Black hole mimickers: from theory to observation

TL;DR

This paper reviews the concept of black hole mimickers, horizonless ultra-compact objects, and discusses how recent observational advances can test their viability as alternatives to traditional black holes.

Contribution

It provides a comprehensive overview of the current state of research on black hole mimickers and outlines future observational strategies to distinguish them from true black holes.

Findings

Recent gravitational-wave observations constrain mimicker models.

Electromagnetic imaging offers new tests for horizonless objects.

Theoretical models of mimickers are increasingly compatible with observations.

Abstract

The black hole paradigm, while remarkably successful, raises fundamental questions-both classical and quantum-about the nature of spacetime, horizons, and singularities. Black hole mimickers, horizonless ultra-compact objects, have emerged as potential alternatives that seek to resolve some of these puzzles while remaining consistent with current observational constraints. Recent breakthroughs in gravitational-wave astronomy and horizon-scale electromagnetic imaging have opened new avenues to test this paradigm-making this an opportune moment to systematically investigate such alternatives. This vision document presents a snapshot of the field as discussed at the Black Hole Mimickers: From Theory to Observation workshop, where experts from gravitational wave astronomy, very long baseline interferometry, numerical and mathematical relativity, and high-energy physics converged to assess the current frontiers. By highlighting key open questions and proposing concrete pathways forward, this document aims to guide future efforts to probe the nature of compact objects. As the field stands at the crossroads of theoretical innovation and observational breakthroughs, we outline strategies to harness upcoming observational capabilities to fundamentally test the black hole paradigm.