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10 pages, 734 KiB

Open AccessFeature PaperArticle

Towards New Constraints in Extended Theories of Gravity: Cosmography and Gravitational-Wave Signals from Neutron Stars

by Álvaro De la Cruz Dombriz

Galaxies 2018, 6(1), 28; https://doi.org/10.3390/galaxies6010028 - 27 Feb 2018

Cited by 1 | Viewed by 3594

Abstract

Combined cosmological, astrophysical and numerical tests may shed some light on the viability of theories of gravity beyond Einsteinian relativity. In this letter, we present two different techniques providing complementary ways of testing new physics beyond the

Λ

CDM cosmological paradigm. First, we [...] Read more.

Combined cosmological, astrophysical and numerical tests may shed some light on the viability of theories of gravity beyond Einsteinian relativity. In this letter, we present two different techniques providing complementary ways of testing new physics beyond the

Λ

CDM cosmological paradigm. First, we shall present some of the latest progress and shortcomings in the cosmographic model-independent approach for several modified gravity theories using supernovae catalogues, baryonic acoustic oscillation data and

H (z)

differential age compilations. Second, we shall show how once the Einsteinian paradigm is abandoned, the phenomenology of neutron stars changes dramatically since neutron-star masses can be much larger than their General Relativity counterparts. Consequently, the total energy available for radiating gravitational waves could be of the order of several solar masses, and thus a merger of these stars constitutes a privileged wave source. Unfortunately at the present time our persisting lack of understanding in the strong interaction sector does not allow to distinguish the alternative theories from the usual General Relativity predictions. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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23 pages, 313 KiB

Open AccessFeature PaperArticle

“All that Matter … in One Big Bang …”, &Other Cosmological Singularities

by Emilio Elizalde

Galaxies 2018, 6(1), 25; https://doi.org/10.3390/galaxies6010025 - 21 Feb 2018

Cited by 4 | Viewed by 4730

Abstract

The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the year 1912. Some of the pieces of evidence presented here have emerged from recent research in [...] Read more.

The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the year 1912. Some of the pieces of evidence presented here have emerged from recent research in the history of science and are not usually shared with the general audiences in popular science books. In particular, the issue of the correct formulation of the original Big Bang concept, according to the precise words of Fred Hoyle, is discussed. Too often, this point is very deficiently explained (when not just misleadingly) in most of the available generalist literature. Other frequent uses of the same words, Big Bang, as to name the initial singularity of the cosmos, and also whole cosmological models, are then addressed, as evolutions of its original meaning. Quantum and inflationary additions to the celebrated singularity theorems by Penrose, Geroch, Hawking and others led to subsequent results by Borde, Guth and Vilenkin. Additionally, corresponding corrections to the Einstein field equations have originated, in particular,

R^{2}

,

f (R)

, and scalar-tensor gravities, giving rise to a plethora of new singularities. For completeness, an updated table with a classification of the same is given. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

8 pages, 201 KiB

Open AccessFeature PaperArticle

Cosmological Constant and Renormalization of Gravity

by Shin’ichi Nojiri

Galaxies 2018, 6(1), 24; https://doi.org/10.3390/galaxies6010024 - 18 Feb 2018

Cited by 3 | Viewed by 2797

Abstract

In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological [...] Read more.

In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological theory. In this model, the divergence of the vacuum energy coming from the quantum corrections from matters can be absorbed into the redefinition of the scalar field. In this paper, we consider the extension of the model in order to apply the mechanism to other kinds of divergences coming from the quantum correction and consider the cosmology in an extended model. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

7 pages, 2281 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Unitary Issues in Some Higher Derivative Field Theories

by Manuel Asorey, Leslaw Rachwal and Ilya Shapiro

Galaxies 2018, 6(1), 23; https://doi.org/10.3390/galaxies6010023 - 14 Feb 2018

Cited by 24 | Viewed by 3486

Abstract

We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of [...] Read more.

We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of finding a consistent quantum field theory of quantum gravity. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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13 pages, 266 KiB

Open AccessFeature PaperArticle

The Avoidance of the Little Sibling of the Big Rip Abrupt Event by a Quantum Approach

by Imanol Albarran, Mariam Bouhmadi-López, Franciso Cabral and Prado Martín-Moruno

Galaxies 2018, 6(1), 21; https://doi.org/10.3390/galaxies6010021 - 6 Feb 2018

Cited by 6 | Viewed by 3081

Abstract

We address the quantisation of a model that induces the Little Sibling of the Big Rip (LSBR) abrupt event, where the dark energy content is described by means of a phantom-like fluid or a phantom scalar field. The quantisation is done in the [...] Read more.

