A. \mathfrak{a} \to A \,.
Definition

(flat \infty-Lie algebroid valued differential forms)

By the characterization of adjunctions in terms of universal factorizations we have by a factorization

X * Π(X) A=X * Π inf(X) ω 𝔞 Π(X) Pexp(ω) A \array{ X &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}(X) &\to& A } \;\;\;\; = \;\;\;\; \array{ X &\to& * \\ \downarrow && \downarrow \\ \mathbf{\Pi}_{inf}(X) &\stackrel{\omega}{\to}& \mathfrak{a} \\ \downarrow && \downarrow \\ \mathbf{\Pi}(X) &\stackrel{P \exp(\int \omega)}{\to}& A }

in H\mathbf{H}

We call Π inf(X)𝔞\mathbf{\Pi}_{inf}(X) \to \mathfrak{a} here flat ∞-Lie algebroid valued differential forms whose integrated parallel transport is the given morphism denoted Pexp(ω):Π(X)AP \exp(\int \omega) : \mathbf{\Pi}(X) \to A.

By the general formula for right adjoints in presheaf categories we have the expression

𝔞lim Π inf,dR(U)AΠ inf,dR(U) \mathfrak{a} \simeq \lim_{\underset{\mathbf{\Pi}_{inf,dR}(U) \to A}{\to}} \mathbf{\Pi}_{inf,dR}(U)

in terms of an (∞,1)-colimit.

This means that 𝔞\mathfrak{a} is the universal object such that morphisms Π inf,dR(U)A\mathbf{\Pi}_{inf,dR}(U) \to A factor through it.

The canonical flat form on an \infty-group GG

We have given above a definition of coefficient objects dRA\mathbf{\flat}_{dR}A for intrinsic nonabelian de Rham cohomology in terms of the homotopy fiber of the morphism AA\mathbf{\flat} A \to A. By the general logic of fiber sequences it follows that when A=BGA = \mathbf{B}G is a delooping of an ∞-group GG we obtain a canonical morphism G dRBGG \to \mathbf{\flat}_{dR} \mathbf{B}G. The following examples show that this is the general abstract generalization of the Maurer-Cartan form on a Lie group.

Definition

Let GHG \in \mathbf{H} be an ∞-group object and write BG\mathbf{B}G for its delooping object.

Consider the pasting of two (∞,1)-pullback diagrams

G * dRBG BG * BG. \array{ G &\to& * \\ \downarrow && \downarrow \\ \mathbf{\flat}_{dR}\mathbf{B}G &\to& \mathbf{\flat}\mathbf{B}G \\ \downarrow && \downarrow \\ * &\to& \mathbf{B}G } \,.

The object dRBG\mathbf{\flat}_{dR}\mathbf{B}G is the lower pullback by definition, the object GG is the upper pullback since by the pasting law for (∞,1)-pullbacks it is also the pullback of the outer rectangle, which is GG by definition of the delooping BG\mathbf{B}G.

The canonical morphism

(θ:G dRBG)H dR(G,BG) (\theta : G \to \mathbf{\flat}_{dR} \mathbf{B}G) \in \mathbf{H}_{dR}(G,\mathbf{B}G)

defined this way, which by the above corresponds to a flat 𝔤\mathfrak{g}-valued differential form datum, we call the \infty-Maurer-Cartan form on GG.

Proposition/Example

For GG an ordinary Lie group regarded naturally as a group object in H=\mathbf{H} = ∞LieGrpd, θ\theta is the ordinarty Maurer-Cartan form θ=g 1dg\theta = g^{-1} d g in that the morphism G dRBGG \to \mathbf{\flat}_{dR}\mathbf{B}G is represented in the model [CartSp op,sSet] proj,cov[CartSp^{op}, sSet]_{proj,cov} by the morphism of sheaves

Hom Diff(,G)Ω flat 1(,𝔤) Hom_{Diff}(-,G) \to \Omega^1_{flat}(-, \mathfrak{g})

given over UCartSpU \in CartSp by

(gC (U,))(g *θ=g 1dgΩ flat 1(U,𝔤), (g \in C^\infty(U,)) \mapsto (g^* \theta = g^{-1} d g \in \Omega^1_{flat}(U,\mathfrak{g}) \,,

where Ω flat 1(,𝔤)\Omega^1_{flat}(-,\mathfrak{g}) is the sheaf of Lie-algebra valued 1-forms whose curvature 2-form vanishes.

Proof

This is discussed at ∞-Lie groupoid in the section Canonical form on a Lie group.

The vertical form on a GG-principal \infty-bundle

We have given above a definition of the canonical 𝔤\mathfrak{g}-valued flat differential Maurer-Cartan form on an ∞-group GG. The construction has a straightforward generalization to canonical form on the fibers of a GG-principal ∞-bundle PP, generalizing the vertical 𝔤\mathfrak{g}-valued forms appearing in Ehresmann connections.

Definition

For PXP \to X the GG-principal ∞-bundle classified by a cocycle XBGX \to \mathbf{B}G, for each point x:*Xx : * \to X the pasting diagram of (∞,1)-pullback squares

GP x P * θ x dRBG At(P) BG * X BG \array{ G \simeq P_x &\to& P &\to& * \\ {}^{\mathllap{\theta}_x}\downarrow && \downarrow && \downarrow \\ \mathbf{\flat}_{dR} \mathbf{B}G &\to& At(P) &\to& \mathbf{\flat} \mathbf{B}G \\ \downarrow && \downarrow && \downarrow \\ * &\to& X &\to& \mathbf{B}G }

exhibits the canonical flat 𝔤\mathfrak{g}-valued vertical 1-form

θ x:P x dRBG \theta_x : P_x \to \mathbf{\flat}_{dR} \mathbf{B}G

on the fiber P xP_x of PP over xx.

Proposition/Example

For GG an ordinary Lie group, PXP \to X an ordinary smooth GG-principal bundle, all regarded naturally as objects in H=\mathbf{H} = ∞LieGrpd this defintiion reproduces the ordinary notion of flat vertical 𝔤\mathfrak{g}-valued differential form.

Basic forms on a GG-principal \infty-bundle

For GG an ordinary Lie group, and p:PXp : P \to X an ordinary GG-principal bundle, a basic form on PP is a differential form ωΩ (P)\omega \in \Omega^\bullet(P) that is invariant under the GG-action on PP. A standard fact is that basic forms are precisely the image of the forms on XX under pullback along pp. The following is the analog of this statement in an (,1)(\infty,1)-topos. This is just a formal tautology, which is however sometimes useful to make explicit.

Definition

(basic forms on a GG-principal \infty-bundle)

For GG an ∞-group in H\mathbf{H} and PXP \to X a GG-principal ∞-bundle, an intrinsic differential form ωH dR(P,A)\omega \in \mathbf{H}_{dR}(P, A) on PP for any coefficient object AA is called basic precisely if it is homotopy GG-invariant in that for its representing morphism ω:P dRA\omega : P \to \mathbf{\flat}_{dR} A there exists an extension to a cocone

P×G×GP×GPω dRA P \times G \times G \stackrel{\to}{\stackrel{\to}{\to}} P \times G \stackrel{\to}{\to} P \stackrel{\omega}{\to} \mathbf{\flat}_{dR} A

under the action-groupoid object of GG acting on PP (see principal ∞-bundle for the details). We write

H dR,basic(P,A):=(P×G ×)/H/ dRA \mathbf{H}_{dR,basic}(P,A) := (P \times G^{\times \bullet})/\mathbf{H}/\mathbf{\flat}_dR A

for the ∞-groupoid of basic AA-valued forms on PP, being the \infty-groupoid of cocones in H\mathbf{H} under the action groupoid object P×G ×P\times G^{\times \bullet} with tip dRA\mathbf{\flat}_{dR} A.

