Universe

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232 KiB

Open AccessArticle

Do We Live in the Best of All Possible Worlds? The Fine-Tuning of the Constants of Nature

by Thomas Naumann

Universe 2017, 3(3), 60; https://doi.org/10.3390/universe3030060 - 1 Aug 2017

Cited by 2 | Viewed by 5385

Abstract

Our existence depends on a variety of constants which appear to be extremely fine-tuned to allow for the existence of life as we know it. These include the number of spatial dimensions, the strengths of the forces, the masses of the particles, the [...] Read more.

Our existence depends on a variety of constants which appear to be extremely fine-tuned to allow for the existence of life as we know it. These include the number of spatial dimensions, the strengths of the forces, the masses of the particles, the composition of the Universe, and others. On the occasion of the 300th anniversary of the death of G.W. Leibniz, we discuss the question of whether we live in the “Best of all possible Worlds”. The hypothesis of a multiverse could explain the mysterious fine tuning of so many fundamental quantities. Anthropic arguments are critically reviewed. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

797 KiB

Open AccessArticle

The First Detection of Gravitational Waves

by Andrzej Królak and Mandar Patil

Universe 2017, 3(3), 59; https://doi.org/10.3390/universe3030059 - 31 Jul 2017

Cited by 5 | Viewed by 7408

Abstract

This article deals with the first detection of gravitational waves by the advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors on 14 September 2015, where the signal was generated by two stellar mass black holes with masses 36

M_{⊙}

and 29 [...] Read more.

This article deals with the first detection of gravitational waves by the advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors on 14 September 2015, where the signal was generated by two stellar mass black holes with masses 36

M_{⊙}

and 29

M_{⊙}

that merged to form a 62

M_{⊙}

black hole, releasing 3

M_{⊙}

energy in gravitational waves, almost 1.3 billion years ago. We begin by providing a brief overview of gravitational waves, their sources and the gravitational wave detectors. We then describe in detail the first detection of gravitational waves from a binary black hole merger. We then comment on the electromagnetic follow up of the detection event with various telescopes. Finally, we conclude with the discussion on the tests of gravity and fundamental physics with the first gravitational wave detection event. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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388 KiB

Open AccessArticle

Entropy Budget for Hawking Evaporation

by Ana Alonso-Serrano and Matt Visser

Universe 2017, 3(3), 58; https://doi.org/10.3390/universe3030058 - 22 Jul 2017

Cited by 3 | Viewed by 3540

Abstract

Blackbody radiation, emitted from a furnace and described by a Planck spectrum, contains (on average) an entropy of

3.9 \pm 2.5

bits per photon. Since normal physical burning is a unitary process, this amount of entropy is compensated by [...] Read more.

Blackbody radiation, emitted from a furnace and described by a Planck spectrum, contains (on average) an entropy of

3.9 \pm 2.5

bits per photon. Since normal physical burning is a unitary process, this amount of entropy is compensated by the same amount of “hidden information” in correlations between the photons. The importance of this result lies in the posterior extension of this argument to the Hawking radiation from black holes, demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget for the evaporation process. In order to carry out this calculation, we adopt a variant of the “average subsystem” approach, but consider a tripartite pure system that includes the influence of the rest of the universe, and which allows “young” black holes to still have a non-zero entropy; which we identify with the standard Bekenstein entropy. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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287 KiB

Open AccessArticle

Modified Gravity with Vector Distortion and Cosmological Applications

by Jose Beltrán Jiménez and Tomi S. Koivisto

Universe 2017, 3(2), 47; https://doi.org/10.3390/universe3020047 - 24 May 2017

Cited by 17 | Viewed by 3371

Abstract

We briefly review the basics of Weyl geometry and its natural extension by a general linear ”distortion” of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl’s semi-metricity condition. We present ghost-free gravitational [...] Read more.

We briefly review the basics of Weyl geometry and its natural extension by a general linear ”distortion” of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl’s semi-metricity condition. We present ghost-free gravitational theories in this geometrical setup and highlight their possible cosmological applications, such as new self-tuning solutions and new bouncing solutions found in the quadratic-curvature theories. The vector distortion can mimic the cosmological effects of dark matter. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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447 KiB

Open AccessArticle

Lemaître Class Dark Energy Model for Relaxing Cosmological Constant

by Irina Dymnikova, Anna Dobosz and Bożena Sołtysek

Universe 2017, 3(2), 39; https://doi.org/10.3390/universe3020039 - 4 May 2017

Cited by 13 | Viewed by 3492

Abstract

Cosmological constant corresponds to the maximally symmetric cosmological term with the equation of state

p = - ρ

. Introducing a cosmological term with the reduced symmetry,

p_{r} = - ρ

in the spherically symmetric case, makes cosmological constant intrinsically variable component [...] Read more.

