10,403 research outputs found
Scalar field black holes
With a suitable decomposition of its energy-momentum tensor into pressureless
matter and a vacuum type term, we investigate the spherical gravitational
collapse of a minimally coupled, self-interacting scalar field, showing that it
collapses to a singularity. The formed blackhole has a mass (in
Planck units), where is the mass of the scalar field. If the latter has the
axion mass, eV, the former has a mass .Comment: 8 pages, no figure
Dental Caries, and Supragingival Plaque and Calculus among Students, Tanga, Tanzania.
The prevalence of dental caries and supragingival plaque and calculus in 785 secondary schools students was assessed. More than half (53.6%) of the students were caries-free, and the majority of those with dental caries experience were aged 14-17 (68.1%) and females (53%). Mean DMFT was 1.26, with mean D-component of 1.05, and molars were most affected. Most students had supragingival plaque (74%) and calculus (56.9%) and more so in males than females (P > 0.05). Less than half of the students had experience of dental caries and those with caries were mostly females and of the younger age group. The low DMFT was contributed to the D-component, and molars were the tooth type most affected.The majority of students had supra-gingival plaque and calculus and more so in males than females
Stability of the Einstein static universe in presence of vacuum energy
The Einstein static universe has played a central role in a number of
emergent scenarios recently put forward to deal with the singular origin of the
standard cosmological model. Here we study the existence and stability of the
Einstein static solution in presence of vacuum energy corresponding to
conformally-invariant fields. We show that the presence of vacuum energy
stabilizes this solution by changing it to a centre equilibrium point, which is
cyclically stable. This allows non-singular emergent cosmological models to be
constructed in which initially the universe oscillates indefinitely about an
initial Einstein static solution and is thus past eternal.Comment: Some references adde
Non-adiabatic Chaplygin gas
The split of a generalised Chaplygin gas with an equation of state p =
-A/\rho^{\alpha} into an interacting mixture of pressureless matter and a
dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda}
implies the existence of non-adiabatic pressure perturbations. We demonstrate
that the square of the effective (non-adiabatic) sound speed c_s of the medium
is proportional to the ratio of the perturbations of the dark energy to those
of the dark matter. Since, as demonstrated explicitly for the particular case
\alpha = -1/2, dark-energy perturbations are negligible compared with
dark-matter perturbations on scales that are relevant for structure formation,
we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities
which have plagued previous adiabatic Chaplygin-gas models.Comment: Version to appear in Physics Letters
- …