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Universe, Volume 2, Issue 1 (March 2016) – 6 articles

Cover Story (view full-size image): As proposed by Jean-Pierre Luminet and collaborators, spatially homogeneous universes with non-trivial topology and small finite volume – popularly known as "wraparound universes" - provide plausible relativistic cosmological models for describing the true shape of our universe. In such spaces, topological lensing may produce "ghost" images of faraway astronomical sources and specific correlations in the anisotropies of the cosmic microwave background. As a pedestrian illustration, this numerical simulation calculates the closest ghost images of the Earth that would be seen in the Weeks space, which has the smallest possible volume of any closed hyperbolic manifold. Recent astronomical observations from WMAP and Planck telescopes give strong constraints on the simplest models of wraparound universes, but do not exclude the full set of multiply-connected topologies. Image courtesy J. Weeks. View this [...] Read more.
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Review
The Scales of Gravitational Lensing
Universe 2016, 2(1), 6; https://doi.org/10.3390/universe2010006 - 14 Mar 2016
Cited by 14 | Viewed by 3456
Abstract
After exactly a century since the formulation of the general theory of relativity, the phenomenon of gravitational lensing is still an extremely powerful method for investigating in astrophysics and cosmology. Indeed, it is adopted to study the distribution of the stellar component in [...] Read more.
After exactly a century since the formulation of the general theory of relativity, the phenomenon of gravitational lensing is still an extremely powerful method for investigating in astrophysics and cosmology. Indeed, it is adopted to study the distribution of the stellar component in the Milky Way, to study dark matter and dark energy on very large scales and even to discover exoplanets. Moreover, thanks to technological developments, it will allow the measure of the physical parameters (mass, angular momentum and electric charge) of supermassive black holes in the center of ours and nearby galaxies. Full article
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Article
Effect of the Cosmological Constant on Light Deflection: Time Transfer Function Approach
Universe 2016, 2(1), 5; https://doi.org/10.3390/universe2010005 - 14 Mar 2016
Cited by 10 | Viewed by 2121
Abstract
We revisit the role of the cosmological constant Λ in the deflection of light by means of the Schwarzschild–de Sitter/Kottler metric. In order to obtain the total deflection angle α, the time transfer function approach is adopted, instead of the commonly used [...] Read more.
We revisit the role of the cosmological constant Λ in the deflection of light by means of the Schwarzschild–de Sitter/Kottler metric. In order to obtain the total deflection angle α, the time transfer function approach is adopted, instead of the commonly used approach of solving the geodesic equation of photon. We show that the cosmological constant does appear in expression of the deflection angle, and it diminishes light bending due to the mass of the central body M. However, in contrast to previous results, for instance, that by Rindler and Ishak (Phys. Rev. D. 2007), the leading order effect due to the cosmological constant does not couple with the mass of the central body M. Full article
Review
Quantum Yang–Mills Dark Energy
Universe 2016, 2(1), 4; https://doi.org/10.3390/universe2010004 - 23 Feb 2016
Cited by 10 | Viewed by 2366
Abstract
In this short review, I discuss basic qualitative characteristics of quantum non-Abelian gauge dynamics in the non-stationary background of the expanding Universe in the framework of the standard Einstein–Yang–Mills formulation. A brief outlook of existing studies of cosmological Yang–Mills fields and their properties [...] Read more.
In this short review, I discuss basic qualitative characteristics of quantum non-Abelian gauge dynamics in the non-stationary background of the expanding Universe in the framework of the standard Einstein–Yang–Mills formulation. A brief outlook of existing studies of cosmological Yang–Mills fields and their properties will be given. Quantum effects have a profound impact on the gauge field-driven cosmological evolution. In particular, a dynamical formation of the spatially-homogeneous and isotropic gauge field condensate may be responsible for both early and late-time acceleration, as well as for dynamical compensation of non-perturbative quantum vacua contributions to the ground state of the Universe. The main properties of such a condensate in the effective QCD theory at the flat Friedmann–Lemaítre–Robertson–Walker (FLRW) background will be discussed within and beyond perturbation theory. Finally, a phenomenologically consistent dark energy can be induced dynamically as a remnant of the QCD vacua compensation arising from leading-order graviton-mediated corrections to the QCD ground state. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
Article
String-Inspired Gravity through Symmetries
Universe 2016, 2(1), 3; https://doi.org/10.3390/universe2010003 - 05 Feb 2016
Cited by 1 | Viewed by 1839
Abstract
We study a string-inspired cosmological model from the symmetries point of view. We start by deducing the form that each physical quantity must take so that the field equations, in the string frame, admit self-similar solutions. In the same way, we formalize the [...] Read more.
We study a string-inspired cosmological model from the symmetries point of view. We start by deducing the form that each physical quantity must take so that the field equations, in the string frame, admit self-similar solutions. In the same way, we formalize the use of power-law solutions (less restrictive than the self-similar ones) by studying the wave equation for the dilaton through the Lie group method. Furthermore, we show how to generate more solutions by using this approach. As examples, we calculate exact solutions to several cosmological models in the four-dimensional NS-NS (Neveu-Schwarz-Neveu-Schwarz) sector of low-energy effective string theory coupled to a dilaton and an axion-like H-field within the string frame background, with FRW and the Bianchi Type II metrics. We also study the existence of Noether symmetries, which allow us to determine the form of the physical quantities in the framework of FRW geometry and to find exact cosmological solutions. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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Editorial
Acknowledgement to Reviewers of Universe in 2015
Universe 2016, 2(1), 2; https://doi.org/10.3390/universe2010002 - 22 Jan 2016
Viewed by 1673
Abstract
The editors of Universe would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2015. [...] Full article
Review
The Status of Cosmic Topology after Planck Data
Universe 2016, 2(1), 1; https://doi.org/10.3390/universe2010001 - 15 Jan 2016
Cited by 18 | Viewed by 3920
Abstract
In the last decade, the study of the overall shape of the universe, called Cosmic Topology, has become testable by astronomical observations, especially the data from the Cosmic Microwave Background (hereafter CMB) obtained by WMAP and Planck telescopes. Cosmic Topology involves both global [...] Read more.
In the last decade, the study of the overall shape of the universe, called Cosmic Topology, has become testable by astronomical observations, especially the data from the Cosmic Microwave Background (hereafter CMB) obtained by WMAP and Planck telescopes. Cosmic Topology involves both global topological features and more local geometrical properties such as curvature. It deals with questions such as whether space is finite or infinite, simply-connected or multi-connected, and smaller or greater than its observable counterpart. A striking feature of some relativistic, multi-connected small universe models is to create multiples images of faraway cosmic sources. While the last CMB (Planck) data fit well the simplest model of a zero-curvature, infinite space model, they remain consistent with more complex shapes such as the spherical Poincaré Dodecahedral Space, the flat hypertorus or the hyperbolic Picard horn. We review the theoretical and observational status of the field. Full article
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