Two approaches to testing general relativity in the strong-field regime

TL;DR

This paper reviews two main methods for testing general relativity in the strong-field regime using observations of compact objects, highlighting their approaches, current status, and insights gained.

Contribution

It compares fundamental modifications of Einstein's equations with phenomenological parametrizations of spacetime, providing a comprehensive overview of recent developments in strong-field tests.

Findings

Both approaches have advanced significantly with recent observations.

Constraints on deviations from general relativity are becoming increasingly stringent.

Lessons learned inform future observational strategies and theoretical models.

Abstract

Observations of compact objects in the electromagnetic spectrum and the detection of gravitational waves from them can lead to quantitative tests of the theory of general relativity in the strong-field regime following two very different approaches. In the first approach, the general relativistic field equations are modified at a fundamental level and the magnitudes of the potential deviations are constrained by comparison with observations. In the second approach, the exterior spacetimes of compact objects are parametrized in a phenomenological way, the various parameters are measured observationally, and the results are finally compared against the general relativistic predictions. In this article, I discuss the current status of both approaches, focusing on the lessons learned from a large number of recent investigations.