Universe

Editorial

2 pages, 162 KiB

Open AccessEditorial

Post-Editorial of the Special Issue “Estate Quantistica Conference—Recent Developments in Gravity, Cosmology, and Mathematical Physics”

by Hermano Velten and Giuseppe Dito

Universe 2020, 6(12), 240; https://doi.org/10.3390/universe6120240 - 13 Dec 2020

Viewed by 1575

Abstract

This editorial summarizes the contributions presented during the Estate Quantistica 2018 and published in a special issue of Universe [...] Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

Research

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40 pages, 2690 KiB

Open AccessArticle

Cosmic Inflation, Quantum Information and the Pioneering Role of John S Bell in Cosmology

by Jérôme Martin

Universe 2019, 5(4), 92; https://doi.org/10.3390/universe5040092 - 15 Apr 2019

Cited by 36 | Viewed by 4207

Abstract

According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave—CMB—anisotropy…) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified [...] Read more.

According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave—CMB—anisotropy…) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified by gravitational instability. At the end of inflation, these perturbations are placed in a two-mode squeezed state with the strongest squeezing ever produced in Nature (much larger than anything that can be made in the laboratory on Earth). This article studies whether astrophysical observations could unambiguously reveal this quantum origin by borrowing ideas from quantum information theory. It is argued that some of the tools needed to carry out this task have been discussed long ago by J. Bell in a, so far, largely unrecognized contribution. A detailled study of his paper and of the criticisms that have been put forward against his work is presented. Although J. Bell could not have realized it when he wrote his letter since the quantum state of cosmological perturbations was not yet fully characterized at that time, it is also shown that Cosmology and cosmic inflation represent the most interesting frameworks to apply the concepts he investigated. This confirms that cosmic inflation is not only a successful paradigm to understand the early Universe. It is also the only situation in Physics where one crucially needs General Relativity and Quantum Mechanics to derive the predictions of a theory and, where, at the same time, we have high-accuracy data to test these predictions, making inflation a playground of utmost importance to discuss foundational issues in Quantum Mechanics. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

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23 pages, 628 KiB

Open AccessArticle

Matter Growth in Imperfect Fluid Cosmology

by Winfried Zimdahl, Hermano E.S. Velten and William C. Algoner

Universe 2019, 5(3), 68; https://doi.org/10.3390/universe5030068 - 4 Mar 2019

Cited by 5 | Viewed by 2775

Abstract

Extensions of Einstein’s General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The [...] Read more.

Extensions of Einstein’s General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard

Λ

-cold-dark-matter (

Λ

CDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the

e_{Φ} Λ

CDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function

f σ_{8}

. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

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21 pages, 309 KiB

Open AccessArticle

The Quantum Nature of Lorentz Invariance

by Richard Kerner

Universe 2019, 5(1), 1; https://doi.org/10.3390/universe5010001 - 20 Dec 2018

Cited by 14 | Viewed by 2961

Abstract

If the reality underlying classical physics is quantum in nature, then it is reasonable to assume that the transformations of fields, currents, energy, and momentum observed macroscopically are the result of averaging of symmetry groups acting in the Hilbert space of quantum states [...] Read more.

If the reality underlying classical physics is quantum in nature, then it is reasonable to assume that the transformations of fields, currents, energy, and momentum observed macroscopically are the result of averaging of symmetry groups acting in the Hilbert space of quantum states of elementary constituents of which classical material bodies are formed. We show how Pauli’s exclusion principle based on the discrete

Z_{2}

symmetry group generates the

S L (2, C)

symmetry of the space of states of an electron endowed with spin. Then, we generalize this reasoning in the case of quark colors and the corresponding

Z_{3}

symmetry. A ternary generalization of Dirac’s equation is proposed, leading to self-confined quarks states. It is shown how certain cubic or quadratic combinations can form freely-propagating entangled states. The entire symmetry of the standard model,

S U (2) \times U (1) \times S U (3)

, is naturally derived, as well. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

6 pages, 253 KiB

Open AccessArticle

Regular Solutions in Higher-Derivative Gravity

by Breno L. Giacchini and Tibério De Paula Netto

Universe 2018, 4(12), 140; https://doi.org/10.3390/universe4120140 - 2 Dec 2018

Cited by 5 | Viewed by 2299

Abstract

Local gravitational theories with more than four derivatives can have remarkable quantum properties. Namely, they can be super-renormalizable and may be unitary in the Lee-Wick sense, if the massive poles of the propagator are complex. It is important, therefore, to also explore the [...] Read more.

Local gravitational theories with more than four derivatives can have remarkable quantum properties. Namely, they can be super-renormalizable and may be unitary in the Lee-Wick sense, if the massive poles of the propagator are complex. It is important, therefore, to also explore the classical aspects of these theories. In this talk we present recent results in this direction. Specifically, we discuss the effect that that higher-order terms can have on the Newtonian potential and related singularities. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

10 pages, 290 KiB

Open AccessArticle

(q, σ, τ)-Differential Graded Algebras

by Viktor Abramov, Olga Liivapuu and Abdenacer Makhlouf

Universe 2018, 4(12), 138; https://doi.org/10.3390/universe4120138 - 1 Dec 2018

Cited by 2 | Viewed by 2600

Abstract

We propose the notion of

(q, σ, τ)

-differential graded algebra, which generalizes the notions of

(σ, τ)

-differential graded algebra and q-differential graded algebra. We construct two examples of [...] Read more.

