Janus Universe
The Big Bang is a mirror point. Time flows in one direction for us; in the opposite direction in a mirror universe.
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
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 claim
The Janus model is named after the Roman god with two faces, looking forward and backward simultaneously. The Big Bang, in this picture, is not a beginning but a 'Janus point' from which two universes emerge, evolving in opposite temporal directions. Our universe expands away from the Janus point into our future. The mirror universe expands away from the Janus point in the opposite direction, into what we would call 'before' but what its inhabitants experience as their future.
This solves a longstanding puzzle in physics: why does time appear to have a preferred direction (an 'arrow of time') when the fundamental laws of physics are time-symmetric? The Janus answer: the laws ARE time-symmetric, and what we call the arrow of time is just the direction away from the Janus point. Observers in the mirror universe see time flowing in their direction with the same conviction we have about ours.
Julian Barbour and his collaborators developed this from work on shape dynamics, an alternative formulation of general relativity. Their model is mathematical and concerned with how to describe a universe whose geometry doesn't include a preferred time direction. The result is that the Big Bang naturally has two evolving sides.
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
- 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
Evidence
- Mathematically elegant solution to the arrow of time problem
- Removes the need for fine-tuned initial conditions to explain low initial entropy
- Consistent with general relativity
Counterpoints
- 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
Variants in this family
▸Go deeperTechnical detail with proper terminology
Barbour, Koslowski, and Mercati formulate the model in terms of shape dynamics, a reformulation of general relativity that focuses on the intrinsic geometry of space rather than spacetime. In their formalism, the universe naturally has two solutions emerging from a common 'shape configuration' at the Big Bang.
The mathematical setup involves the gravitational N-body problem, where it can be shown that under certain conditions, all solutions tend to a 'Janus point' of maximum simplicity. The two halves of the solution are time-reverses of each other.
The proposal is closely related to longstanding debates about the foundations of statistical mechanics. The 'past hypothesis' (the idea that the universe started in a low-entropy state) is essentially the foundation of the arrow of time. The Janus model replaces the past hypothesis with a 'midpoint hypothesis' that may be more physically natural.
References
- EstablishedBarbour, J., Koslowski, T., Mercati, F. (2014). "Identification of a gravitational arrow of time." Phys. Rev. Lett. 113, 181101
- EstablishedBarbour, J. (2020). "The Janus Point: A New Theory of Time." Basic Books
Last reviewed May 14, 2026
Spotted an error? Have a source to add?
Prefer email?
You can also send a prefilled email with the variant URL already filled in.