Delicate curvature bounces in the no-boundary wave function and in the late universe

Abstract: Theoretical considerations motivate us to consider vacuum energy to be able to decay and to assume that the spatial geometry of the universe is closed. Combining both aspects leads to the possibility that the universe, or certain regions thereof, can collapse and subsequently undergo a curvature bounce. This may have occurred in the very early universe, in a pre-inflationary phase. We discuss the construction of the corresponding no-boundary instantons and show that they indeed reproduce a bouncing history of the universe, interestingly with a small and potentially observable departure from classicality during the contracting phase. Such an early bouncing history receives a large weighting and provides competition for a more standard inflationary branch of the wave function. Curvature bounces may also occur in the future. We discuss the conditions under which they may take place, allowing for density fluctuations in the matter distribution in the universe. Overall, we find that curvature bounces require a delicate combination of matter content and initial conditions to occur, though with significant consequences if these conditions are met.