Universe

Research

18 pages, 3794 KiB

Open AccessArticle

Tidal Forces in Majumdar-Papapetrou Spacetimes

by Eduardo Albacete and Maurício Richartz

Universe 2024, 10(2), 62; https://doi.org/10.3390/universe10020062 - 31 Jan 2024

Viewed by 1015

Tidal disruption events occur when astrophysical objects are destroyed by black holes due to strong tidal force effects. Tidal forces have been studied in a variety of black hole spacetimes, including Reissner-Nordström and Kerr spacetimes. Despite the vast literature on the subject, tidal [...] Read more.

Tidal disruption events occur when astrophysical objects are destroyed by black holes due to strong tidal force effects. Tidal forces have been studied in a variety of black hole spacetimes, including Reissner-Nordström and Kerr spacetimes. Despite the vast literature on the subject, tidal forces around black holes in static equilibrium have never been investigated before. The aim of this work is to fill in this gap and explore tidal forces in the Majumdar-Papapetrou spacetime describing two extremely charged binary black holes in equilibrium. We focus on tidal forces associated with radial and circular geodesics of massive neutral particles moving on the plane equidistant to the black holes. In particular, we study the behavior of the tidal forces as a function of the distance from the black holes and as a function of the energy of the geodesics. We also investigate the numerical solutions of the geodesic deviation equation for different initial conditions. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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21 pages, 353 KiB

Open AccessArticle

Efficient Computation of Null Affine Parameters

by Matt Visser

Universe 2023, 9(12), 521; https://doi.org/10.3390/universe9120521 - 18 Dec 2023

Cited by 4 | Viewed by 1347

Abstract

Finding affine parameters for null geodesics is often of considerable physical importance, especially when studying null geodesics or dealing with conservation laws and/or averaged energy conditions. But explicitly finding null affine parameters is also often quite tedious and can sometimes even be somewhat [...] Read more.

Finding affine parameters for null geodesics is often of considerable physical importance, especially when studying null geodesics or dealing with conservation laws and/or averaged energy conditions. But explicitly finding null affine parameters is also often quite tedious and can sometimes even be somewhat tricky. Herein we shall demonstrate that the existence of a conformally related spacetime containing a conformal Killing vector, timelike in the domain of outer communication, is quite sufficient to define a preferred set of spatial three-slices—on which a well-defined “affine” three-metric can be introduced to capture the notion of affine null parameter—before explicitly finding the null geodesics. The construction depends on the properties of conformal transformations and on the conserved quantity associated with the conformal Killing vector. Having the affine null parameter in hand before attempting to find the actual null geodesics often quite radically simplifies other parts of the analysis. We emphasize that the successful identification of affine null parameters is a general-purpose tool of wide applicability in both general relativistic and astrophysical settings. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

15 pages, 318 KiB

Open AccessArticle

Spherically Symmetric Configurations in Unimodular Gravity

by Júlio César Fabris, Mahamadou Hamani Daouda and Hermano Velten

Universe 2023, 9(12), 515; https://doi.org/10.3390/universe9120515 - 14 Dec 2023

Viewed by 979

Abstract

Unimodular gravity (UG) is often deemed comparable to General Relativity (GR) in many respects, despite the theory exhibiting invariance under a more limited set of diffeomorphic transformations. The discussion we propose in this work relies on the criteria for establishing the equivalence between [...] Read more.

Unimodular gravity (UG) is often deemed comparable to General Relativity (GR) in many respects, despite the theory exhibiting invariance under a more limited set of diffeomorphic transformations. The discussion we propose in this work relies on the criteria for establishing the equivalence between these two formulations, specifically exploring UG’s application to static and spherically symmetric configurations with the energy-momentum tensor originating from either a scalar field or an electromagnetic field. We find that the equivalence between UG and GR might be disrupted when scrutinizing the stability of solutions at a perturbative level. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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32 pages, 425 KiB

Open AccessArticle

Schrödinger Symmetry in Gravitational Mini-Superspaces

by Jibril Ben Achour, Etera R. Livine, Daniele Oriti and Goffredo Piani

Universe 2023, 9(12), 503; https://doi.org/10.3390/universe9120503 - 30 Nov 2023

Cited by 8 | Viewed by 1144

Abstract

We prove that the simplest gravitational symmetry-reduced models describing cosmology and black hole mechanics are invariant under the Schrödinger group. We consider the flat FRW cosmology filled with a massless scalar field and the Schwarzschild black hole mechanics and construct their conserved charges [...] Read more.

