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  • Letter
  • Open Access

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

Qing-Hong Cao1,2,3,*, Masanori Tanaka3,†, Jun-Chen Wang1,‡, Ke-Pan Xie4,§, and Jing-Jun Zhang1,∥

  • *Contact author: qinghongcao@pku.edu.cn
  • †Contact author: tanaka@pku.edu.cn
  • ‡Contact author: junchenwang@stu.pku.edu.cn
  • §Contact author: kpxie@buaa.edu.cn
  • ∥Contact author: zhang_jingjun@stu.pku.edu.cn
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Phys. Rev. D 113, L051701 – Published 4 March, 2026

DOI: https://doi.org/10.1103/qxwm-xb8d

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.

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Phys. Rev. D 113, L051701– Published 4 March, 2026
  • Received 4 June 2025
  • Revised 29 September 2025
  • Accepted 13 February 2026
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