  We investigate Euclidean wormholes in Einstein gravity with a massless scalar
field in de Sitter space. Euclidean wormholes are possible due to the analytic
continuation of the time as well as complexification of fields, where we need
to impose the classicality after the Wick-rotation to the Lorentzian
signatures. For some parameters, wormholes are preferred than Hawking-Moss
instantons, and hence wormholes can be more fundamental than Hawking-Moss type
instantons. Euclidean wormholes can be interpreted in three ways: (1) classical
big bounce, (2) either tunneling from a small to a large universe or a creation
of a collapsing and an expanding universe from nothing, and (3) either a
transition from a contracting to a bouncing phase or a creation of two
expanding universes from nothing. These various interpretations shed some light
on challenges of singularities. In addition, these will help to understand
tensions between various kinds of quantum gravity theories.
