Gravitational waves traveling through spacetime is a foundational premise of general relativity and confirmed astrophysical observation, making it a well-established scientific fact that requires no citation.
The claim that gravitational waves travel through spacetime is a foundational and universally accepted concept in general relativity and modern astrophysics, directly demonstrated by gravitational wave detectors like LIGO and Virgo. The retrieved papers themselves study the specific details of this propagation (such as velocity, dispersion, and modifications beyond general relativity), implicitly taking the phenomenon of wave propagation as established fact. Therefore, the claim functions as common scientific knowledge.