## Archive for **November 2012**

## The Dirac operator

We begin by endowing a vector bundle with a Clifford module structure. It is with additional structure that we may define a Dirac operator.

Let be an -dimensional Riemannian manifold with covariant derivative (on ) , and let be a vector bundle.

** Clifford Module Bundles and a Dirac “Type” Operator**

**Definition** (Clifford module)

A **Clifford module** for a real inner product space is a left module over . Equivalently, there is a -algebra homomorphism given by . Since for any (see the Glossary below), one has that satisfies .

**Definition** (Bundle of Clifford Modules)

A bundle (as above) is a **bundle of Clifford modules** if there is a map of bundles of -algebras such that for any section . In other words, for each , is a Clifford module for .

**Definition** (Dirac type operator)

Let be a Clifford module bundle equipped with a covariant derivative . Let be the map defined by the composition

where is the inverse of the bundle isomorphism , and where is Clifford multiplication. We call such a map (which depends on the Clifford module bundle , and ) **a Dirac type operator**.

If we a fix an orthonormal frame for over some neighborhood and, using the metric , let be the corresponding frame for , we may write this composition locally as

That is, a Dirac type operator is locally of the form

.

**Proposition**

A Dirac type operator is a first order differential operator.

**Proof**

Let and . Using the local description above, we compute:

.

In particular, for , so is -linear and hence in . Thus , and as itself is not -linear, is of order 1.

**Remark**

The above proof extends (by incorporating induction) to show that the composition of a – and an order differential operator is a differential operator of order . Here, and are differential operators of order 1 and 0, respectively.

We next show that is elliptic.

**Lemma** (Symbol of a Dirac type operator)

Let be a Dirac type operator and let . Then the symbol of at is given by

,

where is the dual to determined by the metric, i.e., such that .

**Proof**

Fix , and let be an open neighborhood of in . Using choose an orthonormal frame of with dual frame for . Being a bundle homomorphism (over ), is -linear in the -ordinate. Thus it suffices to verify the proposition for ; that is, we wish to show that for .

Choose a local chart such that ; let . Note that is an orthonormal basis for . Let ; then , so .

Thus (see the Glossary below), as is order , we have

,

as required.

** Corollary **

A Dirac type operator is elliptic.

** Proof **

If then has inverse .

** Remark **

Noting that (the product of linear maps), we observe that . Taking , this — apparently — implies that a Dirac type operator is, at the symbol level, the square root of the Laplacian.

** -grading and “a” Dirac Operator**

A Dirac type operator is formally self-adjoint (so that its index is ) if we impose the following further restrictions on the Clifford-module bundle.

**Definition** (Clifford-Compatible)

Let be a bundle of Clifford modules. We say that is **Clifford compatible** if it is equipped with a metric and a covariant derivative such that

(1) is Riemannian, i.e., for all sections :

, and

(2) for all vector fields and for any section :

.

**Definition** (A Dirac operator)

A differential operator is **a Dirac operator** if

(1) is a Dirac type operator, and

(2) is Clifford compatible.

** Lemma**

If is oriented and is a Dirac operator, then is formally self-adjoint. That is,

,

where (and the integration is with respect to the volume form on ).

** Proof **

Omitted ðŸ˜¦

Consequently we have (see Ning’s blog). To make use of the index, then, we introduce a -grading on Clifford module bundles.

**Definition**

Recall that a Clifford algebra is -filtered — with — and -graded — (even and odd products).

A Clifford module bundle is **-graded** if it decomposes into a direct sum of vector bundles such that for each and , one has .

Such a -graded bundle is **compatible** if this decomposition is both orthogonal with respect to and parallel with respect to the covariant derivative , ie. .

** Example **

If is oriented, the Clifford bundle is a -graded compatible Clifford bundle. Some words which may be connected to verify this: Levi Civita connection, induced connection on , lift to principal spin bundle, induced covariant derivative on associated vector bundle, compatibility with the metric.

