896 research outputs found
Probing the Friedmann equation during recombination with future CMB experiments
We show that by combining measurements of the temperature and polarization
anisotropies of the Cosmic Microwave Background (CMB), future experiments will
tightly constrain the expansion rate of the universe during recombination. A
change in the expansion rate modifies the way in which the recombination of
hydrogen proceeds, altering the shape of the acoustic peaks and the level of
CMB polarization. The proposed test is similar in spirit to the examination of
abundances of light elements produced during Big Bang Nucleosynthesis and it
constitutes a way to study possible departures from standard recombination. For
simplicity we parametrize the change in the Friedmann equation by changing the
gravitational constant . The main effect on the temperature power spectrum
is a change in the degree of damping of the acoustic peaks on small angular
scales. The effect can be compensated by a change in the shape of the
primordial power spectrum. We show that this degeneracy between the expansion
rate and the primordial spectrum can be broken by measuring CMB polarization.
In particular we show that the MAP satellite could obtain a constraint for the
expansion rate during recombination of or after observing for four years, whereas Planck could obtain
or within two years, even after
allowing for further freedom in the shape of the power spectrum of primordial
fluctuations.Comment: Replaced to match published PRD version. ACBAR and new Boomerang data
included in analysis. References added. 13 pages, 11 figure
Combining Full-Shape and BAO Analyses of Galaxy Power Spectra: A 1.6% CMB-independent constraint on H0
We present cosmological constraints from a joint analysis of the pre- and
post-reconstruction galaxy power spectrum multipoles from the final data
release of the Baryon Oscillation Spectroscopic Survey (BOSS). Geometric
constraints are obtained from the positions of BAO peaks in reconstructed
spectra, analyzed in combination with the unreconstructed spectra in a
full-shape (FS) likelihood using a joint covariance matrix, giving stronger
parameter constraints than FS-only or BAO-only analyses. We introduce a new
method for obtaining constraints from reconstructed spectra based on a
correlated theoretical error, which is shown to be simple, robust, and
applicable to any flavor of density-field reconstruction. Assuming CDM
with massive neutrinos, we analyze data from two redshift bins
and obtain constraints on the Hubble
constant , using only a single prior on the current baryon density
from Big Bang Nucleosynthesis (BBN) and no knowledge of the power
spectrum slope . This gives , with the inclusion of BAO
data sharpening the measurement by , representing one of the strongest
current constraints on independent of cosmic microwave background data.
Restricting to the best-fit slope from Planck (but without additional
priors on the spectral shape), we obtain a measurement of . We find strong constraints on the
cosmological parameters from a joint analysis of the FS, BAO, and Planck data.
This sets new bounds on the sum of neutrino masses (at confidence) and the effective number of
relativistic degrees of freedom , though
contours are not appreciably narrowed by the inclusion of BAO data.Comment: 42 pages, 12 figures, accepted by JCAP, likelihoods available at
https://github.com/Michalychforever/lss_montepython (minor typo corrected
The morphology of HII regions during reionization
It is possible that the properties of HII regions during reionization depend
sensitively on many poorly constrained quantities (the nature of the ionizing
sources, the clumpiness of the gas in the IGM, the degree to which
photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.),
making it extremely difficult to interpret upcoming observations of this epoch.
We demonstrate that the actual situation is more encouraging, using a suite of
radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc
N-body simulation. Analytic prescriptions are used to incorporate small-scale
structures that affect reionization, yet remain unresolved in the N-body
simulation. We show that the morphology of the HII regions is most dependent on
the global ionization fraction x_i. This is not to say that the bubble
morphology is completely independent of all parameters besides x_i. The next
most important dependence is that of the nature of the ionizing sources. The
rarer the sources, the larger and more spherical the HII regions become. The
typical bubble size can vary by as much as a factor of 4 at fixed x_i between
different possible source prescriptions. The final relevant factor is the
abundance of minihalos or of Lyman-limit systems. These systems suppress the
largest bubbles from growing, and the magnitude of this suppression depends on
the thermal history of the gas as well as the rate at which minihalos are
photo-evaporated. We find that neither source suppression owing to
photo-heating nor gas clumping significantly affect the large-scale structure
of the HII regions. We discuss how observations of the 21cm line with MWA and
LOFAR can constrain properties of reionization, and we study the effect patchy
reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]Comment: 23 pages, 18 figure
Digital learning application for clinical dentistry – a pilot study
Background: Microlearning is a growing trend in education which can have a positive effect on learning motivation and outcome. In health education, it has been shown to increase student knowledge and confidence in clinical procedures. Little research has been done on implementing digital learning solutions specifically aimed for clinical dental studies. Unifractal is an existing digital learning platform on which students and supervisors can easily create and share learning content.
Objective: In the present pilot study, we cooperated with Unifractal to develop and test the value of implementing such a tool in dental education.
Methods: Content, based mainly on clinical instruments, for the application was made by two fourth year dental students. It was tested by third year dental students at the simulation clinic at The Arctic University of Norway – UiT. Also, a questionnaire survey was developed to evaluate user experience. Here, students and supervisors participated.
