Routing protocol

Entropy (arrow of time)

Quantum gravity

Gravitational collapse

Physics

Spacetime

White hole

Cosmological constant

Micro black hole

Cosmology

Friedmann–Lemaître–Robertson–Walker metric

Computer network

Theoretical physics

Mathematical physics

Hawking radiation

Computer science

Quantum cosmology

Black hole (networking)

Black hole information paradox

Quantum mechanics

Routing (electronic design automation)

Classical mechanics

Minkowski space

Quantum

Link-state routing protocol

Journal of High Energy Physics

A quantum extremal island suggests that a region of spacetime is encoded in the quantum state of another system, like the encoding of the black hole interior in Hawking radiation. We study conditions for islands to appear in general spacetimes, with or without black holes. They must violate Bekenstein's area bound in a precise sense, and the boundary of an island must satisfy several other information-theoretic inequalities. These conditions combine to impose very strong restrictions, which we apply to cosmological models. We find several examples of islands in crunching universes. In particular, in the four-dimensional FRW cosmology with radiation and a negative cosmological constant, there is an island near the turning point when the geometry begins to recollapse. In a two-dimensional model of JT gravity in de Sitter spacetime, there are islands inside crunches that are encoded at future infinity or inside bubbles of Minkowski spacetime. Finally, we discuss simple tensor network toy models for islands in cosmology and black holes.

Islands in cosmology