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

Quantum Tunneling Origin· within family

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

1983 · Speculative

Vilenkin's Tunneling from Nothing

1982 · Speculative

Proposed

1983

1982

Key figures

James Hartle, Stephen Hawking

Alexander Vilenkin

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.

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

Predictions

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

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

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.

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

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 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.

Reader vote

0% · 0 votes

100% · 2 votes