8,215 research outputs found
String Cosmology in Anisotropic Bianchi-II Space-time
The present study deals with a spatially homogeneous and anisotropic
Bianchi-II cosmological model representing massive strings. The energy-momentum
tensor, as formulated by Letelier (1983), has been used to construct a massive
string cosmological model for which the expansion scalar is proportional to one
of the components of shear tensor. The Einstein's field equations have been
solved by applying a variation law for generalized Hubble's parameter that
yields a constant value of deceleration parameter in Bianchi-II space-time. A
comparative study of accelerating and decelerating modes of the evolution of
universe has been carried out in the presence of string scenario. The study
reveals that massive strings dominate the early Universe. The strings
eventually disappear from the Universe for sufficiently large times, which is
in agreement with the current astronomical observations.Comment: 11 pages, 6 figures (To appear in Mod. Phys. Lett. A) In this
version, the cosmic string has been directed along z-direction and the
resultant field equations have been solved exactl
Role of NGOs in irrigation development and management in Nepal: Proceedings of the National Workshop on the Role of NGOs in Irrigation Development and Management in Nepal, Nepal Administrative Staff College, Jawalakhel, Lalitpur, Nepal, 29-30 September 1992
Agricultural manpower / Non-governmental organizations / Rural development / Irrigation management / Nepal
Dynamics of Magnetized Bulk Viscous Strings in Brans-Dicke Gravity
We explore locally rotationally symmetric Bianchi I universe in Brans-Dicke
gravity with self-interacting potential by using charged viscous cosmological
string fluid. We use a relationship between the shear and expansion scalars and
also take the power law for scalar field as well as self-interacting potential.
It is found that the resulting universe model maintains its anisotropic nature
at all times due to the proportionality relationship between expansion and
shear scalars. The physical implications of this model are discussed by using
different parameters and their graphs. We conclude that this model corresponds
to an accelerated expanding universe for particular values of the parameters.Comment: 17 pages, 6 figure
Bianchi Type-II String Cosmological Models in Normal Gauge for Lyra's Manifold with Constant Deceleration Parameter
The present study deals with a spatially homogeneous and anisotropic
Bianchi-II cosmological models representing massive strings in normal gauge for
Lyra's manifold by applying the variation law for generalized Hubble's
parameter that yields a constant value of deceleration parameter. The variation
law for Hubble's parameter generates two types of solutions for the average
scale factor, one is of power-law type and other is of the exponential form.
Using these two forms, Einstein's modified field equations are solved
separately that correspond to expanding singular and non-singular models of the
universe respectively. The energy-momentum tensor for such string as formulated
by Letelier (1983) is used to construct massive string cosmological models for
which we assume that the expansion () in the model is proportional to
the component of the shear tensor . This
condition leads to , where A, B and C are the metric coefficients
and m is proportionality constant. Our models are in accelerating phase which
is consistent to the recent observations. It has been found that the
displacement vector behaves like cosmological term in the
normal gauge treatment and the solutions are consistent with recent
observations of SNe Ia. It has been found that massive strings dominate in the
decelerating universe whereas strings dominate in the accelerating universe.
Some physical and geometric behaviour of these models are also discussed.Comment: 24 pages, 10 figure
Elektroaktivnost mangostina i njegovih kompleksa
Biomaterial has occupied a leading position in material science for its various scientific and technological applications. The present work was carried out with a natural gum extracted from raw fruit of mangosteen - an east Indian tree Gercinia mangostana. Solid specimen of the gum was developed following a sol-gel like process. AC and DC electrical analysis on the dried solid specimen of the gum was carried out. It exhibits high electrical conduction, σ ∼ 2 × 10−6 S/cm at room temperature, of which ionic and electronic contributions are 70% and 30%, respectively. The experimental investigations show that origin of high electrical conductivity is due to the presence of a substantial amount of organic acid unit in its polysaccharide background. In fact the observed σ is about 3 times that observed in gum Arabica - a plant gum. Optical absorption of this new biomaterial was also studied using UV-VIS spectro-photometry. The result shows its high absorption coefficient in UV and blue part of the spectrum. The electrical characterization of the material was also made. It has been observed that the electronic conduction can be enhanced to 70% of the total electrical conductivity by forming complex with iodine or an appropriate organic/inorganic acid, e.g. acetic acid. This high-potential material is being studied for the development in device application.Biološki materijali imaju vodeću ulogu u znanosti o materijalima zbog svojih različitih znanstvenih i tehnoloških primjena. U ovom se radu proučava prirodna smola koja je bila izlučena iz sirovog voća magnostina, ploda istočno-indijske voćke Gercinia mangostana. Pripravili smo čvrst uzorak smole sol-gel postupkom. Ispitali smo uzorak izmjeničnom i istosmjernom strujom. Na sobnoj temperaturi našli smo visoku električnu vodljivost, σ ∼ 2×10−6 S/cm, s ionskim i elektronskim doprinosima od 70% odnosno 30%. Mjerenja pokazuju da je visoka električna vodljivost posljedica poveće količine molekula organske kiseline u polisaharidnoj osnovnoj tvari. Vrijednosti σ su oko tri puta veće nego u biljnoj smoli Arabica. Proučavali smo optičku apsorpciju primjenom UV-VIS spektrofotometrije. Ishodi mjerenja pokazuju velik apsorpcijski koeficijent u plavom dijelu spektra. Također smo proučili električne značajke te tvari. Ustanovili smo kako se elektronska vodljivost može povećati na oko 70% ukupne vodljivosti tvorbom kompleksa s jodom ili pogodnom organskom ili anorganskom kiselinom, npr. octenom kiselinom. Ta se tvar velikih mogućnosti proučava radi primjena u raznim napravama
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