Extended AdS spacetime without boundaries and entanglement without holography

TL;DR

The paper constructs a boundary-free, extended AdS spacetime by gluing two AdS copies along their conformal boundaries and shows that coupling an arbitrary number of membranes at antipodal poles leads to a large-N cancellation of UV divergences in entanglement entropy. Using the replica trick in a semiclassical Einstein-gravity regime, the authors derive a finite, universal area law S_E = A(Σ_+∪Σ_-)/(4G) for the entangling surface formed by the antipodal membranes. This demonstrates that RT-like entropy relations can emerge intrinsically from bulk geometry without invoking holography. The approach highlights a potential universal geometric origin for entanglement-area relations and motivates future work on UV completions and deeper interpretations of the large-N mechanism.

Abstract

We glue together two copies of pure AdS spacetime along their conformal boundaries creating a manifold without boundaries. The resulting space, which in dimension $d+2$ we denote by $AdS^{d+2}_\pm$, has the topology of $S^2\timesΣ^d$, where $Σ^d$ is a $d$-manifold without boundary. Acting with $\mathbb Z_n$ on the $S^2$ factor amounts to coupling a pair of membranes at the north and south poles of the 2-sphere. Moreover, extending the domain of the 2-sphere polar coordinate from $[0, π]$ to the interval $[0, (2N-1)π]$, where $N>1$, enables the coupling of one stack of $N$ coincident membranes at each pole of the 2-sphere ($2N$ membranes in total). Assuming the existence of a quantum gravity theory on the glued spacetime, we compute the classical approximation of the entanglement entropy across an entangling surface consisting of the two antipodal stacks of membranes. We find that the resulting entropy exhibits a boundary cutoff divergence that can be canceled by taking the limit of an infinite number of membranes. This large-$N$ cancellation -- possible only in the doubled, extended geometry without boundaries -- yields a finite, universal quarter-area law. The calculation does not require details of the quantum theory other than its infrared limit, which we assume to be Einstein gravity.