We prove that the simplest gravitational symmetry-reduced models describing cosmology and black hole mechanics are invariant under the Schrödinger group. We consider the flat FRW cosmology filled with a massless scalar field and the Schwarzschild black hole mechanics and construct their conserved charges using the Eisenhart–Duval (ED) lift method in order to show that they form a Schrödinger algebra. Our method illustrates how the ED lift and the more standard approach analyzing the geometry of the field space are complementary in revealing different sets of symmetries of these systems. We further identify an infinite-dimensional symmetry for those two models, generated by conserved charges organized in two copies of a

Witt

algebra. These extended charge algebras provide a new algebraic characterization of these homogeneous gravitational sectors. They guide the path to their quantization and open the road to non-linear extensions of quantum cosmology and quantum black hole models in terms of hydrodynamic equations in field space. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

19 pages, 381 KiB

Open AccessArticle

Cosmic Time and the Initial State of the Universe

by Chopin Soo

Universe 2023, 9(12), 489; https://doi.org/10.3390/universe9120489 (registering DOI) - 23 Nov 2023

Cited by 1 | Viewed by 1148

Abstract

The exact solution of the Hamiltonian constraint in canonical gravity and the resultant reduction of Einstein’s theory reveal the synergy between gravitation and the intrinsic cosmic clock of our expanding universe. Intrinsic Time Geometrodynamics advocates a paradigm shift from four covariance to just [...] Read more.

The exact solution of the Hamiltonian constraint in canonical gravity and the resultant reduction of Einstein’s theory reveal the synergy between gravitation and the intrinsic cosmic clock of our expanding universe. Intrinsic Time Geometrodynamics advocates a paradigm shift from four covariance to just spatial diffeomorphism invariance. Consequently, causal time-ordering and quantum Schrödinger–Heisenberg evolution in cosmic time become meaningful. The natural addition of a Cotton–York term to the physical Hamiltonian changes the initial data problem radically. In the classical context, this is studied with the Lichnerowicz–York equation; quantum mechanically, it lends weight to the origin of the universe as an exact Chern–Simons Hartle–Hawking state, which features Euclidean–Lorentzian instanton tunneling. At the level of expectation values, this quantum state yields a low-entropy hot smooth Robertson–Walker beginning in accord with Penrose’s Weyl Curvature Hypothesis. The Chern–Simons Hartle–Hawking state also manifests transverse traceless quantum metric fluctuations, with, at the lowest approximation, scale-invariant two-point correlations as one of its defining characteristics. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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26 pages, 515 KiB

Open AccessArticle

GEMS Embeddings of Hayward Regular Black Holes in Massless and Massive Gravities

by Soon-Tae Hong, Yong-Wan Kim and Young-Jai Park

Universe 2023, 9(11), 486; https://doi.org/10.3390/universe9110486 - 20 Nov 2023

Viewed by 1311

Abstract

After finding a solution for the Hayward regular black hole (HRBH) in massive gravity, we embed the (3+1)-dimensional HRBHs both in massless and in massive gravities into (5+2)- and (6+3)-dimensional Minkowski spacetimes, respectively. Here, massive gravity denotes that a graviton acquires a mass [...] Read more.

After finding a solution for the Hayward regular black hole (HRBH) in massive gravity, we embed the (3+1)-dimensional HRBHs both in massless and in massive gravities into (5+2)- and (6+3)-dimensional Minkowski spacetimes, respectively. Here, massive gravity denotes that a graviton acquires a mass holographically by broken momentum conservation in the HRBH. The original HRBH has no holographically added gravitons, which we call ‘massless’. Making use of newly found embedding coordinates, we obtain desired Unruh temperatures and compare them with the Hawking and local fiducial temperatures, showing that the Unruh effect for a uniformly accelerated observer in a higher-dimensional flat spacetime is equal to the Hawking effect for a fiducial observer in a black hole spacetime. We also obtain freely falling temperatures of the HRBHs in massless and massive gravities seen by freely falling observers, which remain finite even at the event horizons while becoming the Hawking temperatures in asymptotic infinity. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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26 pages, 649 KiB

Open AccessArticle

Hořava–Lifshitz

F (\bar{R})

Theories and the Swampland

by Hugo García-Compeán, Daniel Mata-Pacheco and Luis Zapata

Universe 2023, 9(11), 460; https://doi.org/10.3390/universe9110460 - 26 Oct 2023

Cited by 1 | Viewed by 1191

Abstract

The compatibility between the de Sitter Swampland conjecture and Hořava–Lifshitz

F (\bar{R})

theories with a flat FLRW metric is studied. We first study the standard

f (R)

theories and show that the only way in which the dS [...] Read more.

