Abstract
Primordial blackholes formed in the early Universe via gravitational collapse of over-dense regions may contribute a significant amount to the present dark matter relic density. Inflation provides a natural framework for the production mechanism of primordial blackholes. For example, single field inflation models with a fine-tuned scalar potential may exhibit a period of ultra-slow roll, during which the curvature perturbation may be enhanced to become seeds of the primordial blackholes formed as the corresponding scales reenter the horizon. In this work, we propose an alternative mechanism for the primordial blackhole formation. We consider a model in which a scalar field is coupled to the Gauss-Bonnet term and show that primordial blackholes may be seeded when a scalar potential term and the Gauss-Bonnet coupling term are nearly balanced. Large curvature perturbation in this model not only leads to the production of primordial blackholes but it also sources gravitational waves at the second order. We calculate the present density parameter of the gravitational waves and discuss the detectability of the signals by comparing them with sensitivity bounds of future gravitational wave experiments.
Highlights
Primordial black holes [1,2,3,4] are a viable candidate for dark matter [5,6] and may constitute all or a part of the dark matter relic density today
Primordial black holes may form in the early Universe through the gravitational collapse of large over-dense regions; see, e.g., Refs. [14,15,16,17,18,19] for reviews
We investigate the production of primordial black holes and the scalar-induced second-order gravitational waves in such a setup
Summary
Primordial black holes [1,2,3,4] are a viable candidate for dark matter [5,6] and may constitute all or a part of the dark matter relic density today. Other models for generating large primordial curvature perturbation during inflation include the one with the modified gravity sector, and with a non-canonical form of the inflaton; see e.g., Refs. Inflation models in which primordial black holes are formed via large enhancement of the curvature perturbation inevitably produce the scalar-induced second-order gravitational waves. Other recent studies on the primordial black holes and the scalar-induced second-order gravitational waves include Refs. In the single field inflation models for which the primordial black hole production and the secondary gravitational waves are studied, the gravity sector is usually assumed to be the Einstein gravity. We investigate the production of primordial black holes and the scalar-induced second-order gravitational waves in such a setup.
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