The structure of quantum singularities on a Cauchy horizon

Journal Article · Tue Jul 22 00:00:00 EDT 2025 · Journal of High Energy Physics

DOI:https://doi.org/10.1007/JHEP07(2025)218· OSTI ID:2999773

Shahbazi-Moghaddam, Arvin ^[1]

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1. University of California, Berkeley, CA (United States)

Spacetime singularities pose a long-standing puzzle in quantum gravity. Unlike Schwarzschild, a generic family of black holes gives rise to a Cauchy horizon on which, even in the Hartle-Hawking state, quantum observables such as ⟨T_μν⟩ — the expectation value of the stress-energy tensor — can diverge, causing a breakdown of semiclassical gravity. Because they are diagnosed within quantum field theory (QFT) on a smooth background, these singularities may provide a better-controlled version of the spacetime singularity problem, and merit further study. Here, I highlight a mildness puzzle of Cauchy horizon singularities: the ⟨T_μν⟩ singularity is significantly milder than expected from symmetry and dimensional analysis. I address the puzzle in a simple spacetime $${\mathcal{W}}_P$$, which arises universally near all black hole Cauchy horizons: the past of a codimension-two spacelike plane in flat spacetime. Specifically, I propose an extremely broad QFT construction in which, roughly speaking, Cauchy horizon singularities originate from operator insertions in the causal complement of the spacetime. The construction reproduces well-known outer horizon singularities (e.g., in the Boulware state), and remarkably, when applied to $${\mathcal{W}}_P$$, gives rise to a universal mild singularity structure for robust singularities, ones whose leading singular behavior is state-independent. I make non-trivial predictions for all black hole Cauchy horizon singularities using this, and discuss extending the results beyond robust singularities and the strict near Cauchy horizon limit.

• Research Organization: University of California, Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: SC0019380
• OSTI ID: 2999773
• Alternate ID(s): OSTI ID: 2999785
• Journal Information: Journal of High Energy Physics, Journal Name: Journal of High Energy Physics Journal Issue: 7 Vol. 2025; ISSN 1029-8479
• Publisher: Springer Science and Business Media LLC
• Country of Publication: United States
• Language: English

Related Subjects

black holes
spacetime singularities