On gauge transformations in twistless torsional Newton--Cartan geometry

Abstract: We observe that in type I twistless torsional Newton--Cartan (TTNC) geometry, one can always find (at least locally) a gauge transformation that transforms a specific locally Galilei-invariant function -- that we dub the `locally Galilei-invariant potential' -- to zero, due to the corresponding equation for the gauge parameter taking the form of a Hamilton--Jacobi equation. In the case of type II TTNC geometry, the same gauge fixing may locally be performed by subleading spatial diffeomorphisms. We show (a) how this generalises a classical result in standard Newton--Cartan geometry, and (b) how it allows to parametrise the metric structure of a Galilei manifold as well as the gauge equivalence class of the Bargmann form of TTNC geometry in terms of just the space metric and a unit timelike vector field.