Boyle-Turok CPT-Symmetric Universe
The universe has a mirror image on the other side of the Big Bang.
Placeholder for a 3D visualisation of Time-Symmetric Cosmologies. The interactive scene will land in Phase 3. Standard cosmology treats the Big Bang as a beginning with time flowing only into the future. Time-symmetric models propose that the Big Bang is a mirror point, with a fully separate universe (or set of universes) existing in the "other direction" of time, expanding backward into its own future.
In one sentence
The universe is CPT-symmetric: for every particle after the Big Bang there is an antiparticle mirror before it, making the cosmos a single self-contained quantum object.
The claim
Boyle and Turok propose the universe comes in two sheets joined at the Big Bang. Our sheet runs forward in time; the mirror sheet runs backward. Together they form a CPT-symmetric pair, a universe that requires no external cause or boundary condition because it is self-conjugate under the combined symmetry of charge, parity, and time reversal.
This structure predicts that neutrinos are Majorana particles and fixes the number of right-handed neutrino species to three, making the model unusually predictive for a cosmological proposal. A 2024 extension predicts a quantized set of negative spatial curvature values testable with Planck CMB data.
The family stance
A mirror universe existed before the Big Bang. From its perspective, the Big Bang was also a "beginning," but from our perspective, what we call "before" is its "after." Time has two preferred directions, both running away from the Big Bang event.
Predictions
- Neutrinos are Majorana particles (testable via neutrinoless double beta decay)
- Exactly three right-handed neutrino species
- Small negative spatial curvature Ω_K from a discrete quantized set, distinguishable from zero curvature with next-generation CMB experiments
Evidence
- Deng & Handley (2024) extend the Boyle-Turok framework and predict Ω_K values consistent with current Planck data.
- Deng & Handley (2025) use Planck CMB likelihoods to test the discrete curvature predictions, moving the Boyle-Turok framework into genuinely testable territory.
Counterpoints
- The mirror sheet is unobservable by construction, raising the question of whether the CPT partner is a physical entity or a mathematical convenience.
- The mechanism by which the two sheets exchange information or enforce global CPT symmetry at the bang is not yet fully specified in the original formulation.
Variants in this family
▸Go deeperTechnical detail with proper terminology
The two-sheeted spacetime is a maximal analytic extension of the standard FLRW solution, not a separate postulate, the mirror sheet is the natural analytic continuation across the bang (Boyle & Turok 2021, arXiv:2109.06204).
The quantized curvature prediction arises from imposing symmetric end-of-universe boundary conditions on both sheets simultaneously (Deng & Handley 2024, arXiv:2407.18225).
References
- Needs verificationBoyle, L., Turok, N. (2021). 'Two-Sheeted Universe, Analyticity and the Arrow of Time.'33 citations
- EstablishedDeng, W.-N., Handley, W. (2024). 'Predicting spatial curvature Ω_K in globally CPT-symmetric universes.' Physical Review D 110, 1035286 citations
- EstablishedDeng, W.-N., Handley, W. (2025). 'CMB constraints on quantized spatial curvature Ω_K in globally CPT-symmetric universes.' Physical Review D 113, 0235461 citations
Last reviewed May 16, 2026
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