Cosmological constant corresponds to the maximally symmetric cosmological term with the equation of state

p = - ρ

. Introducing a cosmological term with the reduced symmetry,

p_{r} = - ρ

in the spherically symmetric case, makes cosmological constant intrinsically variable component of a variable cosmological term which describes time-dependent and spatially inhomogeneous vacuum dark energy. Relaxation of the cosmological constant from the big initial value to the presently observed value can be then described in general setting by the spherically symmetric cosmology of the Lemaître class. We outline in detail the cosmological model with the global structure of the de Sitter spacetime distinguished by the holographic principle as the only stable product of quantum evaporation of the cosmological horizon entirely determined by its quantum dynamics. Density of the vacuum dark energy is presented by semiclassical description of vacuum polarization in the spherically symmetric gravitational field, and its initial value is chosen at the GUT scale. The final non-zero value of the cosmological constant is tightly fixed by the quantum dynamics of evaporation and appears in the reasonable agreement with its observational value. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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267 KiB

Open AccessArticle

Effective Gravitational “Constant” in Scalar-(Curvature)Tensor and Scalar-Torsion Gravities

by Laur Järv

Universe 2017, 3(2), 37; https://doi.org/10.3390/universe3020037 - 24 Apr 2017

Cited by 5 | Viewed by 3221

Abstract

In theories where a scalar field couples nonminimally to gravity, the effective gravitational “constant” becomes dependent on the value of the scalar field. This note first gives a brief review on how the cosmological evolution provides a dynamical stabilization for the gravitational “constant” [...] Read more.

In theories where a scalar field couples nonminimally to gravity, the effective gravitational “constant” becomes dependent on the value of the scalar field. This note first gives a brief review on how the cosmological evolution provides a dynamical stabilization for the gravitational “constant” as the system relaxes towards general relativity in matter dominated and potential dominated regimes for scalar-(curvature)tensor and scalar-torsion gravities. Second part summarizes the radius dependence of the gravitational “constant” around a point mass in the parametrized post-Newtonian formalism for scalar-tensor and multiscalar-tensor gravity. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

1273 KiB

Open AccessArticle

Evaluating the New Automatic Method for the Analysis of Absorption Spectra Using Synthetic Spectra

by Matthew B. Bainbridge and John K. Webb

Universe 2017, 3(2), 34; https://doi.org/10.3390/universe3020034 - 5 Apr 2017

Cited by 15 | Viewed by 3138

Abstract

We recently presented a new “artificial intelligence” method for the analysis of high-resolution absorption spectra (Bainbridge and Webb, Mon. Not. R. Astron. Soc. 2017, doi:10.1093/mnras/stx179). This new method unifies three established numerical methods: a genetic algorithm (GVPFIT); non-linear least-squares optimisation with parameter [...] Read more.

We recently presented a new “artificial intelligence” method for the analysis of high-resolution absorption spectra (Bainbridge and Webb, Mon. Not. R. Astron. Soc. 2017, doi:10.1093/mnras/stx179). This new method unifies three established numerical methods: a genetic algorithm (GVPFIT); non-linear least-squares optimisation with parameter constraints (VPFIT); and Bayesian Model Averaging (BMA). In this work, we investigate the performance of GVPFIT and BMA over a broad range of velocity structures using synthetic spectra. We found that this new method recovers the velocity structures of the absorption systems and accurately estimates variation in the fine structure constant. Studies such as this one are required to evaluate this new method before it can be applied to the analysis of large sets of absorption spectra. This is the first time that a sample of synthetic spectra has been utilised to investigate the analysis of absorption spectra. Probing the variation of nature’s fundamental constants (such as the fine structure constant), through the analysis of absorption spectra, is one of the most direct ways of testing the universality of physical laws. This “artificial intelligence” method provides a way to avoid the main limiting factor, i.e., human interaction, in the analysis of absorption spectra. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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268 KiB

Open AccessArticle

From Quantum to Cosmological Regime. The Role of Forcing and Exotic 4-Smoothness

by Jerzy Król, Torsten Asselmeyer-Maluga, Krzysztof Bielas and Paweł Klimasara

Universe 2017, 3(2), 31; https://doi.org/10.3390/universe3020031 - 27 Mar 2017

Cited by 11 | Viewed by 3654

Abstract

Recently, a cosmological model based on smooth open 4-manifolds admitting non-standard smoothness structures was proposed. The manifolds are exotic versions of

R^{4}

and

S^{3} \times R

. The model has been developed further and proven to be capable of obtaining some [...] Read more.

Recently, a cosmological model based on smooth open 4-manifolds admitting non-standard smoothness structures was proposed. The manifolds are exotic versions of

R^{4}

and

S^{3} \times R

. The model has been developed further and proven to be capable of obtaining some realistic cosmological parameters from these exotic smoothings. The important problem of the quantum origins of the exotic smoothness of space-time is addressed here. It is shown that the algebraic structure of the quantum-mechanical lattice of projections enforces exotic smoothness on

R^{n}

. Since the only possibility for such a structure is exotic

R^{4}

, it is found to be a reasonable explanation of the large-scale four-dimensionality of space-time. This is based on our recent research indicating the role of set-theoretic forcing in quantum mechanics. In particular, it is shown that a distributive lattice of projections implies the standard smooth structure on

R^{4}

. Two examples of models valid for cosmology are discussed. The important result that the cosmological constant can be identified with the constant curvature of the embedding

(exotic R^{4}) \to R^{4}

is referred. . The calculations are in good agreement with the observed small value of the dark energy density. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

337 KiB

Open AccessArticle

Spin-Field Correspondence

by Jakub Mielczarek

Universe 2017, 3(2), 29; https://doi.org/10.3390/universe3020029 - 23 Mar 2017

Cited by 10 | Viewed by 3245

Abstract

In the recent article Phys. Lett. B 2016, 759, 424–429, a new class of field theories called Nonlinear Field Space Theory was proposed. In this approach, the standard field theories are considered as linear approximations to some more general theories characterized [...] Read more.

