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NoBoundaryProposal
The no-boundary proposal, introduced by Hartle and Hawking, is a theory in cosmology that describes the universe's origin as a smooth transition from a quantum gravity era, meaning there is no boundary (or singularity) at the beginning of the universe. Instead, time emerges smoothly out of a space-like geometry in a manner analogous to how points emerge from the center of a sphere. This proposal also involves a path integral over compact Euclidean metrics, which in principle sums over all possible past histories of the universe.
Neil Turok, along with Job Feldbrugge and Jean-Luc Lehners, published papers in 2017 and 2018 criticizing the No Boundary Proposal. Their main arguments were:
- The proposal predicted a universe with runaway inflation.
- They claimed the proposal didn't provide a well-defined probability distribution.
- They argued that the Euclidean path integral approach was fundamentally flawed.
- Subsequent work, particularly by Jonathan Halliwell, James Hartle, and Thomas Hertog, showed that Turok's team had used an incomplete formulation of the proposal.
- They demonstrated that a proper implementation of the No Boundary Proposal does not lead to runaway inflation.
- The probability distribution issue was addressed by showing that the correct contour for the path integral leads to well-defined probabilities.
- The No Boundary Proposal remains a viable and influential model in quantum cosmology.
- Turok's objections, while initially seeming significant, have been largely addressed and refuted.
- The debate has actually led to refinements and clarifications of the No Boundary Proposal, strengthening its theoretical foundation.
- Work continues on fully reconciling the proposal with observations of the early universe.
- Efforts are being made to connect the proposal more directly with string theory and other approaches to quantum gravity.
This episode in theoretical physics demonstrates the importance of rigorous analysis and the self-correcting nature of scientific discourse. It also highlights the robustness of the No Boundary Proposal in withstanding and ultimately benefiting from serious challenges.