Fluid nature constrains Horndeski gravity

• DOI: 10.1007/s10714-023-03128-1
• Publication Date: 2023-07-09T13:01:10Z
• Authors: Marcello Miranda; Daniele Vernieri; Salvatore Capozzi...; Valerio Faraoni
• Keywords: High Energy Physics - Theory; High Energy Physics - Theory (hep-th); 0103 physical sciences; FOS: Physical sciences; General Relativity and Quantum Cosmology (gr-qc); 01 natural sciences; General Relativity and Quantum Cosmology

AbstractThe elusive physical nature of Horndeski gravity is elucidated in a new approach depicting this class of theories as a dissipative effective fluid. Requiring the constitutive equations of the latter to be those of a Newtonian fluid restricts the theory to only two disconnected subclasses of “viable” Horndeski gravity. Therefore, a stress-energy tensor of the Horndeski effective fluid, linear in the first derivatives of the fluid’s 4-velocity, is a sufficient condition for gravitational waves to propagate at light speed. All other Horndeski theories correspond to exotic non-Newtonian effective fluids. The two linear Horndeski classes are studied in the framework of first-order thermodynamics of viscous fluids, which further constrains the functional form of the theory.