# Spontaneous growth of vector fields in gravity

## TL;DR

This paper generalizes the concept of spontaneous growth in gravity theories, showing that nonminimal coupling causes instabilities leading to spontaneous tensorization, demonstrated explicitly for vector fields in neutron stars.

## Contribution

It introduces the idea of spontaneous tensorization, extending spontaneous scalarization to vectors and other tensor fields, highlighting a unified mechanism in gravity theories.

## Key findings

- Demonstrates spontaneous vectorization in neutron stars
- Shows nonminimal coupling causes instabilities in various tensor fields
- Proposes a general mechanism for spontaneous tensor growth in gravity theories

## Abstract

We show that the spontaneous scalarization scenario in scalar-tensor theories is a specific case of a more general phenomenon. The key fact is that the instability causing the spontaneous growth in scalars is due to the nonminimal coupling in the theory, and not related to the nature of the scalar. Another field with the same form of coupling undergoes spontaneous growth as well. We explicitly demonstrate this idea for vectors, naming it "spontaneous vectorization", and study spherically symmetric neutron stars in such a theory. We also comment on other tensor fields the idea can be applied, naming the general mechanism "spontaneous tensorization".