We address the quantisation of a model that induces the Little Sibling of the Big Rip (LSBR) abrupt event, where the dark energy content is described by means of a phantom-like fluid or a phantom scalar field. The quantisation is done in the framework of the Wheeler–DeWitt (WDW) equation and imposing the DeWitt boundary condition; i.e., the wave function vanishes close to the abrupt event. We analyse the WDW equation within two descriptions: First, when the dark energy content is described with a perfect fluid. This leaves the problem with the scale factor as the single degree of freedom. Second, when the dark energy content is described with a phantom scalar field in such a way that an additional degree of freedom is incorporated. Here, we have applied the Born–Oppenheimer (BO) approximation in order to simplify the WDW equation. In all cases, the obtained wave function vanishes when the LSBR takes place, thus fulfilling the DeWitt boundary condition. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

7 pages, 1353 KiB

Open AccessFeature PaperArticle

Time in Quantum Cosmology of FRW f(R) Theories

by C. Ramírez and V. Vázquez-Báez

Galaxies 2018, 6(1), 12; https://doi.org/10.3390/galaxies6010012 - 17 Jan 2018

Cited by 1 | Viewed by 3096

Abstract

The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the [...] Read more.

The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the wave function of the universe. Following proposals to assign the clock function to a scalar field, we look at the scalar degree of freedom contained in

f (R)

theories. For this purpose we consider a quadratic

f (R)

theory in an equivalent formulation with a scalar field, with a FRW metric, and consider its Wheeler-DeWitt equation. The wave function is obtained numerically and is consistent with the interpretation of the scalar field as time by means of a conditional probability, from which an effective time-dependent wave function follows. The evolution the scale factor is obtained by its mean value, and the quantum fluctuations are consistent with the Heisenberg relations and a classical universe today. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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17 pages, 267 KiB

Open AccessFeature PaperArticle

Supersymmetric M-brane Solution in a Dynamical Background

by Kengo Maeda and Kunihito Uzawa

Galaxies 2018, 6(1), 11; https://doi.org/10.3390/galaxies6010011 - 16 Jan 2018

Cited by 1 | Viewed by 2644

Abstract

Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution [...] Read more.

Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution have made it clear that one can get more information by asking what happens on supersymmetric systems. In this proceeding, we construct an exact supersymmetric solution of a dynamical M-brane background in the 11-dimensional supergravity and investigate supersymmetry breaking, the geometric features near the singularity and the black hole horizon. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

8 pages, 333 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Ultra Light Axionic Dark Matter: Galactic Halos and Implications for Observations with Pulsar Timing Arrays

by Ivan De Martino, Tom Broadhurst, S.-H. Henry Tye, Tzihong Chiueh, Hsi-Yu Schive and Ruth Lazkoz

Galaxies 2018, 6(1), 10; https://doi.org/10.3390/galaxies6010010 - 16 Jan 2018

Cited by 19 | Viewed by 3960

Abstract

The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of [...] Read more.

The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of experimental detection of Weakly Interacting Massive Particle (WIMP) favors alternative candidates such as light axionic dark matter that naturally arise in string theory. Cosmological N-body simulations have shown that axionic dark matter forms a solitonic core of size of ≃150 pc in the innermost region of the galactic halos. The oscillating scalar field associated to the axionic dark matter halo produces an oscillating gravitational potential that induces a time dilation of the pulse arrival time of ≃400 ns/(m

_{B}

/10

^{- 22}

eV) for pulsar within such a solitonic core. Over the whole galaxy, the averaged predicted signal may be detectable with current and forthcoming pulsar timing array telescopes. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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11 pages, 261 KiB

Open AccessFeature PaperArticle

Wavepacket Evolution in Unimodular Quantum Cosmology

by Natascha Riahi

Galaxies 2018, 6(1), 8; https://doi.org/10.3390/galaxies6010008 - 12 Jan 2018

Cited by 2 | Viewed by 2681

Abstract

The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and [...] Read more.

