This is a sub-entry of infinity-Chern-Simons theory. See there for context.
Contents
Examples
1d Chern-Simons functionals
2d Chern-Simons functionals
Poisson -model
3d Chern-Simons functionals
Ordinary Chern-Simons theory
Lagrangian
Let be a semisimple Lie algebra. For the following computations, choose a basis of and let denote the corresponding degree-shifted basis of .
Notice that in terms of this the differential of the Chevalley-Eilenberg algebra is
and that of the Weil algebra
and
Let be the Killing form invariant polynomial. This being invariant
is equivalent to the fact that the coefficients
are skew-symmetric in and , and therefore skew in all three indices.
Proposition
A Chern-Simons element for the Killing form invariant polynomial is
In particular the Killing form is in transgression with the degree 3-cocycle
Proof
We compute
Under a Lie algebra-valued form
this Chern-Simons element is sent to
If is a matrix Lie algebra then the Killing form is the trace and this is equivalently
This is a traditional incarnation of the standard Chern-Simons form in degree 3.
For a 3-dimensional smooth manifold the corresponding action functional
is the standard action functional of Chern-Simons theory.
Covariant phase space
Obstruction theory
The circle n-bundle with connection given by ordinary Chern-Simons theory is known as the Chern-Simons circle 3-bundle . The non-triviality of its underlying class is the obstruction to lifting the -principal bundle to a string structure. The non-triviality of its connection is the obstruction to having a differential string structure. In general it defines a twisted differential string structure.
Courant -model
4d Chern-Simons functionals
BF-theory and topological Yang-Mills theory
Let be a strict Lie 2-algebra, an L-∞ algebra concentrated in the lowesr two degrees and with the only nontrivial bracket being the binary one.
Proposition
We have
-
every invariant polynomial on is a Chern-Simons element on , exhibiting a transgression to a trivial ∞-Lie algebra cocycle;
-
for a semisimple Lie algebra and the Killing form, the corresponding Chern-Simons action functional on L-∞ algebra valued forms
is the sum of the action functionals of topological Yang-Mills theory with BF-theory with cosmological constant (in the sense of gravity as a BF-theory):
where is the ordinary curvature 2-form of .
This is from (SSSI).
Proof
For a basis of and a basis of we have
Therefore with we have
The right hand is a polynomial in the shifted generators of , and hence an invariant polynomial on . Therefore is a Chern-Simons element for it.
Now for an ∞-Lie algebra-valued form, we have that the 2-form curvature is
Therefore
7d Chern-Simons functionals
7d -Chern-Simons theory
In heterotic string theory Chern-Simons circle 3-bundles appear as the ordinary differential cohomology-incarnation of magnetic charge of NS-fivebranes that twists the Kalb-Ramond field as described by the Green-Schwarz mechanism.
At least when this twist vanishes there is expected to be an electric-magnetic dual description with a Chern-Simons circle 7-bundle.
Lagrangian
Let be the canonical quaternaty invariant polynomial on the special orthogonal Lie algebra . This lifts directly also to an invariant polynomial on the string Lie 2-algebra .
(…)
Obstruction theory
The circle n-bundle with connection given by 7-dimensional Chern-Simons theory is the Chern-Simons circle 7-bundle . The non-triviality of its underlying class is the obstruction to lifting the string 2-group-principal 2-bundle to a fivebrane structure. The non-triviality of its 7-connection is the obstruction to having a differential fivebrane structure. In general it defines a twisted differential fivebrane structure.
Higher dimensional abelian Chern-Simons theory
The line Lie n-algebra carries a canonical invariant polynomial. The -Chern-Simons theory associated with this data is in the literature known as abelian higher dimensional Chern-Simons theory.
Also every higher-degree invariant polynomial on an ordinary Lie algebra gives rise to a higher dimensional Chern-Simons action functional. An concrete example of this is considered (below).
-Dijkgraaf-Witten theory
We consider the case where the target space object is a discrete ∞-groupoid.
with the delooping of an ∞-group Grpd Top.
As we discuss below, -Chern-Simons theory for this setup subsumes and generalizes Dijkgraaf-Witten theory (and the Yetter model in next higher dimension). Therefore we speak of -Dijkgraaf-Witten theory.
