Universe

8 pages, 244 KiB

Open AccessEditorial

Editorial for the Special Issue “Progress in Group Field Theory and Related Quantum Gravity Formalisms”

by Sylvain Carrozza, Steffen Gielen and Daniele Oriti

Universe 2020, 6(1), 19; https://doi.org/10.3390/universe6010019 - 20 Jan 2020

Cited by 7 | Viewed by 2270

This editorial introduces the Special Issue “Progress in Group Field Theory and Related Quantum Gravity Formalisms” which includes a number of research and review articles covering results in the group field theory (GFT) formalism for quantum gravity and in various neighbouring areas of [...] Read more.

This editorial introduces the Special Issue “Progress in Group Field Theory and Related Quantum Gravity Formalisms” which includes a number of research and review articles covering results in the group field theory (GFT) formalism for quantum gravity and in various neighbouring areas of quantum gravity research. We give a brief overview of the basic ideas of the GFT formalism, list some of its connections to other fields, and then summarise all contributions to the Special Issue. Full article

(This article belongs to the Special Issue Progress in Group Field Theory and Related Quantum Gravity Formalisms)

6 pages, 225 KiB

Open AccessEditorial

Acknowledgement to Reviewers of Universe in 2019

by Universe Editorial Office

Universe 2020, 6(1), 18; https://doi.org/10.3390/universe6010018 - 20 Jan 2020

Viewed by 1939

Abstract

The editorial team greatly appreciates the reviewers who have dedicated their considerable time and expertise to the journal’s rigorous editorial process over the past 12 months, regardless of whether the papers are finally published or not [...] Full article

3 pages, 152 KiB

Open AccessEditorial

Post-Editorial of “The Multiverse” Special Volume

by Ana Alonso-Serrano, Mariusz P. Da̧browski and Thomas Naumann

Universe 2020, 6(1), 17; https://doi.org/10.3390/universe6010017 - 20 Jan 2020

Cited by 2 | Viewed by 2340

Abstract

A succesful series of papers devoted to various aspects of an idea of the Multiverse have been gathered together and presented to the readers. In this post-editorial we briefly challenge the content referring to the main issues dealt with by the Authors. We [...] Read more.

A succesful series of papers devoted to various aspects of an idea of the Multiverse have been gathered together and presented to the readers. In this post-editorial we briefly challenge the content referring to the main issues dealt with by the Authors. We hope that this will inspire other investigators for designing future tests which could make this very notion of the Multiverse falsifiable. Full article

(This article belongs to the Special Issue The Multiverse)

12 pages, 475 KiB

Open AccessArticle

Effects of Atomic-Scale Electron Density Profile and a Fast and Efficient Iteration Algorithm for Matter Effect of Neutrino Oscillation

by Mihai Horoi and Adam Zettel

Universe 2020, 6(1), 16; https://doi.org/10.3390/universe6010016 - 18 Jan 2020

Viewed by 2308

Abstract

In a recent article, we noticed that the electron density in condensed matter exhibits large spikes close to the atomic nuclei. We showed that the peak magnitude of these spikes in the electron densities, 3–4 orders larger than the average electron plasma density [...] Read more.

In a recent article, we noticed that the electron density in condensed matter exhibits large spikes close to the atomic nuclei. We showed that the peak magnitude of these spikes in the electron densities, 3–4 orders larger than the average electron plasma density in the Sun’s core, have no effect on the neutrino emission and absorption probabilities or on the neutrinoless double beta decay probability. However, it was not clear if the effect of these spikes is equivalent to that of an average constant electron density in matter. We investigated these effects by a direct integration of the coupled Dirac equations describing the propagation of flavor neutrinos into, through, and out of the matter. We proposed a new iteration-based algorithm for computing the neutrino survival/appearance probability in matter, which we found to be at least 20 times faster than some direct integration algorithms under the same accuracy. With this method, we found little evidence that these spikes affect the standard oscillations probabilities. In addition, we show that the new algorithm can explain the equivalence of using average electron densities instead of the spiked electron densities. The new algorithm is further extended to the case of light sterile neutrinos. Full article

(This article belongs to the Special Issue Neutrino Oscillations)

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22 pages, 5584 KiB

Open AccessArticle

Electrodynamics and Radiation from Rotating Neutron Star Magnetospheres

by Jérôme Pétri

Universe 2020, 6(1), 15; https://doi.org/10.3390/universe6010015 - 15 Jan 2020

Cited by 16 | Viewed by 3290

Abstract

Neutron stars are compact objects rotating at high speed, up to a substantial fraction of the speed of light (up to 20% for millisecond pulsars) and possessing ultra-strong electromagnetic fields (close to and sometimes above the quantum critical field of 4.4 [...] Read more.

