A construction of unitary quantum field theory in curved spacetime and its implications for quantum gravity

Before we ask what is the theory of quantum gravity, it is a legitimate quest to formulate a robust quantum field theory in curved spacetime (QFTCS). Several conceptual problems such as unitarity violation and information paradox have raised concerns over several decades, mainly because no S-matrix formulation is yet found in QFTCS. We argue that the institutional thinking of fixing a spacetime geometry, observers, and light cones, and then quantizing the fields in that "intuitively", or "classically", fixed spacetime is the origin of the problem. In this talk, acknowledging the fact that "time" is a parameter in quantum theory, which is different from its status in the context of General Relativity (GR), we start with a "quantum first approach" and propose a new formulation for QFTCS based on the refined understanding of discrete spacetime transformations. We will address how this new formulation of QFTCS can potentially restore unitarity and observer complementarity in de Sitter and black hole spacetimes. Finally, we shall comment on the implications of our construction to the quantum gravity research that has been happening in the last decades.
The talk is based on arXiv: 2305.06046 [hep-th], arXiv: 2305.06057 [hep-th], arXiv: 2209.03928 [hep-th], and arXiv: 2307.XXXX [hep-th] in collaboration with J. Marto.