Multi-field TDiff theories for cosmology

TL;DR

This paper explores multi-field theories with broken diffeomorphism invariance in cosmology, revealing induced interactions between scalar fields and their potential to model dark sector interactions, supported by observational data comparison.

Contribution

It introduces a novel class of TDiff scalar field models with induced interactions, applied to dark sector dynamics and tested against supernova data.

Findings

Induced interactions arise without explicit Lagrangian terms.

Different domination regimes show energy exchange between fields.

Model predictions align with Type Ia supernova observations.

Abstract

We consider theories which break the invariance under diffeomorphisms (Diff) down to transverse diffeomorphisms (TDiff) in the matter sector, consisting of multiple scalar fields. In particular, we regard shift-symmetric models with two free TDiff scalar fields in a flat Robertson-Walker spacetime (FLRW) and use the perfect fluid approach to study their dynamics. As a consequence of the symmetry breaking, an effective interaction between the fields is induced from the conservation of the total energy-momentum tensor, without the necessity to introduce any explicit interacting term in the Lagrangian. We study the different single-field domination regimes and analyze the energy exchange between the fields. Thereupon, we introduce an application of these models for the description of interactions in the dark sector, and compare the theoretical predictions of our model to observational data from Type Ia supernovae.