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Gravitation
About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.
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TL;DR: In this paper, all stationary, spherically symmetric solutions of Jordan's unified theory of gravity and electromagnetism are constructed and conditions for the solutions are given to represent black holes with nonvanishing mass, electric and magnetic charge.
Abstract: All stationary, spherically symmetric solutions of Jordan's unified theory of gravity and electromagnetism are constructed. Conditions for the solutions are given to represent black holes with nonvanishing mass, electric and magnetic charge.
176 citations
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TL;DR: In this paper, it was shown that no causal Green's function can give the correct linearized response to certain bilocalized distributions, even though these distributions obey the constraints of linearization stability.
Abstract: Classical gravitation on de Sitter space suffers from a linearization instability. One consequence is that the causal response to a spatially localized distribution of positive energy cannot be globally regular. We use this fact to show that no causal Green's function can give the correct linearized response to certain bilocalized distributions, even though these distributions obey the constraints of linearization stability. We avoid the problem by working on the open submanifold spanned by conformal coordinates. The retarded Green's function is first computed in a simple gauge, then the rest of the propagator is inferred by analyticity — up to the usual ambiguity about real, analytic and homogeneous terms. We show that the latter can be chosen so as to give a propagator which does not grow in any direction. The ghost propagator is also given and the interaction vertices are worked out.
176 citations
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TL;DR: In this paper, the general-relativistic equations governing the motion of a large mass under the influence of its own gravitational field and its own pressure have been approximated by finite-difference equations.
Abstract: The general-relativistic equations governing the motion of a large mass under the influence of its own gravitational field and its own pressure have been approximated by finite-difference equations. A spherically symmetric, co-moving frame of reference was used. The pressure was assumed to be zero at the outer boundary. Rest mass was assumed to be conserved and heat transfer by neutrinos, radiation, etc., was not taken into account. Numerical solutions were obtained on a computer for several simplified equations of state, chosen to bracket the behavior of stellar material in late stages of collapse, and several masses. The maximum stable masses obtained were of the same order of magnitude, but somewhat larger than the maximum stable masses calculated statically. The behavior of light signals, of the metric coefficients, and of the hydrodynamic quantities as functions of time is described for collapse past the Schwarzchild radius. Such collapse leads to regions where the surface area of concentric spheres decreases as the rest mass contained by the spheres increases.
176 citations
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TL;DR: In this paper, the authors investigate the influence of photoionization by an ultraviolet radiation background on the formation of galaxies, and they find that it has essentially no effect on the baryonic mass function of galaxies at z = 2.
Abstract: Using cosmological simulations that incorporate gasdynamics and gravitational forces, we investigate the influence of photoionization by an ultraviolet radiation background on the formation of galaxies. In our highest resolution simulations, we find that photoionization has essentially no effect on the baryonic mass function of galaxies at z = 2, down to our resolution limit of ~5 × 109 M☉. We do, however, find a strong interplay between the mass resolution of a simulation and the microphysics included in the computation of heating and cooling rates. At low resolution, a photoionizing background can appear to suppress the formation of even relatively massive galaxies. However, when the same initial conditions are evolved with a factor of 8 improvement in mass resolution, this effect disappears. Our results demonstrate the need for care in interpreting the results of cosmological simulations that incorporate hydrodynamics and radiation physics. For example, we conclude that a simulation with limited resolution may yield more accurate results if it ignores some relevant physical processes, such as photoionization. At higher resolution, the simulated population of massive galaxies is insensitive to the treatment of photoionization or the inclusion of star formation in the simulations, but it does depend significantly on the amplitude of the initial density fluctuations. By z = 2, an Ω = 1 cold dark matter model normalized to produce the observed masses of present-day clusters has already formed galaxies with baryon masses exceeding 1011 M☉.
176 citations
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TL;DR: In this article, the authors used the millisecond pulsar 1937 + 21 to predict a stochastic background of gravitational radiation, which should be observable using the pulsar itself.
Abstract: Breaking a Yang–Mills symmetry at very high energy could lead to macroscopic structures in the vacuum called ‘strings’, which may be the cause of galaxy formation and clustering. Dissipative gravitational interactions of string loops can produce unusual astrophysical side effects. Such scenarios generally predict a stochastic background of gravitational radiation which should be observable using the millisecond pulsar 1937 + 21. Gravitational radiation recoil accelerates loops, but dynamical friction on ordinary matter tends to slow them down so they may be able to accrete matter. Internal oscillations of loops produce a time-varying shear perturbation which may lead to an observable heating of stellar or gaseous systems in their immediate neighbourhood.
176 citations