Axion-driven hybrid inflation over a barrier

We present a novel cosmological scenario that describes both inflation and dark matter. A concrete realization of our scenario is given based on a well-established particle physics model, where an axionlike field drives inflation until a potential barrier, which keeps a waterfall field at the origin, disappears to trigger a waterfall transition. Such a barrier makes the inflaton potential much flatter, improving significantly the naturalness and viability of the otherwise problematic setup adopted previously. The observed spectrum of the cosmic microwave background indicates that the inflationary Hubble scale, which is allowed to span a wide range, uniquely fixes the inflaton mass and decay constant. This raises an intriguing possibility of probing inflation via experimental searches for axionlike particles. Further, our model involves dark matter candidates including the inflaton itself. Also, for a complex waterfall field, we can determine cosmologically the Peccei-Quinn scale associated with the strong CP problem.