We propose the notion of

(q, σ, τ)

-differential graded algebra, which generalizes the notions of

(σ, τ)

-differential graded algebra and q-differential graded algebra. We construct two examples of

(q, σ, τ)

-differential graded algebra, where the first one is constructed by means of the generalized Clifford algebra with two generators (reduced quantum plane), where we use a

(σ, τ)

-twisted graded q-commutator. In order to construct the second example, we introduce the notion of

(σ, τ)

-pre-cosimplicial algebra. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

11 pages, 263 KiB

Open AccessArticle

Generalization of Nambu–Hamilton Equation and Extension of Nambu–Poisson Bracket to Superspace

by Viktor Abramov

Universe 2018, 4(10), 106; https://doi.org/10.3390/universe4100106 - 15 Oct 2018

Cited by 2 | Viewed by 2533

Abstract

We propose a generalization of the Nambu–Hamilton equation in superspace

R^{3 | 2}

with three real and two Grassmann coordinates. We construct the even degree vector field in the superspace

R^{3 | 2}

by means of the right-hand sides of the [...] Read more.

We propose a generalization of the Nambu–Hamilton equation in superspace

R^{3 | 2}

with three real and two Grassmann coordinates. We construct the even degree vector field in the superspace

R^{3 | 2}

by means of the right-hand sides of the proposed generalization of the Nambu–Hamilton equation and show that this vector field is divergenceless in superspace. Then we show that our generalization of the Nambu–Hamilton equation in superspace leads to a family of ternary brackets of even degree functions defined with the help of a Berezinian. This family of ternary brackets is parametrized by the infinite dimensional group of invertible second order matrices, whose entries are differentiable functions on the space

R^{3}

. We study the structure of the ternary bracket in a more general case of a superspace

R^{n | 2}

with n real and two Grassmann coordinates and show that for any invertible second order functional matrix it splits into the sum of two ternary brackets, where one is the usual Nambu–Poisson bracket, extended in a natural way to even degree functions in a superspace

R^{n | 2}

, and the second is a new ternary bracket, which we call the

Ψ

-bracket, where

Ψ

can be identified with an invertible second order functional matrix. We prove that the ternary

Ψ

-bracket as well as the whole ternary bracket (the sum of the

Ψ

-bracket with the usual Nambu–Poisson bracket) is totally skew-symmetric, and satisfies the Leibniz rule and the Filippov–Jacobi identity ( Fundamental Identity). Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

Other

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9 pages, 235 KiB

Open AccessConference Report

Degravitation and the Cascading DGP Model

by Fulvio Sbisà

Universe 2018, 4(12), 136; https://doi.org/10.3390/universe4120136 - 29 Nov 2018

Cited by 3 | Viewed by 2221

Abstract

We consider the 6D Cascading DGP model, a braneworld model which is a promising candidate to realize the phenomenon of the degravitation of vacuum energy. Focusing on a recently proposed thin limit description of the model, we study solutions where the induced metric [...] Read more.

We consider the 6D Cascading DGP model, a braneworld model which is a promising candidate to realize the phenomenon of the degravitation of vacuum energy. Focusing on a recently proposed thin limit description of the model, we study solutions where the induced metric on the codimension-2 brane is of the de Sitter form. While these solutions have already been recovered in the literature imposing by hand the bulk to be flat, we show that it is possible to derive them without making this assumption, by solving a suitably chosen subset of the bulk equations. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

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5 pages, 225 KiB

Open AccessConference Report

The Gravity of Light-Waves

by Jan Willem Van Holten

Universe 2018, 4(10), 110; https://doi.org/10.3390/universe4100110 - 18 Oct 2018

Cited by 4 | Viewed by 2835

Abstract

Light waves carry along their own gravitational field; for simple plain electromagnetic waves, the gravitational field takes the form of a

p p

-wave. I present the corresponding exact solution of the Einstein–Maxwell equations and discuss the dynamics of classical particles and quantum [...] Read more.

Light waves carry along their own gravitational field; for simple plain electromagnetic waves, the gravitational field takes the form of a

p p

-wave. I present the corresponding exact solution of the Einstein–Maxwell equations and discuss the dynamics of classical particles and quantum fields in this gravitational and electromagnetic background. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

7 pages, 4318 KiB

Open AccessConference Report

Using Trajectories in Quantum Cosmology

by Patrick Peter

Universe 2018, 4(8), 89; https://doi.org/10.3390/universe4080089 - 15 Aug 2018

Cited by 4 | Viewed by 3239

Abstract

Quantum cosmology based on the Wheeler De Witt equation represents a simple way to implement plausible quantum effects in a gravitational setup. In its minisuperspace version wherein one restricts attention to FLRW metrics with a single scale factor and only a few degrees [...] Read more.

Quantum cosmology based on the Wheeler De Witt equation represents a simple way to implement plausible quantum effects in a gravitational setup. In its minisuperspace version wherein one restricts attention to FLRW metrics with a single scale factor and only a few degrees of freedom describing matter, one can obtain exact solutions and thus acquire full knowledge of the wave function. Although this is the usual way to treat a quantum mechanical system, it turns out however to be essentially meaningless in a cosmological framework. Turning to a trajectory approach then provides an effective means of deriving physical consequences. Full article

(This article belongs to the Special Issue Estate Quantistica Conference - Recent Developments in Gravity, Cosmology, and Mathematical Physics)

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