******** CTCD/NTHU HEP Seminar ********

Speaker ：Dr. Kaberi Goswami (NTHU)

Title ：The black hole information paradox for small Schwarzschild de
Sitter black holes, quantum extremal surface and islands.

Time ：12:30-13:30, Thursday, 2026/2/26

Place ：R521, 5F, General Building II, Nat'l Tsing Hua Univ.

Host ： Prof. Norihiro Iizuka (NTHU)

Abstract:
In this talk, we will focus on a ‘Small’ Schwarzschild de Sitter black
hole in an effective 2-dimensional dilaton gravity theory and study the
resolution of the black hole information paradox with respect to an
observer considered to be in the static diamond of the background
geometry. The small mass black hole undergoes Hawking evaporation in a
frozen de Sitter background. Considering the purity of the matter state
that formed the black hole, at later times of the evaporation, the
entanglement entropy of Hawking radiation shows an unbounded growth with
time, indicating the black hole information paradox in this background.
Here we will show that a self-consistent island across the black hole
horizon, semi-classically at later times of the evaporation, recovers
expectations on the Page curve. One natural extension to this would be
that the radiation eventually crosses the cosmological horizon and comes
to the future boundary of the Schwarzschild de Sitter background. Next,
we will study the resolution of the black hole information paradox for
the same ‘Small’ mass Schwarzschild de Sitter black hole with respect to
a (meta) observer, which collects the radiation closer to the future
boundary. At later times of the evaporation, the evolution of the
entanglement entropy shows unbounded growth with respect to the spatial
coordinate, indicative of the information paradox. Here, we will also
show that, considering appropriate self-consistent island regions around
the horizons of black hole regions on both left and right cosmological
horizons, the evolution of entanglement entropy of radiation recovers
the Page curve.