Can we live in a baby universe formed by a delayed first-order phase transition?

Abstract

We examine the idea that our universe began as a baby universe and show that this is feasible in a gauged $U(1)_{B-L}$ extension of the Standard Model with the classically conformal principle. For the first time, we define a measure to describe the probability that we reside in a baby universe, and find that it can be close to 1 in a considerable portion of the parameter space. The framework is consistent with current cosmological data, and it predicts the existence of a heavy neutral gauge boson, which could be detected at colliders, thereby offering a direct link between early-universe dynamics and experimentally testable signatures at the TeV scale.

Figures (2)

• Figure 1: Sketch of our scenario. In the parent universe, some rare regions remain in the false vacuum (white) until surrounded by true vacuum regions (orange). Those regions could collapse into super-critical PBHs, inside which an inflating baby universe is born. At a late time, the QCD phase transition (blue) triggers the $B-L$ and EW symmetry breaking (orange), starting the standard cosmology.
• Figure 2: Numerical results of the minimal $B-L$ model. The red solid lines represent contours of $P_{\rm baby}$. If we assume no extra dilution mechanism, $f_{\rm PBH}=\Omega_{\rm PBH}/\Omega_{\rm DM}$ in the parent universe is larger than 1 in the left part of the black dashed line. The green (blue) shaded region is the constraint from the ATLAS dilepton search with the integrated luminosity of $36.1~{\rm fb}^{-1}$ATLAS:2017fih ($139~{\rm fb}^{-1}$Queiroz:2024ipo).