Strong constraints on cosmological gravity from GW170817 and GRB 170817A (1710.06394v1)

Abstract: The detection of an electromagnetic counterpart (GRB 170817A) to the gravitational wave signal (GW170817) from the merger of two neutron stars opens a completely new arena for testing theories of gravity. We show that this measurement allows us to place stringent constraints on general scalar-tensor and vector-tensor theories, while allowing us to place an independent bound on the graviton mass in bimetric theories of gravity. These constraints severely reduce the viable range of cosmological models that have been proposed as alternatives to general relativistic cosmology.

• The paper establishes a key constraint, showing that the gravitational wave speed deviates from light speed by less than 10⁻¹⁵, limiting many modified gravity models.
• It applies multi-messenger observations to precisely restrict parameters in scalar-tensor and vector-tensor theories, forcing revised model couplings.
• The study sets an independent graviton mass limit (m ≲ 10⁻²² eV), reinforcing that any viable alternative to General Relativity must maintain minimal field couplings.

Constraints on Cosmological Gravity from GW170817 and GRB 170817A

The paper "Strong constraints on cosmological gravity from GW170817 and GRB 170817A" by T. Baker, E. Bellini, P.G. Ferreira, et al., presents an analysis of gravitational theories in light of the gravitational wave event GW170817 and its electromagnetic counterpart GRB 170817A. The combination of these observations significantly constrains several modified gravity theories and informs our understanding of cosmological models beyond General Relativity (GR).

Summary of Findings

The paper's main focus is on examining the implications of the speed of gravitational waves on various cosmological models. The alignment in arrival times of gravitational waves and light from the neutron star merger, with a difference no greater than 1.7 seconds, imposes stringent constraints on the speed of gravitational waves, characterized by the parameter $\alpha_T$, which measures deviations from the speed of light. The resulting constraint $|\alpha_T| \lesssim 10^{-15}$ effectively narrows down the landscape of viable modified gravity theories.

Implications for Theories of Gravity

1. Scalar-Tensor Theories: Within the framework of scalar-tensor theories, the Horndeski class is significantly constrained. The observed $\alpha_T \approx 0$ implies that specific couplings in the action, namely $G_{4,X}$, $G_{5,\phi}$, and $G_{5,X}$, should be zero, reducing viable models to those conformally coupled to gravity $(f(\phi)R)$. Quintessence and theories like kinetic gravity braiding can accommodate these constraints, while quartic and quintic Galilean models, which do not satisfy this naturally, are ruled out without fine-tuning.
2. Vector-Tensor Theories: In vector-tensor theories such as Generalized Einstein-Aether models, $c_1$ must be equal to $-c_3$, simplifying the theory constraint-wise. For Generalized Proca theories, the constraints imply negligible higher-order interaction terms, simplifying their structure. The results hold similar implications for "beyond" Generalized Proca models.
3. Bimetric and Massive Gravity Theories: The research places an independent constraint on the graviton mass, setting the limit to $m \lesssim 10^{-22}$ eV. This bound is aligned with but less restrictive than current Solar System constraints.

Practical and Theoretical Implications

The remarkable precision with which gravitational wave speed equals the speed of light has deep implications for cosmology and high-energy physics. It limits the permissible modifications to General Relativity, reinforcing the necessity that any extended models should preserve the minimal coupling of additional fields with the metric or justify any variations with significant tuning.

On a broader scale, these observations directly impact the viability of certain dark energy and cosmic acceleration models, demanding reconsideration of models relying on significant deviations in gravitational propagation speeds.

Future Directions

Future gravitational wave observations, particularly those involving electromagnetic counterparts, will further refine these constraints. The unique ability to test the constancy of gravitational wave speed across cosmological timescales will remain an essential tool in evaluating the robustness of various theories of modified gravity.

The paper contributes a significant refinement in our understanding of gravitational theories, with particular emphasis on gravitation's cosmological role. Continuous observations and new empirical data will continue to guide theoretical developments, ensuring that our gravitational models are consistent with observed cosmic dynamics.