Production of gravitational waves during preheating in the Starobinsky inflationary model

The production of GWs during preheating in the Starobinsky model with a nonminimally coupled auxiliary scalar field is studied through the lattice simulation in this paper. We find that the GW spectrum $$\Omega _{\mathrm{gw}}$$ grows fast with the increase of the absolute value of coupling parameter $$\xi $$. This is because the resonant bands become broad with the increase of $$|\xi |$$. When $$\xi 0$$. $$\Omega _{\mathrm{gw}}$$ reaches the maximum at $$\xi =-20$$ ($$\xi =42$$ for the case $$\xi >0$$) and then decreases with slight oscillation. Furthermore we find that the GWs produced in the era of preheating satisfy the limits from the Planck and next-generation CMB experiments.