Birth of baby universes from gravitational collapse in a modified-gravity scenario

Abstract

We consider equilibrium models of spherical boson stars in Palatini f(ℛ) = ℛ + ξℛ2 gravity and study their collapse when perturbed. The Einstein-Klein-Gordon system is solved using a recently established correspondence in an Einstein frame representation. We find that, in that frame, the endpoint is a nonrotating black hole surrounded by a quasi-stationary cloud of scalar field. However, the dynamics in the f(ℛ) frame is dramatically different. The innermost region of the collapsing object exhibits the formation of a finite-size, exponentially-expanding baby universe connected with the outer (parent) universe via a minimal area surface (a throat or umbilical cord). Our simulations indicate that this surface is at all times hidden inside a horizon, causally disconnecting the baby universe from observers above the horizon. The implications of our findings in other areas of gravitational physics are also discussed.