920 research outputs found
Missing Power vs low-l Alignments in the Cosmic Microwave Background: No Correlation in the Standard Cosmological Model
On large angular scales (greater than about 60 degrees), the two-point
angular correlation function of the temperature of the cosmic microwave
background (CMB), as measured (outside of the plane of the Galaxy) by the
Wilkinson Microwave Anisotropy Probe, shows significantly lower large-angle
correlations than expected from the standard inflationary cosmological model.
Furthermore, when derived from the full CMB sky, the two lowest cosmologically
interesting multipoles, the quadrupole (l=2) and the octopole (l=3), are
unexpectedly aligned with each other. Using randomly generated full-sky and
cut-sky maps, we investigate whether these anomalies are correlated at a
statistically significant level. We conclusively demonstrate that, assuming
Gaussian random and statistically isotropic CMB anisotropies, there is no
statistically significant correlation between the missing power on large
angular scales in the CMB and the alignment of the l=2 and l=3 multipoles. The
chance to measure the sky with both such a lack of large-angle correlation and
such an alignment of the low multipoles is thus quantified to be below 10^{-6}.Comment: 4 Pages, 1 Figur
Quasars and galaxy formation
Quasars are widely believed to be powered by accretion onto supermassive
black holes and there is now considerable evidence for a link between mergers,
quasars and the formation of spheroids. Cattaneo, Haehnelt & Rees (1999) have
demonstrated that a very simple model in which supermassive black holes form
and accrete most of their mass in mergers of galaxies of comparable masses can
reproduce the observed relation of black hole mass to bulge luminosity. Here we
show that this simple model can account for the luminosity function of quasars
and for the redshift evolution of the quasar population provided a few
additional assumptions are made. We use the extended Press-Schechter formalism
to simulate the formation of galaxies in hierarchical models of the formation
of structures and we assume that, when two galaxies of comparable masses merge,
their central black holes coalesce and a fraction of the gas in the merger
remnant is accreted by the supermassive black hole over a time-scale of about
10^7 yr. We find that the decrease in the merging rate with cosmic time and the
depletion in the amount of cold gas available due to the formation of stars are
not sufficient to explain the strong decline in the space density of bright
quasars between z=2 and z=0, since larger and larger structures form, which can
potentially host brighter and brighter quasars. To explain the redshift
evolution of the space density of bright quasars between z=2 and z=0 we need to
assume that there is a dependence on redshift either in the fraction of
available gas accreted or in the time-scale for accretion.Comment: 8 pages, 8 figures, submitted to MNRA
Shape of Cosmic String Loops
Complicated cosmic string loops will fragment until they reach simple,
non-intersecting ("stable") configurations. Through extensive numerical study
we characterize these attractor loop shapes including their length, velocity,
kink, and cusp distributions. We find that an initial loop containing M
harmonic modes will, on average, split into 3M stable loops. These stable loops
are approximately described by the degenerate kinky loop, which is planar and
rectangular, independently of the number of modes on the initial loop. This is
confirmed by an analytic construction of a stable family of perturbed
degenerate kinky loops. The average stable loop is also found to have a 40%
chance of containing a cusp. We examine the properties of stable loops of
different lengths and find only slight variation. Finally we develop a new
analytic scheme to explicitly solve the string constraint equations.Comment: 11 pages, 19 figures. See http://www.phys.cwru.edu/projects/strings/
for more information, movies, code, etc. Minor clarification suggested by
referee. Accepted for publication in Phys. Rev.
Can one reconstruct masked CMB sky?
The CMB maps obtained by observations always possess domains which have to be
masked due to severe uncertainties with respect to the genuine CMB signal.
Cosmological analyses ideally use full CMB maps in order to get e.g. the
angular power spectrum. There are attempts to reconstruct the masked regions at
least at low resolutions, i.e. at large angular scales, before a further
analysis follows. In this paper, the quality of the reconstruction is
investigated for the ILC (7yr) map as well as for 1000 CMB simulations of the
LambdaCDM concordance model. The latter allows an error estimation for the
reconstruction algorithm which reveals some drawbacks. The analysis points to
errors of the order of a significant fraction of the mean temperature
fluctuation of the CMB. The temperature 2-point correlation function C(theta)
is evaluated for different reconstructed sky maps which leads to the conclusion
that it is safest to compute it on the cut-sky
New constraints on Parity Symmetry from a re-analysis of the WMAP-7 low resolution power spectra
The Parity symmetry of the Cosmic Microwave Background (CMB) pattern as seen
by WMAP 7 is tested jointly in temperature and polarization at large angular
scale. A Quadratic Maximum Likelihood (QML) estimator is applied to the WMAP 7
year low resolution maps to compute all polarized CMB angular power spectra.
The analysis is supported by 10000 realistic Monte-Carlo realizations. We
confirm the previously reported Parity anomaly for TT in the range at C.L.. No anomalies have been detected in TT for a
wider range (up to ). No violations have been found for
EE, TE and BB which we test here for the first time. The cross-spectra TB and
EB are found to be consistent with zero. We also forecast {\sc Planck}
capabilities in probing Parity violations on low resolution maps.Comment: 8 pages, 6 figures, 4 tables. Accepted for publication in MNRA
Low-frequency line temperatures of the CMB
Based on SU(2) Yang-Mills thermodynamics we interprete Aracde2's and the
results of earlier radio-surveys on low-frequency CMB line temperatures as a
phase-boundary effect. We explain the excess at low frequencies by evanescent,
nonthermal photon fields of the CMB whose intensity is nulled by that of Planck
distributed calibrator photons. The CMB baseline temperature thus is identified
with the critical temperature of the deconfining-preconfining transition.Comment: v2: 9 pages, 1 figure, extended discussion of why prsent photon mass
bounds are not in contradiction to a low-temperature, low-frequency Meissner
mass responsible for UEGE, matches journal versio
Large Scale Baryon Isocurvature Inhomogeneities
Big bang nucleosynthesis constraints on baryon isocurvature perturbations are
determined. A simple model ignoring the effects of the scale of the
perturbations is first reviewed. This model is then extended to test the claim
that large amplitude perturbations will collapse, forming compact objects and
preventing their baryons from contributing to the observed baryon density. It
is found that baryon isocurvature perturbations are constrained to provide only
a slight increase in the density of baryons in the universe over the standard
homogeneous model. In particular it is found that models which rely on power
laws and the random phase approximation for the power spectrum are incompatible
with big bang nucleosynthesis unless an {\em ad hoc}, small scale cutoff is
included.Comment: 11pages + 8figures, LaTeX (2.09), postscript figures available via
anonymous ftp from oddjob.uchicago.edu:/ftp/ibbn/fig?.ps where ?=1-8 or via
email from [email protected], Fermilab-Pub-94/???-A and UMN-TH-1307/9
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