Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences: (I) Inspiral-Merger (1801.03208v2)

Abstract: The observation of the inspiral and merger of compact binaries by the LIGO/Virgo collaboration ushered in a new era in the study of strong-field gravity. We review current and future tests of strong gravity and of the Kerr paradigm with gravitational-wave interferometers, both within a theory-agnostic framework (the parametrized post-Einsteinian formalism) and in the context of specific modified theories of gravity (scalar-tensor, Einstein-dilaton-Gauss-Bonnet, dynamical Chern-Simons, Lorentz-violating, and extra dimensional theories). In this contribution we focus on (i) the information carried by the inspiral radiation, and (ii) recent progress in numerical simulations of compact binary mergers in modified gravity.

• The paper analyzes how gravitational waves from compact binary mergers test General Relativity in strong gravity, using parameterized (ppE) and specific modified gravity theories.
• It discusses how theories like scalar-tensor, Gauss-Bonnet, Chern-Simons, Lorentz-violating, and extra dimensions predict observable deviations in gravitational waveforms.
• The study highlights the potential of stacking multiple gravitational wave observations, including future LISA data, to constrain modified gravity theories more effectively.

Overview of "Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences: (I) Inspiral-Merger"

The research paper titled "Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences: (I) Inspiral-Merger" by Emanuele Berti, Kent Yagi, and Nicol Yunes presents an in-depth analysis of the capabilities of gravitational wave (GW) observations in testing modifications to General Relativity (GR) within the strong-field regime. The paper focuses on the inspiral and merger phases of compact binary coalescences, a process that offers unprecedented access to dynamics where gravity is both intense and highly non-linear.

Theoretical Framework

The paper leverages advancements in GW astronomy, particularly through the LIGO/Virgo collaborations, to challenge and test Einstein's GR under extreme conditions. The authors discuss a variety of theory-agnostic approaches, such as the parametrized post-Einsteinian (ppE) formalism, along with specific modified gravity theories, namely scalar-tensor, Einstein-dilaton-Gauss-Bonnet, dynamical Chern-Simons, Lorentz-violating theories, and theories with extra dimensions. These frameworks aim to systematically introduce deviations from GR predictions by parameterizing their potential observables in GW signals.

Key Findings and Methodologies

1. Parametrized Post-Einsteinian Approach: The paper utilizes the ppE framework as a tool for consistency testing against GR. The ppE model provides a flexible description of potential deviations across a range of PN (Post-Newtonian) orders in the inspiral waveform. Attention is given to inspired effects such as dipolar radiation and modifications in the propagation speed of GWs.
2. Implications of Different Theories:
   • Scalar-tensor Theories: The paper examines how scalar fields could introduce new dynamics through mechanisms like spontaneous scalarization in neutron stars, which would then be discernible in the emitted GWs.
   • Einstein-dilaton Gauss-Bonnet and Chern-Simons Gravities: These theories introduce corrections that are significant at high curvatures. Gravitational waveforms are analyzed for deviations introduced by these terms, which could manifest differently depending on the mass and spin of the black holes involved.
   • Lorentz-violating Theories and Extra Dimensions: Potential modifications, such as changes in wave propagation speeds and additional massive modes, could lead to observable deviations in GW data that these frameworks predict.
3. Numerical Simulations: Numerical relativity is employed as a critical tool to simulate binary mergers within these alternate theories. The authors highlight the current difficulties due to challenges in the well-posedness of equations governing these modifications.

Observational Prospects and Future Developments

The paper highlights the significance of upcoming GW observations, including those with space-based detectors like LISA, which will probe different frequency bands and offer complementary constraints on deviations from GR. A key methodological insight discussed is the stacking of observations from multiple GW events to improve statistical confidence in detecting or ruling out specific alternative theories.

Conclusions

The paper underscores the transformative potential of gravitational wave astronomy in probing the validity of GR and its alternatives in the most extreme environments. By developing and refining parameterized approaches and specific theory models, the research provides a roadmap for interpreting present and future GW detections in the context of strong-field modifications to GR. Anticipating future advancements in detector sensitivity and observation frequency ranges, the authors propose that combined multi-band GW detections will significantly constrain the parameter space of various modified gravity theories, thereby deepening our understanding of fundamental physics.