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Universe, Volume 4, Issue 6 (June 2018) – 7 articles

Cover Story (view full-size image): In this work we consider the superposition of two semiclassical solutions of the Wheeler-DeWitt equation, describing de Sitter universes coupled to quantized fluctuations of a scalar vacuum. Each solution corresponds to an opposite choice of orientation for the cosmological "arrow of time". On the one hand, the modes of the scalar fluctuations that cross the Hubble horizon act as environmental degrees of freedom, leading to the suppression of the interference between the two arrows of time. On the other hand, this interference is reflected on the field mode correlations observable within the horizon scale, thus leaving its trace in the primordial power spectrum. View the paper here.

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12 pages, 299 KiB

Open AccessArticle

Is it no Longer Necessary to Test Cosmologies with Type Ia Supernovae?

by Ram Gopal Vishwakarma and Jayant V. Narlikar

Universe 2018, 4(6), 73; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060073 - 19 Jun 2018

Cited by 8 | Viewed by 3887

Abstract

We look at the current practice of analyzing the magnitude–redshift relation from the data on Type Ia supernovae. We show that, if the main aim of such analysis were to check the validity of a cosmological model, then the recently advanced arguments do not serve the purpose. Rather, the procedure followed tells us only about the statistical significance of the internal parameters used in the model, whereas the model itself is tacitly assumed to give a good fit to the data. A statistical assessment of the procedure is given and it is argued that given the growing data, the validity of the cosmological model should be checked first rather than the spread of any internal parameters. In passing we also discuss some aspects of the Milne model in the light of the present test. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2018 - Gravitational Physics)

16 pages, 1030 KiB

Open AccessConference Report

The High-Density Symmetry Energy in Heavy-Ion Collisions and Compact Stars

by Hermann Wolter

Universe 2018, 4(6), 72; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060072 - 14 Jun 2018

Cited by 4 | Viewed by 3001

Abstract

High-density nuclear symmetry energy is of crucial importance in astrophysics. Information on such energy has been obtained from mass–radius determinations of neutron stars (NSs), and in the future NS mergers will increasingly contribute. In the laboratory, the symmetry energy can be studied in heavy-ion collisions (HICs) at different incident energies over a large range, from very low to several times higher saturation density. Transport theory is necessary to extract the symmetry energy from the typically non-equilibrated nuclear collisions. In this contribution, we first review the transport approaches, their differences, and recent studies of their reliability. We then discuss several prominent observables, which have been used to determine the symmetry energy at high density: collective flow, light cluster emission, and particle production. It is finally argued that the results of the symmetry energy from microscopic many-body calculations, nuclear structure, nuclear reactions, and astrophysics begin to converge but still need considerable improvements in terms of accuracy. Full article

(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)

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11 pages, 342 KiB

Open AccessArticle

The Decoherence and Interference of Cosmological Arrows of Time for a de Sitter Universe with Quantum Fluctuations

by Marcello Rotondo and Yasusada Nambu

Universe 2018, 4(6), 71; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060071 - 11 Jun 2018

Cited by 3 | Viewed by 3095

Abstract

We consider the superposition of two semiclassical solutions of the Wheeler–DeWitt equation for a de Sitter universe, describing a quantized scalar vacuum propagating in a universe that is contracting in one case and expanding in the other, each identifying the opposite cosmological arrow of time. We discuss the suppression of the interference terms between the two arrows of time due to environment-induced decoherence caused by modes of the scalar vacuum crossing the Hubble horizon. Furthermore, we quantify the effect of the interference on the expectation value of the observable field mode correlations, with respect to an observer that we identify with the spatial geometry. Full article

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12 pages, 258 KiB

Open AccessArticle

High Speed Cylindrical Gravitational Collapse with Anisotropic Pressure

by Quaid Iqbal, Hasrat Hussain Shah and Zahid Ahmad

Universe 2018, 4(6), 70; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060070 - 05 Jun 2018

Cited by 1 | Viewed by 2723

Abstract

This paper focuses on the cylindrical symmetric gravitational collapse in the presence of anisotropic fluid. The high speed approximation scheme was used. In this perspective, the effect of anisotropy of pressure in fluid distribution on the collapsing process with the Equation of State [...] Read more.

p_{t} = λ ρ

and

p_{r} = l ρ, (l + 2 λ < - 1)

. The effect of pressure on collapse in radial and tangential direction was observed for all values of

λ

and l. It is determined that, for some values of constants, i.e.,

λ

and l, collapse results in a Naked Singularity (NS) while, for some values of constants, it does not form NS or Black Hole (BH). This study presents the effect on the collapsing process for all values of

