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Universe
Volume 1
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Universe, Volume 1, Issue 1 (June 2015) – 6 articles , Pages 1-91

Cover Story (view full-size image): The General Theory of Relativity (GTR) strucks for his extreme simplicity and for its not yet fully unfolded harvest of predictions. GTR not only allowed to elegantly explain the anomalous perihelion advance of Mercury, already known at that time, but predicted also an entirely new class of phenomena taking place when light and fast objects move in intense, rapidly varying gravitational fields. Orbital and gyroscope precessions, deflections of electromagnetic waves from otherwise Euclidean paths, temporal retardations and advances in their propagation, extreme spacetime configurations allowing only for unidirectional motions of matter and light, temporal variations of the spacetime fabric, the study of the Universe as a scientific subject and the relentless efforts to put them to the test over the years are just some of the major legacies of GTR reviewed in the present Editorial. View this paper
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254 KiB  
Article
Arctan-Gravity Model
by Sergey I. Kruglov
Universe 2015, 1(1), 82-91; https://doi.org/10.3390/universe1010082 - 22 May 2015
Cited by 3 | Viewed by 3658
Abstract
A new gravity model with the function F(R) = (1/β) arctan (βR – β2R2) instead of the Ricci scalar in the Einstein–Hilbert action, describing inflation of the Universe, is suggested and analyzed. We obtain constant curvature solutions [...] Read more.
A new gravity model with the function F(R) = (1/β) arctan (βR – β2R2) instead of the Ricci scalar in the Einstein–Hilbert action, describing inflation of the Universe, is suggested and analyzed. We obtain constant curvature solutions of the model in the Jordan frame. Performing the conformal transformation of the metric, the potential and the mass of a scalar degree of freedom in the Einstein frame are found. The slow-roll and cosmological parameters of the model are evaluated. It was demonstrated that the index of the scalar spectrum power law, ns, is in agreement with the PLANCK data. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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387 KiB  
Article
Editorial for the Special Issue 100 Years of Chronogeometrodynamics: The Status of the Einstein’s Theory of Gravitation in Its Centennial Year
by Lorenzo Iorio
Universe 2015, 1(1), 38-81; https://doi.org/10.3390/universe1010038 - 27 Apr 2015
Cited by 92 | Viewed by 7796
Abstract
The present Editorial introduces the Special Issue dedicated by the journal Universe to the General Theory of Relativity, the beautiful theory of gravitation of Einstein, a century after its birth. It reviews some of its key features in a historical perspective, and, in [...] Read more.
The present Editorial introduces the Special Issue dedicated by the journal Universe to the General Theory of Relativity, the beautiful theory of gravitation of Einstein, a century after its birth. It reviews some of its key features in a historical perspective, and, in welcoming distinguished researchers from all over the world to contribute it, some of the main topics at the forefront of the current research are outlined. Full article
(This article belongs to the Special Issue 100 Years of Chronogeometrodynamics: the Status of the Einstein's Theory of Gravitation in Its Centennial Year)
250 KiB  
Review
Inhomogeneous Dark Fluid and Dark Matter, Leading to a Bounce Cosmology
by Iver Brevik and Alexander Timoshkin
Universe 2015, 1(1), 24-37; https://doi.org/10.3390/universe1010024 - 16 Mar 2015
Cited by 12 | Viewed by 4430
Abstract
The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for dark energy, when dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in [...] Read more.
The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for dark energy, when dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in terms of the EoS parameters for an exponential, a power-law, or a double-exponential law for the scale factor a. Stability issues are discussed by considering small perturbations around the critical points for the bounce, in the early as well as in the late, universe. The latter part of the paper is concerned with dark energy coupled with dark matter in viscous fluid cosmology. We allow the bulk viscosity ζ = ζ(H, t) to be a function of the Hubble parameter and the time, and consider the Little Rip, the Pseudo Rip, and the bounce universe. Analytic expressions for characteristic properties of these cosmological models are obtained. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
194 KiB  
Article
Cosmological Linear Perturbations in the Models of Dark Energy and Modified Gravity
by Jiro Matsumoto
Universe 2015, 1(1), 17-23; https://doi.org/10.3390/universe1010017 - 13 Feb 2015
Cited by 8 | Viewed by 3599
Abstract
The quasi-static solutions of the matter density perturbation in various dark energy models and modified gravity models have been investigated in numerous papers. However, the oscillating solutions in those models have not been investigated enough so far. In this paper, we review the [...] Read more.
The quasi-static solutions of the matter density perturbation in various dark energy models and modified gravity models have been investigated in numerous papers. However, the oscillating solutions in those models have not been investigated enough so far. In this paper, we review the behavior of the oscillating solutions, which have a possibility to unveil the difference between the models of the late-time accelerated expansion of the Universe, by using appropriate approximations. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
217 KiB  
Article
Observational Constraints on Varying-Alpha Domain Walls
by Pedro P. Avelino and Lara Sousa
Universe 2015, 1(1), 6-16; https://doi.org/10.3390/universe1010006 - 05 Feb 2015
Cited by 6 | Viewed by 3536
Abstract
We consider the possibility that current hints of spatial variations of the fine structure constant at high redshift, based on VLT/UVES and Keck/HIRES observations, could be caused by a biased domain wall network described by a scalar field non-minimally coupled to the electromagnetic [...] Read more.
We consider the possibility that current hints of spatial variations of the fine structure constant at high redshift, based on VLT/UVES and Keck/HIRES observations, could be caused by a biased domain wall network described by a scalar field non-minimally coupled to the electromagnetic field. We show that in order to be responsible for the reported spatial variations of the fine structure constant, the fractional contribution of the domain wall network to the energy density of the Universe should be tightly constrained within the range \(10^{-10} < \Omega_{w0} < 10^{-5}\). We also show that the domain wall dynamics should be essentially frictionless, so that its characteristic scale is in the order of the Hubble radius at the present time. Full article
96 KiB  
Editorial
Universe: An International Multidisciplinary Open Access Journal
by Lorenzo Iorio
Universe 2015, 1(1), 1-5; https://doi.org/10.3390/universe1010001 - 27 Nov 2014
Cited by 2 | Viewed by 4137
Abstract
There is an unescapable connection between mathematics and the Universe [1–4]. It has been so since the dawn of the rationalist view of approaching to the natural world superseding the mythological eras when its various aspects were nothing but manifestations of this or [...] Read more.
There is an unescapable connection between mathematics and the Universe [1–4]. It has been so since the dawn of the rationalist view of approaching to the natural world superseding the mythological eras when its various aspects were nothing but manifestations of this or that godhead. This thread never got broken up over the centuries, surviving to vast and profound changes in the nature of mathematics itself and our concept of Universe, and in the relation of mathematics with the natural world as well. At the times of the Pythagoreans, the Universe was essentially a mathematical construct in the sense that the integer numbers were thought as building blocks of the natural world whose constituents could be put in numerical relationships in terms of ratios of integer numbers. The Pythagoreans were serious with mathematics; when they discovered that the square root of 2 could not be expressed as a rational number, they decided not to divulge such a terrifying discovery, and a strict secret was posed on it. Later, at the time of Ptolemy, the Universe was essentially limited to a part of our Solar System. [...] Full article
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