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.
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.
881828
Search for small trans-Neptunian objects by the TAOS project
The Taiwan-America Occultation Survey (TAOS) aims to determine the number of small icy bodies in the outer reach of the Solar System by means of stellar occultation. An array of 4 robotic small (D=0.5 m), wide-field (f/1.9) telescopes have been installed at Lulin Observatory in Taiwan to simultaneously monitor some thousand of stars for such rare occultation events. Because a typical occultation event by a TNO a few km across will last for only a fraction of a second, fast photometry is necessary. A special CCD readout scheme has been devised to allow for stellar photometry taken a few times per second. Effective analysis pipelines have been developed to process stellar light curves and to correlate any possible flux changes among all telescopes. A few billion photometric measurements have been collected since the routine survey began in early 2005. Our preliminary result of a very low detection rate suggests a deficit of small TNOs down to a few km size, consistent with the extrapolation of some recent studies of larger (30-100 km) TNOs.
119441006
Black hole formation from collapsing dust fluid in a background of dark energy
The gravitational collapse of a spherically symmetric star, made of a dust fluid, rho(DM), in a background of dark energy, p=w rho (w <-1/3) is studied. It is found that when only dark energy is present, black holes are never formed. When both of them are present, black holes can be formed, due to the condensation of the dust fluid. Initially the dust fluid may not play an important role, but, as time increases, it will dominate the collapse and finally lead to formation of black holes. This result remains true even when the interaction between the dust fluid and dark energy does not vanish. When w <-1 (phantoms), some models also can be interpreted as representing the death of a white hole that ejects both dust and phantoms. The ejected matter recollapses to form a black hole.
119810929
Plane walls interacting with gravitational waves and matter fields
Plane walls with reflection symmetry are studied. A method for generating such solutions to the Einstein equations is developed. It is found that plane wall solutions require very strong restrictions on the metric coefficients and on their derivatives. Using the generalized Bianchi identities, the interaction of plane walls with gravitational waves and matter fields is investigated. In particular, it is shown that a plane wall, in general, interacts with both gravitational plane waves and matter fields. But, a plane domain wall interacts only with matter fields. No matter whether the space‐time out of the wall is curved or not, a plane domain wall is always repulsive.
744360
Gravitational collapse of a massless scalar field and radiation fluid
Several classes of conformally-flat and spherically symmetric exact solutions to the Einstein field equations coupled with either a massless scalar field or a radiation fluid are given, and their main properties are studied. It is found that some represent the formation of black holes due to the gravitational collapse of the matter fields. When the spacetimes have continuous self-similarity (CSS), the masses of black holes take a scaling form $M_{BH} \propto (P - P^{*})^{\gamma}$, where $\gamma = 0.5$ for massless scalar field and $\gamma = 1$ for radiation fluid. The reasons for the difference between the values of $\gamma$ obtained here and those obtained previously are discussed. When the spacetimes have neither CSS nor DSS (Discrete self-similarity), the masses of black holes always turn on with finite non-zero values.
119351300
Kink stability of self-similar solutions of scalar field in 2 + 1 gravity
The kink stability of self-similar solutions of a massless scalar field with circular symmetry in 2 + 1 gravity is studied, and found that such solutions are unstable against the kink perturbations along the sonic line (self-similar horizon). However, when perturbations outside the sonic line are considered, and taking the ones along the sonic line as their boundary conditions, we find that non-trivial perturbations do not exist. In other words, the consideration of perturbations in the whole spacetime limits the unstable mode of the perturbations found along the sonic line, and the kink instability rises because of the incomplete treatment of the problem. As a result, the critical solution for the scalar collapse remains critical even after the kink perturbations are taken into account.
30744353
Reconstruction of the deceleration parameter and the equation of state of dark energy
The new 182 gold supernova Ia data, the baryon acoustic oscillation measurement and the shift parameter determined from the Sloan Digital Sky Survey, and the three-year Wilkinson Microwave Anisotropy Probe data are combined to reconstruct the dark energy equation of state parameter $w(z)$ and the deceleration parameter $q(z)$. We find that the strongest evidence of acceleration happens around the redshift $z\ensuremath{\sim}0.2$ and the stringent constraints on $w(z)$ lie in the redshift range $z\ensuremath{\sim}0.2\char21{}0.5$. At the sweet spot, $\ensuremath{-}1.2lw(z)l\ensuremath{-}0.6$ for the dark energy parametrization $w(z)={w}_{0}+{w}_{a}z/(1+z{)}^{2}$ at the $3\ensuremath{\sigma}$ confidence level. The transition redshift ${z}_{t}$ when the Universe underwent the transition from deceleration to acceleration is derived to be ${z}_{t}={0.36}_{\ensuremath{-}0.08}^{+0.23}$. The combined data is also applied to find out the geometry of the Universe, and we find that at the $3\ensuremath{\sigma}$ confidence level, $|{\ensuremath{\Omega}}_{k}|\ensuremath{\lesssim}0.05$ for the simple one-parameter dark energy model, and $\ensuremath{-}0.064l{\ensuremath{\Omega}}_{k}l0.028$ for the $\ensuremath{\Lambda}\mathrm{CDM}$ model.