Neutron stars are compact objects rotating at high speed, up to a substantial fraction of the speed of light (up to 20% for millisecond pulsars) and possessing ultra-strong electromagnetic fields (close to and sometimes above the quantum critical field of 4.4

\times 10^{9}

T

). Moreover, due to copious

e^{\pm}

pair creation within the magnetosphere, the relativistic plasma surrounding the star is forced into corotation up to the light cylinder where the corotation speed reaches the speed of light. The neutron star electromagnetic activity is powered by its rotation which becomes relativistic in the neighborhood of this light cylinder. These objects naturally induce relativistic rotation on macroscopic scales about several thousands of kilometers, a crucial ingredient to trigger the central engine as observed on Earth. In this paper, we elucidate some of the salient features of this corotating plasma subject to efficient particle acceleration and radiation, emphasizing several problems and limitations concerning current theories of neutron star magnetospheres. Relativistic rotation in these systems is indirectly probed by the radiation produced within the magnetosphere. Depending on the underlying assumptions about particle motion and radiation mechanisms, different signatures on their light curves, spectra, pulse profiles and polarization angles are expected in their broadband electromagnetic emission. We show that these measurements put stringent constraints on the way to describe particle electrodynamics in a rotating neutron star magnetosphere. Full article

(This article belongs to the Special Issue Rotation Effects in Relativity)

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89 pages, 1802 KiB

Open AccessFeature PaperReview

In the Quest for Cosmic Rotation

by Vladimir A. Korotky, Eduard Masár and Yuri N. Obukhov

Universe 2020, 6(1), 14; https://doi.org/10.3390/universe6010014 - 15 Jan 2020

Cited by 5 | Viewed by 3115

Abstract

This paper analyzes the problem of global rotation in general relativity (GR) theory. Simple cosmological models with rotation and expansion are presented, which give a natural explanation of the modern values of the acceleration parameter at different red shifts without involving the concepts [...] Read more.

This paper analyzes the problem of global rotation in general relativity (GR) theory. Simple cosmological models with rotation and expansion are presented, which give a natural explanation of the modern values of the acceleration parameter at different red shifts without involving the concepts of “dark energy” and “dark matter”. It is shown that due to the smallness of the cosmological rotation, for its detection one should use observations that do not depend on the magnitude of the angular velocity of the Universe. Such tests include the effects of the cosmic mirror and the cosmic lens. For the first time on the basis of modern electronic catalogs the search on the celestial sphere of images of our Galaxy and other galaxies is made. Viable candidates for both effects have been found. Full article

(This article belongs to the Special Issue Rotation Effects in Relativity)

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15 pages, 397 KiB

Open AccessReview

Review of Charmonium and Bottomonium Quark State Production via Relativistic Heavy Ion Collisions

by Leonard S. Kisslinger

Universe 2020, 6(1), 13; https://doi.org/10.3390/universe6010013 - 10 Jan 2020

Viewed by 2276

Abstract

This is a review of the production of heavy quark states via relativistic heavy ion collisions in RHIC. The heavy quarks here are c, charm quark, and b, bottom quark. The states are charmonium meson states [...] Read more.

This is a review of the production of heavy quark states via relativistic heavy ion collisions in RHIC. The heavy quarks here are c, charm quark, and b, bottom quark. The states are charmonium meson states

Ψ (n S)

, with n = 1,2 and upsilon meson states

Υ (m S)

, with m = 1,2,3. Quantum Chromodynamics (QCD) sum rules were used to derive the result that the

Ψ (2 S)

and

Υ (3 S)

are mixed hybrid states, which increase their production cross sections. We also review the

Ψ (n S)

and

Υ (m S)

production cross sections via Cu-Cu and Au-Au collisions, which are very important for this review of the production of heavy quark states in RHIC. The possible detection of the Quark Gluon Plasma (QGP) is also reviewed. Full article

