# Gravitational Instabilities and Censorship of Large Scalar Field Excursions

## TL;DR

This paper explores how large scalar field variations in flat spacetimes are either censored by black hole formation or stabilized by instabilities in bubble geometries, revealing new mechanisms of censorship in gravitational systems.

## Contribution

It demonstrates that certain smooth Kaluza-Klein bubble geometries can UV-complete large scalar excursions, but these are classically unstable, providing insights into gravitational censorship mechanisms.

## Key findings

- Large scalar excursions can be UV-completed by smooth bubble geometries.
- Kerr KK bubbles are classically unstable, indicating a new censorship mechanism.
- Constructed initial data showing instability at energy maxima.

## Abstract

Large, localized variations of light scalar fields tend to collapse into black holes, dynamically "censoring" distant points in field space. We show that in some cases, large scalar excursions in asymptotically flat spacetimes can be UV-completed by smooth Kaluza-Klein bubble geometries, appearing to circumvent 4d censorship arguments. However, these spacetimes also exhibit classical instabilities related to the collapse or expansion of a bubble of nothing, providing a different censorship mechanism. We show that the Kerr family of static KK bubbles, which gives rise to an infinite scalar excursion upon dimensional reduction, is classically unstable. We construct a family of initial data in which the static bubbles sit at a local maximum of the energy, and we give a general argument that such a property indeed indicates mechanical instability in gravity. We also discuss the behavior of wound strings near a bubble, a local probe of the large traversal through moduli space.