The compatibility between the de Sitter Swampland conjecture and Hořava–Lifshitz

F (\bar{R})

theories with a flat FLRW metric is studied. We first study the standard

f (R)

theories and show that the only way in which the dS conjecture can be made independent of R is by considering a power law of the form

f (R) \sim R^{γ}

. The conjecture and the consistency of the theory puts restrictions on

γ

to be greater but close to one. For

F (\bar{R})

theories described by its two parameters

λ

and

μ

, we use the equations of motion to construct the function starting with an ansatz for the scale factor in the Jordan frame of the power law form. By performing a conformal transformation on the three metric to the Einstein frame, we can obtain an action of gravity plus a scalar field by relating the parameters of the theory. The non-projectable and projectable cases are studied and the differences are outlined. The obtained

F (\bar{R})

function consists of terms of the form

{\bar{R}}^{γ}

with the possibility of having negative power terms. The dS conjecture leads to inequalities for the

λ

parameter; in both versions, it becomes restricted to be greater but close to

1 / 3

. We can also study the general case in which

μ

and

λ

are considered as independent. The obtained F function has the same form as before. The consistency of the theory and the dS conjecture lead to a set of inequalities on both parameters that are studied numerically. In all cases,

λ

is restricted by

μ

around

1 / 3

, and we obtain

λ \to 1 / 3

if

μ \to 0

. We consider the

f (R)

limit

μ, λ \to 1

and we obtain consistent results. Finally, we study the case of a constant Hubble parameter. The dS conjecture can be fulfilled by restricting the parameters of the theory; however, the constraint makes this compatibility exclusive to these kinds of theories. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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13 pages, 316 KiB

Open AccessArticle

Analogue Gravitational Lensing in Bose-Einstein Condensates

by Decheng Ma, Chenglong Jia, Enrique Solano and Lucas Chibebe Céleri

Universe 2023, 9(10), 443; https://doi.org/10.3390/universe9100443 - 1 Oct 2023

Viewed by 1220

Abstract

We consider the propagation of phonons in the presence of a particle sink with radial flow in a Bose–Einstein condensate. Because the particle sink can be used to simulate a static acoustic black hole, the phonon would experience a considerable spacetime curvature at [...] Read more.

We consider the propagation of phonons in the presence of a particle sink with radial flow in a Bose–Einstein condensate. Because the particle sink can be used to simulate a static acoustic black hole, the phonon would experience a considerable spacetime curvature at appreciable distance from the sink. The trajectory of the phonons is bended after passing by the particle sink, which can be used as a simulation of the gravitational lensing effect in a Bose–Einstein condensate. Possible experimental implementations are discussed. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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20 pages, 333 KiB

Open AccessArticle

An Exact Model of a Gravitational Wave in the Bianchi III Universe Based on Shapovalov II Wave Spacetime and the Quadratic Theory of Gravity

by Konstantin Osetrin, Ilya Kirnos and Evgeny Osetrin

Universe 2023, 9(8), 356; https://doi.org/10.3390/universe9080356 - 30 Jul 2023

Cited by 1 | Viewed by 761

Abstract

Exact models of primordial gravitational waves in the Bianchi type-III universe were constructed on the basis of the quadratic theory of gravity with a scalar field and pure radiation in Shapovalov wave spacetimes of type II (subtype 2). Exact solutions of the field [...] Read more.

Exact models of primordial gravitational waves in the Bianchi type-III universe were constructed on the basis of the quadratic theory of gravity with a scalar field and pure radiation in Shapovalov wave spacetimes of type II (subtype 2). Exact solutions of the field equations and scalar equation were obtained. The characteristics of pure radiation were determined. An explicit form of the scalar field functions included in the Lagrangian of the considered quadratic theory of gravity was found. The trajectories of the propagation of light rays in the considered gravitational wave models were obtained. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

10 pages, 359 KiB

Open AccessArticle

First Experimental Survey of a Whole Class of Non-Commutative Quantum Gravity Models in the VIP-2 Lead Underground Experiment

by Kristian Piscicchia, Antonino Marcianò, Andrea Addazi, Diana Laura Sirghi, Massimiliano Bazzi, Nicola Bortolotti, Mario Bragadireanu, Michael Cargnelli, Alberto Clozza, Luca De Paolis, Raffaele Del Grande, Carlo Guaraldo, Mihail Iliescu, Matthias Laubenstein, Simone Manti, Johann Marton, Marco Miliucci, Fabrizio Napolitano, Federico Nola, Alessio Porcelli, Alessandro Scordo, Francesco Sgaramella, Florin Sirghi, Oton Vazquez Doce, Johann Zmeskal and Catalina Curceanu Show full author list Hide full author list

Universe 2023, 9(7), 321; https://doi.org/10.3390/universe9070321 - 4 Jul 2023

Cited by 1 | Viewed by 936

Abstract

This study is aimed to set severe constraints on a whole class of non-commutative space-times scenarios as a class of universality for several quantum gravity models. To this end, slight violations of the Pauli exclusion principle—predicted by these models—are investigated by searching for [...] Read more.

