This paper is a review of ways a mathematical object called a Chern–Simons form shows up in pure math, in condensed‑matter physics, and in a proposal for cosmology. Chern–Simons forms are built from a gauge potential and its curvature. When integrated they give actions that are most natural in odd dimensions, for example in three and five dimensions. The author pays special attention to how three‑dimensional Chern–Simons theory models the integer and fractional quantum Hall effect and outlines a mechanism based on five‑dimensional abelian Chern–Simons theory that might be connected to the observed magnetic fields between galaxies.

At a high level a Chern–Simons action is a compact way to package how a gauge field (think of the electromagnetic potential) interacts with itself. On a space with a boundary such an action is not fully gauge‑invariant, meaning it changes under certain transformations of the potential. The change is a boundary term. In physical systems that boundary term is cancelled by degrees of freedom that live on the edge and are chiral (they move in one direction along the edge). This interplay between a bulk Chern–Simons action and edge modes is a central idea used to describe quantum Hall systems.

The review explains how an abelian Chern–Simons action in three dimensions, written as (σ/2)∫A∧F where A is the electromagnetic potential, F is its field strength, and σ is a real constant, reproduces key laws of the two‑dimensional quantum Hall effect (QHE). From this action one recovers Hall’s law, which relates the electric current to the perpendicular electric field, and a Chern–Simons version of Gauss’s law that says a small change in magnetic flux through the sample induces a proportional change in the charge stored in it. This gives the Středa formula ∆Q = σ_H ∆Φ. On a torus, inserting one quantum of flux (the flux quantum is h/e, where h is Planck’s constant and e the elementary charge) and using the fact that the quantum state returns to itself after such an insertion leads to σ_H = (e^2/h)N when all current carriers have integer electron charge. If the observed Hall conductivity is not such an integer multiple, the argument implies the presence of fractionally charged quasiparticles, a hallmark of the fractional QHE.