In the recent article Phys. Lett. B 2016, 759, 424–429, a new class of field theories called Nonlinear Field Space Theory was proposed. In this approach, the standard field theories are considered as linear approximations to some more general theories characterized by nonlinear field phase spaces. The case of spherical geometry is especially interesting due to its relation with the spin physics. Here, we explore this possibility, showing that classical scalar field theory with such a field space can be viewed as a perturbation of a continuous spin system. In this picture, the spin precession and the scalar field excitations are dual descriptions of the same physics. The duality is studied in the example of the Heisenberg model. It is shown that the Heisenberg model coupled to a magnetic field leads to a non-relativistic scalar field theory, characterized by quadratic dispersion relation. Finally, on the basis of analysis of the relation between the spin phase space and the scalar field theory, we propose the Spin-Field correspondence between the known types of fields and the corresponding spin systems. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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257 KiB

Open AccessArticle

On the Causality and K-Causality between Measures

by Tomasz Miller

Universe 2017, 3(1), 27; https://doi.org/10.3390/universe3010027 - 20 Mar 2017

Cited by 5 | Viewed by 3621

Abstract

Drawing from the optimal transport theory adapted to the Lorentzian setting, we propose and study the extension of the Sorkin–Woolgar causal relation

K^{+}

onto the space of Borel probability measures on a given spacetime. We show that it retains its fundamental properties [...] Read more.

Drawing from the optimal transport theory adapted to the Lorentzian setting, we propose and study the extension of the Sorkin–Woolgar causal relation

K^{+}

onto the space of Borel probability measures on a given spacetime. We show that it retains its fundamental properties of transitivity and closedness. Furthermore, we list and prove several characterizations of this relation, including the “measure-theoretic” analogue of the characterization of

K^{+}

in terms of time functions. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

215 KiB

Open AccessArticle

Cosmology with Varying Constants from a Thermodynamic Viewpoint

by Hussain Gohar

Universe 2017, 3(1), 26; https://doi.org/10.3390/universe3010026 - 17 Mar 2017

Cited by 5 | Viewed by 3690

Abstract

We study the variation of fundamental constants in cosmology while dealing with thermodynamic aspects of gravity. We focus on the variation of the speed of light, c, and Newton’s gravitational constant, G, with respect to cosmic time. We find the constraints [...] Read more.

We study the variation of fundamental constants in cosmology while dealing with thermodynamic aspects of gravity. We focus on the variation of the speed of light, c, and Newton’s gravitational constant, G, with respect to cosmic time. We find the constraints on the possible variation of these constants by comparing varying constants of cosmological models with the latest observational data. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

1066 KiB

Open AccessArticle

Cosmological Perturbations in Phantom Dark Energy Models

by Imanol Albarran, Mariam Bouhmadi-López and João Morais

Universe 2017, 3(1), 22; https://doi.org/10.3390/universe3010022 - 6 Mar 2017

Viewed by 4184

Abstract

The ΛCDM paradigm, characterised by a constant equation of state

w = - 1

for dark energy, is the model that better fits observations. However, the same observations strongly support the possibility of a dark energy content where the corresponding equation of state [...] Read more.

The ΛCDM paradigm, characterised by a constant equation of state

w = - 1

for dark energy, is the model that better fits observations. However, the same observations strongly support the possibility of a dark energy content where the corresponding equation of state is close to but slightly smaller than

- 1

. In this regard, we focus on three different models where the dark energy content is described by a perfect fluid with an equation of state

w ≲ - 1

which can evolve or not. The three proposals show very similar behaviour at present, while the asymptotic evolution of each model drives the Universe to different abrupt events known as (i) Big Rip; (ii) Little Rip (LR); and (iii) Little Sibling of the Big Rip. With the aim of comparing these models and finding possible imprints in their predicted matter distribution, we compute the matter power spectrum and the growth rate

f σ_{8}

. We conclude that the model which induces a LR seems to be favoured by observations. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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713 KiB

Open AccessArticle

3-Form Cosmology: Phantom Behaviour, Singularities and Interactions

by João Morais, Mariam Bouhmadi-López and João Marto

Universe 2017, 3(1), 21; https://doi.org/10.3390/universe3010021 - 3 Mar 2017

Cited by 2 | Viewed by 4071

Abstract

The latest cosmological observations by the Planck collaboration (and combined with others) are compatible with a phantom-like behaviour (

w < - 1

) for the dark energy equation of state that drives the current acceleration of the Universe. With this mindset, we [...] Read more.

