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The Boundary Proposal vs Hartle-Hawking No-Boundary

Quantum Tunneling Origin· within family

	The Boundary Proposal Speculative	Hartle-Hawking No-Boundary Speculative
Proposed	2024	1983
Key figures	Bjoern Hassfeld, Arthur Hebecker	James Hartle, Stephen Hawking
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.	The universe has no temporal boundary at the beginning. In Euclidean time, the universe is a smooth four-dimensional surface with no edge, like asking what is south of the South Pole.
Predictions	Distinct primordial perturbation spectrum from Hartle-Hawking and Vilenkin proposals Specific signatures in the CMB tied to the boundary geometry	No initial [[singularity]] Universe wavefunction smooth at a = 0 Predicts a specific spectrum of cosmological perturbations
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.	Feldbrugge, Lehners, Turok (2017) argue the proposal predicts an unstable universe Direction of "outgoing" vs "incoming" mode is contested A more careful mathematical analysis of the path integral suggests deeper problems, hinting the proposal may predict unstable outcomes (see Go Deeper) Maldacena (2024) re-examines the no-boundary proposal and finds it predicts spatial curvature in conflict with observations, and is non-normalizable for landscape-like potentials. Ivo, Li & Maldacena (2024) show that once you account for the regions of space we cannot observe, the mathematics of the no-boundary state leads to physically unrealistic predictions, sharpening the case against the original proposal.
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 instability problem: a careful redo of the math by Feldbrugge, Lehners, and Turok suggests the no-boundary proposal predicts runaway, ever-growing fluctuations, which would mean an unstable universe rather than the smooth one we see.
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The Boundary Proposal

2024 · Speculative

Hartle-Hawking No-Boundary

1983 · Speculative

Proposed

2024

1983

Key figures

Bjoern Hassfeld, Arthur Hebecker

James Hartle, Stephen Hawking

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.

The universe has no temporal boundary at the beginning. In Euclidean time, the universe is a smooth four-dimensional surface with no edge, like asking what is south of the South Pole.

Predictions

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

No initial [[singularity]]
Universe wavefunction smooth at a = 0
Predicts a specific spectrum of cosmological perturbations

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.

Feldbrugge, Lehners, Turok (2017) argue the proposal predicts an unstable universe
Direction of "outgoing" vs "incoming" mode is contested
A more careful mathematical analysis of the path integral suggests deeper problems, hinting the proposal may predict unstable outcomes (see Go Deeper)
Maldacena (2024) re-examines the no-boundary proposal and finds it predicts spatial curvature in conflict with observations, and is non-normalizable for landscape-like potentials.
Ivo, Li & Maldacena (2024) show that once you account for the regions of space we cannot observe, the mathematics of the no-boundary state leads to physically unrealistic predictions, sharpening the case against the original proposal.

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 instability problem: a careful redo of the math by Feldbrugge, Lehners, and Turok suggests the no-boundary proposal predicts runaway, ever-growing fluctuations, which would mean an unstable universe rather than the smooth one we see.

Reader vote

No votes yet