Polar modes of gravitational waves in Rastall cosmology

In this article, we discuss propagation expressions for polar gravitational waves in the spatially flat Friedmann–Lemaitre–Robertson–Walker spacetime dominated by a perfect fluid in the Rastall theory. We perturb the spatially flat spacetime description by making use of Regge–Wheeler perturbations inducing the polar gravitational waves and formulate the corresponding field equations for both unperturbed and perturbed cases. Then, we focus on these field equations simultaneously to find out the unknown perturbation functions. We attain that the assumed perturbations affect the background matter distribution as well as the four-velocity components. We also investigate the impact of model parameters on the amplitude of the polar gravitational waves. (paper)