When left and right disagree: Entropy and von Neumann algebras in quantum gravity with general AlAdS boundary conditions

Abstract

Euclidean path integrals for UV-completions of $d$-dimensional bulk quantum gravity were studied in [1] by assuming that they satisfy axioms of finiteness, reality, continuity, reflection-positivity, and factorization. Sectors ${\mathcal{H}}_{\mathcal{B}}$ of the resulting Hilbert space were defined for any $(d-2)$-dimensional surface $\mathcal{B}$, where $\mathcal{B}$ may be thought of as the boundary $\partial \Sigma$ of a bulk Cauchy surface in a corresponding Lorentzian description, and where $\mathcal{B}$ includes the specification of boundary conditions for bulk fields. Cases where $\mathcal{B}$ was the disjoint union $B\bigsqcup B$ of two identical $(d-2)$-dimensional surfaces were studied in detail and, after the inclusion of finite-dimensional `hidden sectors,' were shown to provide a Hilbert space interpretation of the associated Ryu-Takayanagi entropy. The analysis was performed by constructing type-I von Neumann algebras ${\mathcal{A}}_L^B,{\mathcal{A}}_R^B$ that act respectively at the left and right copy of $B$ in $B\bigsqcup B$. Below, we consider the case of general $\mathcal{B}=B_L\bigsqcup B_R$ with $B_L,B_R$ distinct. For any $B_R$, we find that the von Neumann algebra at $B_L$ acting on ${\mathcal{H}}_{B_L\bigsqcup B_R}$ is a central projection of the corresponding type-I von Neumann algebra on the `diagonal' Hilbert space ${\mathcal{H}}_{B_L\bigsqcup B_L}$. As a result, the von Neumann algebras ${\mathcal{A}}_L^{B_L},{\mathcal{A}}_R^{B_L}$ defined in [1] using the diagonal Hilbert space coincide precisely with those defined using the full Hilbert space of the theory. A second implication is that, for any ${\mathcal{H}}_{B_L\bigsqcup B_R}$, including the same hidden sectors as in the diagonal case again provides a Hilbert space interpretation of the Ryu-Takayanagi entropy. We also show the above central projections to satisfy consistency conditions that lead to a universal central algebra relevant to all choices of $B_L,B_R$.

35 pages, 4 figures; typos corrected, reference added, comments added in section 1, section 5 and Discussion