Proof
By the -adjoint triple of the ambient cohesive (∞,1)-topos and usinf that is a full and faithful (∞,1)-functor we have
and therefore, using the -zig-zag identity, the constant path inclusion
is an equivalence. Therefore the intrinsic de Rham cohomology of is trivial
and so the intrinsic universal curvature class
is trivial.
3d Dijkgraaf-Witten theory
Dijkgraaf-Witten theory is the analog of
Chern-Simons theory for discrete structure groups. We show that this becomes a precise statement in : the Dijkgraaf-Witten action functional is that induced from applying the -Chern-Simons homomorphism to a characteristic class of the form , for the canonical embedding of discrete ∞-groupoids into all smooth ∞-groupoids.
Lagrangian
Let be a discrete group regarded as an ∞-group object in discrete ∞-groupoids and hence as a smooth ∞-groupoid with discrete smooth cohesion. Write for its delooping in ∞Grpd and for its delooping in Smooth∞Grpd.
We also write . Notice that this is different from , reflecting the fact that has non-discrete smooth structure.
Proposition
For a discrete group, morphisms correspond precisely to cocycles in the ordinary group cohomology of with coefficients in the discrete group underlying the circle group
Proof
By the -adjunction we have
Proposition
For discrete
Proof
By the -adjunction and using that for all .
It follows that for discrete
Proposition
For discrete and any group 3-cocycle, the -Chern-Simons theory action functional on a 3-dimensional manifold
is the action functional of Dijkgraaf-Witten theory.
Proof
By proposition the morphism is given by evaluation of the pullback of the cocycle along a given , on the fundamental homology class of . This is the definition of the Dijkgraaf-Witten action (for instance equation (1.2) in FreedQuinn).
Obstruction theory
The flat Dijkgraaf-Witten circle 3-bundle on is the obstruction to lifting the -principal bundle to a -principal 2-bundle, where is the discrete 2-group classified by the group 3-cocycle.
(…)
4d Yetter model
See
Closed string field theory
For the moment see closed string field theory .
AKSZ theory
We consider symplectic Lie n-algebroids equipped with a canonical invariant polynomial and show that the ∞-Chern-Simons action functional associated to this data is the action functionalof the AKSZ theory sigma model with target space .
This is taken from (FRS11). See there for more details.
Lagrangian
This means that
-
on each coordinate chart of the base manifold of , there is a basis for such that
with and ;
-
the coefficient matrix has an inverse;
-
we have
This -Lie theoretic structure is essentially what in the literature is mostly considered in terms of symplectic dg-geometry :
Observation
We may think of an L-infinity-algebroid as a graded manifold whose global function ring is the graded algebra underlying the Chevalley-Eilenberg algebra
and which is equipped with a vector field of grade 1 whose graded Lie bracket with itself vanishes , given, as a derivation, by the
differential on the Chevalley-Eilenberg algebra:
The pair is a differential graded manifold . In this perspective the graded algebra underlying the Weil algebra of is the de Rham complex of
but the de Rham differential is just , not the full differential on the Weil algebra. The latter is thus a twisted de Rham differential on .
From this perspective all standard constructions of Cartan calculus usefully apply to -algebroids. Notably for any vector field on there is the contraction derivation
and hence the Lie derivative
So in the above notation we have in particular
Definition
For a dg-manifold, let be the vector field which over any coordinate patch is given by the formula
where is a basis of generators and the degree of a generator.
We write
for the Lie derivative of this vector field. The grade of a homogeneous element in is the unique natural number with
Remark.
-
This implies that for an element of grade on , the 1-form is also of grade . This is why we speak of grade (as in “graded manifold”) instead of degree here.
-
The above is indeed well-defined: on overlaps of patches the of positive degree/grade transform by a degreewise linear transformation, which manifestly preserves . Notice that the ordinary (degree-0 coordinates) do not appear in this formula. And indeed the vector field locally defined by (thus including the coordinates of grade 0) does not in general exist globally.
The existence of implies the following useful statement, which is a trivial variant of what in grade 0 would be the standard Poincare lemma.
Observation
On a graded manifold every closed differential form of positive grade is exact: the form
satisfies
Using this differential geometric language we can now capture something very close to def. in more traditional symplectic geometry terms.
Definition
A symplectic dg-manifold of grade is a dg-manifold equipped with 2-form which is
- \item non-degenerate;
- closed;
as usual for symplectic forms, and in addition
- of grade ;
- -invariant: .
