Primordial black holes and oscillating gravitational waves in slow-roll and slow-climb inflation with an intermediate noninflationary phase

We propose a new single field inflation model in which the usual slow-roll inflation is joined to a new period of slow-climb and slow-roll inflation through a short intermediate noninflationary phase. We then show that primordial curvature perturbations can be enhanced at small scales, a sizable amount of primordial black holes (PBHs) can be produced which make up most of dark matter, the gravitational waves (GWs) induced by scalar metric perturbations that accompany with the formation of PBHs can be detectable by future GW experiments, and last but not least, our model is compatible with the latest cosmic microwave background observations. Remarkably, the GW spectrum displays a unique oscillating character in the ultraviolet regions which originates from the short noninflationary phase. A detection of such oscillations in the GW spectrum may suggest the existence of such a noninflationary phase in the whole inflation, thus providing us a chance to reveal an interesting period in the evolution of the early Universe and distinguish our model from others.