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Hartle-Hawking No-Boundary vs The Boundary Proposal

Quantum Tunneling Origin· within family

	Hartle-Hawking No-Boundary Speculative	The Boundary Proposal Speculative
Proposed	1983	2024
Key figures	James Hartle, Stephen Hawking	Bjoern Hassfeld, Arthur Hebecker
In one sentence	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.	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.
Predictions	No initial [[singularity]] Universe wavefunction smooth at a = 0 Predicts a specific spectrum of cosmological perturbations	Distinct primordial perturbation spectrum from Hartle-Hawking and Vilenkin proposals Specific signatures in the CMB tied to the boundary geometry
Where it breaks	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.	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.
Key unresolved problem	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.	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.
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Hartle-Hawking No-Boundary

1983 · Speculative

The Boundary Proposal

2024 · Speculative

Proposed

1983

2024

Key figures

James Hartle, Stephen Hawking

Bjoern Hassfeld, Arthur Hebecker

In one sentence

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.

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.

Predictions

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

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

Where it breaks

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.

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.

Key unresolved problem

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.

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.

Reader vote

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