1999/07/20 by Murat Özer
Physics and Astronomy · #Age of the universe #Astrophysics #Big Rip #Cosmological constant #Cosmology #Cosmology and Gravitation Theories #Dark energy #Dark matter #De Sitter universe #Equation of state #Flatness problem #Galaxies: Formation, Evolution, Phenomena #Galaxy #Metric expansion of space #Phantom energy #Physics #Quantum mechanics #Redshift #Relativity and Gravitational Theory #Scale factor (cosmology) #Theoretical physics #Universe #Vacuum energy #Zero-energy universe #astro-ph
paper · pdf · doi:10.1086/307418
published as Astrophys.J. 520 (1999) 45 · AAS LaTex, 29 pages, published in the Astrophysical Journal, 520, 45, 1999
openalex publication_date 1999/07/20 · arxiv created 1999/11/14 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
It is shown that in cosmological models based on a vacuum energy decaying as a -2 , where a is the scale factor of the universe, the fate of the universe in regard to whether it will collapse in the future or expand forever is determined not by the curvature constant k but by an effective curvature constant k eff . It is argued that a closed universe with k = 1 may expand forever, in other words, simulate the expansion dynamics of a flat or an open universe because of the possibility that k eff = 0 or -1, respectively. Two such models, in one of which the vacuum does not interact with matter and in another of which it does, are studied. It is shown that the vacuum equation of state p vac = -ρ vac may be realized in a decaying vacuum cosmology provided the vacuum interacts with matter. The optical depths for gravitational lensing as a function of the matter density and other parameters in the models are calculated at a source redshift of 2. The age of the universe is discussed and shown to be compatible with the new Hipparcos lower limit of 11 Gyr. The possibility that a time-varying vacuum energy may serve as dark matter is suggested.