Observation

Pullback of forms along the bundle projection p:PXp : P \to X establishes an equivalence between forms on XX and basic forms on PP:

p *:H dR(X,A)H dR,basic(P,A). p^* : \mathbf{H}_{dR}(X,A) \stackrel{\simeq}{\to} \mathbf{H}_{dR, basic}(P,A) \,.
Proof

Up to notation and terminology, this is just a restatement of the fact that XX is the (,1)(\infty,1)-colimit over the action groupoid object

Xlim P×G × X \simeq \lim_\to P \times G^{\times \bullet}

as discussed at principal ∞-bundle.

Tangent \infty-Lie algebroids

Objects in H\mathbf{H} of the form LConstSLConst S for SS \in ∞Grpd have the interpretation of being discrete ∞-Lie groupoids in the sense of discrete Lie groups. The latter have trivial Lie algebras and accordingly the former are to be expected to have trivial ∞-Lie algebroids.

That this is indeed the case follows immediately from the above definitions.

Observation

For H\mathbf{H} a ∞-connected (∞,1)-topos and for *S* \to S \in ∞Grpd a pointed ∞-groupoid, we have

LieLConstS* Lie LConst S \simeq *

in H\mathbf{H}.

Proof

By the assumption that H\mathbf{H} is \infty-connected we have that LConst:GrpdHLConst : \infty Grpd \to \mathbf{H} is a full and faithful (∞,1)-functor and hence that ΓLConstId\Gamma \circ LConst \simeq Id.

It follows that dRLConstS\mathbf{\flat}_{dR} LConst S is the pullback

dRLConstS LConstS LConstS * LConstS \array{ \mathbf{\flat}_{dR} LConst S &\to& \mathbf{\flat} LConst S & \simeq LConst S \\ \downarrow && \downarrow \\ * &\to& LConst S }

and hence itself equivalent to the point. Then so is LieLConstS=Π dR dRLConstSLie LConst S = \mathbf{\Pi}_{dR} \mathbf{\flat}_{dR} LConst S.

While this is almost tautological, it still has noteworthy applications when applied to objects Π(X)=LConstΠ(X)\mathbf{\Pi}(X) = LConst \Pi(X) in the image of the homotopy ∞-groupoid? functor in view of its realization as the path ∞-groupoid.

We first record the following evident statements.

Lemma

If H\mathbf{H} is ∞-connected, then the geometric realization / homotopy \infty-groupoid of Π dR(X)\mathbf{\Pi}_{dR}(X) is trivial:

Π(Π dR(X))*. \Pi(\mathbf{\Pi}_{dR}(X)) \simeq * \,.

Moreover we have for all XHX \in \mathbf{H}

  1. Π dRΠ(X)*\mathbf{\Pi}_{dR}\mathbf{\Pi}(X) \simeq *;

  2. dRΠ(X)*\mathbf{\flat}_{dR} \mathbf{\Pi}(X) \simeq *

Proof

Since Π\Pi is left adjoint it preserves the pushout that defines Π dR(X)\mathbf{\Pi}_{dR}(X) so that the geometric realization is itself the pushout in

Π(X) Π(*) ΠΠ(X) ΠΠ dR(X). \array{ \Pi(X) &\to& \Pi(*) \\ \downarrow && \downarrow \\ \Pi \mathbf{\Pi}(X) &\to& \Pi \mathbf{\Pi}_{dR}(X) } \,.

Now using the assumption that H\mathbf{H} is \infty-connected we have ΠLConstId\Pi \circ LConst \simeq Id and hence Π(*)*\Pi(*) \simeq * and ΠΠ(X)=ΠLContΠ(X)Π(X)\Pi \mathbf{\Pi}(X) = \Pi LCont \Pi(X) \simeq \Pi(X). Therefore the above pushout is equivalent to

Π(X) Π(*) Π(X) ΠΠ dR(X). \array{ \Pi(X) &\to& \Pi(*) \\ \downarrow && \downarrow \\ \Pi(X) &\to& \Pi \mathbf{\Pi}_{dR}(X) } \,.

The right vertical morphism is therefore a pushout of an equivalence and hence itself an equivalence.

Also by preservation of pushout we have

Π(Π dR(X))Π dR(Π(X)). \mathbf{\Pi}(\mathbf{\Pi}_{dR}(X)) \simeq \mathbf{\Pi}_{dR}(\mathbf{\Pi}(X)) \,.

This now implies Π dR(Π(X))*\mathbf{\Pi}_{dR}(\mathbf{\Pi}(X)) \simeq *, which is also immediately seen directly by using again from the ∞-connectedness that ΠΠ(X)Π(X)\mathbf{\Pi} \mathbf{\Pi}(X) \simeq \mathbf{\Pi}(X).

Analogously we have Π(X):=LConstΓLConstΠXLConstΠ(X)=Π(X)\mathbf{\flat} \mathbf{\Pi}(X) := LConst \Gamma LConst \Pi X \simeq LConst \Pi(X) = \mathbf{\Pi}(X) and hence the pullback

dRΠ(X) Π(X) Π(X) * Π(X) \array{ \mathbf{\flat}_{dR} \mathbf{\Pi}(X) &\to& \mathbf{\flat}\mathbf{\Pi}(X) & \simeq \mathbf{\Pi}(X) \\ \downarrow && \downarrow \\ * &\to& \mathbf{\Pi}(X) }

yields dRΠ(X)*\mathbf{\flat}_{dR} \mathbf{\Pi}(X) \simeq *.

Corollary

For *AH* \to A \in \mathbf{H} pointed, we have that the tangent \infty-Lie algebroid

T𝔞:=LieΠ(X)*. T \mathfrak{a} := Lie \mathbf{\Pi}(X) \simeq * \,.

This is a version of the following familiar fact: for 𝔤\mathfrak{g} an L-∞-algebra dual to its Chevalley-Eilenberg algebra CE(𝔤)CE(\mathfrak{g}),
the corresponding Weil algebra W(𝔤)W(\mathfrak{g}), which may be regarded as the Chevalley-Eilenberg algebra of the tangent L L_\infty-algebra T𝔤T \mathfrak{g}, has trivial cohomology.

Intrinsic de Rham theorem

The ordinary de Rham theorem asserts an equivalence between de Rham cohomology? of a smooth manifold and its “ordinary cohomology” (meaning: singular cohomology or anything equivalent to it) with coefficients the additive discrete group \mathbb{R} of real numbers.

The natural generalization of this statement holds for the intrinsic de Rham cohomology and intrinsic cohomology in an \infty-connected (,1)(\infty,1)-topos.

Proposition

(de Rham theorem)

Write RR for the intrinsic line object in H\mathbf{H}, with Π(R)*\Pi(R) \simeq *. Let XHX \in \mathbf{H} be such that \mathbb{R}-valued cohomology on XX is trivial. (For instance in H=\mathbf{H} = ∞-LieGrpd? for XX a connected smooth manifold, but not for instance the path ∞-groupoid of a manifold.)

Write |X||X| for the geometric realization of XX, the image of XX under HΠGrpdTop\mathbf{H} \stackrel{\Pi}{\to} \infty Grpd \stackrel{\simeq}{\to} Top. Notice that by adjunction

H n(|X|,)H(X,B n(R)), H^n(|X|, \mathbb{R}) \simeq H(X, \mathbf{B}^n\flat(R)) \,,

where on the left we have odinary real cohomology of |X||X| (real singular cohomology) and on the right its reflection in H\mathbf{H}.

Then: there is a natural isomorphism of cohomology sets

H n(|X|,)H dR(X,B nR). H^n(|X|, \mathbb{R}) \simeq H_{dR}(X,\mathbf{B}^n R) \,.

