Universe

39 pages, 515 KiB

Open AccessArticle

Simultaneity and Precise Time in Rotation

by Don Koks

Universe 2019, 5(12), 226; https://doi.org/10.3390/universe5120226 - 16 Dec 2019

Cited by 4 | Viewed by 4053

I analyse the role of simultaneity in relativistic rotation by building incrementally on its role in simpler scenarios. Historically, rotation has been analysed in

1 + 1

dimensions; but my stance is that a

2 + 1

-dimensional treatment is necessary. This treatment [...] Read more.

I analyse the role of simultaneity in relativistic rotation by building incrementally on its role in simpler scenarios. Historically, rotation has been analysed in

1 + 1

dimensions; but my stance is that a

2 + 1

-dimensional treatment is necessary. This treatment requires a discussion of what constitutes a frame, how coordinate choices differ from frame choices, and how poor coordinates can be misleading. I determine how precisely we are able to define a meaningful time coordinate on a gravity-free rotating Earth, and discuss complications due to gravity on our real Earth. I end with a critique of several statements made in relativistic precision-timing literature, that I maintain contradict the tenets of relativity. Those statements tend to be made in the context of satellite-based navigation; but they are independent of that technology, and hence are not validated by its success. I suggest that if relativistic precision-timing adheres to such analyses, our civilian timing is likely to suffer in the near future as clocks become ever more precise. Full article

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

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9 pages, 615 KiB

Open AccessArticle

Non-Singular Model of Magnetized Black Hole Based on Nonlinear Electrodynamics

by Sergey I. Kruglov

Universe 2019, 5(12), 225; https://doi.org/10.3390/universe5120225 - 13 Dec 2019

Cited by 9 | Viewed by 2444

Abstract

A new modified Hayward metric of magnetically charged non-singular black hole spacetime in the framework of nonlinear electrodynamics is constructed. When the fundamental length introduced, characterising quantum gravity effects, vanishes, one comes to the general relativity coupled with the Bronnikov model of nonlinear [...] Read more.

A new modified Hayward metric of magnetically charged non-singular black hole spacetime in the framework of nonlinear electrodynamics is constructed. When the fundamental length introduced, characterising quantum gravity effects, vanishes, one comes to the general relativity coupled with the Bronnikov model of nonlinear electrodynamics. The metric can have one (an extreme) horizon, two horizons of black holes, or no horizons corresponding to the particle-like solution. Corrections to the Reissner–Nordström solution are found as the radius approaches infinity. As

r \to 0

the metric has a de Sitter core showing the absence of singularities, the asymptotic of the Ricci and Kretschmann scalars are obtained and they are finite everywhere. The thermodynamics of black holes, by calculating the Hawking temperature and the heat capacity, is studied. It is demonstrated that phase transitions take place when the Hawking temperature possesses the maximum. Black holes are thermodynamically stable at some range of parameters. Full article

(This article belongs to the Special Issue Probing New Physics with Black Holes)

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24 pages, 406 KiB

Open AccessReview

Some Mathematical Aspects of f(R)-Gravity with Torsion: Cauchy Problem and Junction Conditions

by Stefano Vignolo

Universe 2019, 5(12), 224; https://doi.org/10.3390/universe5120224 - 6 Dec 2019

Cited by 5 | Viewed by 2553

Abstract

We discuss the Cauchy problem and the junction conditions within the framework of

f (R)

-gravity with torsion. We derive sufficient conditions to ensure the well-posedness of the initial value problem, as well as general conditions to join together on a [...] Read more.

We discuss the Cauchy problem and the junction conditions within the framework of

f (R)

-gravity with torsion. We derive sufficient conditions to ensure the well-posedness of the initial value problem, as well as general conditions to join together on a given hypersurface two different solutions of the field equations. The stated results can be useful to distinguish viable from nonviable

f (R)

-models with torsion. Full article

(This article belongs to the Special Issue Torsion-Gravity and Spinors in Fundamental Theoretical Physics)

7 pages, 253 KiB

Open AccessCommunication

Free Scalar Fields in Finite Volume Are Holographic

by Csaba Balázs

Universe 2019, 5(12), 223; https://doi.org/10.3390/universe5120223 - 4 Dec 2019

Viewed by 2341

Abstract

This brief note presents a back-of-the-envelope calculation showing that the number of degrees of freedom of a free scalar field in expanding flat space equals the surface area of the Hubble volume in Planck units. The logic of the calculation is the following. [...] Read more.

