Theoretical Physics Implications of the Binary Black-Hole Mergers GW150914 and GW151226 (1603.08955v2)

Abstract: The gravitational wave observations GW150914 and GW151226 by Advanced LIGO provide the first opportunity to learn about physics in the extreme gravity environment of coalescing binary black holes. The LIGO Scientific Collaboration and the Virgo Collaboration have verified that this observation is consistent with General Relativity. This paper expands their analysis to a larger class of anomalies, highlighting the inferences that can be drawn on non-standard theoretical physics mechanisms. We find that these events constrain a plethora of mechanisms associated with the generation and propagation of gravitational waves, including the activation of scalar fields, gravitational leakage into large extra dimensions, the variability of Newton's constant, a modified dispersion relation, gravitational Lorentz violation and the strong equivalence principle. Though other observations limit many of these mechanisms already, GW150914 and GW151226 are unique in that they are direct probes of dynamical strong-field gravity and of gravitational wave propagation. We also show that GW150914 constrains inferred properties of exotic compact object alternatives to Kerr black holes. We argue, however, that the true potential for GW150914 to both rule out exotic objects and constrain physics beyond General Relativity is severely limited by the lack of understanding of the merger regime in almost all relevant modified gravity theories. This event thus significantly raises the bar that these theories have to pass, both in terms of having a sound theoretical underpinning, and being able to solve the equations of motion for binary merger events. We conclude with a discussion of the additional inferences that can be drawn if the lower-confidence observation of an electromagnetic counterpart to GW150914 holds true; this would provide dramatic constraints on the speed of gravity and gravitational Lorentz violation.

• The paper analyzes gravitational waveforms with the gIMR and ppE frameworks to test General Relativity in strong, dynamic gravitational fields.
• It constrains alternative theories like Einstein-dilaton Gauss-Bonnet gravity and Lorentz-violating models using precise merger data.
• The study underscores that enhanced detector sensitivity in future events will sharpen constraints on deviations from Einstein's theory.

Theoretical Implications of Binary Black-Hole Merger Events GW150914 and GW151226

This paper explores the theoretical implications of the binary black-hole merger events GW150914 and GW151226, as observed by the Advanced LIGO detectors. The research focuses on understanding the implications of these events for extreme gravity, testing general relativity (GR), and constraining alternative gravitational theories.

The gravitational wave detections of GW150914 and GW151226 provided a unique opportunity to test Einstein's theory of General Relativity in the extreme gravity regime, i.e., where gravitational fields are both strong and dynamic. One of the key hypotheses is that these observations can help discriminate between GR and alternative models that predict deviations from the standard theory in such regimes.

Parametric Deformations and Constraints on Alternative Theories

The authors analyze the gravitational waveforms using a generalized inspiral-merger-ringdown (gIMR) model, which incorporates potential deviations from GR predicted by alternative theories. The waveforms are analyzed using the parameterized post-Einsteinian (ppE) framework to assess how well GR holds up against a broad class of modified gravity theories.

The paper presents constraints on various physical mechanisms that might be active during the generation and propagation of gravitational waves:

1. Dipole Radiation and Scalar Fields: The paper examines theories like Einstein-dilaton Gauss-Bonnet (EdGB) gravity and scalar-tensor theories, which predict scalar dipole radiation. However, due to degeneracies with binary parameters, constraints from the observed events are generally weaker than existing bounds from other astrophysical sources.
2. Lorentz Violations and Extra Dimensions: The research constrains Einstein-\AE ther and khronometric gravity, which imply Lorentz-violating effects, as well as scenarios predicting extra spatial dimensions. Yet, these constraints are less stringent compared to existing solar system and binary pulsar limits.
3. Propagation Speed of Gravitational Waves: Modified dispersion relations of gravitational waves are tested, providing stringent bounds on graviton mass and multifractional spacetime theories. The gravitational wave data offer unique insights into gravitational wave propagation that complements or, in some cases, strengthens existing bounds from other astrophysical and planetary tests.

Implications for Exotic Compact Objects

Further, the paper explores theoretical implications for exotic compact objects and their potential deviation from the Kerr black hole hypothesis. It examines how well GW150914 constrains non-Kerr spacetimes and other exotic objects like boson stars or gravastars, primarily through their ringdown phase.

The paper addresses the effective bulk and shear viscosities required for exotic compact objects to match the rapid damping observed in the GW150914 event. It finds that the viscosities needed are consistent with black holes but not easily with other compact star models.

Future Prospects

The authors conclude that while current observations place limited constraints on some gravitational theories, the unique nature of black hole mergers could significantly enhance the capacity to test GR and explore alternative theories as detector sensitivity improves and more events are detected.

Future detections involving different sources, such as neutron star binaries, and higher signal-to-noise ratios, will potentially lead to stronger constraints on deviations from GR, sharpen existing bounds on alternative scenarios, and further elucidate the properties of compact objects in extreme gravity regimes.

Conclusion

Overall, the paper underscores the importance of gravitational wave astronomy in testing the foundations of gravity. The detections of GW150914 and GW151226 serve as powerful tests of GR and provide a platform for probing the validity of theoretical models predicting deviations in extreme gravitational conditions. The research sets a path for ongoing and future inquiry into the behavior of gravity near black holes and the fundamental nature of spacetime.