This even comes from a natural equivalence of cocycle \infty-groupoids

H(X,(R))H dR(X,B nR). \mathbf{H}(X, \mathbf{\flat}(R)) \simeq \mathbf{H}_{dR}(X,\mathbf{B}^n R) \,.
Proof

By the definition of intrinsic de Rham cohomology, this is essentially a tautology, since by the assumption that H(X,B nR)*\mathbf{H}(X,\mathbf{B}^n R) \simeq * we have

H(X,B nR)H(X, dRB nR). \mathbf{H}(X, \mathbf{\flat} \mathbf{B}^n R) \simeq \mathbf{H}(X, \mathbf{\flat}_{dR} \mathbf{B}^n R) \,.
Remark

While this is a trivial formal consequence of the abstract definition of intrinsic de Rham cohomology, the point is that this abstract defrinition indeed captures ordinaty de Rham cohomology.

Differential cohomology with groupal coefficients

We define and study now differential cohomology in H\mathbf{H} for the case that the coefficient object AHA \in \mathbf{H} is a group object, so that its delooping object BA\mathbf{B}A exists. In this case there is a characteristic class curv A:A dRBAcurv_A : A \to \mathbf{\flat}_{dR}\mathbf{B}A with values in the intrinsic de Rham cohomology with values in BA\mathbf{B}A that measures curvature? of AA-cocycles (the canonical Maurer-Cartan form on AA), and we define differential AA-cohomology to be the twisted cohomology induced by this class.

It turns out that this twisted cohomology is an exact obstruction theory: an AA-cocycle g:XAg : X \to A has an extension to flat differential cohomology if and only if its curvature class curv A(g)curv_A(g) is trivial in cohomology.

The same is not true if AA is not groupal, i.e. if we are looking at differentuial refinements of fully nonabelian cohomology. Below in the section Differential cohomology with non-groupal coefficients we consider a more general but less exact curvature characteristic class that applies to arbitrary coefficients – hence also to fully nonabelian cohomology and its differential refinements – and which is in some sense the universal best approximation to the curvature class that we consider here.

Curvature classes

We produce now a characteristic class, naturally defined on groupal objects AHA \in \mathbf{H}, that measures the obstructions to lifts of AA-cocycles through (A)A\mathbf{\flat}(A) \to A to flat differential AA-cocycles.

Observation

(differential fibration sequence)

If AA is a group object with delooping BA\mathbf{B}A, then we have a fiber sequence

dR(A)AA dRBABABA. \cdots \to \mathbf{\flat}_{dR}(A) \to \mathbf{\flat} A \to A \to \mathbf{\flat}_{dR}\mathbf{B}A \to \mathbf{\flat}\mathbf{B}A \to \mathbf{B}A \,.
Definition

For groupal AA, we call the morphism

curv A:A dRBA curv_A : A \to \mathbf{\flat}_{dR}\mathbf{B}A

from the above diagram the curvature characteristic class of AA.

This is just the observation of the Maurer-Cartan form repeated – but one degree up: typically we have A=BGA = \mathbf{B}G for GG an ∞-group so that AA-cohomology classifies GG-principal ∞-bundles. Then BA=B 2G\mathbf{B}A = \mathbf{B}^2 G is the 2-fold delooping of GG and the curvature characteristic form of GG is

(curv G:BG dR(B 2G))H dR(BG,B 2G), (curv_G : \mathbf{B}G \to \mathbf{\flat}_{dR}(\mathbf{B}^2 G)) \in \mathbf{H}_{dR}(\mathbf{B}G, \mathbf{B}^2 G) \,,

which is the Maurer-Cartan form of the \infty-group BG\mathbf{B}G.

While formally trivial, this definition has the following important consequence.

Remark

By the fact that the (,1)(\infty,1)-hom-functor preserves (∞,1)-limits, it follows that for every XHX \in \mathbf{H} we have a fiber sequence of cocycle ∞-groupoids

H flat(X,A)H(X,A)H dR(X,BA) \cdots \to \mathbf{H}_{flat}(X,A) \to \mathbf{H}(X,A) \to \mathbf{H}_{dR}(X, \mathbf{B}A ) \to \cdots

and hence a long exact sequence of pointed cohomology sets

H flat(X,A)H(X,A)H dR(X,BA). \cdots \to H_flat(X,A) \to H(X,A) \to H_{dR}(X, \mathbf{B}A) \,.

This says exactly that for g:XAg : X \to A an AA-cocycle, the obstruction to lifting it to a flat differential AA cocycle through H flat(X,A)H(X,A)H_{flat}(X,A) \to H(X,A) is the nontriviality of its image under H(X,A)H dR(X,BA)H(X,A) \to H_{dR}(X,\mathbf{B}A).

Notice that if A=B nKA = \mathbf{B}^n K is an Eilenberg-MacLane object for an abelian goup object KK, the above long exact sequence may be written in possibly more familiar form equivalently as

H flat n(X,K)H n(X,K)H dR n+1(X,K). \cdots \to H^n_{flat}(X,K) \to H^n(X,K) \to H_{dR}^{n+1}(X,K) \to \cdots \,.

By the intrinsic de Rham theorem, we may think of the morphism H n(X,K)H dR n+1(X,K)H^n(X,K) \to H_{dR}^{n+1}(X,K) here as being the generalization of producing an image of an integral cohomology class in real cohomology. From classical theory we expect that this image is measured by curvature data, and that curvature is the obstruction to flatness. This is the situation that the above fiber sequence formalizes. For a detailed discussion of how this connects to standard theory see the examples.

Differential cohomology

Above we defined for an object AHA \in \mathbf{H} with delooping BA\mathbf{B}A the curvature class curv:A dRBAcurv : A \to \mathbf{\flat}_{dR}\mathbf{B}A. This we now use to define differential cohomology with coefficients in AA.

Definition

(differential cohomology with groupal coefficients)

For AHA \in \mathbf{H} a group object write H diff(,A){H}_{diff}(-,A) for the twisted cohomology induced by the curvature characteristic class A dRBAA \to \mathbf{\flat}_{dR}\mathbf{B}A, i.e. for the connected components H diff(X,A):=π 0H diff(X,A)H_{diff}(X,A) := \pi_0 \mathbf{H}_{diff}(X,A) of the homotopy pullback

H diff(,A) F H dR(,BA) η H(,A) curv H dR(,BA). \array{ \mathbf{H}_{diff}(-,A) &\stackrel{F}{\to}& H_{dR}(-,\mathbf{B}A) \\ {}^{\mathllap{\eta}}\downarrow && \downarrow \\ \mathbf{H}(-,A) &\stackrel{curv}{\to}& \mathbf{H}_{dR}(-,\mathbf{B}A) } \,.

For cH diff(X,A)c \in \mathbf{H}_{diff}(X,A) a cocycle, we call

  • F(c)H dR(X,BA)F(c) \in H_{dR}(X,\mathbf{B}A) the curvature class of cc;

  • [η(c)]H(X,A)[\eta(c)] \in H(X,A) the underlying class in AA-cohomology.

Recall that H dR(X,A):=H(X, dRA)H_{dR}(X,A) := H(X, \mathbf{\flat}_{dR} A) denotes the intrinsic de Rham cohomology of H\mathbf{H}, as discussed in the section on intrinsic de Rham cohomology.

Lemma

(differential fiber sequence)

Differential cohomology fits into a fiber sequence

H(X,ΩA)H dR(X,A)H diff(X,A)H(X,A). \cdots \to \mathbf{H}(X, \Omega A) \to \mathbf{H}_{dR}(X, A) \to \mathbf{H}_{diff}(X, A) \to \mathbf{H}(X, A) \,.
Proof

This is a general statement about the definition of twisted cohomology: consider the diagram

H(X, dRΩBA) H diff(X,A) H(X, dRBA) * H(X,A) curv H(X, dRBA). \array{ \mathbf{H}(X,\mathbf{\flat}_{dR} \Omega \mathbf{B} A) &\to& \mathbf{H}_{diff}(X,A) &\to & H(X, \mathbf{\flat}_{dR} \mathbf{B}A) \\ \downarrow && \downarrow && \downarrow \\ * &\to& \mathbf{H}(X, A) &\stackrel{curv}{\to}& \mathbf{H}(X, \mathbf{\flat}_{dR} \mathbf{B} A) } \,.