The latest cosmological observations by the Planck collaboration (and combined with others) are compatible with a phantom-like behaviour (

w < - 1

) for the dark energy equation of state that drives the current acceleration of the Universe. With this mindset, we look into models where dark energy is described by a 3-form field minimally coupled to gravity. When compared to a scalar field, these models have the advantage of more naturally accommodating a cosmological-constant and phantom-like behaviours. We show how the latter happens for a fairly general class of positive-valued potentials, and through a dynamical system approach, we find that in such cases the 3-form field leads the Universe into a Little Sibling of the Big Rip singular event into the future. In this work, we explore the possibility of avoiding such singularity via an interaction in the dark sector between cold dark matter and the 3-form field. For the kind of interactions considered, we deduce a condition for replacing the LSBR by a late time de Sitter phase. For specific examples of interactions that meet this condition, we look for distinctive imprints in the statefinder hierarchy

{{S_{3}}^{(1)}; {S_{4}}^{(1)}}

,

{{S_{3}}^{(1)}; {S_{5}}^{(1)}}

, and in the growth rate of matter,

ϵ (z)

, through the composite null diagnostic (CND). Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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236 KiB

Open AccessArticle

Non-Standard Hierarchies of the Runnings of the Spectral Index in Inﬂation

by Chris Longden

Universe 2017, 3(1), 17; https://doi.org/10.3390/universe3010017 - 2 Mar 2017

Cited by 4 | Viewed by 3416

Abstract

Recent analyses of cosmic microwave background surveys have revealed hints that there may be a non-trivial running of the running of the spectral index. If future experiments were to conﬁrm these hints, it would prove a powerful discriminator of inﬂationary models, ruling out [...] Read more.

Recent analyses of cosmic microwave background surveys have revealed hints that there may be a non-trivial running of the running of the spectral index. If future experiments were to conﬁrm these hints, it would prove a powerful discriminator of inﬂationary models, ruling out simple single ﬁeld models. We discuss how isocurvature perturbations in multi-ﬁeld models can be invoked to generate large runnings in a non-standard hierarchy, and ﬁnd that a minimal model capable of practically realising this would be a two-ﬁeld model with a non-canonical kinetic structure. We also consider alternative scenarios such as variable speed-of-light models and canonical quantum gravity effects and their implications for runnings of the spectral index. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

721 KiB

Open AccessArticle

Bell Violation in Primordial Cosmology

by Sayantan Choudhury, Sudhakar Panda and Rajeev Singh

Universe 2017, 3(1), 13; https://doi.org/10.3390/universe3010013 - 17 Feb 2017

Cited by 33 | Viewed by 3832

Abstract

In this paper, we have worked on the possibility of setting up an Bell’s inequality violating experiment in the context of primordial cosmology following the fundamental principles of quantum mechanics. To set up this proposal, we have introduced a model-independent theoretical framework using [...] Read more.

In this paper, we have worked on the possibility of setting up an Bell’s inequality violating experiment in the context of primordial cosmology following the fundamental principles of quantum mechanics. To set up this proposal, we have introduced a model-independent theoretical framework using which we have studied the creation of new massive particles for the scalar fluctuations in the presence of an additional time-dependent mass parameter. Next we explicitly computed the one-point and two-point correlation functions from this setup. Then, we comment on the measurement techniques of isospin breaking interactions of newly introduced massive particles and its further prospects. After that, we give an example of the string theory-originated axion monodromy model in this context. Finally, we provide a bound on the heavy particle mass parameter for any arbitrary spin field. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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277 KiB

Open AccessArticle

Nonlinear Gravitational Waves as Dark Energy in Warped Spacetimes

by Reinoud Jan Slagter

Universe 2017, 3(1), 11; https://doi.org/10.3390/universe3010011 - 15 Feb 2017

Viewed by 3957

Abstract

We find an azimuthal-angle dependent approximate wave like solution to second order on a warped five-dimensional manifold with a self-gravitating U(1) scalar gauge field (cosmic string) on the brane using the multiple-scale method. The spectrum of the several orders of approximation show maxima [...] Read more.

We find an azimuthal-angle dependent approximate wave like solution to second order on a warped five-dimensional manifold with a self-gravitating U(1) scalar gauge field (cosmic string) on the brane using the multiple-scale method. The spectrum of the several orders of approximation show maxima of the energy distribution dependent on the azimuthal-angle and the winding numbers of the subsequent orders of the scalar field. This breakup of the quantized flux quanta does not lead to instability of the asymptotic wavelike solution due to the suppression of the n-dependency in the energy momentum tensor components by the warp factor. This effect is triggered by the contribution of the five dimensional Weyl tensor on the brane. This contribution can be understood as dark energy and can trigger the self-acceleration of the universe without the need of a cosmological constant. There is a striking relation between the symmetry breaking of the Higgs field described by the winding number and the SO(2) breaking of the axially symmetric configuration into a discrete subgroup of rotations of about 180

