Primordial Black Holes Generated by Fast-roll Mechanism in Noncanonical Natural Inflation

Abstract

In this work, a new fast-roll (FR) mechanism to generate primordial black holes (PBHs) and gravitational waves (GWs) in generalized noncanonical natural inflation is introduced. In this model, choosing a suitable function for noncanonical mass scale parameter M ( ϕ ) gives rise to produce a cliff-like region in the field evolution path. When inflaton rolls down the steep cliff, its kinetic energy during an FR stage increases in comparison with a slow-roll stage. Hence, seeds of PBH production are born in this transient FR stage. Depending on the position of the cliff, appropriate cases of PBHs for explaining total dark matter, microlensing effects, LIGO–Virgo events, and NANOGrav 15 yr data can be formed. The density spectrum of GWs related to one case of the model lies in the NANOGrav 15 yr domain and behaves like ${{\rm{\Omega }}}_{{\mathrm{GW}}_{0}}\sim {f}^{5-\gamma }$ . The spectral index γ = 3.42 for this case satisfies the NANOGrav 15 yr constraint. Moreover, regarding reheating considerations, it is demonstrated that PBHs are born in the radiation-dominated era. Furthermore, viability of the model in light of theoretical swampland criteria and observational constraints on cosmic microwave background scales are illustrated.

Keywords

• Cosmic inflation
• Primordial black holes
• Gravitational waves