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

**Authors:** Andreu Masó-Ferrando, Nicolas Sanchis-Gual, José A. Font, Gonzalo J. Olmo  
**arXiv:** [2304.12018](https://arxiv.org/abs/2304.12018)  
**Date:** 2023-04-24  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.12018v1/

## Key points

- Study investigates equilibrium models of spherical boson stars in Palatini $f(\mathcal{R})=\mathcal{R}+\xi \mathcal{R}^2$ gravity
- Authors solve the Einstein-Klein-Gordon system using a correspondence established in an Einstein frame representation
- In the Einstein frame, endpoint of collapse is a nonrotating black hole surrounded by a quasi-stationary cloud of scalar field
- Significant differences observed in dynamics when considering the $f(\mathcal{R})$ frame
- Formation of finite-size, exponentially-expanding "baby universe" observed during collapse
- Baby universe connected to outer universe through minimal area surface (throat or umbilical cord)
- Surface remains hidden inside horizon, causally disconnecting baby universe from observers above horizon
- Findings have implications for understanding singularities and bounce mechanisms in general relativity and quantum gravity
- Connections noted to loop quantization of black holes, heuristic black bounce models, and other static solutions
- Study provides new insights into nonsingular collapse processes and highlights qualitative distinctions between different frames

## Summary

, , , , 

This study investigates equilibrium models of spherical boson stars in Palatini $f(\mathcal{R})=\mathcal{R}+\xi \mathcal{R}^2$ gravity and explores their collapse behavior when perturbed. The authors solve the Einstein-Klein-Gordon system using a correspondence established in an Einstein frame representation. In the Einstein frame, they find that the endpoint of the collapse is a nonrotating black hole surrounded by a quasi-stationary cloud of scalar field. However, they observe significant differences in the dynamics when considering the $f(\mathcal{R})$ frame. During the collapse, they observe the formation of a finite-size, exponentially-expanding "baby universe" in the innermost region of the collapsing object. This baby universe is connected to the outer (parent) universe through a minimal area surface, which can be thought of as a throat or umbilical cord. Importantly, this surface remains hidden inside a horizon throughout all times, causally disconnecting the baby universe from observers above the horizon. These findings have implications beyond just understanding boson star collapses in modified gravity theories and could shed light on singularities and bounce mechanisms in general relativity and quantum gravity. The authors also note connections to loop quantization of black holes, heuristic black bounce models, and other static solutions. Overall, this study provides new insights into nonsingular collapse processes and highlights how different frames can lead to qualitatively distinct descriptions of gravitational phenomena.