(This article belongs to the Special Issue Heavy Ion Collisions)

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17 pages, 323 KiB

Open AccessArticle

Gravity and Nonlinear Symmetry Realization

by Andrej Arbuzov and Boris Latosh

Universe 2020, 6(1), 12; https://doi.org/10.3390/universe6010012 - 10 Jan 2020

Cited by 6 | Viewed by 2230

Abstract

Application of nonlinear symmetry realization technique to gravity is studied. We identify the simplest extensions of the Poincare group suitable for nonlinear realization at the level of physical fields. Two simple models are proposed. The first one introduces additional scalar degrees of freedom [...] Read more.

Application of nonlinear symmetry realization technique to gravity is studied. We identify the simplest extensions of the Poincare group suitable for nonlinear realization at the level of physical fields. Two simple models are proposed. The first one introduces additional scalar degrees of freedom that may be suitable for driving inflation. The second one describes states with well-defined mass that lack a linear interaction with matter states. We argue that this phenomenon points out a necessity to draw a distinction between gravitational states with well-defined masses and states that participate in interaction with matter. Full article

(This article belongs to the Special Issue Spontaneous Breaking of Conformal Symmetry)

9 pages, 8658 KiB

Open AccessArticle

Curvature Invariants for Lorentzian Traversable Wormholes

by Brandon Mattingly, Abinash Kar, William Julius, Matthew Gorban, Cooper Watson, MD Ali, Andrew Baas, Caleb Elmore, Bahram Shakerin, Eric Davis and Gerald Cleaver

Universe 2020, 6(1), 11; https://doi.org/10.3390/universe6010011 - 9 Jan 2020

Cited by 12 | Viewed by 4822

Abstract

The curvature invariants of three Lorentzian wormholes are calculated and plotted in this paper. The plots may be inspected for discontinuities to analyze the traversability of a wormhole. This approach was formulated by Henry, Overduin, and Wilcomb for black holes (Henry et al., [...] Read more.

The curvature invariants of three Lorentzian wormholes are calculated and plotted in this paper. The plots may be inspected for discontinuities to analyze the traversability of a wormhole. This approach was formulated by Henry, Overduin, and Wilcomb for black holes (Henry et al., 2016). Curvature invariants are independent of coordinate basis, so the process is free of coordinate mapping distortions and the same regardless of your chosen coordinates (Christoffel, E.B., 1869; Stephani, et al., 2003). The four independent Carminati and McLenaghan (CM) invariants are calculated and the nonzero curvature invariant functions are plotted (Carminati et al., 1991; Santosuosso et al., 1998). Three traversable wormhole line elements analyzed include the (i) spherically symmetric Morris and Thorne, (ii) thin-shell Schwarzschild wormholes, and (iii) the exponential metric (Visser, M., 1995; Boonserm et al., 2018). Full article

(This article belongs to the Section Cosmology)

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13 pages, 1278 KiB

Open AccessArticle

Investigation of Infrasound Background Noise at Mátra Gravitational and Geophysical Laboratory (MGGL)

by Edit Fenyvesi, József Molnár and Sándor Czellár

Universe 2020, 6(1), 10; https://doi.org/10.3390/universe6010010 - 3 Jan 2020

Cited by 2 | Viewed by 2785

Abstract

Infrasound and seismic waves are supposed to be the main contributors to the gravity-gradient noise (Newtonian noise) of the third-generation subterranean gravitational wave detectors. This noise will limit the sensitivity of the instrument at frequencies below 20 Hz. Investigation of its origin and [...] Read more.

Infrasound and seismic waves are supposed to be the main contributors to the gravity-gradient noise (Newtonian noise) of the third-generation subterranean gravitational wave detectors. This noise will limit the sensitivity of the instrument at frequencies below 20 Hz. Investigation of its origin and the possible methods of mitigation have top priority during the designing period of the detectors. Therefore, long-term site characterizing measurements are needed at several subterranean sites. However, at some sites, mining activities can occur. These activities can cause sudden changes (transients) in the measured signal, and increase the continuous background noise, too. We have developed an algorithm based on discrete Haar transform to find these transients in the infrasound signal. We found that eliminating the transients decreases the variation of the noise spectra, and therefore results a more accurate characterization of the continuous background noise. We carried out experiments for controlling the continuous noise. Machines operating at the mine were turned on and off systematically in order to see their effect on the noise spectra. These experiments showed that the main contributor of the continuous noise is the ventilation system of the mine. We also estimated the contribution of infrasound Newtonian noise at MGGL to the strain noise of a subterranean GW detector similar to Einstein Telescope. Full article