^{\circ}

. The discrete sequence of non-axially symmetric deviations, cancelled by the emission of gravitational waves in order to restore the SO(2) symmetry, triggers the pressure

T_{z z}

for discrete values of the azimuthal-angle. There could be a possible relation between the recently discovered angle-preferences of polarization axes of quasars on large scales and our theoretical predicted angle-dependency and this could be evidence for the existence of cosmic strings. Careful comparison of this spectrum of extremal values of the first and second order φ-dependency and the distribution of the alignment of the quasar polarizations is necessary. This can be accomplished when more observational data become available. It turns out that, for late time, the vacuum 5D spacetime is conformally invariant if the warp factor fulfils the equation of a vibrating “drum”, describing standing normal modes of the brane. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

251 KiB

Open AccessArticle

The Relation between Fundamental Constants and Particle Physics Parameters

by Rodger I. Thompson

Universe 2017, 3(1), 6; https://doi.org/10.3390/universe3010006 - 24 Jan 2017

Cited by 3 | Viewed by 3865

Abstract

The observed constraints on the variability of the proton to electron mass ratio μ and the fine structure constant α are used to establish constraints on the variability of the Quantum Chromodynamic Scale and a combination of the Higgs Vacuum Expectation Value and [...] Read more.

The observed constraints on the variability of the proton to electron mass ratio μ and the fine structure constant α are used to establish constraints on the variability of the Quantum Chromodynamic Scale and a combination of the Higgs Vacuum Expectation Value and the Yukawa couplings. Further model dependent assumptions provide constraints on the Higgs VEV and the Yukawa couplings separately. A primary conclusion is that limits on the variability of dimensionless fundamental constants such as μ and α provide important constraints on the parameter space of new physics and cosmologies. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Review

Jump to: Research, Other

246 KiB

Open AccessReview

A Review on the Cosmology of the de Sitter Horndeski Models

by Nelson J. Nunes, Prado Martín-Moruno and Francisco S. N. Lobo

Universe 2017, 3(2), 33; https://doi.org/10.3390/universe3020033 - 31 Mar 2017

Cited by 7 | Viewed by 3037

Abstract

We review the most general scalar-tensor cosmological models with up to second-order derivatives in the field equations that have a fixed spatially flat de Sitter critical point independent of the material content or vacuum energy. This subclass of the Horndeski Lagrangian is capable [...] Read more.

We review the most general scalar-tensor cosmological models with up to second-order derivatives in the field equations that have a fixed spatially flat de Sitter critical point independent of the material content or vacuum energy. This subclass of the Horndeski Lagrangian is capable of dynamically adjusting any value of the vacuum energy of the matter fields at the critical point. We present the cosmological evolution of the linear models and the non-linear models with shift symmetry. We come to the conclusion that the shift symmetric non-linear models can deliver a viable background compatible with current observations. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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286 KiB

Open AccessReview

The Geometry of Noncommutative Spacetimes

by Michał Eckstein

Universe 2017, 3(1), 25; https://doi.org/10.3390/universe3010025 - 16 Mar 2017

Cited by 4 | Viewed by 4315

Abstract

We review the concept of ‘noncommutative spacetime’ approached from an operational stand-point and explain how to endow it with suitable geometrical structures. The latter involves i.a. the causal structure, which we illustrate with a simple—‘almost-commutative’—example. Furthermore, we trace the footprints of noncommutive geometry [...] Read more.

We review the concept of ‘noncommutative spacetime’ approached from an operational stand-point and explain how to endow it with suitable geometrical structures. The latter involves i.a. the causal structure, which we illustrate with a simple—‘almost-commutative’—example. Furthermore, we trace the footprints of noncommutive geometry in the foundations of quantum field theory. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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280 KiB

Open AccessReview

Dark Energy and Spacetime Symmetry

by Irina Dymnikova

Universe 2017, 3(1), 20; https://doi.org/10.3390/universe3010020 - 3 Mar 2017

Cited by 15 | Viewed by 4252

Abstract

The Petrov classification of stress-energy tensors provides a model-independent definition of a vacuum by the algebraic structure of its stress-energy tensor and implies the existence of vacua whose symmetry is reduced as compared with the maximally symmetric de Sitter vacuum associated with the [...] Read more.

The Petrov classification of stress-energy tensors provides a model-independent definition of a vacuum by the algebraic structure of its stress-energy tensor and implies the existence of vacua whose symmetry is reduced as compared with the maximally symmetric de Sitter vacuum associated with the Einstein cosmological term. This allows to describe a vacuum in general setting by dynamical vacuum dark fluid, presented by a variable cosmological term with the reduced symmetry which makes vacuum fluid essentially anisotropic and allows it to be evolving and clustering. The relevant solutions to the Einstein equations describe regular cosmological models with time-evolving and spatially inhomogeneous vacuum dark energy, and compact vacuum objects generically related to a dark energy: regular black holes, their remnants and self-gravitating vacuum solitons with de Sitter vacuum interiors—which can be responsible for observational effects typically related to a dark matter. The mass of objects with de Sitter interior is generically related to vacuum dark energy and to breaking of space-time symmetry. In the cosmological context spacetime symmetry provides a mechanism for relaxing cosmological constant to a needed non-zero value. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Other

Jump to: Research, Review

804 KiB

Open AccessConference Report

Observational Consequences of an Interacting Multiverse

by Salvador J. Robles-Pérez

Universe 2017, 3(2), 49; https://doi.org/10.3390/universe3020049 - 25 May 2017

Cited by 4 | Viewed by 3423

Abstract

The observability of the multiverse is at the very root of its physical significance as a scientific proposal. In this conference we present, within the third quantization formalism, an interacting scheme between the wave functions of different universes and analyze the effects of [...] Read more.