The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and a spatially flat Friedmann universe. We show that the longterm behaviour of the expectation values of the canonical quantities corresponds to the evolution of the classical variables. The solutions provided in an explicit example can be continued beyond the singularity at

t = 0

, passing a finite minimal extension of the universe. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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7 pages, 222 KiB

Open AccessFeature PaperArticle

A Proposal of a Regular Black Hole Satisfying the Weak Energy Condition

by Stefano Chinaglia

Galaxies 2018, 6(1), 4; https://doi.org/10.3390/galaxies6010004 - 4 Jan 2018

Cited by 6 | Viewed by 2664

Abstract

We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior. [...] Read more.

We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

289 KiB

Open AccessFeature PaperArticle

Electromagnetic Casimir Effect in AdS Spacetime

by Anna S. Kotanjyan, Aram A. Saharian and Astghik A. Saharyan

Galaxies 2017, 5(4), 102; https://doi.org/10.3390/galaxies5040102 - 19 Dec 2017

Cited by 6 | Viewed by 3080

Abstract

We investigate the vacuum expectation value (VEV) of the energy-momentum tensor for the electromagnetic field in anti-de Sitter (AdS) spacetime in the presence of a boundary parallel to the AdS horizon. On the boundary, the field obeys the generalized perfect conductor boundary condition. [...] Read more.

We investigate the vacuum expectation value (VEV) of the energy-momentum tensor for the electromagnetic field in anti-de Sitter (AdS) spacetime in the presence of a boundary parallel to the AdS horizon. On the boundary, the field obeys the generalized perfect conductor boundary condition. The VEV of the energy-momentum tensor is decomposed into the boundary-free and boundary-induced contributions. In this way, for points away from the boundary, the renormalization is reduced to that for AdS spacetime without the boundary. The boundary-induced energy density is negative everywhere, and the normal stress is positive in the region between the boundary and the AdS boundary and is negative in the region between the boundary and the AdS horizon. Near both the AdS boundary and horizon, the boundary-induced VEV decays exponentially as a function of the corresponding proper distance. Applications are given for even and odd vector fields in Randall–Sundrum model with a single brane. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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1279 KiB

Open AccessFeature PaperArticle

Bell Inequality and Its Application to Cosmology

by Sugumi Kanno and Jiro Soda

Galaxies 2017, 5(4), 99; https://doi.org/10.3390/galaxies5040099 - 12 Dec 2017

Cited by 5 | Viewed by 4001

Abstract

One of the cornerstones of inflationary cosmology is that primordial density fluctuations have a quantum mechanical origin. However, most physicists consider that such quantum mechanical effects disappear in CMB data due to decoherence. In this conference report, we show that the violation of [...] Read more.

One of the cornerstones of inflationary cosmology is that primordial density fluctuations have a quantum mechanical origin. However, most physicists consider that such quantum mechanical effects disappear in CMB data due to decoherence. In this conference report, we show that the violation of Bell inequalities in an initial state of our universe increases exponentially with the number of modes to measure in inflation. This indicates that some evidence that our universe has a quantum mechanical origin may survive in CMB data, even if quantum entanglement decays exponentially afterward due to decoherence. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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231 KiB

Open AccessFeature PaperArticle

Cosmological Effects of Quantum Vacuum Condensates

by Antonio Capolupo

Galaxies 2017, 5(4), 98; https://doi.org/10.3390/galaxies5040098 - 12 Dec 2017

Cited by 8 | Viewed by 3137

Abstract

In quantum field theory, many phenomena are characterized by a condensed structure of their vacua. Such a structure is responsible of a non trivial vacuum energy. Here we analyze disparate systems and we show that the thermal vacuum state for hot plasmas, the [...] Read more.

In quantum field theory, many phenomena are characterized by a condensed structure of their vacua. Such a structure is responsible of a non trivial vacuum energy. Here we analyze disparate systems and we show that the thermal vacuum state for hot plasmas, the vacuum for boson field in curved space and the vacuum for mixed neutrinos have the state equation of dark matter,

w = 0

, and values of the energy density which are in agreement with the one estimated for dark matter. Moreover, we show that the vacuum of axions mixed with photons has the state equation of the cosmological constant

w = - 1

and a value of the energy density compatible with the one of dark energy. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

299 KiB

Open AccessArticle

Exploring String Axions with Gravitational Waves

by Jiro Soda and Daiske Yoshida

Galaxies 2017, 5(4), 96; https://doi.org/10.3390/galaxies5040096 - 11 Dec 2017

Cited by 13 | Viewed by 3634

Abstract

We explore the string axion dark matter with gravitational waves in Chern–Simons gravity. We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation. Remarkably, the circular polarization of gravitational waves is induced by the parity violating Chern–Simons [...] Read more.

