Testing the nature of dark compact objects: a status report (1904.05363v3)

Abstract: Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.

• The paper outlines testing methods using gravitational-wave ringdown, multipole moments, and tidal effects to verify the Kerr black hole model.
• It details observational techniques including quasinormal mode analysis and null tests that distinguish standard black holes from exotic compact objects.
• Implications involve refining General Relativity predictions and guiding future research with advanced detectors like LISA to probe strong-field gravity.

Overview of "Testing the Nature of Dark Compact Objects: A Status Report"

Introduction

This review paper by Vitor Cardoso and Paolo Pani thoroughly investigates the theoretical frameworks and observational methods for testing the nature of compact astrophysical objects, specifically scrutinizing whether these objects are indeed black holes (BHs), as described by the Kerr paradigm of General Relativity (GR), or if they could be alternative exotic compact objects (ECOs). Fundamental physics puzzles, including the nature of dark matter, spacetime singularities, and issues surrounding Hawking radiation and quantum effects on gravity, are considered. The paper aims to quantify the evidence supporting the existence of BHs and identify any phenomenological signatures indicative of non-Kerr solutions.

Significance of Compact Objects

Historically, black holes have been understood within GR as regions of spacetime where gravity is so intense that nothing can escape beyond the event horizon. With the advent of new astronomical tools, such as gravitational-wave (GW) detectors and very long baseline interferometry, the opportunity to empirically probe the characteristics of these dense objects in strong-field regimes has expanded. The confirmation of black holes' properties is crucial not only for validating GR but also for leading potential discoveries that urge modifications or extensions to the current models.

Testing the Black Hole Hypothesis

The paper explores multiple testing approaches to assess whether gravitationally compact objects are indeed Kerr black holes:

• Ringdown Analysis: Observing quasinormal modes through the ringdown signals of post-merger events offers insights into the properties of the final mass and spin, crucial for confirming the Kerr nature of black holes.
• Multipole Moments and Null Tests: The Kerr spacetime is characterized by unique multipole moments that depend only on the mass and spin. Any measured deviations in higher-order multipoles would provide strong evidence against GR or the nature of black holes.
• Gravitational Waves and Echoes: GW observations, especially post-merger, could reveal echoes—a possible signature of structures or surfaces replacing the event horizon. The detection of such echoes could suggest alternatives to Kerr black holes or unknown physics modifying the event horizon structure.
• Tidal Effects and Heating: Tidal Love numbers, quantifying the deformability of compact objects under external gravitational fields, are zero for black holes. Non-zero measurements would challenge the black hole model.

Observational Campaigns and Findings

Several observational campaigns using both EM and GW signals to test the aforementioned predictions have been ongoing. The paper reviews these efforts and refrains from sensationalizing findings, acknowledging both the potential of these tests to authenticate black hole models and highlight potential anomalies suggestive of new physics. Constraints on ECO models, particularly those approaching the black hole limit, are significant in understanding the nature of spacetime near these compact objects.

Future Directions and Challenges

The paper underscores the necessity of advancing both observational technologies and theoretical frameworks to enhance precision in testing GR's predictions about black holes. Challenges remain in:

• Developing precise models of ECOs to better predict their observational signatures.
• Uncovering the full implications of possible non-GR corrections through high-fidelity data analysis from current and future detectors, such as LISA and third-generation ground-based observatories.

Conclusions

"Testing the Nature of Dark Compact Objects: A Status Report" brings into focus the multifaceted approach required to probe the frontier of astrophysics concerning the nature of black holes versus their alternatives. While strongly supporting traditional models, it poses an open challenge to researchers to continuously explore these enigmatic objects across all plausible scenarios, utilizing all available observational data to potentially uncover new insights into the cosmos. The paper concludes with detailed quantitative constraints and a discussion of methodologies that serve as a prudent guide for ongoing and future explorations in strong-field gravity.