(This article belongs to the Special Issue Black Hole Physics and Astrophysics)

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46 pages, 10736 KiB

Open AccessFeature PaperEditor’s ChoiceReview

The Legacy of Einstein’s Eclipse, Gravitational Lensing

by Jorge L. Cervantes-Cota, Salvador Galindo-Uribarri and George F. Smoot

Universe 2020, 6(1), 9; https://doi.org/10.3390/universe6010009 - 31 Dec 2019

Cited by 12 | Viewed by 11846

Abstract

A hundred years ago, two British expeditions measured the deflection of starlight by the Sun’s gravitational field, confirming the prediction made by Einstein’s General Theory of Relativity. One hundred years later many physicists around the world are involved in studying the consequences and [...] Read more.

A hundred years ago, two British expeditions measured the deflection of starlight by the Sun’s gravitational field, confirming the prediction made by Einstein’s General Theory of Relativity. One hundred years later many physicists around the world are involved in studying the consequences and use as a research tool, of the deflection of light by gravitational fields, a discipline that today receives the generic name of Gravitational Lensing. The present review aims to commemorate the centenary of Einstein’s Eclipse expeditions by presenting a historical perspective of the development and milestones on gravitational light bending, covering from early XIX century speculations, to its current use as an important research tool in astronomy and cosmology. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2019 - Gravitational Physics)

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12 pages, 299 KiB

Open AccessArticle

Regular Black Holes with Asymptotically Minkowski Cores

by Alex Simpson and Matt Visser

Universe 2020, 6(1), 8; https://doi.org/10.3390/universe6010008 - 31 Dec 2019

Cited by 106 | Viewed by 3235

Abstract

Standard models of “regular black holes” typically have asymptotically de Sitter regions at their cores. Herein, we shall consider novel “hollow” regular black holes, those with asymptotically Minkowski cores. The reason for doing so is twofold: First, these models greatly simplify the physics [...] Read more.

Standard models of “regular black holes” typically have asymptotically de Sitter regions at their cores. Herein, we shall consider novel “hollow” regular black holes, those with asymptotically Minkowski cores. The reason for doing so is twofold: First, these models greatly simplify the physics in the deep core, and second, one can trade off rather messy cubic and quartic polynomial equations for somewhat more elegant special functions such as exponentials and the increasingly important Lambert W function. While these “hollow” regular black holes share many features with the Bardeen/Hayward/Frolov regular black holes, there are also significant differences. Full article

(This article belongs to the Special Issue Singularities in Spacetime)

26 pages, 681 KiB

Open AccessArticle

Quantum Mixmaster as a Model of the Primordial Universe

by Hervé Bergeron, Ewa Czuchry, Jean Pierre Gazeau and Przemysław Małkiewicz

Universe 2020, 6(1), 7; https://doi.org/10.3390/universe6010007 - 31 Dec 2019

Cited by 18 | Viewed by 2766

Abstract

The Mixmaster solution to Einstein field equations was examined by C. Misner in an effort to better understand the dynamics of the early universe. We highlight the importance of the quantum version of this model for the early universe. This quantum version and [...] Read more.

The Mixmaster solution to Einstein field equations was examined by C. Misner in an effort to better understand the dynamics of the early universe. We highlight the importance of the quantum version of this model for the early universe. This quantum version and its semi-classical portraits are yielded through affine and standard coherent state quantizations and more generally affine and Weyl–Heisenberg covariant integral quantizations. The adiabatic and vibronic approximations widely used in molecular physics can be employed to qualitatively study the dynamics of the model on both quantum and semi-classical levels. Moreover, the semi-classical approach with the exact anisotropy potential can be effective in the numerical integration of some solutions. Some promising physical features such as the singularity resolution, smooth bouncing, the excitation of anisotropic oscillations and a substantial amount of post-bounce inflation as the backreaction to the latter are pointed out. Finally, a realistic cosmological scenario based on the quantum mixmaster model, which includes the formation and evolution of local structures is outlined. Full article

