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

We consider equilibrium models of spherical boson stars in Palatini $f(\mathcal{R})=\mathcal{R}+\xi \mathcal{R}^2$ gravity and study their collapse when perturbed. The Einstein-Klein-Gordon system is solved using a recently established correspondence an Einstein frame representation. find that, that frame, the endpoint nonrotating black hole surrounded by quasi-stationary cloud scalar field. However, dynamics $f(\mathcal{R})$ dramatically different. innermost region collapsing object exhibits formation finite-size, exponentially-expanding $\textit{ baby universe}$ connected with outer (parent) universe via minimal area surface (a throat or umbilical cord). Our simulations indicate this at all times hidden inside horizon, causally disconnecting from observers above horizon. implications our findings other areas gravitational physics are also discussed.