For every particle after the Bang, there's an antiparticle before it. Perfect symmetry.
Boyle-Turok CPT-Symmetric Universe
The universe has a mirror image on the other side of the Big Bang.
Looping ambient scene for Time-Symmetric Cosmologies. 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.
§1 · The claim, 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.
§2 · Why it might be true
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.
§2.5 · Evidence
- Deng & Handley (2024) extend the Boyle-Turok framework and predict spatial-curvature (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.
§3 · What you'd need to test it
- Neutrinos are Majorana particles (testable via neutrinoless double beta decay)
- Exactly three right-handed neutrino species
- A small negative spatial curvature (Ω_K, the measure of how far space departs from perfectly flat), taking specific quantized values that next-generation CMB experiments could tell apart from a flat universe
§4 · Where it breaks
- 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.
Go deeper
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).
▸§5 · Who built it, and when(3 sources, 2 established, 1 needs verification)
- Needs verificationBoyle & Turok (2021). 'Two-Sheeted Universe, Analyticity and the Arrow of Time.' arXiv:2109.0620433 citations
- EstablishedDeng & Handley (2024). 'Predicting spatial curvature Ω_K in globally CPT-symmetric universes.' Phys. Rev. D 110, 1035287 citations
- EstablishedDeng & Handley (2025). 'CMB constraints on quantized spatial curvature Ω_K in globally CPT-symmetric universes.' Phys. Rev. D 113, 0235462 citations
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