(This article belongs to the Special Issue Quantum Models for Cosmology)

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35 pages, 856 KiB

Open AccessArticle

Baryon Physics and Tight Coupling Approximation in Boltzmann Codes

by Masroor C. Pookkillath, Antonio De Felice and Shinji Mukohyama

Universe 2020, 6(1), 6; https://doi.org/10.3390/universe6010006 - 31 Dec 2019

Cited by 15 | Viewed by 2838

Abstract

We provide two derivations of the baryonic equations that can be straightforwardly implemented in existing Einstein–Boltzmann solvers. One of the derivations begins with an action principle, while the other exploits the conservation of the stress-energy tensor. While our result is manifestly covariant and [...] Read more.

We provide two derivations of the baryonic equations that can be straightforwardly implemented in existing Einstein–Boltzmann solvers. One of the derivations begins with an action principle, while the other exploits the conservation of the stress-energy tensor. While our result is manifestly covariant and satisfies the Bianchi identities, we point out that this is not the case for the implementation of the seminal work by Ma and Bertschinger and in the existing Boltzmann codes. We also study the tight coupling approximation up to the second order without choosing any gauge using the covariant full baryon equations. We implement the improved baryon equations in a Boltzmann code and investigate the change in the estimate of cosmological parameters by performing an MCMC analysis. With the covariantly correct baryon equations of motion, we find

1 %

deviation for the best fit values of the cosmological parameters that should be taken into account. While in this paper, we study the

Λ

CDM model only, our baryon equations can be easily implemented in other models and various modified gravity theories. Full article

(This article belongs to the Section Cosmology)

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8 pages, 364 KiB

Open AccessArticle

β⁻-Decay Half-Lives of Even-Even Nuclei Using the Recently Introduced Phase Space Recipe

by Jameel-Un Nabi, Mavra Ishfaq, Ovidiu Niţescu, Mihail Mirea and Sabin Stoica

Universe 2020, 6(1), 5; https://doi.org/10.3390/universe6010005 - 26 Dec 2019

Cited by 2 | Viewed by 2947

Abstract

In this paper, we present the

β

-decay half-lives calculation for selected even-even nuclei that decay through electron emission. The kinematical portion of the half-life calculation was performed using a recently introduced technique for computation of phase space factors (PSFs). The dynamical portion [...] Read more.

In this paper, we present the

β

-decay half-lives calculation for selected even-even nuclei that decay through electron emission. The kinematical portion of the half-life calculation was performed using a recently introduced technique for computation of phase space factors (PSFs). The dynamical portion of our calculation was performed within the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. Six nuclei (

^{20}

O,

^{24}

Ne,

^{34}

Si,

^{54}

Ti,

^{62}

Fe and

^{98}

Zr) were selected for the present calculation. We compare the calculated PSFs for these cases against the traditionally used recipe. In our new approach, the Dirac equation was numerically solved by employing a Coulomb potential. This potential was adopted from a more realistic proton distribution of the daughter nucleus. Thus, the finite size of the nucleus and the diffuse nuclear surface corrections are taken into account. Moreover, a screened Coulomb potential was constructed to account for the effect of atomic screening. The power series technique was used for the numerical solution. The calculated values of half-lives, employing the recently developed method for computation of PSFs, were in good agreement with the experimental data. Full article

(This article belongs to the Special Issue Relativistic Astrophysics)

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7 pages, 560 KiB

Open AccessFeature PaperArticle

Ultraperipheral vs. Ordinary Nuclear Interactions

by I. M. Dremin

Universe 2020, 6(1), 4; https://doi.org/10.3390/universe6010004 - 25 Dec 2019

Cited by 4 | Viewed by 1995

Abstract

It is argued that the cross sections of ultraperipheral interactions of heavy nuclei can become comparable in value to those of their ordinary hadronic interactions at high energies. Simple estimates of corresponding “preasymptotic energy thresholds” are provided. The method of equivalent photons is [...] Read more.

