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Universe, Volume 2, Issue 3 (September 2016) – 9 articles

Cover Story (view full-size image): The cover picture represents an artistic depiction of "the ballet of two black holes" about to merge. The full “ballet” produces some of the most powerful gravitational waves in the universe. Black hole merging often takes place very far away, and by the time the signal reaches our planet, it has diminished to a small whisper, also depicted in the picture. The theory behind gravitational waves was first established by Einstein's theory of general relativity in 1916, but the definite proof of its existence remained elusive until September 2015. The confirmation was announced until February 2016, after analysis by a team of more than 1,000 scientists. This year in May 31 it was announced the successful detection of gravitational waves for the third time in human history. A hundred years have passed, from Einstein´s General Relativity to the dawn of the birth of gravitational wave [...] Read more.
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Review
A Brief History of Gravitational Waves
Universe 2016, 2(3), 22; https://doi.org/10.3390/universe2030022 - 13 Sep 2016
Cited by 25 | Viewed by 11057
Abstract
This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the [...] Read more.
This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the subsequent successful discovery. Full article
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Review
Testing General Relativity with the Radio Science Experiment of the BepiColombo mission to Mercury
Universe 2016, 2(3), 21; https://doi.org/10.3390/universe2030021 - 12 Sep 2016
Cited by 13 | Viewed by 3431
Abstract
The relativity experiment is part of the Mercury Orbiter Radio science Experiment (MORE) on-board the ESA/JAXA BepiColombo mission to Mercury. Thanks to very precise radio tracking from the Earth and accelerometer, it will be possible to perform an accurate test of General Relativity, [...] Read more.
The relativity experiment is part of the Mercury Orbiter Radio science Experiment (MORE) on-board the ESA/JAXA BepiColombo mission to Mercury. Thanks to very precise radio tracking from the Earth and accelerometer, it will be possible to perform an accurate test of General Relativity, by constraining a number of post-Newtonian and related parameters with an unprecedented level of accuracy. The Celestial Mechanics Group of the University of Pisa developed a new dedicated software, ORBIT14, to perform the simulations and to determine simultaneously all the parameters of interest within a global least squares fit. After highlighting some critical issues, we report on the results of a full set of simulations, carried out in the most up-to-date mission scenario. For each parameter we discuss the achievable accuracy, in terms of a formal analysis through the covariance matrix and, furthermore, by the introduction of an alternative, more representative, estimation of the errors. We show that, for example, an accuracy of some parts in 10 6 for the Eddington parameter β and of 10 5 for the Nordtvedt parameter η can be attained, while accuracies at the level of 5 × 10 7 and 1 × 10 7 can be achieved for the preferred frames parameters α 1 and α 2 , respectively. Full article
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Article
Warm Inflation
Universe 2016, 2(3), 20; https://doi.org/10.3390/universe2030020 - 06 Sep 2016
Cited by 8 | Viewed by 1882
Abstract
I show here that there are some interesting differences between the predictions of warm and cold inflation models focusing in particular upon the scalar spectral index n s and the tensor-to-scalar ratio r. The first thing to be noted is that the warm inflation models in general predict a vanishingly small value of r. Cold inflationary models with the potential V = M 4 ( ϕ / M P ) p and a number of e-folds N = 60 predict δ n s C 1 n s ( p + 2 ) / 120 , where n s is the scalar spectral index, while the corresponding warm inflation models with constant value of the dissipation parameter Γ predict δ n s W = [ ( 20 + p ) / ( 4 + p ) ] / 120 . For example, for p = 2 this gives δ n s W = 1.1 δ n s C . The warm polynomial model with Γ = V seems to be in conflict with the Planck data. However, the warm natural inflation model can be adjusted to be in agreement with the Planck data. It has, however, more adjustable parameters in the expressions for the spectral parameters than the corresponding cold inflation model, and is hence a weaker model with less predictive force. However, it should be noted that the warm inflation models take into account physical processes such as dissipation of inflaton energy to radiation energy, which is neglected in the cold inflationary models. Full article
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Article
The Teleparallel Equivalent of General Relativity and the Gravitational Centre of Mass
Universe 2016, 2(3), 19; https://doi.org/10.3390/universe2030019 - 31 Aug 2016
Cited by 6 | Viewed by 1510
Abstract
We present a brief review of the teleparallel equivalent of general relativity and analyse the expression for the centre of mass density of the gravitational field. This expression has not been sufficiently discussed in the literature. One motivation for the present analysis is [...] Read more.
We present a brief review of the teleparallel equivalent of general relativity and analyse the expression for the centre of mass density of the gravitational field. This expression has not been sufficiently discussed in the literature. One motivation for the present analysis is the investigation of the localization of dark energy in the three-dimensional space, induced by a cosmological constant in a simple Schwarzschild-de Sitter space-time. We also investigate the gravitational centre of mass density in a particular model of dark matter, in the space-time of a point massive particle and in an arbitrary space-time with axial symmetry. The results are plausible, and lead to the notion of gravitational centre of mass (COM) distribution function. Full article
Article
Symplectic Structure of Intrinsic Time Gravity
Universe 2016, 2(3), 18; https://doi.org/10.3390/universe2030018 - 30 Aug 2016
Cited by 1 | Viewed by 1615
Abstract
The Poisson structure of intrinsic time gravity is analysed. With the starting point comprising a unimodular three-metric with traceless momentum, a trace-induced anomaly results upon quantization. This leads to a revision of the choice of momentum variable to the (mixed index) traceless momentric. [...] Read more.
