| 141. |
- Lueben, Marvin, et al.
(författare)
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Bimetric cosmology is compatible with local tests of gravity
- 2020
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 37:4
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Tidskriftsartikel (refereegranskat)abstract
- Recently, Kenna-Allison et al claimed that bimetric gravity cannot give rise to a viable cosmological expansion history while at the same time being compatible with local gravity tests. In this note we review that claim and combine various results from the literature to provide several simple counter examples. We conclude that the results of Kenna-Allison et al cannot hold in general.
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| 142. |
- McCormick, Stephen
(författare)
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On the charged Riemannian Penrose inequality with charged matter
- 2020
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Ingår i: Classical and quantum gravity. - : IOP PUBLISHING LTD. - 0264-9381 .- 1361-6382. ; 37:1
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Tidskriftsartikel (refereegranskat)abstract
- Throughout the literature on the charged Riemannian Penrose inequality, it is generally assumed that there is no charged matter present; that is, the electric field is divergence-free. The aim of this article is to clarify when the charged Riemannian Penrose inequality holds in the presence of charged matter, and when it does not. First we revisit Jang's proof of the charged Riemannian Penrose inequality to show that under suitable conditions on the charged matter, this argument still carries though. In particular, a charged Riemannian Penrose inequality is obtained from this argument when charged matter is present provided that the charge density does not change sign. Moreover, we show that such hypotheses on the sign of the charge are in fact required by constructing counterexamples to the charged Riemannian Penrose inequality when these conditions are violated. We conclude by noting that one of these counterexamples contradicts a version of the charged Penrose inequality existing in the literature, and explain how this existing result can be repaired.
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| 143. |
- Rosswog, Stephan, et al.
(författare)
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Detectability of compact binary merger macronovae
- 2017
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 34:10
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Tidskriftsartikel (refereegranskat)abstract
- We study the optical and near-infrared luminosities and detectability of radioactively powered electromagnetic transients ('macronovae') occuring in the aftermath of binary neutron star and neutron star black hole mergers. We explore the transients that result from the dynamic ejecta and those from different types of wind outflows. Based on full nuclear network simulations we calculate the resulting light curves in different wavelength bands. We scrutinize the robustness of the results by comparing (a) two different nuclear reaction networks and (b) two macronova models. We explore in particular how sensitive the results are to the production of alpha-decaying trans-lead nuclei. We compare two frequently used mass models: the finite-range Droplet model (FRDM) and the nuclear mass model of Duflo and Zuker (DZ31). We find that the abundance of alpha-decaying trans-lead nuclei has a significant impact on the observability of the resulting macronovae. For example, the DZ31 model yields considerably larger abundances resulting in larger heating rates and thermalization efficiencies and therefore predicts substantially brighter macronova transients. We find that the dynamic ejecta from NSNS models can reach peak K-band magnitudes in excess of -15 while those from NSBH cases can reach beyond -16. Similar values can be reached by some of our wind models. Several of our models (both wind and dynamic ejecta) yield properties that are similar to the transient that was observed in the aftermath of the short GRB 130603B. We further explore the expected macronova detection frequencies for current and future instruments such as VISTA, ZTF and LSST.
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| 144. |
- Rosswog, Stephan, et al.
(författare)
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SPHINCS_BSSN : a general relativistic smooth particle hydrodynamics code for dynamical spacetimes
- 2021
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 38:11
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Tidskriftsartikel (refereegranskat)abstract
- We present a new methodology for simulating self-gravitating general-relativistic fluids. In our approach the fluid is modelled by means of Lagrangian particles in the framework of a general-relativistic (GR) smoothed particle hydrodynamics (SPH) formulation, while the spacetime is evolved on a mesh according to the Baumgarte–Shapiro–Shibata–Nakamura (BSSN) formulation that is also frequently used in Eulerian GR-hydrodynamics. To the best of our knowledge this is the first Lagrangian fully general relativistic hydrodynamics code (all previous SPH approaches used approximations to GR-gravity). A core ingredient of our particle–mesh approach is the coupling between the gas (represented by particles) and the spacetime (represented by a mesh) for which we have developed a set of sophisticated interpolation tools that are inspired by other particle–mesh approaches, in particular by vortex-particle methods. One advantage of splitting the methodology between matter and spacetime is that it gives us more freedom in choosing the resolution, so that—if the spacetime is smooth enough—we obtain good results already with a moderate number of grid cells and can focus the computational effort on the simulation of the matter. Further advantages of our approach are the ease with which ejecta can be tracked and the fact that the neutron star surface remains well-behaved and does not need any particular treatment. In the hydrodynamics part of the code we use a number of techniques that are new to SPH, such as reconstruction, slope limiting and steering dissipation by monitoring entropy conservation. We describe here in detail the employed numerical methods and demonstrate the code performance in a number of benchmark problems ranging from shock tube tests, over Cowling approximations to the fully dynamical evolution of neutron stars in self-consistently evolved spacetimes.
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| 145. |
- Sedda, Manuel Arca, et al.
