# Electromagnetism and hidden vector fields in modified gravity theories: spontaneous and induced vectorization

## TL;DR

This paper explores how nonminimal coupling between gravity and a massless vector field can lead to nonperturbative effects in compact stars, revealing new solutions and potential instabilities in modified gravity theories.

## Contribution

It introduces novel solutions for compact stars with nontrivial vector fields arising from strong-gravity modifications, including both electromagnetic and hidden vector fields.

## Key findings

- Existence of star solutions with nontrivial vector configurations
- Identification of instabilities associated with certain solutions
- Potential implications for electromagnetic and hidden vector fields in gravity

## Abstract

In general relativity, Maxwell's equations are embedded in curved spacetime through the minimal prescription, but this could change if strong-gravity modifications are present. We show that with a nonminimal coupling between gravity and a massless vector field, nonperturbative effects can arise in compact stars. We find solutions describing stars with nontrivial vector field configurations, some of which are associated to an instability, while others are not. The vector field can be interpreted either as the electromagnetic field, or as a hidden vector field weakly coupled with the standard model.