This brief note presents a back-of-the-envelope calculation showing that the number of degrees of freedom of a free scalar field in expanding flat space equals the surface area of the Hubble volume in Planck units. The logic of the calculation is the following. The amount of energy in the Hubble volume scales with its linear size, consequently the volume can only contain a finite number of quantized field modes. Since the momentum of the lowest energy mode scales inversely with the linear size of the volume, the maximal number of such modes in the volume scales with its surface area. It is possible to show that when the number of field modes is saturated the modes are confined to the surface of the volume. Gravity only enters this calculation as a regulator, providing a finite volume that contains the field, the entire calculation is done in flat space. While this toy model is bound to be incomplete, it is potentially interesting because it reproduces the defining aspects of holography, and advocates a regularization of the quantum degrees of freedom based on Friedmann’s equation. Full article

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

13 pages, 541 KiB

Open AccessArticle

Monitoring Jovian Orbital Resonances of a Spacecraft: Classical and Relativistic Effects

by Luis Acedo

Universe 2019, 5(12), 222; https://doi.org/10.3390/universe5120222 - 3 Dec 2019

Cited by 1 | Viewed by 2873

Abstract

Orbital resonances continue to be one of the most difficult problems in celestial mechanics. They have been studied in connection with the so-called Kirkwood gaps in the asteroid belt for many years. On the other hand, resonant trans-Neptunian objects are also an active [...] Read more.

Orbital resonances continue to be one of the most difficult problems in celestial mechanics. They have been studied in connection with the so-called Kirkwood gaps in the asteroid belt for many years. On the other hand, resonant trans-Neptunian objects are also an active area of research in Solar System dynamics, as are the recently discovered resonances in extrasolar planetary systems. A careful monitoring of the trajectories of these objects is hindered by the small size of asteroids or the large distances of the trans-Neptunian bodies. In this paper, we propose a mission concept, called CHRONOS (after the greek god of time), in which a spacecraft could be sent to with the initial condition of resonance with Jupiter in order to study the future evolution of its trajectory. We show that radio monitoring of these trajectories could allow for a better understanding of the initial stages of the evolution of resonant trajectories and the associated relativistic effects. Full article

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

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Graphical abstract

15 pages, 420 KiB

Open AccessEditor’s ChoiceArticle

Towards a Fisher-Information Description of Complexity in de Sitter Universe

by Chong-Bin Chen and Fu-Wen Shu

Universe 2019, 5(12), 221; https://doi.org/10.3390/universe5120221 - 29 Nov 2019

Cited by 4 | Viewed by 2895

Abstract

Recent developments on holography and quantum information physics suggest that quantum information theory has come to play a fundamental role in understanding quantum gravity. Cosmology, on the other hand, plays a significant role in testing quantum gravity effects. How to apply this idea [...] Read more.

Recent developments on holography and quantum information physics suggest that quantum information theory has come to play a fundamental role in understanding quantum gravity. Cosmology, on the other hand, plays a significant role in testing quantum gravity effects. How to apply this idea to a realistic universe is still unknown. Here, we show that some concepts in quantum information theory have cosmological descriptions. Particularly, we show that the complexity of a tensor network can be regarded as a Fisher information measure (FIM) of a dS universe, followed by several observations: (i) the holographic entanglement entropy has a tensor-network description and admits a information-theoretical interpretation, (ii) on-shell action of dS spacetime has a same description of FIM, (iii) complexity/action(CA) duality holds for dS spacetime. Our result is also valid for

f (R)

gravity, whose FIM exhibits the same features of a recent proposed

L^{n}

norm complexity. Full article

(This article belongs to the Special Issue Inflation, Black Holes and Gravitational Waves)

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13 pages, 391 KiB

Open AccessEditor’s ChoiceArticle

EHT Constraint on the Ultralight Scalar Hair of the M87 Supermassive Black Hole

by Pedro V. P. Cunha, Carlos A. R. Herdeiro and Eugen Radu

Universe 2019, 5(12), 220; https://doi.org/10.3390/universe5120220 - 27 Nov 2019

Cited by 113 | Viewed by 3825

Abstract

Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such a process, however, is only efficient if resonant—when the Compton wavelength [...] Read more.

Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such a process, however, is only efficient if resonant—when the Compton wavelength of the field approximately matches the gravitational scale of the black hole. For a complex-valued field, the process can form a stationary, bosonic field black hole equilibrium state—a black hole with synchronised hair. For sufficiently massive black holes, such as the one at the centre of the M87 supergiant elliptic galaxy, the hairy black hole can be robust against its own superradiant instabilities, within a Hubble time. Studying the shadows of such scalar hairy black holes, we constrain the amount of hair which is compatible with the Event Horizon Telescope (EHT) observations of the M87 supermassive black hole, assuming the hair is a condensate of ultralight scalar particles of mass

μ \sim 10^{- 20}

eV, as to be dynamically viable. We show the EHT observations set a weak constraint, in the sense that typical hairy black holes that could develop their hair dynamically, are compatible with the observations, when taking into account the EHT error bars and the black hole mass/distance uncertainty. Full article

(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)

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Universe, Volume 5, Issue 12 (December 2019) – 7 articles

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