  Quantum field theory (QFT) in Rindler spacetime is a gateway to understanding
unitarity and information loss paradoxes in curved spacetime. Rindler
coordinates map Minkowski spacetime onto regions with horizons, effectively
dividing accelerated observers into causally disconnected sectors. Employing
standard quantum field theory techniques and Bogoliubov transformations between
Minkowski and Rindler coordinates yields entanglement between states across
these causally separated regions of spacetime. This results in a breakdown of
unitarity, implying that information regarding the entangled partner may be
irretrievably lost beyond the Rindler horizon. As a consequence, one has a
situation of pure states evolving into mixed states. In this paper, we
introduce a novel framework for comprehending this phenomenon using a recently
proposed formulation of direct-sum quantum field theory (DQFT), which is
grounded in superselection rules formulated by the parity and time reversal
($\mathcal{P}\mathcal{T}$) symmetry of Minkowski spacetime. In the context of
DQFT applied to Rindler spacetime, we demonstrate that each Rindler observer
can, in principle, access pure states within the horizon, thereby restoring
unitarity. However, our analysis also reveals the emergence of a thermal
spectrum of Unruh radiation. This prompts a reevaluation of entanglement in
Rindler spacetime, where we propose a novel perspective on how Rindler
observers may reconstruct complementary information beyond the horizon.
Furthermore, we revisit the implications of the Reeh-Schlieder theorem within
the framework of DQFT. Lastly, we underscore how our findings contribute to
ongoing efforts aimed at elucidating the role of unitarity in quantum field
theory within the context of de Sitter and black hole spacetimes.
