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 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, 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. I will address how this new formulation of QFTCS can potentially restore Unitarity and observer complementarity in de Sitter spacetime and black hole physics. I will discuss in detail how one can develop a notion of S-matrix in curved spacetime. Finally, I shall comment on the implications of our construction to the quantum gravity research that has been happening in the last decades.