Spectral asymmetry

Last updated June 13, 2020

In mathematics and physics, the spectral asymmetry is the asymmetry in the distribution of the spectrum of eigenvalues of an operator. In mathematics, the spectral asymmetry arises in the study of elliptic operators on compact manifolds, and is given a deep meaning by the Atiyah-Singer index theorem. In physics, it has numerous applications, typically resulting in a fractional charge due to the asymmetry of the spectrum of a Dirac operator. For example, the vacuum expectation value of the baryon number is given by the spectral asymmetry of the Hamiltonian operator. The spectral asymmetry of the confined quark fields is an important property of the chiral bag model. For fermions, it is known as the Witten index, and can be understood as describing the Casimir effect for fermions.

Definition

Given an operator with eigenvalues $\omega _{n}$ , an equal number of which are positive and negative, the spectral asymmetry may be defined as the sum

B=\lim _{t\to 0}{\frac {1}{2}}\sum _{n}\operatorname {sgn}(\omega _{n})\exp(-t|\omega _{n}|)

where $\operatorname {sgn}(x)$ is the sign function. Other regulators, such as the zeta function regulator, may be used.

The need for both a positive and negative spectrum in the definition is why the spectral asymmetry usually occurs in the study of Dirac operators.

Example

As an example, consider an operator with a spectrum

\omega _{n}=n+\alpha

where n is an integer, ranging over all positive and negative values. One may show in a straightforward manner that in this case $B(\alpha )$ obeys $B(\alpha )=B(\alpha +m)$ for any intger $m$ , and that for $0<\alpha <1$ we have $B(\alpha )=1/2-\alpha$ . The graph of $B(\alpha )$ is therefore a periodic sawtooth curve.

Discussion

Related to the spectral asymmetry is the vacuum expectation value of the energy associated with the operator, the Casimir energy, which is given by

E=\lim _{t\to 0}{\frac {1}{2}}\sum _{n}|\omega _{n}|\exp(-t|\omega _{n}|)

This sum is formally divergent, and the divergences must be accounted for and removed using standard regularization techniques.

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References

MF Atiyah, VK Patodi and IM Singer, Spectral asymmetry and Riemannian geometry I, Proc. Camb. Phil. Soc., 77 (1975), 43-69.
Linas Vepstas, A.D. Jackson, A.S. Goldhaber, Two-phase models of baryons and the chiral Casimir effect, Physics Letters B140 (1984) p. 280-284.
Linas Vepstas, A.D. Jackson, Justifying the Chiral Bag, Physics Reports, 187 (1990) p. 109-143.

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v t e Spectral theory and ^*-algebras
Basic concepts	Involution/-algebra Banach algebra B-algebra C-algebra Noncommutative topology Projection-valued measure Spectrum Spectrum of a C-algebra Spectral radius Operator space
Main results	Gelfand–Mazur theorem Gelfand–Naimark theorem Gelfand representation Polar decomposition Singular value decomposition Spectral theorem Spectral theory of normal C*-algebras
Special Elements/Operators	Isospectral Normal operator Hermitian/Self-adjoint operator Unitary operator Unit
Spectrum	Krein–Rutman theorem Normal eigenvalue Spectrum of a C*-algebra Spectral radius Spectral asymmetry Spectral gap
Decomposition of a spectrum	(Continuous Point Residual) Approximate point Compression Discrete Spectral abscissa
Spectral Theorem	Borel functional calculus Min-max theorem Projection-valued measure Riesz projector Rigged Hilbert space Spectral theorem Spectral theory of compact operators Spectral theory of normal C*-algebras
Special algebras	Amenable Banach algebra With an Approximate identity Banach function algebra Disk algebra Uniform algebra
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Generalizations	Dirac spectrum Essential spectrum Pseudospectrum Structure space (Shilov boundary)
Miscellaneous	Abstract index group Banach algebra cohomology Cohen–Hewitt factorization theorem Extensions of symmetric operators Limiting absorption principle Unbounded operator
Examples	Wiener algebra
Applications	Almost Mathieu operator Corona theorem Hearing the shape of a drum (Dirichlet eigenvalue) Heat kernel Kuznetsov trace formula Lax pair Proto-value function Ramanujan graph Rayleigh–Faber–Krahn inequality Spectral geometry Spectral method Spectral theory of ordinary differential equations Sturm–Liouville theory Superstrong approximation Transfer operator Transform theory Weyl law