This study is aimed to set severe constraints on a whole class of non-commutative space-times scenarios as a class of universality for several quantum gravity models. To this end, slight violations of the Pauli exclusion principle—predicted by these models—are investigated by searching for Pauli forbidden K

_{α}

and K

_{β}

transitions in lead. The selection of a high atomic number target material allows to test the energy scale of the space-time non-commutativity emergence at high atomic transition energies. As a consequence, the measurement is very sensitive to high orders in the power series expansion of the Pauli violation probability, which allows to set the first constraint to the “triply special relativity” model proposed by Kowalski-Glikman and Smolin. The characteristic energy scale of the model is bound to

Λ > 5.6 \cdot 10^{- 9}

Planck scales. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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15 pages, 399 KiB

Open AccessArticle

Matching Slowly Rotating Spacetimes Split by Dynamic Thin Shells

by Jonas P. Pereira and Jorge A. Rueda

Universe 2023, 9(7), 305; https://doi.org/10.3390/universe9070305 - 24 Jun 2023

Cited by 1 | Viewed by 762

Abstract

We investigated within the Darmois–Israel thin-shell formalism the match of neutral and asymptotically flat, slowly rotating spacetimes (up to second order in the rotation parameter) when their boundaries are dynamic. It has several important applications in general relativistic systems, such as black holes [...] Read more.

We investigated within the Darmois–Israel thin-shell formalism the match of neutral and asymptotically flat, slowly rotating spacetimes (up to second order in the rotation parameter) when their boundaries are dynamic. It has several important applications in general relativistic systems, such as black holes and neutron stars, which we exemplify. We mostly focused on the stability aspects of slowly rotating thin shells in equilibrium and the surface degrees of freedom on the hypersurfaces splitting the matched slowly rotating spacetimes, e.g., surface energy density and surface tension. We show that the stability upon perturbations in the spherically symmetric case automatically implies stability in the slow rotation case. In addition, we show that, when matching slowly rotating Kerr spacetimes through thin shells in equilibrium, the surface degrees of freedom can decrease compared to their Schwarzschild counterparts, meaning that the energy conditions could be weakened. The frame-dragging aspects of the match of slowly rotating spacetimes are also briefly discussed. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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11 pages, 316 KiB

Open AccessArticle

Limitations in Testing the Lense–Thirring Effect with LAGEOS and the Newly Launched Geodetic Satellite LARES 2

by Lorenzo Iorio

Universe 2023, 9(5), 211; https://doi.org/10.3390/universe9050211 - 28 Apr 2023

Cited by 1 | Viewed by 1755

Abstract

The new geodetic satellite LARES 2, cousin of LAGEOS and sharing with it almost the same orbital parameters apart from the inclination, displaced by 180 deg, was launched last year. Its proponents suggest using the sum of the nodes of LAGEOS and of [...] Read more.

The new geodetic satellite LARES 2, cousin of LAGEOS and sharing with it almost the same orbital parameters apart from the inclination, displaced by 180 deg, was launched last year. Its proponents suggest using the sum of the nodes of LAGEOS and of LARES 2 to measure the sum of the Lense–Thirring node precessions independently of the systematic bias caused by the even zonal harmonics of the geopotential, claiming a final ≃0.2 percent total accuracy. In fact, the actual orbital configurations of the two satellites do not allow one to attain the sought for mutual cancellation of their classical node precessions due to the Earth’s quadrupole mass moment, as their sum is still ≃5000 times larger than the added general relativistic rates. This has important consequences. One is that the current uncertainties in the eccentricities and the inclinations of both satellites do not presently allow the stated accuracy goal to be met, needing improvements of 3–4 orders of magnitude. Furthermore, the imperfect knowledge of the Earth’s angular momentum S impacts the uncancelled sum of the node precessions, from 150 to 4900 percent of the relativistic signal depending on the uncertainty assumed in S. It is finally remarked that the real breakthrough in reliably testing the gravitomagnetic field of the Earth would consist in modeling it and simultaneously estimating one or more dedicated parameter(s) along with other ones characterising the geopotential, as is customarily performed for any other dynamical feature. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Gravitation)

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