28 research outputs found
Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes
We review, compare and extend recent studies searching for evidence for a
preferred cosmological axis. We start from the Union2 SnIa dataset and use the
hemisphere comparison method to search for a preferred axis in the data. We
find that the hemisphere of maximum accelerating expansion rate is in the
direction (\omm=0.19) while the hemisphere of
minimum acceleration is in the opposite direction
(\omm=0.30). The level of anisotropy is described by the normalized
difference of the best fit values of \omm between the two hemispheres in the
context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of
\frac{\Delta \ommax}{\bomm}=0.43\pm 0.06. Such a level does not necessarily
correspond to statistically significant anisotropy because it is reproduced by
about of simulated isotropic data mimicking the best fit Union2 dataset.
However, when combined with the axes directions of other cosmological
observations (bulk velocity flow axis, three axes of CMB low multipole moments
and quasar optical polarization alignment axis), the statistical evidence for a
cosmological anisotropy increases dramatically. We estimate the probability
that the above independent six axes directions would be so close in the sky to
be less than . Thus either the relative coincidence of these six axes is a
very large statistical fluctuation or there is an underlying physical or
systematic reason that leads to their correlation.Comment: 10 pages, 7 figures. Accepted in JCAP (to appear). Extended analysis
with redshift tomography of SnIa, included errorbars and increased number of
axes. The Mathematica 7 files with the data used for the production of the
figures along with a Powerpoint file with additional figures may be
downloaded from http://leandros.physics.uoi.gr/anisotrop
Light propagation in statistically homogeneous and isotropic universes with general matter content
We derive the relationship of the redshift and the angular diameter distance
to the average expansion rate for universes which are statistically homogeneous
and isotropic and where the distribution evolves slowly, but which have
otherwise arbitrary geometry and matter content. The relevant average expansion
rate is selected by the observable redshift and the assumed symmetry properties
of the spacetime. We show why light deflection and shear remain small. We write
down the evolution equations for the average expansion rate and discuss the
validity of the dust approximation.Comment: 42 pages, no figures. v2: Corrected one detail about the angular
diameter distance and two typos. No change in result
Pulmonary responses to lower body negative pressure and fluid loading during head-down tilt bedrest.
Exposure to microgravity redistributes body fluids with important secondary effects on cardiovascular function. We tested the hypothesis that fluid shifts also affect pulmonary gas exchange. Microgravity was simulated in six male volunteers by a 10-day period of bedrest at 6° head-down tilt (HDT). Lower body negative pressure (LBNP) and intravenous saline loading superimposed acute changes in fluid distribution on the prolonged effects of HDT. HDT produced relative dehydration and hypovolemia with decreased pulmonary blood flow and diffusing capacity. Blood bedrest, pulmonary blood flow decreased by 24% during LBNP and diffusing capacity by 7%, while functional residual capacity increased by 14% (p < 0.05). Intravenous saline loading caused a 24% increase in pulmonary blood-flow (p < 0.05). Functional residual capacity decreased by 10% and diffusing capacity by 6% (p < 0.05). Lung tissue volume did not change significantly. Head-down tilt had only minor effects on the responses to LBNP and saline loading. We conclude that LBNP and intravenous saline loading produce major changes in pulmonary blood-flow and minor effects on pulmonary gas exchange, and that the response to acute changes in fluid distribution is not significantly altered during simulated microgravity