** Examples of Dirac Operators**

We now look at four examples of Dirac operators. The first two are familiar; here we reinterpret them in terms of Clifford modules.

**Example 1: The De Rham Operator**

Recall that the filtered algebra has associated graded algebra . (See the Glossary below.)

** Lemma**

Let be the map defined by

.

Then is

(1) an isomorphism of vector spaces

(2) filtration preserving, i.e., , and

(3) -equivariant, i.e., for and .

** Proof**

(1) and (2). Let is an orthonormal basis for . Since (by an equivalent definition of the exterior algebra) , we see that is induced by the map taking to and descending to ; that is, . It follows that is an isomorphism and preserves the grading.

(3) Using that , I feel like we need to be working with here. Please comment!

** Corollary**

The exterior algebra bundle over a(n oriented?) manifold and the clifford bundles are isomorphic as vector bundles.

** Proof **

Let denote the principal -bundle associated to . By parts (1) and (3) of the lemma, the map above induces a vector bundle isomorphism .

** Theorem 2.5.12 (Lawson, Michelsohn)**

Under this bundle isomorphism , the de Rham operator corresponds to the Dirac operator .

** Corollary**

Since we have already established (see Hailiang’s(?) blog post) that the Euler characteristic of is equal to , the theorem (along with the grading-preserving property of ) implies we may also compute it as .

** Example 2: The Signature Operator**

We now look to reinterpret the signature operator in terms of Clifford bundles.

Recall (see Hailiang’s blog) in the case that and is even, the Hodge star operator is an involution so we can decompose into the and eigenspaces of . We defined the signature operator . Since , we saw that took to and letting , we found that . The signature of was defined to be the signature of the quadratic form on given by .

In the case that is odd, we had to modify the construction. We complexified, taking , and defined . Then the above paragraph went through with replacing and replacing .

Let be an oriented orthonormal basis for . Let in . Then by the lemma above and the corresponding properties of the volume form in , we obtain that is a basis-independent section of .

** Lemma **

(1) We have

(2) If is even and , then .

** Proof **

(1) We compute . Writing for , one finds that is even if and only if or .

(2) It suffices to verify for . We have and . (Here, a hat indicates that the element be omitted from the product.) Since is even, .

Now, acts on any Clifford module via , and by part (1) of the lemma this defines an involution in the case ; in the case , defines an involution. Compare with the Hodge star operator recalled above. So define

then . Thus if is an oriented manifold and is a Clifford module bundle of , putting , we have

if : , or

if :

** Corollary **

If is even then is -graded, i.e., for and , one has .

** Proof **

By part (2) of the lemma, for we have .

Thus if (i.e. ) then , so .

** Proposition **

If is even and is -graded compatible, then the associated Dirac operator splits as

.

In particular, if , by Theorem 2.5.12 we have .

**Proof**

Since is -graded and is compatible (so in particular, the covariant derivative preserves , the Dirac operator

takes to .

** Example 3: twisted Dirac Operators **

** Preliminary:** If and are vector bundles over with covariant derivatives and , respectively, then the tensor product bundle has covariant derivative

.

**Fact:** If is a compatible -graded Clifford module bundle and is a Riemannian bundle (see Property (1) of a compatible Clifford Bundle above for the definition), then is a compatible -graded Clifford module bundle (with Clifford multiplication for , , ). In the case that , we call the Dirac operator on a **twisted Dirac operator**.

**Fact:** (Apparently from topological K-theory) If the Index theorem holds for any twisted *Signature* operator then it holds for all elliptic differential operators.

** Example 4: Spin Manifolds and The Atiyah-Singer Dirac Operator**

Recall (see Prasit’s blog) that there is an isomorphism . So since

is an module, one has that is an -module via , for and . To avoid confusion, let us call with this module structure .

Now, any -module is isomorphic to , so it follows that any -module is isomorphic to .

