Pure de Sitter space and the island moving back in time

Understand

Observers in de Sitter space can only access the space up to their cosmological horizon.

• Assuming thermal equilibrium, we use the quantum Ryu-Takayanagi or island formula to compute the entanglement entropy between the states inside the cosmological horizon and states outside, as a function of time.
• We obtain a Page curve that is bound at a value corresponding to the Gibbons-Hawking entropy.
• At this transition an 'island' forms, which is in a significantly different location as compared to when considering black hole horizons and even moves back in time.