# Baby universes A baby universe describes a disconnected component of a spatial time slice in gravity that "branches off" from the parent space. They play a role in understanding various aspects of quantum gravity such as the [[0131 Information paradox|information paradox]], the [[0249 Factorisation problem|factorisation problem]], and etc. ## Refs - 1988: [[GiddingsStrominger1988]][](https://www.sciencedirect.com/science/article/abs/pii/0550321388901095) and [[Coleman1988]][](https://www.sciencedirect.com/science/article/abs/pii/0550321388901101) - Giddings and Strominger says there is loss of quantum coherence, but Coleman said no. - [[2020#Marolf, Maxfield (a)]] - propose a construction for a Hilbert space of baby universes out of the gravitational path integral - diffeomorphism invariance drastically reduces the dimensionality of this Hilbert space <- quotienting of null states - [[McNamaraVafa2020]] - formulated a stronger baby universe hypothesis: the Hilbert space of baby universes in a unitary theory of quantum gravity in d > 3 spacetime dimensions has dimension one <!-- (Don) it's a desire rather than an argument --> - [[2020#Gardiner, Megas]] - extension of MM model to TQFTs - [[2020#Gesteau, Kang]] - give primordial role to algebra of observable rather than the Hilbert space - [[2020#Anous, Kruthoff, Mahajan]]: Bra-ket wormholes - [[CasaliMarolfMaxfieldRangamani2021]][](https://arxiv.org/pdf/2101.12221.pdf) - [[GodetMarteau2021]][](https://arxiv.org/abs/2103.13422) - [[PengTianYu2021]][](https://arxiv.org/pdf/2111.14856.pdf) - with matters - [[2022#Post, van der Heijden, Verlinde]] - relation to [[0050 JT gravity|JT gravity]] - in JT gravity (see also [[0195 Firewall]]) - [[2022#Stanford, Yang]]: genus-1 (emitting one baby universe) - [[2024#Zolfi]]: higher genus (emitting more than one baby universe) - [[2024#Blommaert, Chen, Nomura]]: exponentially late time; resum the expansion in the number of wormholes; appearance of [[0117 Shockwave|shockwaves]] - [[2024#Cui, Rozali]]: JT with matter - [[2025#Zolfi, Alishahiha]]: higher genus and multiple geodesic boundaries - Antonini-Rath puzzle - [[2025#Engelhardt, Gesteau]] - [[2025#Higginbotham]]