Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories (1710.05893v3)

Abstract: The LIGO/VIRGO collaboration has recently announced the detection of gravitational waves from a neutron star-neutron star merger (GW170817) and the simultaneous measurement of an optical counterpart (the gamma-ray burst GRB 170817A). The close arrival time of the gravitational and electromagnetic waves limits the difference in speed of photons and gravitons to be less than about one part in $10^{15}$. This has three important implications for cosmological scalar-tensor gravity theories that are often touted as dark energy candidates and alternatives to $\Lambda$CDM. First, for the most general scalar-tensor theories---beyond Horndeski models---three of the five parameters appearing in the effective theory of dark energy can now be severely constrained on astrophysical scales; we present the results of combining the new gravity wave results with galaxy cluster observations. Second, the combination with the lack of strong equivalence principle violations exhibited by the supermassive black hole in M87, constrains the quartic galileon model to be cosmologically irrelevant. Finally, we derive a new bound on the disformal coupling to photons that implies that such couplings are irrelevant for the cosmic evolution of the field.

• The paper constrains scalar-tensor models by showing that gravitational waves travel at nearly the same speed as light.
• The analysis rules out significant portions of beyond Horndeski and quartic Galileon parameter space for dark energy.
• Novel bounds on disformal photon couplings underscore the need for multimessenger tests in cosmological theory validation.

Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories

The paper by Sakstein and Jain explores the implications of the gravitational wave event GW170817 for scalar-tensor theories in cosmology, specifically those theories considered as candidates for dark energy and alternatives to the standard ΛCDM model. Gravitational waves (GWs) originating from a neutron star-neutron star merger, identified as GW170817, in conjunction with gamma-ray burst observations (GRB 170817A), have provided precise constraints on the relative speed of photons and gravitons, which turns out to be nearly identical. The analysis deduced from this has profound consequences for models of modified gravity, particularly for beyond Horndeski theories.

Scalar-tensor theories generally forecast that the speed of GWs differs from that of light within the cosmological background. GW170817's observation, with negligible time lag between gravitational and electromagnetic wave detections, constrains this speed difference to less than a part in 15. The paper explores how this affects the space of viable scalar-tensor theories, putting stringent limits on modifications to General Relativity (GR) on astrophysical scales within beyond Horndeski models.

Major Findings and Implications

1. Constraints on Beyond Horndeski Theories:
   • Within these theories, cosmological deviations from ΛCDM can be described by a set of parameters in the effective theory of dark energy: $$\{\alpha_M, \alpha_K, \alpha_B, \alpha_H, \alpha_T\}$$.
   • The parameter $\alpha_T$, representing the fractional difference between the speed of gravitons and photons, is now constrained to be approximately zero. This stark constraint severely restricts the parameter space of beyond Horndeski models.
   • Azimuthal constraints from astrophysical tests apply additional pressure on these parameters, leading to narrowed viable regions in $\alpha_B$ and $\alpha_H$ planes, as illustrated comprehensively in the paper.
2. Excluded Domains for Galileon Models:
   • The authors derived significant implications for vanilla quartic Galileon models, noting that the newly derived bounds render them essentially irrelevant cosmologically.
   • The covariant quartic Galileon model's contribution to dark energy is largely ruled out due to the incompatibility between the required gravitational wave speed deviation and lack of evidence for strong equivalence principle violations.
3. Disformal Coupling of Photons:
   • A novel bound on disformal coupling to photons emerges, indicating that such couplings cannot substantially influence the cosmos's evolution.

Speculations and Future Directions

The paper suggests potential directions for further research, particularly enhancements in observational strategies that could impose even tighter constraints on dark energy models outlined by these modified-gravity theories. Continued GW observations would further refine the estimation of these model parameters, supporting the convergence toward a more comprehensive understanding of cosmic acceleration phenomena beyond the simple cosmological constant $\Lambda$.

This paper exemplifies how multimessenger astrophysics—combining gravitational and electromagnetic observations—creates robust tests for cosmological theories traditionally insulated from direct empirical scrutiny. The authors insightfully align theoretical modeling with empirical observations, advocating for synergies between particle physics, cosmology, and gravitation fields. The implications underscore the importance of an interdisciplinary approach in advancing the cosmological paradigm, a probable pathway towards unraveling the nature of dark energy in forthcoming investigations.