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Vilenkin's Tunneling from Nothing vs Hartle-Hawking No-Boundary

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Quantum Tunneling Origin· within family

	Vilenkin's Tunneling from Nothing Speculative	Hartle-Hawking No-Boundary Speculative
Proposed	1982	1983
Key figures	Alexander Vilenkin	James Hartle, Stephen Hawking
In one sentence	Vilenkin proposed in 1982 that the universe quantum-tunneled into existence from a state with no space, no time, no fields, literal nothing.	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	[[Inflation]] begins naturally with high probability after tunneling Universe is initially small, closed, and de Sitter-like No contracting phase before the Bang	No initial [[singularity]] Universe wavefunction smooth at a = 0 Predicts a specific spectrum of cosmological perturbations
Where it breaks	Krauss: still requires quantum laws, so not really nothing Hawking & Bousso (1995): catastrophic particle production Penrose: ignores Weyl curvature hypothesis	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 laws-from-nothing problem: the quantum tunneling that creates the universe still needs quantum laws and a space of possible states to work in, so the proposal has not truly started from nothing.	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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Vilenkin's Tunneling from Nothing

1982 · Speculative

Hartle-Hawking No-Boundary

1983 · Speculative

Proposed

1982

1983

Key figures

Alexander Vilenkin

James Hartle, Stephen Hawking

In one sentence

Vilenkin proposed in 1982 that the universe quantum-tunneled into existence from a state with no space, no time, no fields, literal nothing.

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

[[Inflation]] begins naturally with high probability after tunneling
Universe is initially small, closed, and de Sitter-like
No contracting phase before the Bang

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

Where it breaks

Krauss: still requires quantum laws, so not really nothing
Hawking & Bousso (1995): catastrophic particle production
Penrose: ignores Weyl curvature hypothesis

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 laws-from-nothing problem: the quantum tunneling that creates the universe still needs quantum laws and a space of possible states to work in, so the proposal has not truly started from nothing.

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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