The square on the right is a pullback by definition of twisted cohomology in general and our special case of differential cohomology in particular. Take the left square to be the pullback of the middle vertical morphism to the point and deduce the top left object from that: by the pasting law for (∞,1)-pullbacks this top left object is the pullback of the total diagram. But by the definition of H(X, dRBA)H(X,\mathbf{\flat}_{dR}\mathbf{B}A) as the set of connected components of H(X, dRBA)\mathbf{H}(X,\mathbf{\flat}_{dR}\mathbf{B}A) it follows that the pullback of the outer diagram is

ΩH(X, dRBA) H(X, dRBA) * H(X, dRBA). \array{ \Omega \mathbf{H}(X, \mathbf{\flat}_{dR} \mathbf{B}A) &\to& H(X,\mathbf{\flat}_{dR} \mathbf{B}A) \\ \downarrow && \downarrow \\ * &\to& \mathbf{H}(X,\mathbf{\flat}_{dR} \mathbf{B} A) } \,.

Finally using that (as discussed at cohomology and at fiber sequence) ΩH(X, dRBA)H(X,Ω dRBA)\Omega \mathbf{H}(X,\mathbf{\flat}_{dR} \mathbf{B}A) \simeq \mathbf{H}(X,\Omega \mathbf{\flat}_{dR} \mathbf{B}A) and then using the above observation that Ω dRBA dRΩBA\Omega \mathbf{\flat}_{dR} \mathbf{B}A \simeq \mathbf{\flat}_{dR} \Omega \mathbf{B}A and finally the defining equivalence ΩBAA\Omega \mathbf{B}A \simeq A the claim follows.

Corollary

Let B nK\mathbf{B}^n K be an Eilenberg-MacLane object in H\mathbf{H}, then differential cohomology in H\mathbf{H} fits into a short exact sequence

0H dR n(X,K)/H n1(X,K)H diff n(X,K)H n(X,K)0. 0 \to H_{dR}^n(X,K)/H^{n-1}(X,K) \to H_{diff}^n(X,K) \to H^n(X,K) \to 0 \,.
Proof

The above fiber sequence yields (as recalled there) a long exact sequence of pointed cohomology sets

H(X,ΩA)H(X, dRA)H diff(X,A)H(X,A). \cdots \to H(X, \Omega A) \to H(X,\mathbf{\flat}_{dR} A) \to H_{diff}(X, A) \to H(X, A) \,.

If A=B nKA = \mathbf{B}^n K is an Eilenberg-MacLane object on an abelian group object KK, then this reads

H n1(X,K)H dR n(X,K)H diff n(X,K)H n(X,K). \cdots \to H^{n-1}(X,K) \to H^{n}_{dR}(X,K) \to H_{diff}^n(X, K) \to H^n(X, K) \,.

Moreover observing that by construction the last morphism H diff n(X,K)H n(X,K)H_{diff}^n(X,K) \to H^n(X,K) is surjective (because in the defining (,1)(\infty,1)-pullback for H diff\mathbf{H}_{diff} the right vertical morphism is evidently surjective on connected components) this yields the short exact sequence as claimed.

Warning. This is essentially verbatim the expected short exact sequence familiar from ordinary generalized differential cohomology only up to the following slight nuances in notation:

  1. The cohomology groups of the short exact sequence above denote the groups obtained in the given (∞,1)-topos H\mathbf{H}, not in Top. Notably for H=\mathbf{H} = ∞LieGrpd and |X|Top|X| \in Top the geometric realization of a paracompact manifold XX, we have that H 1(X,U(1))H^1(X,U(1)) above is H sing 2(|X|,)H^2_{sing}(|X|,\mathbb{Z}) and not H sing 1(|X|,U(1))H^1_{sing}(|X|,U(1)).

  2. The fact that on the left of the short exact sequence we find the intrinsic de Rham cohomology set H dR n(X,A)H_{dR}^n(X,A) instead of something like the set of all flat forms as familiar from discussions of ordinary generalized differential cohomology is due to the fact that, following the general abstract procedure of twisted cohomology, we defined H diff(X,A)\mathbf{H}_{diff}(X,A) as the (,1)(\infty,1)-pullback of the inclusion H(X, dRA)H(X, dRA)H(X,\mathbf{\flat}_{dR}A ) \to \mathbf{H}(X,\mathbf{\flat}_{dR} A) of the set of connected components of curvature classes into the cocycle ∞-groupoid. This is the only natural thing to do in a fully natural (∞,1)-categorical setup. However, in applications we typically have a concrete model for the ∞-groupoid H(X, dRA)\mathbf{H}(X,\mathbf{\flat}_{dR} A) in mind, and then can consider the inclusion H(X, dRA) 0H(X, dRA)\mathbf{H}(X,\mathbf{\flat}_{dR} A)_0 \hookrightarrow \mathbf{H}(X,\mathbf{\flat}_{dR} A) of all of its objects. While this does not make intrinsic sense – it is a bit evil – , this is what is effectively done in ordinary generalized differential cohomology, and doing so in our definition changes the above short exact sequence slightly to become exactly the familiar sequence, for the suitable special cases.

A detailed discussion of how ordinary differential cohomology is reproduced from the above definition is in the examples.

Connections

Above we have given a general abstract definition of differential cocycles in terms of certain homotopy fibers. It is useful to concretely model these cocyclea in terms of intrinsic differential form data on the underlying underlying base XX space or the underlying principal ∞-bundle PXP \to X. Such data is called a connection on PP.

We describe now a way to encode differential cocycles with groupal coefficients AA in terms of explicit Cech cocycles with values in “pseudo-connection”-forms that are constrained to take values in genuine connection forms.

The following makes statements about explicit representatives of differential cocycles in H\mathbf{H} in terms of the model by simplicial presheaves [C op,sSet Quillen] proj,cov[C^{op}, sSet_{Quillen}]_{proj,cov}. Moreover, we invoke the model for the arrow category of H\mathbf{H}: write I=Δ[1]I = \Delta[1] for the interval, regarded as a category with two objects and a single nontrivial morphism. Then on the functor category [I,[C op,sSet]][I,[C^{op}, sSet]] we have the global projective model structure on functors [I,[C op,sSet] proj,cov] cov[I,[C^op, sSet]_{proj,cov}]_{cov}.

An object in this model is cofibrant precisely if it is given by a morphism in [C op,sSet] proj,cov[C^{op}, sSet]_{proj,cov} which is a cofibration between cofibrant objects. We use in particular that for UCU \in C a representable simplicial presheaf, the morphism UΠ R(U)U \to \mathbf{\Pi}_R(U) is such a cofibration between cofibrant objects, as discussed at path ∞-groupoid.

An object in this model is fibrant if it is a morphism between fibrant objects. We assume we have fixed a fibrant model for AA in [C op,sSet] proj,cov[C^{op}, sSet]_{proj,cov} which we denote by the same letter. We write BA\mathbf{B}A for its fibrant delooping model and EA:=(BA) I× BA*\mathbf{E}A := (\mathbf{B}A)^I \times_{\mathbf{B}A} * for the standard model for the point, that makes EABA\mathbf{E}A \to \mathbf{B}A a fibration. We use the standard properties of this resolution as described at category of fibrant objects.

