# Gauge theory (mathematics)

In mathematics, and especially differential geometry and mathematical physics, gauge theory is the general study of connections on vector bundles, principal bundles, and fibre bundles. Gauge theory in mathematics should not be confused with the closely related concept of a gauge theory in physics, which is a field theory which admits gauge symmetry. In mathematics theory means a mathematical theory, encapsulating the general study of a collection of concepts or phenomena, whereas in the physical sense a gauge theory is a physical model of some natural phenomenon.

Gauge theory in mathematics is typically concerned with the study of gauge-theoretic equations. These are differential equations involving connections on vector bundles or principal bundles, or involving sections of vector bundles, and so there are strong links between gauge theory and geometric analysis. These equations are often physically meaningful, corresponding to important concepts in quantum field theory or string theory, but also have important mathematical significance. For example, the Yang–Mills equations are a system of partial differential equations for a connection on a principal bundle, and in physics solutions to these equations correspond to vacuum solutions to the equations of motion for a classical field theory, particles known as instantons.

Gauge theory has found uses in constructing new invariants of smooth manifolds, the construction of exotic geometric structures such as hyperkähler manifolds, as well as giving alternative descriptions of important structures in algebraic geometry such as moduli spaces of vector bundles and coherent sheaves.

Gauge theory has its origins as far back as the formulation of Maxwell's equations describing classical electromagnetism, which may be phrased as a gauge theory with structure group the circle group. Work of Paul Dirac on magnetic monopoles and relativistic quantum mechanics encouraged the idea that bundles and connections were the correct way of phrasing many problems in quantum mechanics. Gauge theory in mathematical physics arose as a significant field of study with the seminal work of Robert Mills and Chen-Ning Yang on so-called Yang–Mills gauge theory, which is now the fundamental model that underpins the standard model of particle physics.[1]

Significant breakthroughs encouraging the development of mathematical gauge theory occurred in the early 1980s. At this time the important work of Atiyah and Raoul Bott about the Yang–Mills equations over Riemann surfaces showed that gauge theoretic problems could give rise to interesting geometric structures, spurring the development of infinite-dimensional moment maps, equivariant Morse theory, and relations between gauge theory and algebraic geometry.[6] Important analytical tools in geometric analysis were developed at this time by Karen Uhlenbeck, who studied the analytical properties of connections and curvature proving important compactness results.[7] The most significant advancements in the field occurred due to the work of Simon Donaldson and Edward Witten.

Witten similarly observed the power of gauge theory to describe topological invariants, by relating quantities arising from Chern–Simons theory in three dimensions to the Jones polynomial, an invariant of knots.[10] This work and the discovery of Donaldson invariants, as well as novel work of Andreas Floer on Floer homology, inspired the study of topological quantum field theory.

After the discovery of the power of gauge theory to define invariants of manifolds, the field of mathematical gauge theory expanded in popularity. Further invariants were discovered, such as Seiberg–Witten invariants and Vafa–Witten invariants.[11][12] Strong links to algebraic geometry were realised by the work of Donaldson, Uhlenbeck, and Shing-Tung Yau on the Kobayashi–Hitchin correspondence relating Yang–Mills connections to stable vector bundles.[13][14] Work of Nigel Hitchin and Carlos Simpson on Higgs bundles demonstrated that moduli spaces arising out of gauge theory could have exotic geometric structures such as that of hyperkähler manifolds, as well as links to integrable systems through the Hitchin system.[15][16] Links to string theory and mirror symmetry were realised, where gauge theory is essential to phrasing the homological mirror symmetry conjecture and the AdS/CFT correspondence.

The fundamental objects of interest in gauge theory are connections on vector bundles and principal bundles. In this section we briefly recall these constructions, and refer to the main articles on them for details. The structures described here are standard within the differential geometry literature, and an introduction to the topic from a gauge-theoretic perspective can be found in the book of Donaldson and Peter Kronheimer.[17]

The central objects of study in gauge theory are principal bundles and vector bundles. The choice of which to study is essentially arbitrary, as one may pass between them, but principal bundles are the natural objects from the physical perspective to describe gauge fields, and mathematically they more elegantly encode the corresponding theory of connections and curvature for vector bundles associated to them.

There are various notational conventions used for connections on vector bundles and principal bundles which will be summarised here.

The mathematical and physical fields of gauge theory involve the study of the same objects, but use different terminology to describe them. Below is a summary of how these terms relate to each other.

As a demonstration of this dictionary, consider an interacting term of an electron-position particle field and the electromagnetic field in the Lagrangian of quantum electrodynamics:[18]

The predominant theory that occurs in mathematical gauge theory is Yang–Mills theory. This theory involves the study of connections which are critical points of the Yang–Mills functional defined by

These critical points are characterised as solutions of the associated Euler–Lagrange equations, the Yang–Mills equations

The moduli space of solutions to the Nahm equations has the structure of a hyperkähler manifold.

Hitchin's work was subsequently vastly generalised by Carlos Simpson, and the correspondence between solutions to Hitchin's equations and Higgs bundles over an arbitrary Kähler manifold is known as the nonabelian Hodge theorem.[25][26][27][28][29]

Chern–Simons theory in 3 dimensions is a topological quantum field theory with an action functional proportional to the integral of the Chern–Simons form, a three-form defined by

In analogy with instanton Floer homology one may define Seiberg–Witten Floer homology where instantons are replaced with solutions of the Seiberg–Witten equations. By work of Clifford Taubes this is known to be isomorphic to embedded contact homology and subsequently Heegaard Floer homology.

Gauge theory has been most intensively studied in four dimensions. Here the mathematical study of gauge theory overlaps significantly with its physical origins, as the standard model of particle physics can be thought of as a quantum field theory on a four-dimensional spacetime. The study of gauge theory problems in four dimensions naturally leads to the study of topological quantum field theory. Such theories are physical gauge theories that are insensitive to changes in the Riemannian metric of the underlying four-manifold, and therefore can be used to define topological (or smooth structure) invariants of the manifold.

Cobordism given by moduli space of anti-self-dual connections in Donaldson's theorem

An extension of these ideas leads to Donaldson theory, which constructs further invariants of smooth four-manifolds out of the moduli spaces of connections over them. These invariants are obtained by evaluating cohomology classes on the moduli space against a fundamental class, which exists due to analytical work showing the orientability and compactness of the moduli space by Karen Uhlenbeck, Taubes, and Donaldson.

The effectiveness of solutions of the Yang–Mills equations in defining invariants of four-manifolds has led to interest that they may help distinguish between exceptional holonomy manifolds such as G2 manifolds in dimension 7 and Spin(7) manifolds in dimension 8, as well as related structures such as Calabi–Yau 6-manifolds and nearly Kähler manifolds.[35][36]

New gauge-theoretic problems arise out of superstring theory models. In such models the universe is 10 dimensional consisting of four dimensions of regular spacetime and a 6-dimensional Calabi–Yau manifold. In such theories the fields which act on strings live on bundles over these higher dimensional spaces, and one is interested in gauge-theoretic problems relating to them. For example, the limit of the natural field theories in superstring theory as the string radius approaches zero (the so-called large volume limit) on a Calabi–Yau 6-fold is given by Hermitian Yang–Mills equations on this manifold. Moving away from the large volume limit one obtains the deformed Hermitian Yang–Mills equation, which describes the equations of motion for a D-brane in the B-model of superstring theory. Mirror symmetry predicts that solutions to these equations should correspond to special Lagrangian submanifolds of the mirror dual Calabi–Yau.[37]