1,393 research outputs found
Numerical Methods in Cosmological Global Texture Simulations
Numerical simulations of the evolution of a global topological defect field
have two characteristic length scales --- one macrophysical, of order the field
correlation length, and the other microphysical, of order the field width. The
situation currently of most interest to particle cosmologists involves the
behaviour of a GUT-scale defect field at the epoch of decoupling, where the
ratio of these scales is typically of order . Such a ratio is
unrealisable in numerical work, and we consider the approximations which may be
employed to deal with this. Focusing on the case of global texture we outline
the implementation of the associated algorithms, and in particular note the
subtleties involved in handling texture unwinding events. Comparing the results
in each approach then establishes that, subject to certain constraints on the
minimum grid resolution, the methods described are both robust and consistent
with one another.Comment: LaTeX, IMPERIAL/TP/93-94/2
Estimate of the Cosmological Bispectrum from the MAXIMA-1 Cosmic Microwave Background Map
We use the measurement of the cosmic microwave background taken during the
MAXIMA-1 flight to estimate the bispectrum of cosmological perturbations. We
propose an estimator for the bispectrum that is appropriate in the flat sky
approximation, apply it to the MAXIMA-1 data and evaluate errors using
bootstrap methods. We compare the estimated value with what would be expected
if the sky signal were Gaussian and find that it is indeed consistent, with a
per degree of freedom of approximately unity. This measurement places
constraints on models of inflation.Comment: 5 pages, 2 figures. New version to match paper accepted for
publication in Phys. Rev. Lett. Non-diagonal terms included leading to new
limits on f_N
Measurement of a Peak in the Cosmic Microwave Background Power Spectrum from the North American test flight of BOOMERANG
We describe a measurement of the angular power spectrum of anisotropies in
the Cosmic Microwave Background (CMB) from 0.3 degrees to ~10 degrees from the
North American test flight of the BOOMERANG experiment. BOOMERANG is a
balloon-borne telescope with a bolometric receiver designed to map CMB
anisotropies on a Long Duration Balloon flight. During a 6-hour test flight of
a prototype system in 1997, we mapped > 200 square degrees at high galactic
latitudes in two bands centered at 90 and 150 GHz with a resolution of 26 and
16.6 arcmin FWHM respectively. Analysis of the maps gives a power spectrum with
a peak at angular scales of ~1 degree with an amplitude ~70 uK.Comment: 5 pages, 1 figure LaTeX, emulateapj.st
Defect Production in Slow First Order Phase Transitions
We study the formation of vortices in a U(1) gauge theory following a
first-order transition proceeding by bubble nucleation, in particular the
effect of a low velocity of expansion of the bubble walls. To do this, we use a
two-dimensional model in which bubbles are nucleated at random points in a
plane and at random times and then expand at some velocity .
Within each bubble, the phase angle is assigned one of three discrete values.
When bubbles collide, magnetic `fluxons' appear: if the phases are different, a
fluxon--anti-fluxon pair is formed. These fluxons are eventually trapped in
three-bubble collisions when they may annihilate or form quantized vortices. We
study in particular the effect of changing the bubble expansion speed on the
vortex density and the extent of vortex--anti-vortex correlation.Comment: 13 pages, RevTeX, 15 uuencoded postscript figure
Determining Foreground Contamination in CMB Observations: Diffuse Galactic Emission in the MAXIMA-I Field
Observations of the CMB can be contaminated by diffuse foreground emission
from sources such as Galactic dust and synchrotron radiation. In these cases,
the morphology of the contaminating source is known from observations at
different frequencies, but not its amplitude at the frequency of interest for
the CMB. We develop a technique for accounting for the effects of such emission
in this case, and for simultaneously estimating the foreground amplitude in the
CMB observations. We apply the technique to CMB data from the MAXIMA-1
experiment, using maps of Galactic dust emission from combinations of IRAS and
DIRBE observations, as well as compilations of Galactic synchrotron emission
observations. The spectrum of the dust emission over the 150--450 GHz observed
by MAXIMA is consistent with preferred models but the effect on CMB power
spectrum observations is negligible.Comment: 19 pages, 8 figures, accepted for publication in the Astrophysical
Journal. Monor changes to match the published versio
Making Maps Of The Cosmic Microwave Background: The MAXIMA Example
This work describes Cosmic Microwave Background (CMB) data analysis
algorithms and their implementations, developed to produce a pixelized map of
the sky and a corresponding pixel-pixel noise correlation matrix from time
ordered data for a CMB mapping experiment. We discuss in turn algorithms for
estimating noise properties from the time ordered data, techniques for
manipulating the time ordered data, and a number of variants of the maximum
likelihood map-making procedure. We pay particular attention to issues
pertinent to real CMB data, and present ways of incorporating them within the
framework of maximum likelihood map-making. Making a map of the sky is shown to
be not only an intermediate step rendering an image of the sky, but also an
important diagnostic stage, when tests for and/or removal of systematic effects
can efficiently be performed. The case under study is the MAXIMA data set.
However, the methods discussed are expected to be applicable to the analysis of
other current and forthcoming CMB experiments.Comment: Replaced to match the published version, only minor change
Frequentist Estimation of Cosmological Parameters from the MAXIMA-1 Cosmic Microwave Background Anisotropy Data
We use a frequentist statistical approach to set confidence intervals on the
values of cosmological parameters using the MAXIMA-1 and COBE measurements of
the angular power spectrum of the cosmic microwave background. We define a
statistic, simulate the measurements of MAXIMA-1 and COBE,
determine the probability distribution of the statistic, and use it and the
data to set confidence intervals on several cosmological parameters. We compare
the frequentist confidence intervals to Bayesian credible regions. The
frequentist and Bayesian approaches give best estimates for the parameters that
agree within 15%, and confidence interval-widths that agree within 30%. The
results also suggest that a frequentist analysis gives slightly broader
confidence intervals than a Bayesian analysis. The frequentist analysis gives
values of \Omega=0.89{+0.26\atop -0.19}, \Omega_{\rm B}h^2=0.026{+0.020\atop
-0.011} and n=1.02{+0.31\atop -0.10}, and the Bayesian analysis gives values of
\Omega=0.98{+0.14\atop -0.19}, \Omega_{\rm B}h^2=0.0.029{+0.015\atop-0.010},
and , all at the 95% confidence level.Comment: 10 pages, 9 Postscript figures, changes made to reflect published
versio
Cosmic string loops and large-scale structure
We investigate the contribution made by small loops from a cosmic string
network as seeds for large-scale structure formation. We show that cosmic
string loops are highly correlated with the long-string network on large scales
and therefore contribute significantly to the power spectrum of density
perturbations if the average loop lifetime is comparable to or above one Hubble
time. This effect further improves the large-scale bias problem previously
identified in earlier studies of cosmic string models.Comment: 5 pages, 5 figure
Analytical modeling of large-angle CMBR anisotropies from textures
We propose an analytic method for predicting the large angle CMBR temperature
fluctuations induced by model textures. The model makes use of only a small
number of phenomenological parameters which ought to be measured from simple
simulations. We derive semi-analytically the -spectrum for together with its associated non-Gaussian cosmic variance error bars. A
slightly tilted spectrum with an extra suppression at low is found, and we
investigate the dependence of the tilt on the parameters of the model. We also
produce a prediction for the two point correlation function. We find a high
level of cosmic confusion between texture scenarios and standard inflationary
theories in any of these quantities. However, we discover that a distinctive
non-Gaussian signal ought to be expected at low , reflecting the prominent
effect of the last texture in these multipoles
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