We explore the string axion dark matter with gravitational waves in Chern–Simons gravity. We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation. Remarkably, the circular polarization of gravitational waves is induced by the parity violating Chern–Simons coupling. In fact, the gravitational waves should be enhanced ten times every

10^{- 8} pc

propagation in the presence of the axion dark matter with mass

10^{- 10} eV

provided the coupling constant

ℓ = 10^{8} km

. Hence, after

10 kpc

propagation, the amplitude of gravitational waves is enhanced significantly and the polarization of gravitational waves becomes circular. However, we have never observed these signatures. This implies that the Chern–Simons coupling constant and/or the abundance of string axions should be constrained much stronger than the current limits. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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772 KiB

Open AccessFeature PaperArticle

Reheating via Gravitational Particle Production in Simple Models of Quintessence or ΛCDM Inflation

by Jaume De Haro and Llibert Aresté Saló

Galaxies 2017, 5(4), 78; https://doi.org/10.3390/galaxies5040078 - 14 Nov 2017

Cited by 1 | Viewed by 2618

Abstract

We have tested some simple

Λ

CDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via [...] Read more.

We have tested some simple

Λ

CDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via gravitational particle production after inflation, supporting the nucleosynthesis success. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

Other

Jump to: Research

8 pages, 403 KiB

Open AccessFeature PaperConference Report

Galactic Structures from Gravitational Radii

by Salvatore Capozziello, Duško Borka, Vesna Borka Jovanović and Predrag Jovanović

Galaxies 2018, 6(1), 22; https://doi.org/10.3390/galaxies6010022 - 7 Feb 2018

Cited by 2 | Viewed by 3159

Abstract

We demonstrate that the existence of a Noether symmetry in

f (R)

theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain [...] Read more.

We demonstrate that the existence of a Noether symmetry in

f (R)

theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain the baryonic Tully-Fisher relation and the rotation curve of gas-rich galaxies without the dark matter hypothesis. Furthermore, under the same standard, the Fundamental Plane of elliptical galaxies can be addressed. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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8 pages, 549 KiB

Open AccessFeature PaperConference Report

The Third Quantization: To Tunnel or Not to Tunnel?

by Mariam Bouhmadi-López, Manuel Krämer, João Morais and Salvador Robles-Pérez

Galaxies 2018, 6(1), 19; https://doi.org/10.3390/galaxies6010019 - 2 Feb 2018

Cited by 2 | Viewed by 3247

Abstract

Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions [...] Read more.

Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions for the evolution of the scale factor in the Lorentzian and Euclidean regions as well as the probability that the baby universe indeed crosses the wormhole when it reaches its maximum size. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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8 pages, 242 KiB

Open AccessFeature PaperConference Report

Quantum-Gravitational Effects on Primordial Power Spectra in Slow-Roll Inflationary Models

by David Brizuela and Manuel Krämer

Galaxies 2018, 6(1), 6; https://doi.org/10.3390/galaxies6010006 - 8 Jan 2018

Cited by 2 | Viewed by 2778

Abstract

We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt [...] Read more.

We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt equation describing a perturbed inflationary universe. This expansion leads to a hierarchy of equations at consecutive orders of the approximation and allows us to write down a corrected Schrödinger equation that encodes information about quantum-gravitational effects. The analytical dependence of the correction to the power spectrum on the wavenumber is obtained. Nonetheless, some numerical work is needed in order to obtain its precise value. Finally, it is shown that the correction turns out to be positive, which leads to an enhancement of the power spectrum especially prominent for large scales. We will also discuss whether this correction leads to a measurable effect in the cosmic microwave background anisotropies. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

935 KiB

Open AccessFeature PaperConference Report

The Mimetic Born-Infeld Gravity: The Primordial Cosmos and Spherically Symmetric Solutions

by Che-Yu Chen, Mariam Bouhmadi-López and Pisin Chen

Galaxies 2017, 5(4), 87; https://doi.org/10.3390/galaxies5040087 - 27 Nov 2017

Cited by 2 | Viewed by 2967

Abstract

The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in [...] Read more.

The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in the EiBI model are found to be avoidable for some bouncing solutions. For a vacuum, static and spherically symmetric geometry, a new branch of solutions in which the black hole singularity that is replaced with a lightlike singularity is found. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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