Bounding the speed of gravity with gravitational wave observations (1707.06101v2)

Abstract: The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO collaboration we constrain the gravitational waves propagation speed c_gw to the 90% credible interval 0.55 c < c_gw < 1.42 c, where c is the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order twenty detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.

• The paper constrains the gravitational wave speed to a 90% credible interval of 0.55 c to 1.42 c using LIGO observations.
• It employs a Bayesian framework that utilizes time delays from the first three LIGO detections to derive these bounds.
• Forecasts indicate that with future detections and expanded networks, the accuracy of c₍gw₎ measurements could improve to within 1% of the speed of light.

Analyzing the Propagation Speed of Gravitational Waves: Constraints from LIGO Observations

The paper entitled "Bounding the speed of gravity with gravitational wave observations" by Cornish, Blas, and Nardini presents a rigorous analysis of the propagation speed of gravitational waves (GWs) using data from the first three detections reported by the LIGO Scientific and Virgo Collaborations. Through a Bayesian framework, this research provides bounds on the speed of gravitational wave propagation, denoted by $c_{\rm gw}$, contributing to the ongoing efforts in testing the fundamental predictions of General Relativity (GR).

Summary of Key Findings

1. Results and Implications: The paper sets the propagation speed of GWs within a 90% credible interval of $0.55 \, c < c_{\rm gw} < 1.42 \, c$, where $c$ is the speed of light in a vacuum. This range represents both a conservative and a more stringent assessment of $c_{\rm gw}$ compared to theoretical expectations of $c_{\rm gw} \geq c$. The results are significant since they present empirical constraints on the speed of gravity derived directly from observational data, which has implications for GR and alternative theories of gravity.
2. Bayesian Analysis Approach: The authors adopted a Bayesian methodology that incorporates the time delay between gravitational wave signals at different detector sites. This delay was used to infer constraints on $c_{\rm gw}$. By leveraging the first three gravitational wave events detected by LIGO (GW150914, GW151226, and GW170104), the analysis explores the full posterior distribution of $c_{\rm gw}$ while accounting for experimental uncertainties in the time delay measurements.
3. Predictive Power of Larger Detector Networks: The paper explores potential improvements in constraints with additional GW event detections and the inclusion of new detectors, such as those in the KAGRA and future LIGO-India projects. Preliminary forecasts suggest that with about twenty detections by the LIGO network alone, $c_{\rm gw}$ could be constrained to within 20% of $c$. Moreover, with only five detections by the combined LIGO-Virgo-Kagra network, the constraint could narrow to within 1% of $c$.
4. Theoretical and Observational Context: The research provides an essential context by comparing these observational constraints with the requirements from theories predicting gravitational Cherenkov radiation, which demand $c_{\rm gw} \ge c$. It also contrasts the level of constraint achievable through GW observations to other tests of GR such as post-Newtonian parameters and cosmological datasets, thereby situating the findings within a broader scope of gravitational physics.

Implications and Future Directions

These findings hold substantial implications for theoretical and observational astrophysics. A more precise measurement of $c_{\rm gw}$ not only tests GR’s predictions but also constrains alternative gravity theories that predict modifications to $c_{\rm gw}$. This could potentially open new avenues in understanding fundamental forces and the structure of spacetime. Moreover, the results highlight the necessity of advancing detection capabilities and increasing event sample sizes to enhance constraints on gravitational theories.

With further implementation of broader detector networks and more sophisticated analysis techniques, the speed of gravitational waves' propagation can be more tightly bound, thus playing a critical role in refining current models of gravitation and potentially uncovering new physics beyond GR. Future observations, especially those involving coincident electromagnetic and GW detections, could yield even more stringent constraints, significantly impacting theoretical models.

In conclusion, this paper showcases the utility of GW astronomy in probing fundamental physics and the promising outlook for future research that builds on these foundational results.