Islands in Kerr-de Sitter spacetime and their flat limit

We use the quantum extremal island method to study the information paradox in certain cosmological setups known as three dimensional Kerr???de Sitter spacetimes. To do so, we couple an auxiliary flat bath system to this spacetime in timelike singularity and measure entropy of Hawking radiation in its asymptotic regions where the gravity is weak. We show that adding the island regions to the entanglement wedge of the radiation causes its entropy to obey Page curve. The boundary of the island, i.e., quantum extremal surface is located outside the cosmological horizon in the region connected directly to the bath. Taking the flatspace limit from the location of the island and its related calculation in the Kerr???de Sitter spacetime results in the flat-space cosmology (FSC) island, scrambling time and also Page time that were obtained in our previous paper. We repeat the same calculation for the pure de Sitter spacetime and show that our setup which neglects the effect of backreaction, leads also to a quantum extremal surface outside the cosmological horizon. We calculate the scrambling time and confirm the idea that pure de Sitter spacetime is a fast scrambler.

Automated summary

In this study, we used the quantum extremal island method to investigate the information paradox in three-dimensional Kerr-de Sitter spacetimes. By coupling an auxiliary flat bath system to the spacetime and measuring the entropy of Hawking radiation in the weak gravity asymptotic regions, we found that adding island regions to the entanglement wedge of the radiation leads to the Page curve. We also determined the location of the quantum extremal surface outside the cosmological horizon and calculated the scrambling time.