On the stability and causality of scalar-vector theories

Understand

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).