{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,14]],"date-time":"2026-01-14T16:02:16Z","timestamp":1768406536114,"version":"3.49.0"},"reference-count":72,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2024,2,6]],"date-time":"2024-02-06T00:00:00Z","timestamp":1707177600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Deutsche Forschungsgemeinschaft","award":["40401154"],"award-info":[{"award-number":["40401154"]}]},{"name":"Deutsche Forschungsgemeinschaft","award":["315477589\u2013TRR 211"],"award-info":[{"award-number":["315477589\u2013TRR 211"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>The de-Sitter spacetime is a maximally symmetric Lorentzian manifold with constant positive scalar curvature that plays a fundamental role in modern cosmology. Here, we investigate bulk-viscosity-assisted quasi de-Sitter inflation, that is the period of accelerated expansion in the early universe during which \u2212H\u02d9\u226aH2, with H(t) being the Hubble expansion rate. We do so in the framework of a causal theory of relativistic hydrodynamics, which takes into account non-equilibrium effects associated with bulk viscosity, which may have been present as the early universe underwent an accelerated expansion. In this framework, the existence of a quasi de-Sitter universe emerges as a natural consequence of the presence of bulk viscosity, without requiring introducing additional scalar fields. As a result, the equation of state, determined by numerically solving the generalized momentum-conservation equation involving bulk viscosity pressure turns out to be time dependent. The transition timescale characterising its departure from an exact de-Sitter phase is intricately related to the magnitude of the bulk viscosity. We examine the properties of the new equation of state, as well as the transition timescale in the presence of bulk viscosity pressure. In addition, we construct a fluid description of inflation and demonstrate that, in the context of the causal formalism, it is equivalent to the scalar field theory of inflation. Our analysis also shows that the slow-roll conditions are realised in the bulk-viscosity-supported model of inflation. Finally, we examine the viability of our model by computing the inflationary observables, namely the spectral index and the tensor-to-scalar ratio of the curvature perturbations, and compare them with a number of different observations, finding good agreement in most cases.<\/jats:p>","DOI":"10.3390\/sym16020194","type":"journal-article","created":{"date-parts":[[2024,2,6]],"date-time":"2024-02-06T07:50:12Z","timestamp":1707205812000},"page":"194","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Sustaining Quasi De-Sitter Inflation with Bulk Viscosity"],"prefix":"10.3390","volume":"16","author":[{"given":"Sayantani","family":"Lahiri","sequence":"first","affiliation":[{"name":"Center of Applied Space Technology and Microgravity (ZARM), University of Bremen, 28359 Bremen, Germany"},{"name":"Institut f\u00fcr Theoretische Physik, Goethe Universit\u00e4t Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1330-7103","authenticated-orcid":false,"given":"Luciano","family":"Rezzolla","sequence":"additional","affiliation":[{"name":"Institut f\u00fcr Theoretische Physik, Goethe Universit\u00e4t Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany"},{"name":"School of Mathematics, Trinity College Dublin, D02 PN40 Dublin, Ireland"},{"name":"Frankfurt Institute for Advanced Studies, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2024,2,6]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"347","DOI":"10.1103\/PhysRevD.23.347","article-title":"Inflationary universe: A possible solution to the horizon and flatness problems","volume":"23","author":"Guth","year":"1981","journal-title":"Phys. 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