I will describe extremal surfaces in de Sitter space anchored at the future boundary I+. Since such surfaces do not return, they require extra data in the past. In entirely Lorentzian dS, this leads to future-past timelike surfaces stretching between I+/I-, with pure imaginary area (relative to spacelike surfaces in AdS). With a no-boundary type boundary condition, the top half of these joins with a spacelike part on the hemisphere giving a complex-valued area. These can be thought of as certain analytic continuations from AdS while also amounting to space-time rotations. The areas are best interpreted as pseudo-entropy or time-entanglement (entanglement-like structures with timelike separations). I will also briefly discuss multiple subregions, entropy relations, the pseudo-entanglement wedge, a heuristic Lewkowycz-Maldacena formulation, as well as aspects in toy models in quantum mechanics, involving the time evolution operator, reduced transition amplitudes, and future-past entangle...
We consider linear superpositions of single particle excitations in a scalar field theory on AdS3 and evaluate their contribution to the bulk entanglement entropy across the Ryu-Takayanagi surface. We compare the entanglement entropy of these excitations obtained using the Faulkner-Lewkowycz-Maldacena formula to the entanglement entropy of linear superposition of global descendants of a conformal primary in a large c CFT obtained using the replica trick. We show that the closed from expressions for the entanglement entropy in the small interval expansion both in gravity and the CFT precisely agree. The agreement serves as a non-trivial check of the FLM formula for the quantum corrections to holographic entropy which also involves a contribution from the back reacted minimal area. Our checks includes an example in which the state is time dependent and spatially in-homogenous as well another example involving a coherent state with a Bañados geometry as its holographic dual.
The emergent geometry from large N matrix models is shown to be naturally granular, with a short distance cut-off proportional to 1/N. This is explicitly demonstrated for matrix quantum mechanics which is exactly mapped to a lattice boson with lattice spacing 1/N. In case of the double scaled c=1 matrix model, even though N is infinite, the exact boson theory has an effective short distance cutoff given by a scaled quantity proportional to the string coupling. This explains the finite entanglement entropy and finite S matrix elements of the 2D string theory in contrast with collective field theory where these quantities are divergent. We also briefly discuss a lattice boson representation of time-dependent unitary matrix models.
In recent years, it has been realized that algebraic techniques can be used to compute formal entropy differences for semiclassical black holes in quantum gravity, and that these entropy differences are consistent with the Bekenstein-Hawking formula. I will explain how to remove the word "formal" from the previous sentence, by showing that the algebraic entropy differences have an interpretation in terms of microstate counting that is consistent with our usual ideas about statistical mechanics. Based on 2404.16098 with Akers.
What is the bulk Hilbert space of quantum gravity? In this paper, we resolve this problem in 2d JT gravity, both with and without matter, providing the first example of an explicit definition of a non-perturbative Hilbert space specified in terms of metric variables. The states are wavefunctions of the length and matter state, but with a non-trivial and highly degenerate inner product. We explicitly identify the null states, and discuss their importance for defining operators non-perturbatively. To highlight the power of the formalism we developed, we study the non-perturbative effects for two bulk linear operators that may serve as proxies for the experience of an observer falling into a two-sided black hole: one captures the length of an Einstein-Rosen bridge and the other captures the center-of-mass collision energy between two particles falling from opposite sides. We track the behavior of these operators up to times of order eSBH, at which point the wavefunction spreads to the com...
