Formation of trapped vacuum bubbles during inflation, and consequences for PBH scenarios

Understand

A class of inflationary scenarios for primordial black hole (PBH) formation include a small barrier in the slope of the potential.

• There, the inflaton slows down, generating an enhancement of primordial perturbations.
• Moreover, the background solution overcomes the barrier at a very low speed, and large backward quantum fluctuations can prevent certain regions from overshooting the barrier.
• This leads to localized bubbles where the field remains trapped behind the barrier.