The Poisson structure of intrinsic time gravity is analysed. With the starting point comprising a unimodular three-metric with traceless momentum, a trace-induced anomaly results upon quantization. This leads to a revision of the choice of momentum variable to the (mixed index) traceless momentric. This latter choice unitarily implements the fundamental commutation relations, which now take on the form of an affine algebra with SU(3) Lie algebra amongst the momentric variables. The resulting relations unitarily implement tracelessness upon quantization. The associated Poisson brackets and Hamiltonian dynamics are studied. Full article
Article
What Is the Validity Domain of Einstein’s Equations? Distributional Solutions over Singularities and Topological Links in Geometrodynamics
Universe 2016, 2(3), 17; https://doi.org/10.3390/universe2030017 - 29 Aug 2016
Cited by 2 | Viewed by 1869
Abstract
The existence of singularities alerts that one of the highest priorities of a centennial perspective on general relativity should be a careful re-thinking of the validity domain of Einstein’s field equations. We address the problem of constructing distinguishable extensions of the smooth spacetime [...] Read more.
The existence of singularities alerts that one of the highest priorities of a centennial perspective on general relativity should be a careful re-thinking of the validity domain of Einstein’s field equations. We address the problem of constructing distinguishable extensions of the smooth spacetime manifold model, which can incorporate singularities, while retaining the form of the field equations. The sheaf-theoretic formulation of this problem is tantamount to extending the algebra sheaf of smooth functions to a distribution-like algebra sheaf in which the former may be embedded, satisfying the pertinent cohomological conditions required for the coordinatization of all of the tensorial physical quantities, such that the form of the field equations is preserved. We present in detail the construction of these distribution-like algebra sheaves in terms of residue classes of sequences of smooth functions modulo the information of singular loci encoded in suitable ideals. Finally, we consider the application of these distribution-like solution sheaves in geometrodynamics by modeling topologically-circular boundaries of singular loci in three-dimensional space in terms of topological links. It turns out that the Borromean link represents higher order wormhole solutions. Full article
Article
Predictions for Bottomonia Suppression in 5.023 TeV Pb-Pb Collisions
Universe 2016, 2(3), 16; https://doi.org/10.3390/universe2030016 - 25 Aug 2016
Cited by 50 | Viewed by 2390
Abstract
We compute the suppression of the bottomonia states Υ ( 1 S ) , Υ ( 2 S ) , Υ ( 3 S ) , χ b ( 1 P ) , χ b ( 2 P ) , and χ b ( 3 P ) states in Large Hadron Collider (LHC) s N N = 5.023 TeV Pb-Pb collisions. For the background evolution we use 3+1d anisotropic hydrodynamics with conditions extrapolated from s N N = 2.76 TeV and we self-consistently compute bottomonia decay rates including non-equilibrium corrections to the interaction potential. For our final results, we make predictions for R A A as function of centrality, rapidity, and p T for the Υ ( 1 S ) and Υ ( 2 S ) states, including feed down effects. In order to assess the dependence on some of the model assumptions, we vary the shear viscosity-to-entropy density ratio, 4 π η / s { 1 , 2 , 3 } , and the initial momentum-space anisotropy parameter, ξ 0 { 0 , 10 , 50 } , while holding the total light hadron multiplicity fixed. Full article
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Review
Loop Quantum Cosmology, Modified Gravity and Extra Dimensions
Universe 2016, 2(3), 15; https://doi.org/10.3390/universe2030015 - 10 Aug 2016
Viewed by 1732
Abstract
Loop quantum cosmology (LQC) is a framework of quantum cosmology based on the quantization of symmetry reduced models following the quantization techniques of loop quantum gravity (LQG). This paper is devoted to reviewing LQC as well as its various extensions including modified gravity [...] Read more.
Loop quantum cosmology (LQC) is a framework of quantum cosmology based on the quantization of symmetry reduced models following the quantization techniques of loop quantum gravity (LQG). This paper is devoted to reviewing LQC as well as its various extensions including modified gravity and higher dimensions. For simplicity considerations, we mainly focus on the effective theory, which captures main quantum corrections at the cosmological level. We set up the basic structure of Brans–Dicke (BD) and higher dimensional LQC. The effective dynamical equations of these theories are also obtained, which lay a foundation for the future phenomenological investigations to probe possible quantum gravity effects in cosmology. Some outlooks and future extensions are also discussed. Full article
(This article belongs to the Special Issue Loop Quantum Cosmology and Quantum Black Holes)
Article
Starobinsky-Like Inflation and Running Vacuum in the Context of Supergravity
Universe 2016, 2(3), 14; https://doi.org/10.3390/universe2030014 - 26 Jul 2016
Cited by 30 | Viewed by 1881
Abstract
We describe the primeval inflationary phase of the early Universe within a quantum field theoretical (QFT) framework that can be viewed as the effective action of vacuum decay in the early times. Interestingly enough, the model accounts for the “graceful exit” of the [...] Read more.
We describe the primeval inflationary phase of the early Universe within a quantum field theoretical (QFT) framework that can be viewed as the effective action of vacuum decay in the early times. Interestingly enough, the model accounts for the “graceful exit” of the inflationary phase into the standard radiation regime. The underlying QFT framework considered here is supergravity (SUGRA), more specifically an existing formulation in which the Starobinsky-type inflation (de Sitter background) emerges from the quantum corrections to the effective action after integrating out the gravitino fields in their (dynamically induced) massive phase. We also demonstrate that the structure of the effective action in this model is consistent with the generic idea of re-normalization group (RG) running of the cosmological parameters; specifically, it follows from the corresponding RG equation for the vacuum energy density as a function of the Hubble rate, ρ Λ ( H ) . Overall, our combined approach amounts to a concrete-model realization of inflation triggered by vacuum decay in a fundamental physics context, which, as it turns out, can also be extended for the remaining epochs of the cosmological evolution until the current dark energy era. Full article
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