TITLE: Mathematical framework of the Koopman operator
QUESTION [10 upvotes]: I'm trying to understand the relation between the study of a dynamical system, seen as a pair $(X,\varphi_t)$ where $\varphi_t: X \rightarrow X$ represents a dynamics on the (measurable) space of states $X$ and the 'spectrum' of the associated Koopman operator
$$f \longmapsto f \circ \varphi$$
Basically the Koopman operator allow us to see the states involved in terms of their measurements.
Where $f$ is an observable, namely a linear positive functional $f: X \rightarrow \mathbb{C}$. Physically, I undestand that the dynamics $\varphi_t(x) := (x_t)_{t \in \mathbb{R}}$ describes the transitions of the physical system by time $t$ evolution running on $\mathbb{R}$, for example the position and velocity of gas particles in a three-dimensional space. And the observable $f$ is just a measurement of some quantity, for instance the temperature.
What I'm trying to understand is, what is the main 'mathematical' advantage and relation of studying operator properties of $\{T_{\varphi_t}(f) \, : \, t \in \mathbb{R}\}$ and why are we interested on studying the spectrum of such a thing?

REPLY [3 votes]: Perhaps this is too obvious but I believe it is worth stating: The Koopman operator allows one to attach a linear system (albeit acting on a big space) to a possibly nonlinear system in accordance with general categorical principles (pullbacks in this case). This is done to get extra structure to use; in this case e.g. spectral theory.

This fits into the framework of attaching to an anonymous dynamical system some other  not-as-anonymous dynamical system. For $X$ measurable there are essentially three popular such categorical constructions. There is:

An induced action on the space of probability measures on $X$ by affine automorphisms,
An induced action on some space of observables on $X$ by (typically bounded) linear operators, given by the Koopman operator,
An induced action on the Boolean $\sigma$-algebra of $X$ by Boolean $\sigma$-algebra automorphisms.

(Halmos in his Lectures on Ergodic Theory calls the first the geometric approach, the second the analytic approach and the third the algebraic approach (p. 42).)

Likewise, for instance if $X$ has some topological structure one looks at associated homological/homotopical actions and if $X$ has some differential structure one looks at actions on vector fields or forms. Often there are many structures one considers in tandem as these different structures have implications for each other and the original anonymous dynamics.