λ

and l. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

21 pages, 1051 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

New Exact Solutions of Relativistic Hydrodynamics for Longitudinally Expanding Fireballs

by Tamás Csörgő, Gábor Kasza, Máté Csanád and Zefang Jiang

Universe 2018, 4(6), 69; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060069 - 01 Jun 2018

Cited by 17 | Viewed by 2909

Abstract

We present new, exact, finite solutions of relativistic hydrodynamics for longitudinally expanding fireballs for arbitrary constant value of the speed of sound. These new solutions generalize earlier, longitudinally finite, exact solutions, from an unrealistic to a reasonable equation of state, characterized by a temperature independent (average) value of the speed of sound. Observables such as the rapidity density and the pseudorapidity density are evaluated analytically, resulting in simple and easy to fit formulae that can be matched to the high energy proton–proton and heavy ion collision data at RHIC and LHC. In the longitudinally boost-invariant limit, these new solutions approach the Hwa–Bjorken solution and the corresponding rapidity distributions approach a rapidity plateaux. Full article

(This article belongs to the Special Issue BGL2017: 10th Bolyai-Gauss-Lobachevsky Conference on Non-Euclidean Geometry and Its Applications)

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19 pages, 516 KiB

Open AccessArticle

Near-Horizon Geodesics for Astrophysical and Idealised Black Holes: Coordinate Velocity and Coordinate Acceleration

by Petarpa Boonserm, Tritos Ngampitipan and Matt Visser

Universe 2018, 4(6), 68; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060068 - 28 May 2018

Cited by 3 | Viewed by 3319

Abstract

Geodesics (by definition) have an intrinsic 4-acceleration zero. However, when expressed in terms of coordinates, the coordinate acceleration

d^{2} x^{i} / d t^{2}

can very easily be non-zero, and the coordinate velocity

d x^{i} / d t

can behave [...] Read more.

Geodesics (by definition) have an intrinsic 4-acceleration zero. However, when expressed in terms of coordinates, the coordinate acceleration

d^{2} x^{i} / d t^{2}

can very easily be non-zero, and the coordinate velocity

d x^{i} / d t

can behave unexpectedly. The situation becomes extremely delicate in the near-horizon limit—for both astrophysical and idealised black holes—where an inappropriate choice of coordinates can quite easily lead to significant confusion. We shall carefully explore the relative merits of horizon-penetrating versus horizon-non-penetrating coordinates, arguing that in the near-horizon limit the coordinate acceleration

d^{2} x^{i} / d t^{2}

is best interpreted in terms of horizon-penetrating coordinates. Full article

(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)

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35 pages, 594 KiB

Open AccessArticle

Towards a Unified Quark-Hadron-Matter Equation of State for Applications in Astrophysics and Heavy-Ion Collisions

by Niels-Uwe F. Bastian, David Blaschke, Tobias Fischer and Gerd Röpke

Universe 2018, 4(6), 67; https://0-doi-org.brum.beds.ac.uk/10.3390/universe4060067 - 25 May 2018

Cited by 35 | Viewed by 4017

Abstract

We outline an approach to a unified equation of state for quark-hadron matter on the basis of a

Φ -

derivable approach to the generalized Beth-Uhlenbeck equation of state for a cluster decomposition of thermodynamic quantities like the density. To this end we [...] Read more.

We outline an approach to a unified equation of state for quark-hadron matter on the basis of a

Φ -

derivable approach to the generalized Beth-Uhlenbeck equation of state for a cluster decomposition of thermodynamic quantities like the density. To this end we summarize the cluster virial expansion for nuclear matter and demonstrate the equivalence of the Green’s function approach and the

Φ -

derivable formulation. As an example, the formation and dissociation of deuterons in nuclear matter is discussed. We formulate the cluster

Φ -

derivable approach to quark-hadron matter which allows to take into account the specifics of chiral symmetry restoration and deconfinement in triggering the Mott-dissociation of hadrons. This approach unifies the description of a strongly coupled quark-gluon plasma with that of a medium-modified hadron resonance gas description which are contained as limiting cases. The developed formalism shall replace the common two-phase approach to the description of the deconfinement and chiral phase transition that requires a phase transition construction between separately developed equations of state for hadronic and quark matter phases. Applications to the phenomenology of heavy-ion collisions and astrophysics are outlined. Full article

(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)

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Universe, Volume 4, Issue 6 (June 2018) – 7 articles

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