ECO-spotting: looking for extremely compact objects with bosonic fields

TL;DR

This paper explores the maximum compactness of self-gravitating bosonic field solutions, revealing they cannot reach black hole levels of compactness, thus informing the search for exotic compact objects that mimic black holes.

Contribution

It investigates the influence of self-interacting potentials on the compactness of bosonic stars and their connection to hairy black hole solutions, providing new insights into potential black hole mimickers.

Findings

Bosonic stars can have light rings but are less compact than black holes.

Self-interacting potentials influence the structure and compactness of solutions.

Connecting stars to hairy black holes as zero-mass black holes offers new theoretical insights.

Abstract

Black holes are thought to describe the geometry of massive, dark compact objects in the universe. To further support and quantify this long-held belief requires knowledge of possible, if exotic alternatives. Here, we wish to understand how compact can self-gravitating solutions be. We discuss theories with a well-posed initial value problem, consisting in either a single self-interacting scalar, vector or both. We focus on spherically symmetric solutions, investigating the influence of self-interacting potentials into the compactness of the solutions, in particular those that allow for flat-spacetime solutions. We are able to connect such stars to hairy black hole solutions, which emerge as a zero-mass black hole. We show that such stars can have light rings, but their compactness is never parametrically close to that of black holes. The challenge of finding black hole mimickers to investigate full numerical-relativity binary setups remains open.