Physical predictions in closed quantum gravity
5 affiliations
- 1University of California
- 2Lawrence Berkeley National Laboratory
- 3RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS)
- 4UTIAS, The University of Tokyo
- 5Rikkyo University
Abstract
A bstract : Recent developments in gravitational path integrals indicate that the nonperturbative physical Hilbert space of a closed universe is one-dimensional within each superselection sector. This raises a basic puzzle: how can a unique quantum-gravity state give rise to semiclassical physics, measurement outcomes, and classical probabilities? In this paper, we develop a framework in which nontrivial and statistically stable predictions emerge despite the one-dimensionality of the fully constrained Hilbert space. The key idea is to extract physical predictions in an enlarged, unconstrained Hilbert space by conditioning on observational data. We show that partial observability — reflecting the limited access of observers to the degrees of freedom of the universe — suppresses ensemble fluctuations associated with microscopic structure in the gravitational path integral, thereby restoring semiclassical predictability with exponential accuracy. We formulate the construction explicitly including contributions from the Hartle-Hawking no-boundary state, define a gauge-invariant Hilbert space for observations via a density operator, and generalize the formalism to conditioning on histories, clarifying the emergence of classical probabilities and an effective arrow of time. Finally, we explore whether this framework can support a realistic cosmology and identify assumptions that the underlying theory of quantum gravity must satisfy.
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BibTeX
@article{nomura2026physical,
title = {{Physical predictions in closed quantum gravity}},
author = {Yasunori Nomura and Tomonori Ugajin},
journal = {Journal of High Energy Physics},
year = {2026},
month = nov,
doi = {10.1007/jhep11(2026)030},
url = {https://link.springer.com/article/10.1007/JHEP11(2026)030},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Physical predictions in closed quantum gravity
AU - Yasunori Nomura
AU - Tomonori Ugajin
PY - 2026
DA - 2026/11/04
JO - Journal of High Energy Physics
DO - 10.1007/jhep11(2026)030
UR - https://link.springer.com/article/10.1007/JHEP11(2026)030
AB - A bstract : Recent developments in gravitational path integrals indicate that the nonperturbative physical Hilbert space of a closed universe is one-dimensional within each superselection sector. This raises a basic puzzle: how can a unique quantum-gravity state give rise to semiclassical physics, measurement outcomes, and classical probabilities? In this paper, we develop a framework in which nontrivial and statistically stable predictions emerge despite the one-dimensionality of the fully constrained Hilbert space. The key idea is to extract physical predictions in an enlarged, unconstrained Hilbert space by conditioning on observational data. We show that partial observability — reflecting the limited access of observers to the degrees of freedom of the universe — suppresses ensemble fluctuations associated with microscopic structure in the gravitational path integral, thereby restoring semiclassical predictability with exponential accuracy. We formulate the construction explicitly including contributions from the Hartle-Hawking no-boundary state, define a gauge-invariant Hilbert space for observations via a density operator, and generalize the formalism to conditioning on histories, clarifying the emergence of classical probabilities and an effective arrow of time. Finally, we explore whether this framework can support a realistic cosmology and identify assumptions that the underlying theory of quantum gravity must satisfy.
SN - 1029-8479
ER -