Lemma

We have

  1. The object A[C op,sSet]A \in [C^{op}, sSet] is isomorphic to the simplicial presheaf

    A:U[I,[C op,sSet]]([U Π R(U)],[A *]). A : U \mapsto [I,[C^{op}, sSet]]\left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right], \left[ \array{ A \\ \downarrow \\ * } \right] \right) \,.
  2. The object A\mathbf{\flat}A is represented by the simplicial presheaf

    A:U[I,[C op,sSet]]([U Π R(U)],[A Id A]). \mathbf{\flat}A : U \mapsto [I,[C^{op}, sSet]]\left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right], \left[ \array{ A \\ \downarrow^{\mathrlap{Id}} \\ A } \right] \right) \,.

    and similarly BA\mathbf{\flat}\mathbf{B}A by

    BA:U[I,[C op,sSet]]([U Π R(U)],[BA Id BA]). \mathbf{\flat}\mathbf{B}A : U \mapsto [I,[C^{op}, sSet]]\left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right], \left[ \array{ \mathbf{B}A \\ \downarrow^{\mathrlap{Id}} \\ \mathbf{B}A } \right] \right) \,.
  3. The object dRBA\mathbf{\flat}_{dR}\mathbf{B}A is represented by the simplicial presheaf

    dRBA:U[I,[C op,sSet]]([U Π R(U)],[* BA]). \mathbf{\flat}_{dR}\mathbf{B}A : U \mapsto [I,[C^{op}, sSet]]\left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right], \left[ \array{ * \\ \downarrow \\ \mathbf{B}A } \right] \right) \,.
  4. The simplicial presheaf A diffA_{diff} defined by

    A diff:U[I,[C op,sSet]]([U Π R(U)],[A EG]) A_{diff} : U \mapsto [I,[C^{op}, sSet]]\left( \left[ \array{ U \\ \downarrow \\ \mathbf{\Pi}_R(U) } \right], \left[ \array{ A \\ \downarrow \\ \mathbf{E}G } \right] \right)

    is weakly equivalent to AA in [C op,sSet] proj[C^{op}, sSet]_{proj}, where the weak equivalence A diffAA_{diff} \stackrel{\simeq}{\to} A is induced by postcomposition with the morphism

    A Id A EA * \array{ A &\stackrel{Id}{\to}& A \\ \downarrow && \downarrow \\ \mathbf{E}A &\stackrel{\simeq}{\to}& * }

    in [I,[C op,sSet]] proj[I,[C^{op}, sSet]]_{proj}.

  5. In terms of these models the curvature characteristic class curv A:A dRBAcurv_A : A \to \mathbf{\flat}_{dR}\mathbf{B}A is modeled by the span

    curv A:AA diff dRBA curv_A : A \stackrel{\simeq}{\leftarrow} A_{diff} \to \mathbf{\flat}_{dR} \mathbf{B}A

    where the morphism on the right is given by postcomposition with the morphism

    A * EA BA \array{ A &\to& * \\ \downarrow && \downarrow \\ \mathbf{E}A &\to& \mathbf{B}A }

    in the arrow category.

Proof

We observe that the defining pasting diagram of (∞,1)-pullbacks

A * dRBA BA * BA \array{ A &\to& * \\ \downarrow && \downarrow \\ \mathbf{\flat}_{dR}\mathbf{B}A &\to& \mathbf{\flat} \mathbf{B}A \\ \downarrow && \downarrow \\ * &\to& \mathbf{B}A }

in H\mathbf{H} is modeled by the pasting diagram of ordinary pullbacks

[I,[C op,sSet]]([() Π R()],(AEA) (EAEA) (*BA) (BABA) (**) (BA*)). [I,[C^{op},sSet]] \left( \left[ \array{ (-) \\ \downarrow \\ \mathbf{\Pi}_R(-) } \right] \;\;\; , \;\;\; \array{ (A \to \mathbf{E}A) &\to& (\mathbf{E}A \to \mathbf{E}A) \\ \downarrow && \downarrow \\ (* \to \mathbf{B}A) &\to& (\mathbf{B}A\to \mathbf{B}A) \\ \downarrow && \downarrow \\ (* \to *) &\to& (\mathbf{B}A \to *) } \right) \,.

Here we use that the diagram of arrows on the right is a pullback diagram of fibrant objects in [I,[C op,sSet] proj,cov] proj[I,[C^{op}, sSet]_{proj, cov}]_{proj} with the two right vertical morphisms being fibrations, and that homming a cofibrant arrow (UΠ R(U))(U \to \mathbf{\Pi}_R(U)) into this makes this (by the defining property of the sSet-enriched model category [I,[C op,sSet] proj,cov] proj[I,[C^{op}, sSet]_{proj,cov}]_{proj}) a diagram with the same properties in sSet QuillensSet_{Quillen}. Therefore the entrie diagram above is in turn a pullback diagram of fibrant objects with the two right morphisms being fibrations in [C op,sSet] proj,cov[C^{op}, sSet]_{proj,cov}.

Now the right bottom corner of this diagram in the model clearly models the right bottom corner of the above diagram in H\mathbf{H}. Its pullback is therefore the correct homotopy pullback modelling the bottom (∞,1)-pullback. Analogously then for the top pullback diagram.

The next lemma asserts that the curvature characteristic morphism is also globally modeled by just precomposition with the fibration A * EABA\array{ A &\to& * \\ \downarrow && \downarrow \\ \mathbf{E}A \to \mathbf{B}A} in the arrow category.

Lemma

For any Y[C op,sSet] projY \in [C^{op}, sSet]_{proj} cofibrant, the morphism

[C op,sSet](Y,A diff)[C op,sSet](Y, dRBA) [C^{op},sSet](Y, A_{diff}) \to [C^{op},sSet](Y, \mathbf{\flat}_{dR}\mathbf{B}A)

in sSet induced by the curvature characteristic class AA diff dRBAA \leftarrow A_{diff} \to \mathbf{\flat}_{dR}\mathbf{B}A as modeled above is given by the morphism

[I,[C op,sSet]]([Y Π R(Y)],[A * EA BA],):[I,[C op,sSet]]([Y Π R(Y)],[A EA],)[I,[C op,sSet]]([Y Π R(Y)],[* BA],) [I,[C^{op},sSet]] \left( \left[ \array{ Y \\ \downarrow \\ \mathbf{\Pi}_R(Y) } \right], \left[ \array{ A &\to& * \\ \downarrow && \downarrow \\ \mathbf{E}A &\to & \mathbf{B}A } \right], \right) : [I,[C^{op},sSet]] \left( \left[ \array{ Y \\ \downarrow \\ \mathbf{\Pi}_R(Y) } \right], \left[ \array{ A \\ \downarrow \\ \mathbf{E}A } \right], \right) \to [I,[C^{op},sSet]] \left( \left[ \array{ Y \\ \downarrow \\ \mathbf{\Pi}_R(Y) } \right], \left[ \array{ * \\ \downarrow \\ \mathbf{B}A } \right], \right)

in that domain and codomain objects are isomorphic and the two morphisms are equal under this isomorphism.

Proof

Using end-calculus for describing the sSet-hom-objects in the enriched functor categories we have

sPSh(Y,A diff) = UCsSet(Y(U),[I,sPSh](U Π R(U),A EA)) = UCsSet(Y(U), iI(U i,A i)) = iI UCsSet(Y(U),(U i,A i)), \begin{aligned} sPSh(Y,A_{diff}) & = \int^{U \in C} sSet( Y(U), [I,sPSh] ( \array{U \\ \downarrow \\ \mathbf{\Pi}_R(U)}, \array{A \\ \downarrow \\ \mathbf{E}A} ) ) \\ & = \int^{U \in C} sSet( Y(U), \int^{i \in I} ( U_i, A_i ) ) \\ &= \int^{i \in I} \int^{U \in C} sSet( Y(U), ( U_i, A_i ) ) \end{aligned} \,,

where we set U 0:=UU_0 := U, U 1:=Π R(U)U_1 := \mathbf{\Pi}_R(U), A 0:=AA_0 := A and A 1:=EAA_1 := \mathbf{E}A and where we have used that the enriched hom preserves ends in the second argument and that by the Fubini theorem for ends the two ends commute.

