Compare · Before the Universe

The Boundary Proposal vs Gott-Li Self-Creating Universe

Quantum Tunneling Origin· within family

	The Boundary Proposal Speculative	Gott-Li Self-Creating Universe Historical
Proposed	2024	1998
Key figures	Bjoern Hassfeld, Arthur Hebecker	J. Richard Gott, Li-Xin Li
In one sentence	An alternative to Hartle-Hawking and Vilenkin in which the universe begins with a finite spacelike spherical boundary that can dominate over the no-boundary instanton.	Gott and Li proposed in 1998 that the early universe contained a closed time loop, allowing the universe to literally create itself.
Predictions	Distinct primordial perturbation spectrum from Hartle-Hawking and Vilenkin proposals Specific signatures in the CMB tied to the boundary geometry	The very early universe contained closed timelike curves A region of CTC geometry preceded the standard cosmological era Some specific predictions about the early universe's geometry differ from standard Big Bang cosmology
Where it breaks	Requires specifying the boundary geometry as input, which some critics view as no improvement over the boundary conditions it replaces. The phenomenological implications are still being worked out and have not yet been compared to Planck data.	Most physicists view closed timelike curves as unphysical or as a sign that general relativity breaks down The proposal requires specific energy conditions that may not be physically realizable No direct observational test Stephen Hawking proposed a "Chronology Protection Conjecture" that argues physics conspires to prevent CTCs from forming
Key unresolved problem	The input-geometry problem: the proposal has to be handed the size of its starting boundary surface from outside, which critics say just moves the fine-tuning problem somewhere else rather than removing it.	The chronology protection problem: as Hawking argued, quantum effects likely pile up runaway energy wherever a closed timelike curve, a path that loops back in time, would form, destroying the time loop this proposal needs before it can exist.
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The Boundary Proposal

2024 · Speculative

Gott-Li Self-Creating Universe

1998 · Historical

Proposed

2024

1998

Key figures

Bjoern Hassfeld, Arthur Hebecker

J. Richard Gott, Li-Xin Li

In one sentence

An alternative to Hartle-Hawking and Vilenkin in which the universe begins with a finite spacelike spherical boundary that can dominate over the no-boundary instanton.

Gott and Li proposed in 1998 that the early universe contained a closed time loop, allowing the universe to literally create itself.

Predictions

Distinct primordial perturbation spectrum from Hartle-Hawking and Vilenkin proposals
Specific signatures in the CMB tied to the boundary geometry

The very early universe contained closed timelike curves
A region of CTC geometry preceded the standard cosmological era
Some specific predictions about the early universe's geometry differ from standard Big Bang cosmology

Where it breaks

Requires specifying the boundary geometry as input, which some critics view as no improvement over the boundary conditions it replaces.
The phenomenological implications are still being worked out and have not yet been compared to Planck data.

Most physicists view closed timelike curves as unphysical or as a sign that general relativity breaks down
The proposal requires specific energy conditions that may not be physically realizable
No direct observational test
Stephen Hawking proposed a "Chronology Protection Conjecture" that argues physics conspires to prevent CTCs from forming

Key unresolved problem

The input-geometry problem: the proposal has to be handed the size of its starting boundary surface from outside, which critics say just moves the fine-tuning problem somewhere else rather than removing it.

The chronology protection problem: as Hawking argued, quantum effects likely pile up runaway energy wherever a closed timelike curve, a path that loops back in time, would form, destroying the time loop this proposal needs before it can exist.

Reader vote

No votes yet