10,553 research outputs found
Constraints on dark energy and cosmic topology
A non-trivial spatial topology of the Universe is a potentially observable
attribute, which can be probed through the circles-in-the-sky for all locally
homogeneous and isotropic universes with no assumptions on the cosmological
parameters. We show how one can use a possible circles-in-the-sky detection of
the spatial topology of globally homogeneous universes to set constraints on
the dark energy equation of state parameters.Comment: 6 pages, 1 figure. To appear in Int. J. Mod. Phys. A (2009). From a
talk presented at the Seventh Alexander Friedmann International Seminar on
Gravitation and Cosmolog
CMB Polarization Experiments
We discuss the analysis of polarization experiments with particular emphasis
on those that measure the Stokes parameters on a ring on the sky. We discuss
the ability of these experiments to separate the and contributions to
the polarization signal. The experiment being developed at Wisconsin university
is studied in detail, it will be sensitive to both Stokes parameters and will
concentrate on large scale polarization, scanning a degree ring. We will
also consider another example, an experiment that measures one of the Stokes
parameters in a ring. We find that the small ring experiment will be able
to detect cosmological polarization for some models consistent with the current
temperature anisotropy data, for reasonable integration times. In most
cosmological models large scale polarization is too small to be detected by the
Wisconsin experiment, but because both and are measured, separate
constraints can be set on and polarization.Comment: 27 pages with 12 included figure
Inhomogeneous reionization and the polarization of the cosmic microwave background
In a universe with inhomogeneous reionization, the ionized patches create a
second order signal in the cosmic microwave background polarization anisotropy.
This signal originates in the coupling of the free electron fluctuation to the
quadruple moment of the temperature anisotropy. We examine the contribution
from a simple inhomogeneous reionization model and find that the signal from
such a process is below the detectable limits of the Planck Surveyor mission.
However t he signal is above the fundamental uncertainty limit from cosmic
variance, so th at a future detection with a high accuracy experiment on
sub-arcminute scales is possible.Comment: 10 pages, 2 eps figures, final version accepted for publication in
ApJ Letter
Secondary CMB anisotropies in a universe reionized in patches
In a universe reionized in patches, the Doppler effect from Thomson
scattering off free electrons generates secondary cosmic microwave background
(CMB) anisotropies. For a simple model with small patches and late
reionization, we analytically calculate the anisotropy power spectrum. Patchy
reionization can, in principle, be the main source of anisotropies on arcminute
scales. On larger angular scales, its contribution to the CMB power spectrum is
a small fraction of the primary signal and is only barely detectable in the
power spectrum with even an ideal, i.e. cosmic variance limited, experiment and
an extreme model of reionization. Consequently patchy reionization is unlikely
to affect cosmological parameter estimation from the acoustic peaks in the CMB.
Its detection on small angles would help determine the ionization history of
the universe, in particular the typical size of the ionized region and the
duration of the reionization process.Comment: 7 pages, 2 figures, submitted to Ap
Age problem in holographic dark energy
We study the age problem of the universe with the holographic DE model
introduced in [21], and test the model with some known old high redshift
objects (OHRO). The parameters of the model have been constrained using the
SNIa, CMB and BAO data set. We found that the age of the old quasar APM 08
279+5255 at z = 3.91 can be described by the model.Comment: 13 page
Testing Gaussian random hypothesis with the cosmic microwave background temperature anisotropies in the three-year WMAP data
We test the hypothesis that the temperature of the cosmic microwave
background is consistent with a Gaussian random field defined on the celestial
sphere, using de-biased internal linear combination (DILC) map produced from
the 3-year WMAP data. We test the phases for spherical harmonic modes with l <=
10 (which should be the cleanest) for their uniformity, randomness, and
correlation with those of the foreground templates. The phases themselves are
consistent with a uniform distribution, but not for l <= 5, and the differences
between phases are not consistent with uniformity. For l=3 and l=6, the phases
of the CMB maps cross-correlate with the foregrounds, suggestion the presence
of residual contamination in the DLC map even on these large scales. We also
use a one-dimensional Fourier representation to assemble a_lm into the \Delta
T_l(\phi) for each l mode, and test the positions of the resulting maxima and
minima for consistency with uniformity randomness on the unit circle. The
results show significant departures at the 0.5% level, with the one-dimensional
peaks being concentrated around \phi=180 degs. This strongly significant
alignment with the Galactic meridian, together with the cross-correlation of
DILC phases with the foreground maps, strongly suggests that even the lowest
spherical harmonic modes in the map are significantly contaminated with
foreground radiation.Comment: submitted to ApJL, one paragraph is added in Section 3 and some more
in the Referenc
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