Now we compute for i=0i = 0 in the integrand

UCsSet(Y(U),sPSh(U,A)) = UCsPSh(Y(U)U,A) =sPsh( UCY(U)U,A) =sPSh(Y,A), \begin{aligned} \int^{U \in C} sSet(Y(U), sPSh(U, A)) &= \int^{U \in C} sPSh(Y(U)\cdot U, A) \\ &= sPsh(\int_{U \in C} Y(U)\cdot U , A) \\ &= sPSh(Y,A) \end{aligned} \,,

where in the first step we use the tensoring of sPSh(C)sPSh(C) over sSetsSet, in the second step that the enriched hom takes coends in the first argument to ends, and finally the co-Yoneda lemma that expresses a presheaf as a coend over representables.

Analogously we compute for i=1i = 1

UCsSet(Y(U),sPSh(Π R(U),EA)) = UCsPSh(Y(U)Π R(U),EA) =sPsh( UCY(U)Π R(U),EA) =sPSh(Π R(U),EA), \begin{aligned} \int^{U \in C} sSet(Y(U), sPSh(\mathbf{\Pi}_R(U), \mathbf{E}A)) &= \int^{U \in C} sPSh(Y(U)\cdot \mathbf{\Pi}_R(U), \mathbf{E}A) \\ &= sPsh(\int_{U \in C} Y(U)\cdot \mathbf{\Pi}_R(U) , \mathbf{E}A) \\ &= sPSh(\mathbf{\Pi}_R(U),\mathbf{E}A) \end{aligned} \,,

where now in the last step we used the definition of the path ∞-groupoid functor Π R\mathbf{\Pi}_R by left Kan extension.

This shows the isomorphism of the simplicial sets in question. Since all of this is entirely natural in all variables involved, we have the equality of morphism as claimed.

Corollary

(characterization of differential cocycles)

Let A[C op,sSet] proj,covA \in [C^{op},sSet]_{proj,cov} be a fibrant representative of a group object in H\mathbf{H} and let Y[C op,sSet]Y \in [C^{op},sSet] be a cofibrant representative of an object XHX \in \mathbf{H}.

Let [P]H dR(X,BA)[P] \in H_{dR}(X,\mathbf{B}A) be a fixed curvature class, represented by some fixed P:Y dRBAP : Y \to \mathbf{\flat}_{dR}\mathbf{B}A. Then the cocycles and coboundaries in the differential cohomology of XX with coefficients in AA for curvature characteristic PP, i.e. the (∞,1)-pullback

H diff [P](X,A) * H(X,A) curv H dR(X,BA) \array{ \mathbf{H}_{diff}^{[P]}(X,A) &\to& * \\ \downarrow && \downarrow \\ \mathbf{H}(X,A) &\stackrel{curv}{\to}& \mathbf{H}_{dR}(X,\mathbf{B}A) }

is given as follows:

  • a cocycle is a commuting diagram

    Y g A Π R(Y) EA \array{ Y &\stackrel{g}{\to}& A \\ \downarrow && \downarrow^{} \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla}{\to}& \mathbf{E}A }

    in [C op,sSet][C^{op}, sSet] such that the bottom composite in

    Y g A * Π R(Y) EA BA \array{ Y &\stackrel{g}{\to}& A &\to& * \\ \downarrow && \downarrow^{} && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla}{\to}& \mathbf{E}A &\stackrel{}{\to}& \mathbf{B}A }

    equals PP.

    The underlying cocycle is η(g,)=g:YA\eta(g,\nabla) = g : Y \to A.

    We call Π R(Y)EA\mathbf{\Pi}_R(Y) \to \mathbf{E}A the connection encoded by the differential cocycle and P jP_j the curvature of this connection.

  • A coboundary between two such cocycles is a strictly commuting diagram of 2-cells in sPSh(C)sPSh(C) of the form

    Y A Π R(Y) EA \array{ Y &\stackrel{\nearrow \searrow}{\to}& A \\ \downarrow && \downarrow^{\mathrlap{}} \\ \mathbf{\Pi}_R(Y) &\stackrel{\nearrow \searrow}{\to}& \mathbf{E}A }

    such that this induces the identity on PP, i.e. such that

    Y A i A Π R(Y) EA Id Π R(Y) P BA \array{ Y &\stackrel{\nearrow \searrow}{\to}& A \\ \downarrow && \downarrow^{\mathrlap{i_A}} \\ \mathbf{\Pi}_R(Y) &\stackrel{\nearrow \searrow}{\to}& \mathbf{E}A \\ \downarrow^{\mathrlap{Id}} && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{P}{\to}& \mathbf{B}A }

    commutes strictly.

Proof

Using the above lemmas we have that the (∞,1)-pullback in question is modeled in sSet QuillensSet_{Quillen} by the ordinary pullback

[I,[C op,sSet]]([Y Π R(Y)],[A EA])[I,[C op,sSet]]([Y Π R(Y)],[* BA]). [I,[C^{op}, sSet]]\left( \left[ \array{Y \\ \downarrow \\ \mathbf{\Pi}_R(Y)} \right] , \left[ \array{A \\ \downarrow^{} \\ \mathbf{E}A} \right] \right) \to [I,[C^{op}, sSet]]\left( \left[ \array{Y \\ \downarrow \\ \mathbf{\Pi}_R(Y)} \right] , \left[ \array{* \\ \downarrow \\ \mathbf{B}A} \right] \right) \,.

All homs involved here go from cofibrant to fibrant objects. Since in a simplicial model category the enriched hom out of a cofibrant object preserves fibrations, this is a fibration in sSet, i.e. a Kan fibration. Therefore the pullback diagram of the point along this morphism is a fibrant diagram and the homotopy pullback in question is computed by the ordinary pullback in sSetsSet. The cells of this ordinary pullback in degrees 0 and 1 is what the above proposition describes.

Remark

(connections)

Conceptually the crucial aspect here is the way A diffA_{diff} serves to produce the span AA diff dRBAA \stackrel{\simeq}{\leftarrow} A_{diff} \to \mathbf{\flat}_{dR}\mathbf{B}A and induced from that the span

sPSh(Y,A)sPSh(Y,A diff)sPSh(Y, dRBA) sPSh(Y,A) \stackrel{\simeq}{\leftarrow} sPSh(Y,A_{diff}) \to sPSh(Y,\mathbf{\flat}_{dR}\mathbf{B}A)

that exhibits a map from AA-cocycles to de Rham-cocycles obtained by first lifting any given AA-cocycle g:YAg : Y \to A to an A diffA_{diff}-cocycle (g,)(g,\nabla) and then computing from the lift \nabla the corresponding characteristic class [P()]H(X, dRBA)[P(\nabla)] \in H(X,\mathbf{\flat}_{dR}\mathbf{B}A).

This lift is a choice of connection on the underlying AA-cocycle gg, respectively on the principal ∞-bundle that it classifies. With this interpretation we can label the components of the diagram that characterizes a differential cocycles according to the above corollary as follows

Y g A underlyingcocycle Π R(Y) EA connection Π R(Y) BA curvature. \array{ Y &\stackrel{g}{\to}& A &&& underlying \; cocycle \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla}{\to}& \mathbf{E}A &&& connection \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\to& \mathbf{B} A &&& curvature } \,.

The fact that sPSh(Y,A diff)sPSh(Y,A)sPSh(Y,A_{diff}) \to sPSh(Y,A) is a weak equivalence says that every equivalence class of AA-cocycles admits a connection. But in fact this morphism is even an acyclic fibration, since A diffAA_{diff} \to A is, and therefore surjective on objects. This implies in particular that every single AA-cocycle admits a lift to A diffA_{diff}, which in light of the above we may state as:

Every AA-principal ∞-bundle admits a connection.