The observability of the multiverse is at the very root of its physical significance as a scientific proposal. In this conference we present, within the third quantization formalism, an interacting scheme between the wave functions of different universes and analyze the effects of some particular values of the coupling function. One of the main consequences of the interaction between universes can be the appearance of a pre-inflationary stage in the evolution of the universes that might leave observable consequences in the properties of the CMB. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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815 KiB

Open AccessConference Report

Quantum Cosmology and Varying Physical Constants

by Katarzyna Leszczyńska

Universe 2017, 3(2), 46; https://doi.org/10.3390/universe3020046 - 23 May 2017

Cited by 1 | Viewed by 3831

Abstract

The main task of this review is to discuss quantum cosmology minisuperspace models based on the Wheeler–DeWitt equation, which apart from the standard matter and 3-geometry configuration degrees of freedom, allow those related to the variability of physical constants—varying speed of light (VSL) [...] Read more.

The main task of this review is to discuss quantum cosmology minisuperspace models based on the Wheeler–DeWitt equation, which apart from the standard matter and 3-geometry configuration degrees of freedom, allow those related to the variability of physical constants—varying speed of light (VSL) c and varying gravitational constant G. The tunneling probability of the universe “from nothing” to the Friedmann phase will be given for such varying constants minisuperspace models. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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2701 KiB

Open AccessConference Report

Blind Spots for Direct Detection with Simplified DM Models and the LHC

by Arghya Choudhury, Kamila Kowalska, Leszek Roszkowski, Enrico Maria Sessolo and Andrew J. Williams

Universe 2017, 3(2), 41; https://doi.org/10.3390/universe3020041 - 11 May 2017

Cited by 8 | Viewed by 3348

Abstract

Using the existing simplified model framework, we build several dark matter models which have suppressed spin-independent scattering cross section. We show that the scattering cross section can vanish due to interference effects with models obtained by simple combinations of simplified models. For weakly [...] Read more.

Using the existing simplified model framework, we build several dark matter models which have suppressed spin-independent scattering cross section. We show that the scattering cross section can vanish due to interference effects with models obtained by simple combinations of simplified models. For weakly interacting massive particle (WIMP) masses ≳10 GeV, collider limits are usually much weaker than the direct detection limits coming from LUX or XENON100. However, for our model combinations, LHC analyses are more competitive for some parts of the parameter space. The regions with direct detection blind spots can be strongly constrained from the complementary use of several Large Hadron Collider (LHC) searches like mono-jet, jets + missing transverse energy, heavy vector resonance searches, etc. We evaluate the strongest limits for combinations of scalar + vector, “squark” + vector, and scalar + “squark” mediator, and present the LHC 14 TeV projections. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Nucleosynthesis Predictions and High-Precision Deuterium Measurements

by Signe Riemer-Sørensen and Espen Sem Jenssen

Universe 2017, 3(2), 44; https://doi.org/10.3390/universe3020044 - 9 May 2017

Cited by 22 | Viewed by 4141

Abstract

Two new high-precision measurements of the deuterium abundance from absorbers along the line of sight to the quasar PKS1937–1009 were presented. The absorbers have lower neutral hydrogen column densities (N(HI) ≈ 18 cm

^{- 2}

) than for previous high-precision measurements, boding well [...] Read more.

Two new high-precision measurements of the deuterium abundance from absorbers along the line of sight to the quasar PKS1937–1009 were presented. The absorbers have lower neutral hydrogen column densities (N(HI) ≈ 18 cm

^{- 2}

) than for previous high-precision measurements, boding well for further extensions of the sample due to the plenitude of low column density absorbers. The total high-precision sample now consists of 12 measurements with a weighted average deuterium abundance of D/H =

2.55 \pm 0.02 \times 10^{- 5}

. The sample does not favour a dipole similar to the one detected for the fine structure constant. The increased precision also calls for improved nucleosynthesis predictions. For that purpose we have updated the public AlterBBN code including new reactions, updated nuclear reaction rates, and the possibility of adding new physics such as dark matter. The standard Big Bang Nucleosynthesis prediction of D/H =

2.456 \pm 0.057 \times 10^{- 5}

is consistent with the observed value within 1.7 standard deviations. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Quantum Cosmology of the Big Rip: Within GR and in a Modified Theory of Gravity

by Mariam Bouhmadi-López, Imanol Albarran and Che-Yu Chen

Universe 2017, 3(2), 36; https://doi.org/10.3390/universe3020036 - 14 Apr 2017

Cited by 5 | Viewed by 4837

Abstract

Quantum gravity is the theory that is expected to successfully describe systems that are under strong gravitational effects while at the same time being of an extreme quantum nature. When this principle is applied to the universe as a whole, we use what [...] Read more.

