Zoology of instanton solutions in flat potential barriers

Understand

We perform a detailed study of the existence and the properties of O(4)-invariant instanton solutions in Einstein-scalar theory in the presence of flat potential barriers, i.e.

• barriers where the second derivative of the potential is small at the top of the barrier.
• We find a whole zoo of solutions: Hawking-Moss, Coleman-De Luccia (CdL), oscillating instantons, asymmetric CdL as well as other non-standard CdL-like solutions with additional negative modes in their spectrum of fluctuations.
• Our work shows how these different branches of solutions are connected to each other via "critical" instantons possessing an extra zero mode fluctuation.