44854065
Gravitational Collapse of Cylindrical Shells Made of Counter-Rotating Dust Particles
Gravitational collapse of a realistic body has been one of the most thorny and important problems in Einstein’s theory of General Relativity. Due to the complexity of the Einstein field equations, the problem even in simple cases, such as, spacetimes with spherical symmetry, is still not well understood [1], and new phenomena keep emerging [2]. Particularly, in 1991 Shapiro and Teukolsky [3] studied numerically the problem of a dust spheroid, and found that only the spheroid is compact enough, a black hole can be formed. Otherwise, the collapse most likely ends with a naked singularity. Later, Barrabés, Israel and Letelier constructed an analytical model of a collapsing convex thin shell and found that in certain cases no apparent horizons are formed, too [4]. Their results were soon generalized to more general cases [5]. However, since in all the cases considered by them, the external gravitational field of the collapsing shell is not known, one cannot exclude, similar to the ST case, the formation of an outer event horizon [4,5]. Since then, the gravitational collapse with non-spherical symmetry has been attracting more and more attention. In particular, by studying the collapse of a cylindrical shell that is made of counter-rotating particles, Apostolatos and Thorne (AT) showed analytically that the centrifugal forces associated with an arbitrarily small amount of rotation, by themselves, without the aid of any pressure, can halt the collapse at some non-zero, minimum radius, and the shell will then oscillate until it settles down at some final, finite radius, whereby a spacetime singularity is prevented from forming on the symmetry axis [6]. Soon after AT’s work, Shapiro and Teukolsky studied numerically the gravitational collapse of rotating spheroids, and found that the rotation indeed significantly modifies the evolution when it is sufficiently large. However, for small enough angular momentum, their simulations showed that spindle singularities appeared to arise without apparent horizons, too. Hence, it is possible that even spheroids with some angular momentum may still form naked singularities [7]. It should be noted that in the AT work it was considered only the case where the shell has zero total angular momentum and is momentarily static and radiation-free. In a realistic case, the spacetime has neither cylindrical symmetry nor zero angular momentum, and gravitational and particle radiations are always expected to occur. As a generalization of the AT work, in this paper we shall consider the case where cylindrical shell radiates gravitational waves and massless particles, as it is collapsing, while still keep the requirement that the total angular momentum of the shell be zero. Specifically, the paper is organized as follows: In Sec. II, the formulas for a general dynamic timelike thin shell that connects two arbitrary cylindrical regions are given, using Israel’s formula [8], while in Sec. III, a collapsing thin shell made of counter-rotating dust particles is studied. To model the particle radiation of the
2444705
Gravitational and particle radiation from long cosmic strings
Gravitational and massless particle radiation from infinitely long cosmic strings with finite thickness is studied by constructing analytic solutions to the Einstein field equations with certain forms of energy - momentum tensor. Using Thorne's -energy arguments, it is shown that the radiation always carries away energy. As a result, the angle defect outside the string is less than that in the non-radiation case. Since the solutions are exact, the back reaction of the radiation is automatically taken into account. In fact, it is shown that because of this back reaction the spacetime is always singular on the symmetry axis. The singularity is a scalar one and not removable.
109935503
No-Go theorem in spacetimes with two commuting spacelike killing vectors
Four-dimensional Riemannian spacetimes with two commuting spacelike Killing vectors are studied in Einstein's theory of gravity, and found that no outer apparent horizons exist, provided that the dominant energy condition holds.
10393301
On the sources of static plane symmetric vacuum space-times
Abstract The sources of static vacuum plane space-times with reflection symmetry are considered. It is argued that the counterpart of the Einstein theory to the gravitational field of a massive Newtonian plane should be described by the Rindler solution, which represents also a uniform gravitational field, and the test particles moving in which have constant accelerations.
119227824
Cosmological constant and late transient acceleration of the universe in the Horava-Witten Heterotic M-Theory on S^{1}/Z_{2}
Abstract Orbifold branes are studied in the framework of the 11-dimensional Horava–Witten heterotic M-theory. It is found that the effective cosmological constant can be easily lowered to its current observational value by the mechanism of large extra dimensions. The domination of this constant over the evolution of the universe is only temporary. Due to the interaction of the bulk and the branes, the universe will be in its decelerating expansion phase again in the future, whereby all problems connected with a far future de Sitter universe are resolved.
119168760
Homothetic self-similar solutions of three-dimensional Brans-Dicke gravity
All homothetic self-similar solutions of the Brans-Dicke scalar field in three-dimensional spacetime with circular symmetry are found in closed form.
81234
Levi-Civita solutions coupled with electromagnetic fields
The local and global properties of the Levi-Civita (LC) solutions coupled with an electromagnetic field are studied and some limits to the vacuum LC solutions are given, from which the physical and geometrical interpretations of the free parameters involved in the solutions are made clear. Sources for both the LC vacuum solutions and the LC solutions coupled with an electromagnetic field are studied, and in particular it is found that all the LC vacuum solutions with σ ≥ 0 can be produced by cylindrically symmetric thin shells that satisfy all the energy conditions, weak, dominant and strong. When the electromagnetic field is present, the situation changes dramatically. In the case of a purely magnetic field, all the solutions can be produced by physically acceptable cylindrical thin shells, while in the case of a purely electric field, no such shells are found for any choice of the free parameters involved in the solutions.
123322377
PLANAR DOMAIN WALLS COUPLED WITH PURE RADIATION FIELDS
All the exact solutions of the Einstein-Maxwell-Weyl equations are given, which represent planar domain walls coupled with electromagnetic and neutrino radiation fields. Some specific solutions are studied.
118411966
Elastic Scattering of Positive Kaons on Polarized Deuterons at 1.5 Gev/c.
121101159
Effect of a preshower detector on calorimetry performance
Abstract Test beam results obtained with a preshower detector in a SDC calorimetry configuration are presented. The benefits of independently instrumenting the first scintillating tile of each tower of the tile/fiber electromagnetic calorimeter, forming the analogue of the “Massless Gap” in LAr calorimeters, are demonstrated. Electron and pion data were taken with a calorimeter configured with one or two radiation lengths of inert material in front, a scintillating fiber preshower detector, a tile/fiber electromagnetic calorimeter, a shower maximum detector, and a tile/fiber hadron calorimeter. The energy deposited in the scintillating fibers of the preshower detector was summed to simulate the preshower energy that would be detected by the first scintillating tile of an EMC tower. The data were collected in the MP9 beam line at Fermilab during the Summer and Fall of 1991. The relative degradation in electromagnetic energy resolution due to the inert material in front of the electromagnetic calorimeter was measured. The resolution is improved by adding in the properly weighted preshower energy. The charged pion rejection versus electron acceptance is presented for combined cuts using the electromagnetic and hadronic calorimeters, with the energy deposited in the preshower and shower maximum detectors. We show that, even after calorimetric and shower maximum cuts, the pion rejection is further improved by using the preshower energy.
197512779
Stable Gravastars of Phantom Energy