On the stability of Einstein static universe at background level in massive bigravity

We study the static cosmological solutions and their stability at background level in the framework of massive bigravity theory with Friedmann-Robertson-Walker (FRW) metrics. By the modification proposed in the cosmological equations subject to a perfect fluid we obtain new solutions interpreted as the Einstein static universe. It turns out that the non-vanishing size of initial scale factor of Einstein static universe depends on the non-vanishing three-dimensional spatial curvature of FRW metrics and also the graviton's mass. By dynamical system approach and numerical analysis, we find that the extracted solutions for closed and open universes can be stable for some viable ranges of equation of state parameter, viable values of fraction of two scale factors, and viable values of graviton's mass obeying the hierarchy $m << M_Pl$ which is more cosmologically motivated.Comment: 14 pages, 8 figures, 5 tables, major revisio

Classical and quantum cosmology of minimal massive bigravity

In a Friedmann-Robertson-Walker (FRW) space-time background we study the classical cosmological models in the context of recently proposed theory of nonlinear minimal massive bigravity. We show that in the presence of perfect fluid the classical field equations acquire contribution from the massive graviton as a cosmological term which is positive or negative depending on the dynamical competition between two scale factors of bigravity metrics. We obtain the classical field equations for flat and open universes in the ordinary and Schutz representation of perfect fluid. Focusing on the Schutz representation for flat universe, we find classical solutions exhibiting singularities at early universe with vacuum equation of state. Then, in the Schutz representation, we study the quantum cosmology for flat universe and derive the Schrodinger-Wheeler-DeWitt equation. We find its exact and wave packet solutions and discuss on their properties to show that the initial singularity in the classical solutions can be avoided by quantum cosmology. Similar to the study of Hartle-Hawking no-boundary proposal in the quantum cosmology of de Rham, Gabadadze and Tolley (dRGT) massive gravity, it turns out that the mass of graviton predicted by quantum cosmology of the minimal massive bigravity is large at early universe. This is in agreement with the fact that at early universe the cosmological constant should be large.Comment: 13 pages, 6 figures, published online, Phys. Lett. B (2016

Head lice infestation in school children of a low socioeconomy area of Tabriz city, Iran

Head lice infestation (HLI) caused by Pediculus humanus var capitis (Pediculus capitis) is a world wild public health concern that affects mostly school aged children. HLI does not produce an illness, but itis physically unpleasant and an unbearable social stigma. Over a five months period from July to November, 2006, 2795 school children from a low socio-economy area of Tabriz city enrolled in 13 elementary and high schools (governmental school) including 1948 girls (69.69 %) and 847 boys (30.30%) were examined for the presence of Pediculus capitis (nits, adults and immature). During this study all the infested cases were seen among girls and there was no infestation in boys. School girls aged 10-14 years constituted the highest infestation rate (6.5%) and the lowest infestation rate was seen in girls aged 15-18 years old (1.6%). The infestation rate in 5-9 years old children was 5.7%. The overall infestation rate in the studied population was 3.64%. There was a significant difference between infestation rates among the age groups X2 = 15.43, df = 2, p = 0.0004). But there was no significantdifference between the infestation rates of 5-9 and 10-14  groups(X2=0.36, df=1, p=0.55). Pediculosis is a public health issue in many parts of the world. Certainly, personal hygiene practices and socioeconomic status influence the level of prevalence of pediculosis. The lowest infestation rate in 15-18 years old children in this study may indicates that the better personal hygiene practices includingregular combing and washing of the hair is the main reason for reducing the head lice infestation rate in this group in comparison with the two other groups. The overall infestation rate in the present study isprobably one of the lowest infestation rates in Iran and in the region

Quantum Dynamics in a Time-dependent Hard-Wall Spherical Trap

Exact solution of the Schr\"{o}dinger equation is given for a particle inside a hard sphere whose wall is moving with a constant velocity. Numerical computations are presented for both contracting and expanding spheres. The propagator is constructed and compared with the propagator of a particle in an infinite square well with one wall in uniform motion.Comment: 6 pages, 4 figures, Accepted by Europhys. Let