It is argued that the cross sections of ultraperipheral interactions of heavy nuclei can become comparable in value to those of their ordinary hadronic interactions at high energies. Simple estimates of corresponding “preasymptotic energy thresholds” are provided. The method of equivalent photons is compared with the perturbative approach. The situation at NICA/FAIR energies is discussed. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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21 pages, 4264 KiB

Open AccessArticle

Decline of the Current Quadrupole Moment during the Merger Phase of Binary Black Hole Coalescence

by Fan Zhang

Universe 2020, 6(1), 3; https://doi.org/10.3390/universe6010003 - 25 Dec 2019

Cited by 2 | Viewed by 3215

Abstract

Utilizing the tools of tendex and vortex, we study the highly dynamic plunge and merger phases of several

π

-symmetric binary black hole coalescences. In particular, we observe a decline of the strength of the current quadrupole moment compared to that of the [...] Read more.

Utilizing the tools of tendex and vortex, we study the highly dynamic plunge and merger phases of several

π

-symmetric binary black hole coalescences. In particular, we observe a decline of the strength of the current quadrupole moment compared to that of the mass quadrupole moment during the merger phase, contrary to a naive estimate according to the dependence of these moments on the separation between the black holes. We further show that this decline of the current quadrupole moment is achieved through the remnants of the two individual spins becoming nearly aligned or anti-aligned with the total angular momentum. We also speculate on the ability to achieve a consistency between the electric and magnetic parity quasinormal modes. Full article

(This article belongs to the Special Issue Black Hole Physics and Astrophysics)

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18 pages, 1043 KiB

Open AccessArticle

Theoretical Study on Spin-Selective Coherent Electron Transfer in a Quantum Dot Array

by Shumpei Masuda, Kuan Yen Tan and Mikio Nakahara

Universe 2020, 6(1), 2; https://doi.org/10.3390/universe6010002 - 22 Dec 2019

Cited by 3 | Viewed by 2966

Abstract

Recently, we proposed the spin-selective coherent electron transfer in a silicon-quantum-dot array. It requires temporal tuning of two pulses of an oscillating magnetic field and gate voltage control. This paper proposes a simpler method that requires a single pulse of oscillating magnetic field [...] Read more.

Recently, we proposed the spin-selective coherent electron transfer in a silicon-quantum-dot array. It requires temporal tuning of two pulses of an oscillating magnetic field and gate voltage control. This paper proposes a simpler method that requires a single pulse of oscillating magnetic field and gate voltage control. We examined the robustness of the control against the error in the pulse amplitude and the effect of the excited states relaxation to the control efficiency. In addition, we propose a novel control method based on a shortcuts-to-adiabaticity protocol, which utilizes two pulses but requires temporal control of the pulse amplitude for only one of them. We compared their efficiencies under the effect of realistic pulse amplitude errors and relaxation. Full article

(This article belongs to the Special Issue Quantum Dynamics and Applications)

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44 pages, 774 KiB

Open AccessArticle

A Modified Dynamical Model of Cosmology I Theory

by Hossein Shenavar and Kurosh Javidan

Universe 2020, 6(1), 1; https://doi.org/10.3390/universe6010001 - 19 Dec 2019

Cited by 5 | Viewed by 3865

Abstract

Wheeler (1964) had formulated Mach’s principle as the boundary condition for general relativistic field equations. Here, we use this idea and develop a modified dynamical model of cosmology based on imposing Neumann boundary condition on cosmological perturbation equations. Then, it is shown that [...] Read more.

Wheeler (1964) had formulated Mach’s principle as the boundary condition for general relativistic field equations. Here, we use this idea and develop a modified dynamical model of cosmology based on imposing Neumann boundary condition on cosmological perturbation equations. Then, it is shown that a new term appears in the equation of motion, which leads to a modified Poisson equation. In addition, a modified Hubble parameter is derived due to the presence of the new term. Moreover, it is proved that, without a cosmological constant, such a model has a late time-accelerated expansion with an equation of state converging to

w < - 1

. Also, the luminosity distance in the present model is shown to differ from that of the

Λ C D M

model at high redshifts. Furthermore, it is found that the adiabatic sound speed squared is positive in radiation-dominated era and then converges to zero at later times. Theoretical implications of the Neumann boundary condition have been discussed, and it is shown that, by fixing the value of the conjugate momentum (under certain conditions), one could derive a similar version of modified dynamics. In a future work, we will confine the free parameters of the Neumann model based on hype Ia Supernovae, Hubble parameter data, and the age of the oldest stars. Full article

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