Notice from the above, following the logic of twisted cohomology, that a choice of connection may be understood as an attempt to extend the underlying cocycle g:YAg : Y \to A to a flat differential cocycle Π R(Y)A\mathbf{\Pi}_R(Y) \to A. Because the attempt may fail, \nabla takes values not in AA, but in the contractible EA\mathbf{E}A. The composite curvature F():Π R(Y)EAp ABAF(\nabla) : \mathbf{\Pi}_R(Y) \stackrel{\nabla}{\to} \mathbf{E}A \stackrel{p_A}{\to} \mathbf{B}A projects out, precisely up to equivalence, the failure of this attempt:

by the general logic of twisted cohomology, an AA-cocycle gH(X,A)g \in \mathbf{H}(X,A) admits a lift through H(Π R(X),A)H(X,A)\mathbf{H}(\mathbf{\Pi}_R(X), A) \to \mathbf{H}(X,A) to a flat differential cocycle – i.e. the principal ∞-bundle it classifies admits a flat connection – precisely if its curvature characteristic class, its image [P(A)][P(A)] under the morphism H(X,A)H(X, dRBA)\mathbf{H}(X,A) \to H(X,\mathbf{\flat}_{dR}\mathbf{B}A), is the trivial class.

Universal connections

For mathbB nA\mathb{B}^n A an Eilenberg-MacLane object it typically happens that H(B nA, dR)B n+1A)\mathbf{H}(\mathbf{B}^n A, \mathbf{\flat}_{dR}) \mathbf{B}^{n+1}A) is 0-truncated. Equivalently, that all the intrinsic de Rham cohomologies in degree k<n+1k \lt n+1 are trivial, H(B nA, dRB kA)=*H(\mathbf{B}^n A, \mathbf{\flat}_{dR}\mathbf{B}^k A) = *.

Example. This is the case in H=\mathbf{H} = ∞LieGrpd for A=U(1)A = U(1) the circle group. The discussion is at Intrinsic de Rham cohomology of Bu U(1).

In this case we have a 2-cell

H(B nA, dRB n+1A) H(B nA, dRB n+1A) H(B nA, dRB n+1A). \array{ \mathbf{H}(\mathbf{B}^n A, \mathbf{\flat}_{dR} \mathbf{B}^{n+1}A) &\to& H(\mathbf{B}^n A, \mathbf{\flat}_{dR} \mathbf{B}^{n+1}A) \\ & \searrow & \downarrow \\ && \mathbf{H}(\mathbf{B}^n A, \mathbf{\flat}_{dR} \mathbf{B}^{n+1}A) } \,.

By the (,1)(\infty,1)-pullback definition of differental cohomology, this induces a morphism

H(B nA,B nA)H diff(B nA,H(B nA). \mathbf{H}(\mathbf{B}^n A, \mathbf{B}^n A) \to \mathbf{H}_{diff}(\mathbf{B}^n A, \mathbf{H}(\mathbf{B}^n A) \,.

This image of Id B nAId_{\mathbf{B}^n A} under this map we call the universal B n1A\mathbf{B}^{n-1}A-connection on B nA\mathbf{B}^n A:

univ:{Id}H(BA,BA)H diff(B nA,B nA). \nabla_{univ} : \{Id\} \hookrightarrow \mathbf{H}(\mathbf{B}A, \mathbf{B}A) \to \mathbf{H}_{diff}(\mathbf{B}^n A, \mathbf{B}^n A) \,.

For c:BGB nAc : \mathbf{B}G \to \mathbf{B}^n A any morphism, we get an induced diagram

H(B nA,B nA) H dR(B nA,B n+1A) H dR(B nA,B n+1A) H(BG,B nA) H dR(BG,B n+1A) H dR(BG,B n+1A) \array{ \mathbf{H}(\mathbf{B}^n A, \mathbf{B}^n A) &\to& \mathbf{H}_{dR}(\mathbf{B}^n A, \mathbf{B}^{n+1} A) &\leftarrow& H_{dR}(\mathbf{B}^n A, \mathbf{B}^{n+1} A) \\ \downarrow && \downarrow && \downarrow \\ \mathbf{H}(\mathbf{B}G, \mathbf{B}^n A) &\to& \mathbf{H}_{dR}(\mathbf{B}G, \mathbf{B}^{n+1} A) &\leftarrow& H_{dR}(\mathbf{B}G, \mathbf{B}^{n+1} A) }

in H\mathbf{H}. This morphism of diagrams induces a morphism of the (∞,1)-pullbacks

H diff(B nA,B nA)H diff(BG,B nA) \mathbf{H}_{diff}(\mathbf{B}^n A, \mathbf{B}^n A) \to \mathbf{H}_{diff}(\mathbf{B}G, \mathbf{B}^n A)

and the composite

c * univ:{Id} univH diff(B nA,B nA)H diff(BG,B nA) c^* \nabla_{univ} : \{Id\} \stackrel{\nabla_{univ}}{\to} \mathbf{H}_{diff}(\mathbf{B}^n A, \mathbf{B}^n A) \to \mathbf{H}_{diff}(\mathbf{B}G, \mathbf{B}^n A)

produces a canonical B n1A\mathbf{B}^{n-1}A-connection on BG\mathbf{B}G.

Example See circle n-bundle with connection.

Holonomy

An ordinary connection on a bundle (P,)(P,\nabla) over a space XX induces a holonomy-map from loops in XX to the structure group: the parallel transport of the connection along the loops. One may think of the holonomy of a loop γ:[0,1]X\gamma : [0,1] \to X as associated to the pullback bundle with connection (γ *P,γ *)(\gamma^* P , \gamma^* \nabla) over S 1S^1. From this perspective the fact that a notion of holonomy exists and is well defined is crucially related to the fact that the curvature of γ *\gamma^* \nabla necessarily vanishes. Similarly, a connection \nabla on a principal 2-bundle PP induces a notion of holonomy over 2-dimensional surfaces ϕ:Σ 2X\phi : \Sigma_2 \to X, and this too may be understood as induced from the flat pullback 2-connection ϕ *\phi^* \nabla.

The following definition generalizes this to a notion of holonomy of differential cocycles with groupal coefficients in an \infty-connected (,1)(\infty,1)-topos H\mathbf{H}. That in H=\mathbf{H} = ∞LieGrpd this reproduces the ordinary notion of holonomy of circle bundles and bundle nn-gerbes is discussed at differential cohomology in an (∞,1)-topos -- examples.

Consider Σ,X,AA\Sigma, X, A \in \mathbf{A} such that the intrinsic de Rham cohomology of Σ\Sigma with coefficients in BA\mathbf{B}A is trivial,

H dR(Σ,BA)*. \mathbf{H}_{dR}(\Sigma, \mathbf{B}A) \simeq * \,.

Then every morphism phi:ΣXphi : \Sigma \to X induces a morphism

hol ϕ:H diff(X,A)H flat(X,A) hol_{\phi} : \mathbf{H}_{diff}(X,A) \to H_{flat}(X,A)

from the differential cohomology of XX to the flat differential cohomology of Σ\Sigma, from the diagram

H diff(X,A) H dR(X,BA) H flat(Σ,A) * H(X,A) H dR(X,BA) H(Σ,A) H dR(Σ,BA). \array{ \mathbf{H}_{diff}(X,A) &\to& &\to& H_{dR}(X,\mathbf{B}A) \\ \downarrow &\searrow& && \downarrow & \searrow \\ && \mathbf{H}_{flat}(\Sigma,A) &\to& &\to& * \\ \downarrow && \downarrow && \downarrow && \downarrow \\ \mathbf{H}(X,A) &\to& &\to& \mathbf{H}_{dR}(X,\mathbf{B}A) \\ &\searrow& \downarrow && &\searrow & \downarrow \\ && \mathbf{H}(\Sigma,A) &\to& &\to& \mathbf{H}_{dR}(\Sigma,\mathbf{B}A) } \,.

