On the stability and causality of scalar-vector theories

TL;DR

This paper investigates scalar-vector inflationary models, deriving necessary conditions for their stability and causality, which are crucial for their physical viability and consistency with cosmological observations.

Contribution

It provides the first systematic analysis of stability and causality constraints in scalar-vector theories with non-standard couplings.

Findings

Derived necessary conditions for stability (Hamiltonian boundedness)

Established criteria for causality (hyperbolic equations of motion)

Identified restrictions on model parameters for viable theories

Abstract

Various extensions of standard inflationary models have been proposed recently by adding vector fields. Because they are generally motivated by large-scale anomalies, and the possibility of statistical anisotropy of primordial fluctuations, such models require to introduce non-standard couplings between vector fields on the one hand, and either gravity or scalar fields on the other hand. In this article, we study models involving a vector field coupled to a scalar field. We derive restrictive necessary conditions for these models to be both stable (Hamiltonian bounded by below) and causal (hyperbolic equations of motion).