6,668 research outputs found
Can a Kasner Universe with a Viscous Cosmological Fluid be Anisotropic?
A Bianchi type -I metric of Kasner form is considered, when the space is
filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the
Kasner metric to be anisotropic provided that the fluid satisfies the
Zel'dovich equation of state, the viscous fluid does not permit the Kasner
metric to be anisotropic at all. In the latter case, we calculate the Kasner
(isotropic) metric expressed by the fluid's density, pressure, and bulk
viscosity, at some chosen instant . The equation of state is also
calculated. The present paper is related to a recent Comment of Cataldo and del
Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the
present authors [Phys. Rev. D {\bf 56}, 3322 (1997)].Comment: 8 pages, LaTeX, no figures. To appear in PR
Loop quantum cosmology of Bianchi type IX models
The loop quantum cosmology "improved dynamics" of the Bianchi type IX model
are studied. The action of the Hamiltonian constraint operator is obtained via
techniques developed for the Bianchi type I and type II models, no new input is
required. It is shown that the big bang and big crunch singularities are
resolved by quantum gravity effects. We also present the effective equations
which provide modifications to the classical equations of motion due to quantum
geometry effects.Comment: 20 page
Quantum Cosmology of Kantowski-Sachs like Models
The Wheeler-DeWitt equation for a class of Kantowski-Sachs like models is
completely solved. The generalized models include the Kantowski-Sachs model
with cosmological constant and pressureless dust. Likewise contained is a
joined model which consists of a Kantowski-Sachs cylinder inserted between two
FRW half--spheres. The (second order) WKB approximation is exact for the wave
functions of the complete set and this facilitates the product structure of the
wave function for the joined model. In spite of the product structure the wave
function can not be interpreted as admitting no correlations between the
different regions. This problem is due to the joining procedure and may
therefore be present for all joined models. Finally, the {s}ymmetric {i}nitial
{c}ondition (SIC) for the wave function is analyzed and compared with the ``no
bouindary'' condition. The consequences of the different boundary conditions
for the arrow of time are briefly mentioned.Comment: 21 pages, uses LaTeX2e, epsf.sty and float.sty, three figures (50
kb); changes: one figure added, new interpretation of quantizing procedure
for the joined model and many minor change
Practicum 2012 - 2013: LIFT Boston Client Well Being Study
A Boston based non-profit and a team of public policy PhD students engaged in several months of collaborative problem identification and goal setting focused on the effectiveness of the organization’s unique service delivery model. The nonprofit uses volunteer advocates and a goal-oriented process with no eligibility criteria to assist clients in distress. We collected administrative data, administered a survey, and conducted interviews to explore client well-being
Galaxy Mergers at z>1 in the HUDF: Evidence for a Peak in the Major Merger Rate of Massive Galaxies
We present a measurement of the galaxy merger fraction and number density
from observations in the Hubble Ultra Deep Field for 0.5<z<2.5. We fit the
combination of broadband data and slitless spectroscopy of 1308 galaxies with
stellar population synthesis models to select merging systems based on a
stellar mass of >10^10 M_sol. When correcting for mass incompleteness, the
major merger fraction is not simply proportional to (1+z)^m, but appears to
peak at z_frac~=1.3+-0.4. From this merger fraction, we infer that ~42% of
massive galaxies have undergone a major merger since z~1. We show that the
major merger number density peaks at z_dens~1.2, which marks the epoch where
major merging of massive galaxies is most prevalent. This critical redshift is
comparable to the peak of the cosmic star formation rate density, and occurs
roughly 2.6 Gyr earlier in cosmic time than the peak in the number density of
X-ray selected active galactic nuclei. These observations support an indirect
evolutionary link between merging, starburst, and active galaxies.Comment: Accepted to ApJ. 7 pages, 6 figures, 1 table. Uses and includes
emulateapj.cls. In the initial submission, Figures 1 and 2 where switche
Comments on Closed Bianchi Models
We show several kinematical properties that are intrinsic to the Bianchi
models with compact spatial sections. Especially, with spacelike hypersurfaces
being closed, (A) no anisotropic expansion is allowed for Bianchi type V and
VII(A\not=0), and (B) type IV and VI(A\not=0,1) does not exist. In order to
show them, we put into geometric terms what is meant by spatial homogeneity and
employ a mathematical result on 3-manifolds. We make clear the relation between
the Bianchi type symmetry of space-time and spatial compactness, some part of
which seem to be unnoticed in the literature. Especially, it is shown under
what conditions class B Bianchi models do not possess compact spatial sections.
Finally we briefly describe how this study is useful in investigating global
dynamics in (3+1)-dimensional gravity.Comment: 14 pages with one table, KUCP-5
Isolated OB Associations in Stripped HI Gas Clouds
HST ACS/HRC images in UV (F250W), V (F555W), and I (F814W) resolve three
isolated OB associations that lie up to 30 kpc from the stellar disk of the S0
galaxy NGC 1533. Previous narrow-band Halpha imaging and optical spectroscopy
showed these objects as unresolved intergalactic HII regions having Halpha
luminosities consistent with single early-type O stars. These young stars lie
in stripped HI gas with column densities ranging from 1.5 - 2.5 * 10^20 cm^-2
and velocity dispersions near 30 km s^-1. Using the HST broadband colors and
magnitudes along with previously-determined Halpha luminosities, we place
limits on the masses and ages of each association, considering the importance
of stochastic effects for faint (M_V >-8) stellar populations. The upper limits
to their stellar masses range from 600 M_sun to 7000 M_sun, and ages range from
2 - 6 Myrs. This analysis includes an updated calculation of the conversion
factor between the ionizing luminosity and the total number of main sequence O
stars contained within an HII region. The photometric properties and sizes of
the isolated associations and other objects in the HRC fields are consistent
with those of Galactic stellar associations, open clusters and/or single O and
B stars. We interpret the age-size sequence of associations and clustered field
objects as an indication that these isolated associations are most likely
rapidly dispersing. Furthermore, we consider the possibility that these
isolated associations represent the first generation of stars in the HI ring
surrounding NGC 1533. This work suggests star formation in the unique
environment of a galaxy's outermost gaseous regions proceeds similarly to that
within the Galactic disk and that star formation in tidal debris may be
responsible for building up a younger halo component.Comment: 21 pages, 9 figures, 6 tables; accepted for publication in Ap
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