2020/11/30 by Nobuyoshi KOMATSU, Nobuyoshi Komatsu · 8 citations
Physics and Astronomy · #Age of the universe #Apparent horizon #Astrophysics #Black Holes and Theoretical Physics #Cosmology #Cosmology and Gravitation Theories #Dark energy #De Sitter universe #Dissipative system #Entropy (arrow of time) #Event horizon #Galaxies: Formation, Evolution, Phenomena #Horizon #Hubble's law #Mathematical physics #Physics #Quantum mechanics #Theoretical physics #Type (biology) #Universe #astro-ph.CO #gr-qc #hep-ph
paper · pdf · doi:10.1103/physrevd.103.023534
published in Physical review. D/Physical review. D. 103(2) (American Physical Society) · Final version accepted for publication in PRD. A typo is corrected. [17 pages, 8 figures]
arxiv created 2021/01/14 · openalex publication_date 2021/01/25 · arxiv updated 2021/01/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
Density perturbations related to structure formations are expected to be different in dissipative and nondissipative universes, even if the background evolution of the two universes is the same. To clarify the difference between the two universes, first-order density perturbations are studied, using two types of holographic cosmological models. The first type is a ``\mathrm\ensuremathΛ(t) model'' similar to a time-varying \mathrm\ensuremathΛ(t) cosmology for the nondissipative universe. The second type is a ``BV model'' similar to a bulk viscous cosmology for the dissipative universe. To systematically examine the two different universes, a power-law term proportional to H^\ensuremathα is applied to the \mathrm\ensuremathΛ(t) and BV (bulk-viscous-cosmology-like) models, assuming a flat Friedmann-Robertson-Walker model for the late universe. Here, H is the Hubble parameter and \ensuremathα is a free parameter whose value is a real number. The \mathrm\ensuremathΛ(t)\text\ensuremath-H^\ensuremathα and BV\text\ensuremath-H^\ensuremathα models are used to examine first-order density perturbations for matter, in which the background evolution of the two models is equivalent. In addition, thermodynamic constraints on the two models are discussed, with a focus on the maximization of entropy on the horizon of the universe, extending previous analyses [Phys. Rev. D 100, 123545 (2019); Phys. Rev. D102, 063512 (2020)]. Consequently, the \mathrm\ensuremathΛ(t)\text\ensuremath-H^\ensuremathα model for small |\ensuremathα| values is found to be consistent with observations and satisfies the thermodynamic constraints, compared with the BV\text\ensuremath-H^\ensuremathα model. The results show that the nondissipative universe described by the \mathrm\ensuremathΛ(t)\text\ensuremath-H^\ensuremathα model similar to lambda cold dark matter models is likely favored.