(författare)
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The missing link in gravitational-wave astronomy : discoveries waiting in the decihertz range
- 2020
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 37:21
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Tidskriftsartikel (refereegranskat)abstract
- The gravitational-wave astronomical revolution began in 2015 with LIGO's observation of the coalescence of two stellar-mass black holes. Over the coming decades, ground-based detectors like laser interferometer gravitational-wave observatory (LIGO), Virgo and KAGRA will extend their reach, discovering thousands of stellar-mass binaries. In the 2030s, the space-basedlaser interferometer space antenna(LISA) will enable gravitational-wave observations of the massive black holes in galactic centres. Between ground-based observatories and LISA lies the unexplored dHz gravitational-wave frequency band. Here, we show the potential of adecihertz observatory(DO) which could cover this band, and complement discoveries made by other gravitational-wave observatories. The dHz range is uniquely suited to observation of intermediate-mass (similar to 10(2)-10(4)M(circle dot)) black holes, which may form the missing link between stellar-mass and massive black holes, offering an opportunity to measure their properties. DOs will be able to detect stellar-mass binaries days to years before they merge and are observed by ground-based detectors, providing early warning of nearby binary neutron star mergers, and enabling measurements of the eccentricity of binary black holes, providing revealing insights into their formation. Observing dHz gravitational-waves also opens the possibility of testing fundamental physics in a new laboratory, permitting unique tests of general relativity (GR) and the standard model of particle physics. Overall, a DO would answer outstanding questions about how black holes form and evolve across cosmic time, open new avenues for multimessenger astronomy, and advance our understanding of gravitation, particle physics and cosmology.
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| 146. |
- Semrén, Philip, et al.
(författare)
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Perturbations of a class of locally rotationally symmetric cosmologies with applications to dissipative fluids
- 2022
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 39:23
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Tidskriftsartikel (refereegranskat)abstract
- A gauge invariant perturbation theory, based on the 1 + 1 + 2 covariant split of spacetime, is used to study first order perturbations on a class of anisotropic cosmological backgrounds. The perturbations as well as the energy-momentum tensor are kept general, giving a system of equations on which different physical situations may be imposed. Through a harmonic decomposition, the system is then transformed to evolution equations in time and algebraic constraints. This result is then applied to dissipative one-component fluids, and on using the simplified acausal Eckart theory the system is reduced to two closed subsystems, governed by four and eight harmonic coefficients for the odd and even sectors respectively. The system is also seen to close in a simplified causal theory. It is then demonstrated, within the Eckart theory, how vorticity can be generated from viscosity.
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| 147. |
- Skagerstam, Bo Sture K., et al.
(författare)
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Quantum field theory with classical sources - Linearized quantum gravity
- 2019
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Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 36:1
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Tidskriftsartikel (refereegranskat)abstract
- In a previous work (Skagerstam 2018 arXiv:1801.09947v1 [quant-ph]) and in terms of an exact quantum-mechanical framework, h-independent causal and retarded expectation values of the second-quantized electro-magnetic fields in the Coulomb gauge were derived in the presence of a conserved classical electric current. The classical h-independent Maxwells equations then naturally emerged. In the present work, we extend these considerations to linear gravitational quantum deviations around a flat Minkowski space-time in a Coulomb-like gauge. The emergence of the classical, causal, and properly retarded linearized classical theory of general relativity with a conserved classical energymomentum tensor is then outlined. The quantum-mechanical framework also provides for a simple approach to classical quadrupole gravitational radiation of Einstein and microscopic spontaneous graviton emission and/or absorption processes.
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| 148. |
- Thaller, Maximilian, 1990
(författare)
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Rotating clouds of charged Vlasov matter in general relativity
- 2020
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Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 37:3
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Tidskriftsartikel (refereegranskat)abstract
- The existence of stationary solutions of the Einstein-Vlasov-Maxwell system which are axially symmetric but not spherically symmetric is proven by means of the implicit function theorem on Banach spaces. The proof relies on the methods of Andreasson et al (2014 Commun. Math. Phys. 329 787-808) where a similar result is obtained for uncharged particles. Among the solutions constructed in this article there are rotating and non-rotating ones. Static solutions exhibit an electric but no magnetic field. In the case of rotating solutions, in addition to the electric field, a purely poloidal magnetic field is induced by the particle current. The existence of toroidal components of the magnetic field turns out to be not possible in this setting.
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| 149. |
- Torsello, Francesco, et al.
(författare)
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Covariant BSSN formulation in bimetric relativity
- 2020
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 37:2
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Tidskriftsartikel (refereegranskat)abstract
- Numerical integration of the field equations in bimetric relativity is necessary to obtain solutions describing realistic systems. Thus, it is crucial to recast the equations as a well-posed problem. In general relativity, under certain assumptions, the covariant BSSN formulation is a strongly hyperbolic formulation of the Einstein equations, hence its Cauchy problem is well-posed. In this paper, we establish the covariant BSSN formulation of the bimetric field equations. It shares many features with the corresponding formulation in general relativity, but there are a few fundamental differences between them. Some of these differences depend on the gauge choice and alter the hyperbolic structure of the system of partial differential equations compared to general relativity. Accordingly, the strong hyperbolicity of the system cannot be claimed yet, under the same assumptions as in general relativity. In the paper, we stress the differences compared with general relativity and state the main issues that should be tackled next, to draw a roadmap towards numerical bimetric relativity.
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| 150. |
- Torsello, Francesco
(författare)
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The mean gauges in bimetric relativity
- 2019
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 36:23
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Tidskriftsartikel (refereegranskat)abstract
- The choice of gauge in numerical relativity is crucial in avoiding coordinate and curvature singularities. In addition, the gauge can affect the well-posedness of the system. In this work, we consider the mean gauges, established with respect to the geometric mean metric in bimetric relativity. We consider three gauge conditions widely used in numerical relativity, and compute them with respect to the geometric mean: The gauge condition and the maximal slicing for the lapse function of , and the -driver gauge condition for the shift vector of . In addition, in the bimetric covariant BSSN formalism, there are other arbitrary choices to be made before evolving the system. We show that it is possible to make them by using the geometric mean metric, which is determined dynamically by the system, rather than using an arbitrary external metric, as in general relativity. These choices represent opportunities to recast the system in a well-posed form.
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