Quantum gravity is the theory that is expected to successfully describe systems that are under strong gravitational effects while at the same time being of an extreme quantum nature. When this principle is applied to the universe as a whole, we use what is commonly named “quantum cosmology”. So far we do not have a definite quantum theory of gravity or cosmology, but we have several promising approaches. Here we will review the application of the Wheeler–DeWitt formalism to the late-time universe, where it might face a Big Rip future singularity. The Big Rip singularity is the most virulent future dark energy singularity which can happen not only in general relativity but also in some modified theories of gravity. Our goal in this paper is to review two simple setups of the quantisation of the Big Rip in a Friedmann–Lemaître–Robertson–Walker universe within general relativity and in a modified theory of gravity. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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How to Reconstruct a Varying Speed of Light Signal from Baryon Acoustic Oscillations Surveys

by Vincenzo Salzano

Universe 2017, 3(2), 35; https://doi.org/10.3390/universe3020035 - 11 Apr 2017

Cited by 3 | Viewed by 2630

Abstract

We describe an alternative way to use future Baryon Acoustic Oscillation observations to perform non-mainstream research. We focus on the so-called Varying Speed of Light theories, in which the speed of light is made to vary in time. Using prescriptions from future BAO [...] Read more.

We describe an alternative way to use future Baryon Acoustic Oscillation observations to perform non-mainstream research. We focus on the so-called Varying Speed of Light theories, in which the speed of light is made to vary in time. Using prescriptions from future BAO surveys (BOSS, DESI, WFirst-2.4 and SKA), we show that, within such surveys, a 1% Varying Speed of Light (VSL) signal could be detected at 3 sigmas confidence level, in the redshift interval [0.75, 1.45]. Smaller signals will be hardly detected. We also discuss some possible problems related to such kinds of observation, in particular, the degeneracy between a VSL signal and a non-null spatial curvature. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Probing the Gravitational Dependence of the Fine-Structure Constant from Observations of White Dwarf Stars

by Matthew B. Bainbridge, Martin A. Barstow, Nicole Reindl, W.-Ü Lydia Tchang-Brillet, Thomas R. Ayres, John K. Webb, John D. Barrow, Jiting Hu, Jay B. Holberg, Simon P. Preval, Wim Ubachs, Vladimir A. Dzuba, Victor V. Flambaum, Vincent Dumont and Julian C. Berengut

Universe 2017, 3(2), 32; https://doi.org/10.3390/universe3020032 - 30 Mar 2017

Cited by 30 | Viewed by 7590

Abstract

Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic [...] Read more.

Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling, and fundamental physics in the search for variation in the fine structure constant. This variation manifests as shifts in the observed wavelengths of absorption lines, such as quadruply ionized iron (FeV) and quadruply ionized nickel (NiV), when compared to laboratory wavelengths. Berengut et al. (Phys. Rev. Lett. 2013, 111, 010801) demonstrated the validity of such an analysis using high-resolution Space Telescope Imaging Spectrograph (STIS) spectra of G191-B2B. We have made three important improvements by: (a) using three new independent sets of laboratory wavelengths; (b) analysing a sample of objects; and (c) improving the methodology by incorporating robust techniques from previous studies towards quasars (the Many Multiplet method). A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. Here we describe our approach and present preliminary results from nine objects using both FeV and NiV. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Dark Energy Constraints from Espresso Tests of the Stability of Fundamental Couplings

by Ana C. O. Leite, Carlos J. A. P. Martins and Paolo Molaro

Universe 2017, 3(2), 30; https://doi.org/10.3390/universe3020030 - 24 Mar 2017

Cited by 3 | Viewed by 2897

Abstract

ESPRESSO is a high-resolution-ultra-stable spectrograph for the Very Large Telescope (VLT), whose commissioning will start in 2017. One of its key science goals is to test the stability of nature’s fundamental couplings with unprecedented accuracy and control of possible systematics. A total of [...] Read more.

ESPRESSO is a high-resolution-ultra-stable spectrograph for the Very Large Telescope (VLT), whose commissioning will start in 2017. One of its key science goals is to test the stability of nature’s fundamental couplings with unprecedented accuracy and control of possible systematics. A total of 27 nights of the ESPRESSO Consortium’s guaranteed time observations (GTO) will be spent on testing the stability of the fine-structure constant and other fundamental couplings. A set of 14 priority optimal targets have been selected for the GTO period. In this work, we discuss the criteria underlying this selection, describe the selected targets, and present some forecasts of the impact of these measurements on fundamental physics and cosmology, focusing on dark energy constraints and using future supernova type Ia surveys as a comparison point. This report is a summary of the results reported in Phys. Rev. D 2016, 94, 123512, to which we refer the reader for further details. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Quantum Entanglement in the Multiverse

by Sugumi Kanno

Universe 2017, 3(2), 28; https://doi.org/10.3390/universe3020028 - 23 Mar 2017

Cited by 5 | Viewed by 5523

Abstract

In this report, we consider cosmological implications of quantum entanglement between two causally disconnected universes in the multiverse. Supposing that our universe was initially entangled with a causally separated universe, we compute the spectrum of vacuum fluctuations of our universe. To clearly see [...] Read more.

