Compare · Before the Universe

Janus Universe vs Boyle-Turok CPT-Symmetric Universe

Time-Symmetric Cosmologies· within family

	Janus Universe Speculative	Boyle-Turok CPT-Symmetric Universe Speculative
Proposed	2018	2021 / 2024
Key figures	Julian Barbour, Tim Koslowski, Flavio Mercati	Latham Boyle, Neil Turok
In one sentence	Barbour and collaborators proposed in 2018 that the Big Bang is the central point of two universes, with time flowing forward into ours and backward into a mirror universe that we can never directly observe.	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.
Predictions	A mirror universe exists with reversed time arrow relative to ours [[Entropy]] increases from the Janus point in both directions, never inversely No direct observational signatures from the mirror universe (it's causally disconnected from us) The arrow of time arises naturally from cosmic geometry rather than from special initial conditions	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
Where it breaks	No direct observational test possible Other approaches to the arrow of time (statistical thermodynamics, decoherence) don't require a mirror universe The mirror universe is undetectable by definition Some critics argue the mathematics is more complicated than necessary	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.
Key unresolved problem	The untestable twin: the mirror universe in the Janus model can never be detected, so there is no experiment that could tell it apart from simpler explanations for why time flows one way.	The missing rulebook: the original model never fully spells out what forces the two mirror sheets of spacetime to obey CPT symmetry, a mirror-flip of matter, charge, and time, at the Big Bang when they cannot communicate.
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Janus Universe

2018 · Speculative

Boyle-Turok CPT-Symmetric Universe

2021 / 2024 · Speculative

Proposed

2018

2021 / 2024

Key figures

Julian Barbour, Tim Koslowski, Flavio Mercati

Latham Boyle, Neil Turok

In one sentence

Barbour and collaborators proposed in 2018 that the Big Bang is the central point of two universes, with time flowing forward into ours and backward into a mirror universe that we can never directly observe.

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.

Predictions

A mirror universe exists with reversed time arrow relative to ours
[[Entropy]] increases from the Janus point in both directions, never inversely
No direct observational signatures from the mirror universe (it's causally disconnected from us)
The arrow of time arises naturally from cosmic geometry rather than from special initial conditions

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

Where it breaks

No direct observational test possible
Other approaches to the arrow of time (statistical thermodynamics, decoherence) don't require a mirror universe
The mirror universe is undetectable by definition
Some critics argue the mathematics is more complicated than necessary

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.

Key unresolved problem

The untestable twin: the mirror universe in the Janus model can never be detected, so there is no experiment that could tell it apart from simpler explanations for why time flows one way.

The missing rulebook: the original model never fully spells out what forces the two mirror sheets of spacetime to obey CPT symmetry, a mirror-flip of matter, charge, and time, at the Big Bang when they cannot communicate.

Reader vote

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