Noether symmetries for fields and branes in backgrounds with Killing vectors

TL;DR

This paper extends the Belinfante energy-momentum tensor construction to curved backgrounds, analyzes Noether symmetries for fields and branes, and clarifies the role of Killing vectors in conserved currents within fixed or dynamic spacetimes.

Contribution

It generalizes the Belinfante tensor to curved backgrounds and explores Noether symmetries for fields and branes, highlighting the role of Killing vectors in conserved quantities.

Findings

Belinfante tensor is covariantly conserved in curved backgrounds.

Killing symmetries induce Noether conserved currents in fixed backgrounds.

Extended objects exhibit target spacetime and world volume diffeomorphism symmetries.

Abstract

We show that Belinfante construction of an improved energy-momentum tensor can be carried over to curved backgrounds, in analogy to the case of flat spacetime. The results hold irrespective of the background being dynamical or a fixed, non-backreacting one. It turns out that the analogous would-be canonical energy-momentum tensor is not covariantly conserved in general, but its Belinfante "improvement" is. We relate this last tensor with the Hilbert tensor obtained by functionally derivating the Lagrangian with respect to the metric. When the background in non-dynamical, we discuss some issues concerning the Noether conserved currents associated with its Killing symmetries and the role played by the Belinfante tensor. Next we study extended objects ($p$-branes) either in a dynamic or in a fixed background, and obtain the Noether identities associated both with target spacetime and world volume diffeomorphisms. We show that in field theory as well as with extended objects, the Killing symmetries of the background become ordinary rigid Noether symmetries of the theory in this fixed background. With the example of Maxwell theory in Minkowski spacetime we show in an appendix the role of the Belinfante tensor in the construction of these symmetries.