In this report, we consider cosmological implications of quantum entanglement between two causally disconnected universes in the multiverse. Supposing that our universe was initially entangled with a causally separated universe, we compute the spectrum of vacuum fluctuations of our universe. To clearly see the effect of entanglement, we compare it with the spectrum of an initially non-entangled state. It is found that, due to quantum interference, scale-dependent modulations may enter the spectrum for the case of an initially non-entangled state. We discuss that the existence of causally disconnected universes may be experimentally tested by analyzing correlators in detail. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Infinitesimal Structure of Singularities

by Michael Heller and Jerzy Król

Universe 2017, 3(1), 16; https://doi.org/10.3390/universe3010016 - 27 Feb 2017

Cited by 5 | Viewed by 3571

Abstract

Some important problems of general relativity, such as the quantisation of gravity or classical singularity problems, crucially depend on geometry on very small scales. The so-called synthetic differential geometry—a categorical counterpart of the standard differential geometry—provides a tool to penetrate infinitesimally small portions [...] Read more.

Some important problems of general relativity, such as the quantisation of gravity or classical singularity problems, crucially depend on geometry on very small scales. The so-called synthetic differential geometry—a categorical counterpart of the standard differential geometry—provides a tool to penetrate infinitesimally small portions of space-time. We use this tool to show that on any “infinitesimal neighbourhood” the components of the curvature tensor are themselves infinitesimal, and construct a simplified model in which the curvature singularity disappears, owing to this effect. However, one pays a price for this result. Using topoi as a generalisation of spaces requires a weakening of arithmetic (the existence of infinitesimals) and of logic (to the intuitionistic logic). Is this too high a price to pay for acquiring a new method of solving unsolved problems in physics? Without trying, we shall never know the answer. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Quantum Correlations in de Sitter Space

by Jiro Soda, Sugumi Kanno and Jonathan P. Shock

Universe 2017, 3(1), 2; https://doi.org/10.3390/universe3010002 - 1 Jan 2017

Cited by 8 | Viewed by 3827

Abstract

We study quantum correlation of a massive scalar field in a maximally entangled state in de Sitter space. We prepare two observers, one in a global chart and the other in an open chart of de Sitter space. We find that the state [...] Read more.

We study quantum correlation of a massive scalar field in a maximally entangled state in de Sitter space. We prepare two observers, one in a global chart and the other in an open chart of de Sitter space. We find that the state becomes less entangled as the curvature of the open chart gets larger. In particular, for the cases of a massless and a conformally coupled scalar field, the quantum entanglement vanishes in the limit of infinite curvature. However, we find that the quantum discord never disappears, even in the limit that entanglement disappears. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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New Constraints on Spatial Variations of the Fine Structure Constant from Clusters of Galaxies

by Ivan De Martino, Carlos J. A. P. Martins, Harald Ebeling and Dale Kocevski

Universe 2016, 2(4), 34; https://doi.org/10.3390/universe2040034 - 21 Dec 2016

Cited by 13 | Viewed by 3976

Abstract

We have constrained the spatial variation of the fine structure constant using multi-frequency measurements of the thermal Sunyaev-Zeldovich effect of 618 X-ray selected clusters. Although our results are not competitive with the ones from quasar absorption lines, we improved by a factor 10 [...] Read more.

We have constrained the spatial variation of the fine structure constant using multi-frequency measurements of the thermal Sunyaev-Zeldovich effect of 618 X-ray selected clusters. Although our results are not competitive with the ones from quasar absorption lines, we improved by a factor 10 and ∼2.5 previous results from Cosmic Microwave Background power spectrum and from galaxy clusters, respectively. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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Experimental Studies on the Lorentz Symmetry in Post-Newtonian Gravity with Pulsars

by Lijing Shao

Universe 2016, 2(4), 29; https://doi.org/10.3390/universe2040029 - 1 Dec 2016

Cited by 3 | Viewed by 3545

Abstract

Local Lorentz invariance (LLI) is one of the most important fundamental symmetries in modern physics. While the possibility of LLI violation (LLIv) was studied extensively in flat spacetime, its counterpart in gravitational interaction also deserves significant examination from experiments. In this contribution, I [...] Read more.

Local Lorentz invariance (LLI) is one of the most important fundamental symmetries in modern physics. While the possibility of LLI violation (LLIv) was studied extensively in flat spacetime, its counterpart in gravitational interaction also deserves significant examination from experiments. In this contribution, I review several recent studies of LLI in post-Newtonian gravity, using powerful tools of pulsar timing. It shows that precision pulsar timing experiments hold a unique position to probe LLIv in post-Newtonian gravity. Full article

(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)

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