Let be a Clifford module bundle. From the above paragraph we see that each fiber (a -module) is isomorphic (via , say) to , where is a copy of . We may then

ask if this splitting extends over the whole bundle; that is, is there a Clifford module bundle and a bundle isomorphism which restricts fiberwise to an isomorphism .

In turns out the answer is a resounding “Yes” if is a spin manifold.

**Definition** (Spin Structure)

Let is an -dimensional vector bundle. A **spin structure** on is a principal -bundle together with a bundle isomorphism . (Then is the associated vector bundle for ). Using classifying space theory, we may reinterpret this

to say that a spin structure on is a lift of the classifying map to .

We may break up the existence of a spin structure into pieces as follows.

After choosing a metric on , we may first reduce the structure group of to . (The only obstruction to doing so is the paracompactness of .) So we’re left to lift a map to .

Since is the universal cover of , we may first try to lift to .

The short exact sequence of groups induces a fibration of classifying spaces

.

It turns out that the map lifts to if and only if the composite is nullhomotopic. Since , there is an element that vanishes if and only if lifts. We call the first Stiefel Whitney class of .

Similarly, the map lifts to if and only if the composition is nullhomotopic; we call the corresponding element in (that vanishes iff lifts) the second Stiefel Whitney class of .

**Definition** (Spin manifold)

We will call an oriented manifold (so ) a **spin manifold** if its tangent bundle admits a spin structure (i.e., ). It can be shown that this is equivalent to the existence of a trivialization of over the -skeleton of . (Compare with the fact that is orientable if and only if is trivializable over the -skeleton.)

**Definition** (The Atiyah-Singer Dirac Operator)

Suppose has a spin structure with principal -bundle (associated to ). Since acts on on the left and (where has general element with , ) is a subgroup of the group of units , we may define

the -bundle associated to by . Since is a module, is a Clifford module bundle.

Some words: By lifting the Levi-Civita connection on one obtains a connection on , and hence (see who’s blog?) a covariant derivative on which makes it Clifford compatible

as a graded Clifford module bundle.

We may then define the ** Atiyah-Singer Dirac Operator** by

.

Some more words:

If then is called a harmonic spinor.

If has positive scalar curvature, then is injective. So if we have ways to compute the index (using the ASHI theorem, for example), we may be able to deduce that does not admit a metric of positive scalar curvature.

**Glossary**

(to include links to other blog posts)

** Differential Operator (global definition) **

If and are vector bundles over (of the same dimension), we define the family of differential operators of order from to by

with . In particular, .

** Symbol of a differential operator**

(cf. Juanita’s blog) We recall the definition of the symbol of an order- differential operator . Denote , , so . Let . Let . The symbol of at is the homomorphism defined by

where , and is such that .

** Covariant Derivative**

A covariant derivative on a vector bundle is a map , where is an -linear map satisfying the Leibnitz rule for and .

** Graded Algebras **

(cf Prasit’s blog) A -algebra is -graded if such that . A -algebra is filtered over

if such that and . A graded algebra defines a filtered algebra by taking , and conversely a filtered algebra defines a graded algebra by taking (with ).

** Tensor Algebra and the Clifford Algebra **

If is a vector space, the tensor algebra has multiplication defined by concatenation, i.e., . Thus is a graded algebra with , and filtered with .

Recall that with . (Here, is a quadratic form; sometimes it is convenient to refer instead to the associated symmetric bilinear form .) For any , so maps to under the quotient . This sets up a natural identification between and . Whence the associated graded algebra for is isomorphic to . In particular, they are isomorphic as vector spaces, with dimension .

** Clifford bundle**

The Clifford bundle has fibers (), where is a Riemannian metric on . (The latter isomorphism is given by identifying an orthonormal (with respect to ) basis for with the standard generators .) Just as is the -bundle associated to the orthogonal frame bundle (of the tangent bundle) over , is the associated -bundle to .