Cosmological Constraints on Horndeski Gravity in Light of GW170817

TL;DR

This paper uses multiple cosmological data sets to constrain Horndeski gravity models after GW170817, finding tight bounds on modifications to gravitational wave propagation and implications for primordial B-mode signals.

Contribution

It provides the first comprehensive cosmological constraints on Horndeski gravity parameters considering the GW170817 speed constraint.

Findings

95% CL upper bound on Hubble friction coefficient is 2.38

Suppression of primordial B-mode amplitude is limited to less than 0.8%

Scalar ISW effect primarily constrains the model parameters

Abstract

The discovery of the electromagnetic counterpart to GW170817 severely constrains the tensor mode propagation speed, eliminating a large model space of Horndeski theory. We use the cosmic microwave background data from Planck and the joint analysis of the BICEP2/Keck Array and Planck, galaxy clustering data from the SDSS LRG survey, BOSS baryon acoustic oscillation data, and redshift space distortion measurements to place constraints on the remaining Horndeski parameters. We evolve the Horndeski parameters as power laws with both the amplitude and power law index free. We find a 95% CL upper bound on the present-day coefficient of the Hubble friction term in the cosmological propagation of gravitational waves is 2.38, whereas General Relativity gives 2 at all times. While an enhanced friction suppresses the amplitude of the reionization bump of the primordial B-mode power spectrum at $\ell < 10$, our result limits the suppression to be less than 0.8%. This constraint is primarily due to the scalar integrated Sachs-Wolfe effect in temperature fluctuations at low multipoles.