In this talk, we present how the recent proposal of direct-sum quantum field theory (DQFT) in curved spacetime offers unitarity and observer complementarity. We discuss, in particular, quantum aspects of the de Sitter Universe and Schwarzschild black hole. We demonstrate that one can have Hawking radiation with pure states without firewalls. Furthermore, we illustrate the way DQFT ideas can be traced back to the initial thoughts by Einstein and Rosen in 1935, Schrodinger in 1956, and 't Hooft in 2016. We also show the analogies between the (quantum) physics of inverted harmonic oscillator and quantum fields in curved spacetime. Finally, we show compelling evidence for DQFT treatment of inflationary quantum fluctuations (called Direct-sum inflation) in explaining large-scale (parity-asymmetric) features of the cosmic microwave background (CMB). Direct-sum inflation uncovers the interplay between gravity and quantum mechanics, fitting the CMB data 650 times better than standard inflationary quantum fluctuations. 

The talk will be based on a series of works in JCAP 06 (2024) 001, Universe 2024, 10(8), 320, Gen.Rel.Grav. 56 (2024) 12, 143, PTEP, 12, 2024, 123E01; ptae176, Symmetry 2025, 17(1), 29, Preprint: 202410.0190/v1, arXiv: 2209.03928