Example. It follows that a symplectic dg-manifold of grade 0 is the same as an ordinary symplectic manifold. In the following we are mostly interested in the case of positive grade.
Observation
The function algebra of a symplectic dg-manifold of grade is naturally equipped with a Poisson bracket
which decreases grade by . On a local coordinate patch this is given by
where is the inverse matrix to .
Observation
For and we say that fv_ or equivalently that
_
vf$ if
Proposition
There is a of symplectic dg-manifolds of grade into symplectic Lie -algebroids.
Proof
The dg-manifold itself is identified with an -algebroid as in observation . For a symplectic form, the conditions and imply and hence that under the identification this is an invariant polynomial on .
It remains to observe that the -algebroid is in fact a Lie -algebroid. This is implied by the fact that is of grade and non-degenerate: the former condition implies that it has no components in elements of grade and the latter then implies that all such elements vanish.
Proposition
Let be a symplectic Lie -algebroid for positive in the image of the embedding of prop. . Then it carries the canonical -algebroid cocycle
which moreover is the Hamiltonian, according to def. , of .
Proof
The required condition from def. holds by observation .
Our central observation now is the following.
Proposition
The cocycle from prop. is in transgression with the invariant polynomial . A Chern-Simons element witnessing the transgression according to def. is
Proof
It is clear that . So it remains to check that . Notice that
by Cartan calculus. Using this we compute the first summand in :
The second summand is simply
since is a cocycle.
Proposition
For a symplectic Lie -algebroid coming from a symplectic dg-manifold by prop. , the higher Chern-Simons action functional associated with its canonical Chern-Simons element from prop. is the AKSZ Lagrangean:
Proof
We work in local coordinates where
and the Chern-Simons element is
We want to substitute here . Notice that in coordinates the equation
becomes
Therefore
Hence
This means that for an -dimensional manifold and
a -valued differential form on we have
This is indeed .
Remark The AKSZ -model action functional interpretation of -Chern-Weil functionals for binary invariant polynomials on -algebroids from prop. gives rise to the following dictionary of concepts\
Covariant phase space
Proposition
The of with target is the space of those whose -form vanishes
The on this space is
Proof
This is a special case of prop. , prop. in view of corollary , using that, by definition of , is a binary and non-degenerate .
– The topological particle
For a we may regard its cotangent bundle as a Lie 0-algebroid and the canonical 2-form as a binary invariant polynomial in degree 2.
The Chern-Simons element is the canonical 1-form which in local coordinates is .
The corresponding action functional on the line
is the familiar term for the action functional of the particle (missing the kinetic term, which makes it “topological”).
– The Poisson -model
Let be a . Over a Darboux chart the corresponding has coordinates of degree 0 and of degree 1. We have
where and
The Hamiltonian cocycle from prop. is
and the Chern-Simons element from prop. is
In terms of instead of this is
So for
a Poisson-Lie algebroid valued differential form – which in components is a function and a 1-form – the corresponding Chern-Simons form is
This is the Lagrangean of the Poisson -model [CattaneoFelder].
– Ordinary Chern-Simons theory
We show how the ordinary arises from this perspective. So let be a and its Killing form invariant polynomial. For a dual basis for we have
where and
where . The Hamiltonian cocycle from prop. is
Therefore in this case the Chern-Simons element from def. becomes
This is indeed the familiar standard choice of Chern-Simons element on a Lie algebra. Notice that evaluated on a -valued form
this is
If is a matrix Lie algebra then the Killing form is proportional to the trace of the matrix product: . In this case we have
and hence
Often this is written in terms of the de Rham differential 2-form instead of the curvature 2-form . Since the former is the image under of we can alternatively write
Hence
– The Courant -model
(…)
Supergravity
Lagrangians
References
The notion of Chern-Simons elements for -algebras and the associated -Chern-Simons Lagrangians is due to
- , , , -connections (web)
The induced construction of the with special attention to the and the is in
In the general context of -Chern-Simons theory is discussed in section 4.3 of
The case of the is discussed in
Discussion of s is in
-
, Courant algebroids, derived brackets and even symplectic supermanifolds PhD thesis (arXiv)
On the structure of graded symplectic supermanifolds and Courant algebroids (arXiv)