{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,19]],"date-time":"2026-02-19T05:40:59Z","timestamp":1771479659270,"version":"3.50.1"},"reference-count":104,"publisher":"MDPI AG","issue":"1","license":[{"start":{"date-parts":[[2024,12,27]],"date-time":"2024-12-27T00:00:00Z","timestamp":1735257600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"JSPS and KAKENHI Grant-in-Aid for Scientific Research","award":["JP20F20320"],"award-info":[{"award-number":["JP20F20320"]}]},{"name":"JSPS and KAKENHI Grant-in-Aid for Scientific Research","award":["JP21H00069"],"award-info":[{"award-number":["JP21H00069"]}]},{"name":"Royal Society","award":["JP20F20320"],"award-info":[{"award-number":["JP20F20320"]}]},{"name":"Royal Society","award":["JP21H00069"],"award-info":[{"award-number":["JP21H00069"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>Before we ask what the quantum gravity theory is, there is a legitimate quest to formulate a robust quantum field theory in curved spacetime (QFTCS). Several conceptual problems, especially unitarity loss (pure states evolving into mixed states), have raised concerns over several decades. In this paper, 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 \u201cquantum first approach\u201d and propose a new formulation for QFTCS based on the discrete spacetime transformations which offer a way to achieve unitarity. We rewrite the QFT in Minkowski spacetime with a direct-sum Fock space structure based on the discrete spacetime transformations and geometric superselection rules. Applying this framework to QFTCS, in the context of de Sitter (dS) spacetime, we elucidate how this approach to quantization complies with unitarity and the observer complementarity principle. We then comment on understanding the scattering of states in de Sitter spacetime. Furthermore, we discuss briefly the implications of our QFTCS approach to future research in quantum gravity.<\/jats:p>","DOI":"10.3390\/sym17010029","type":"journal-article","created":{"date-parts":[[2024,12,27]],"date-time":"2024-12-27T09:13:32Z","timestamp":1735290812000},"page":"29","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Towards a Unitary Formulation of Quantum Field Theory in Curved Spacetime: The Case of de Sitter Spacetime"],"prefix":"10.3390","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3126-8195","authenticated-orcid":false,"given":"K. Sravan","family":"Kumar","sequence":"first","affiliation":[{"name":"Institute of Cosmology & Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3974-9177","authenticated-orcid":false,"given":"Jo\u00e3o","family":"Marto","sequence":"additional","affiliation":[{"name":"Departamento de F\u00edsica, Centro de Matem\u00e1tica e Aplica\u00e7\u00f5es (CMA-UBI), Universidade da Beira Interior, Rua Marqu\u00eas D\u2019\u00c1vila e Bolama, 6201-001 Covilha, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2024,12,27]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Wald, R.M. (1984). General Relativity, Chicago University Press.","DOI":"10.7208\/chicago\/9780226870373.001.0001"},{"key":"ref_2","unstructured":"Coleman, S. (2018). Lectures of Sidney Coleman on Quantum Field Theory, WSP."},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Roberts, B.W. 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