- The paper demonstrates that GW170817’s precise measurement of gravitational wave speed forces dark energy models to adhere to strict parameter relationships.
- It employs Effective Field Theory to constrain key operator coefficients, ruling out significant deviations within Horndeski and beyond-Horndeski frameworks.
- The findings motivate further experimental investigations to test theoretical refinements and reveal potential deeper symmetry principles in cosmology.
Implications of GW170817 on Theories of Dark Energy and Modified Gravity
The detection of the gravitational wave event GW170817 and its electromagnetic counterpart has provided significant insights into the speed of gravitational waves (GWs) and its implications for theories of dark energy and modified gravity, particularly those characterized by a single scalar degree of freedom. This paper by Paolo Creminelli and Filippo Vernizzi addresses the constraints that arise from the empirical observation that gravitational waves travel at the speed of light, to a high precision of less than a few parts in 10−15. This measurement has substantial consequences for theoretical models and necessitates a reformulation of several existing frameworks.
Constraints on Theoretical Models
The authors discuss the impact of GW170817 within the context of the Effective Field Theory (EFT) of Dark Energy and its covariant counterpart, the Horndeski and beyond Horndeski theories. To align with the observed universality of the speed of gravitational waves, it becomes necessary for the parameters within these theoretical frameworks to satisfy precise relationships. Most notably, in the broader class of quartic and quintic beyond Horndeski theories, only a single function remains viable and must conform to the standard conformal coupling to the Ricci scalar in Horndeski theories. This dramatic simplification implies that any potential deviations must remain exceedingly small, ensuring stability under quantum corrections. Therefore, radiative stability is crucial, as it maintains the absence of tuning over perturbation theory orders, supporting the naturalness of these parameter constraints.
Implications for Effective Field Theory
In the EFT approach, the primary challenge is ensuring that the conditions imposed by the observational data are robust against small variations in the cosmological history, such as changes in the dark matter abundance. The decomposition of gravitational wave dynamics in terms of the EFT coefficients reveals that only specific combinations of operator parameters are allowed, which stabilizes the predicted speed of gravitational waves. For example, the cubic and quartic operators that typically drive the Vainshtein mechanism must be absent to maintain compatibility with GW observations. This exclusion has direct consequences on screening mechanisms and potentially observable signatures at cosmological scales.
Covariant Theory and Beyond Horndeski
When translating these constraints to the covariant theory, the authors find that the functions governing Horndeski theories must satisfy specific relations to ensure the same speed of gravitational waves across different cosmological backgrounds. Namely, functions related to the beyond Horndeski terms must be constrained or absent, aligning with the empirical data. Additionally, disformal transformations, which modify the coupling of matter without affecting gravitational wave trajectories, provide an alternative method to understand the coupling in these models, ensuring observational consistency.
Future Directions
The paper suggests several future research directions. One intriguing prospect is exploring whether the relations necessitated by gravitational wave observations can derive from deeper symmetries or principles within the theory. On the experimental side, further gravitational wave detections, particularly those covering longer distances and varying frequencies, could provide greater robustness to the current constraints and offer additional tests of theoretical models.
Ultimately, the empirical insights provided by GW170817 have necessitated a reevaluation of existing theories of dark energy and modified gravity, emphasizing the importance of precise theoretical tuning to align with observed cosmic phenomena. Such requirements not only refine our understanding of the universe's expansion dynamics but also underscore the synergy between observational astrophysics and theoretical physics in contemporary cosmological research.