For H diff(X,A)\nabla \in \mathbf{H}_{diff}(X,A) a differential cocycle, we call hol ϕ()hol_\phi(\nabla) the holonomy of \nabla along ϕ\phi.

Differential cohomology with general coefficients

Above we described differential cohomology in an with ∞-connected (∞,1)-topos H\mathbf{H} with groupal coefficients AHA \in \mathbf{H} as the twisted cohomology for the canonical curvature characteristic class A dRBAA \to \mathbf{\flat}_{dR} \mathbf{B}A. We showed that this is an exact obstruction theory, in that the curvature class XAcurv A dRAX \to A \stackrel{curv_A}{\to} \mathbf{\flat}_{dR}A of an AA-cocycle g:XAg : X \to A is trivial in cohomology if and only if gg extends to flat differential cohomology.

If AA fails to be deloopable, we can find various approximations to obstruction classes against flat refinements.

Curvature characteristic classes

A characteristic class on BG\mathbf{B}G is simply any morphism

c:BGB nK c : \mathbf{B}G \to \mathbf{B}^n K

in H\mathbf{H}.

The composition operation with the canonical curvature characteristic form curv B nKcurv_{\mathbf{B}^{n} K}

B nK c curv B nK BG curv B nKc dRB nK \array{ && \mathbf{B}^n K \\ & {}^{\mathllap{c}}\nearrow & \Downarrow^{\mathrlap{\simeq}} & \searrow^{\mathrlap{curv_{\mathbf{B}^n K}}} \\ \mathbf{B}G &&\underset{curv_{\mathbf{B}^n K}\circ c}{\to}&& \mathbf{\flat}_{dR}\mathbf{B}^n K }

induces a square

H(X,BG) H dR n+1X H(X,B nU(1)) curv H dR(X,B n+1K). \array{ \mathbf{H}(X,\mathbf{B}G) &\to& H_{dR}^{n+1}{X} \\ \downarrow &{}^{\mathllap{\simeq}}\swArrow& \downarrow \\ \mathbf{H}(X, \mathbf{B}^n U(1)) &\stackrel{curv}{\to}& \mathbf{H}_{dR}(X,\mathbf{B}^{n+1}K) } \,.

By the universal propery of the (∞,1)-pullback this yields a morphism

H(X,BG)H diff(X,B nK) \mathbf{H}(X,\mathbf{B}G) \to \mathbf{H}_{diff}(X,\mathbf{B}^n K)

from GG-principal ∞-bundles to abelian differential cohomology.

This is the curvature characteristic of the given characteristic class.

Connections

Above in Connections we had seen that cocycles in differential cohomology with grobal coefficients may be modeled in [C op,sSet][C^{op}, sSet] by connections

Y B nK Π R(Y) EB nK. \array{ Y &\to& \mathbf{B}^n K \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\stackrel{\nabla}{\to}& \mathbf{E}\mathbf{B}^n K } \,.

For BGA\mathbf{B}G \to A a given curvature characteristic class there typically is a groupal model for the universal G-principal ∞-bundle EG\mathbf{E}G and a diagram

B^G B nK BEG EB nK \array{ \hat \mathbf{B}G &\to& \mathbf{B}^n K \\ \downarrow && \downarrow \\ \mathbf{B}\mathbf{E}G &\to& \mathbf{E}\mathbf{B}^n K }

covering the model for the characteristic class. In this case the curvature characteristic class is modeled by a diagram

Y B^G underlyingcocycle Π R(Y) BEG connectionandcurvature Π R(Y) B n+1K curvaturecharacteristicform. \array{ Y &\to& \hat \mathbf{B}G &&& underlying\;cocycle \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\to& \mathbf{B}\mathbf{E}G &&& connection\;and\;curvature \\ \downarrow && \downarrow \\ \mathbf{\Pi}_R(Y) &\to& \mathbf{B}^{n+1}K &&& curvature\;characteristic\;form } \,.
Example

(\infty-Chern-Weil homomorphism)

In H=\mathbf{H} = ∞LieGrpd such constructions may be obtained by Lie integration. This is described at ∞-Chern-Weil homomorphism.

The intrinsic Chern character

We say a line object RR in an ∞-connected (∞,1)-topos H\mathbf{H} is compatible with the \infty-connectivity if its geometric realization is contractible

Π(R)*. \Pi(R) \simeq * \,.

Being in particular an abelian ∞-group-object, the line object induces a notion of RR-cohomology

H n(,R):=π 0H(,B nR). H^n(-,R) := \pi_0 \mathbf{H}(-,\mathbf{B}^n R) \,.

Say that a moprhism f:XYf : X \to Y in H\mathbf{H} is trivial in RR-cohomology if

H n(f,R):H n(X,R)H n(Y,R) H^n(f,R) : H^n(X,R) \to H^n(Y,R)

is an isomorphism for all nn \in \mathbb{N}. We have the universal localization of an (∞,1)-category at these morphisms, giving a reflective sub-(∞,1)-category

(L Ri R):Li RL RH. (L_R \dashv i_R) : \mathbf{L} \stackrel{\overset{L_R}{\leftarrow}}{\underset{i_R}{\hookrightarrow}} \mathbf{H} \,.

We say that this exhibits on H\mathbf{H} the structure of rational homotopy theory in an (∞,1)-topos if combined with the terminal geometric morphism

Li RL RHΓLConstΠGrpd \mathbf{L} \stackrel{\overset{L_R}{\leftarrow}}{\underset{i_R}{\hookrightarrow}} \mathbf{H} \stackrel{\overset{\Pi}{\to}}{\stackrel{\overset{LConst}{\leftarrow}}{\underset{\Gamma}{\to}}} \infty Grpd

we have that the composite (∞,1)-adjunction

GrpdLConstHL RLi RHΓGrpd \infty Grpd \stackrel{LConst}{\to} \mathbf{H} \stackrel{L_R}{\to} \mathbf{L} \stackrel{i_R}{\to} \mathbf{H} \stackrel{\Gamma}{\to} \infty Grpd

is rationalization XXRX \mapsto X \otimes R over the field RR.

We have then the triple composite adjunction

Π()R:HΠGrpdLConstHL RLi RHΓGrpdLConstH. \mathbf{\Pi}(-)\otimes R : \mathbf{H} \stackrel{\Pi}{\to} \infty Grpd \stackrel{LConst}{\to} \mathbf{H} \stackrel{L_R}{\to} \mathbf{L} \stackrel{i_R}{\to} \mathbf{H} \stackrel{\Gamma}{\to} \infty Grpd \stackrel{LConst}{\to} \mathbf{H} \,.

We say that its unit

ch A:AΠ(A)R:=LConst(Π(A)R) ch_A : A \to \mathbf{\Pi}(A) \otimes R := LConst(\Pi(A)\otimes R)

is the intrinsic Chern-character on AA in H\mathbf{H}.

Twisted differential cohomology

For f:BCf : B \to C a morphism – a characteristic class – with homotopy fiber ABA \to B, we may think of the H(X,A)× H(X,B)H(X,B)\mathbf{H}(X,A) \times_{\mathbf{H}(X,B)} H(X,B) as the ff-twisted cohomology with coefficients in AA.

Above we conceived differential cohomology as curvature-twisted or Chern-character-twisted cohomology. This may be paired with the twisting ff simply by forming the product characteristic class

(ch B,f):B( dRΠ dRB)×C. (ch_B, f) : B \to (\mathbf{\flat}_{dR}\mathbf{\Pi}_{dR}B ) \times C \,.

For suitable choices of ff this then discribes twisted bundles with connection, twisted 2-bundles with connection, etc.

This has been considered in

where connection data on twisted String-principal bundles and twisted Fivebrane-principal bundles is worked out.

(…)


Context

  1. survey

  2. general structures

  3. paths

  4. Lie theory

  5. differential cohomology

  6. examples

  7. references

Revised on October 13, 2010 20:55:04 by Urs Schreiber (131.211.232.122)