Results: The survey analysis showed that students appreciated the content provided and experienced the application as a potentially useful learning tool. However, the use of the application seemed to be limited by the relatively sparse amount of content available.
Conclusion: This study shows that a digital learning platform, such as Unifractal, holds the potential to be an important and useful digital learning tool for dental students. However, more content needs to be developed in cooperation with students and supervisors to reach a satisfactory level of usefulness
Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array
We forecast the sensitivity with which the Murchison Widefield Array (MWA)
can measure the 21 cm power spectrum of cosmic hydrogen, using radiative
transfer simulations to model reionization and the 21 cm signal. The MWA is
sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}),
with foreground contamination precluding measurements on larger scales, and
thermal detector noise limiting the small scale sensitivity. This amounts
primarily to constraints on two numbers: the amplitude and slope of the 21 cm
power spectrum on the scales probed. We find, however, that the redshift
evolution in these quantities can yield important information about
reionization. Although the power spectrum differs substantially across
plausible models, a generic prediction is that the amplitude of the 21 cm power
spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM)
is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales
flattens as the ionization fraction increases and the sizes of the HII regions
grow. Considering detection sensitivity, we show that the optimal MWA antenna
configuration for power spectrum measurements would pack all 500 antenna tiles
as close as possible in a compact core. The MWA is sensitive enough in its
optimal configuration to measure redshift evolution in the slope and amplitude
of the 21 cm power spectrum. Detecting the characteristic redshift evolution of
our models will confirm that observed 21 cm fluctuations originate from the
IGM, and not from foregrounds, and provide an indirect constraint on the
volume-filling factor of HII regions during reionization. After two years of
observations under favorable conditions, the MWA can constrain the filling
factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.Comment: 14 pages, 9 figures, submitted to Ap
Métricas para simplificaçao de malhas triangulares
Orientador: Hélio PedriniDissertaçao (mestrado) - Universidade Federal do Paraná, Setor de Ciencias Exatas, Programa de Pós-Graduaçao em Informática. Defesa: Curitiba, 2005Inclui bibliografia e apendiceResumo: Malhas triangulares são uma das representações de dados espaciais mais utilizadas, pois possibilitam a manipulação e visualização de superfícies de alta complexidade, além de apresentarem diversas vantagens, como suporte direto em software e hardware. O problema de como transformar conjuntos de dados espaciais distintos, entre eles modelos de terrenos, conjuntos de pontos tridimensionais e dados volumétricos, em malhas triangulares é amplamente estudado. Além disso, com o aumento da capacidade de coleta desse tipo de dados, torna-se cada vez mais comum a necessidade de manipulação de modelos com altíssima resolução. Uma abordagem para a solução desses dois problemas é a simplificação de malhas triangulares. Esta consiste em primeiramente transformar um determinado conjunto de dados em uma malha triangular e, então, simplificá-la reduzindo-se o número de vértices, arestas e triângulos, de forma que a malha simplificada aproxime a malha original com o menor erro possível. Assim, torna-se possível manipular os dados em questão e, aplicando-se o método de simplificação repetidamente com diferentes requisitos de qualidade, obtêm-se representações da malha triangular em diversas resoluções. Dentro do âmbito da simplificação de malhas triangulares, este trabalho se propõe a verificar experimentalmente quais métricas de caráter local existentes na literatura possibilitam uma simplificação rápida e gerando aproximações de alta qualidade. Além disso, este trabalho também propõe duas novas métricas que geram aproximações de grande qualidade e são uma alternativa mais adequada para certas aplicações. Outros aspectos dos métodos de simplificação de malhas triangulares também são investigados, bem como o seu efeito na qualidade das aproximações geradas. Para que as diferentes opções do método de simplificação pudessem ser investigadas sem a interferência de implementações distintas, um programa para a simplificação de malhas triangulares foi implementado, de forma que o resultado da mudança de apenas opções específicas do método de simplificação é adequadamente verificado. Também são discutidos os diversos trabalhos da literatura que utilizam as operações de simplificação para a construção de uma representação em múltiplas resoluções, da qual 'e possível então extrair uma malha triangular com determinado nível de detalhe, sem a necessidade da aplicação de um método de simplificação.Abstract: Triangle meshes are one of the mostly employed spatial data representations, since they allow the manipulation and visualization of highly complex surfaces, apart from presenting many advantages, including direct software and hardware support. The problem of converting distinct spatial datasets, among them terrain models, tridimensional point sets, and volumetric data, into triangle meshes is widely studied. Furthermore, with the increase in the collection capacity of such type of data, the manipulation of models with high resolution has become a common task. One approach for the solution of these two problems is triangle mesh simplification, which consists in firstly converting a specific dataset into a triangle mesh and then simplifying it to reduce the number of vertices, edges, and faces, in a way that the simplified mesh approximates the original mesh with the lowest error. Therefore, it is possible to manipulate the data and, by applying repeatedly the simplification method with different quality requirements, a representation in multiple resolutions is obtained. In the scope of triangle mesh simplification, this work presents an experimental study to establish which local metrics that exist in the literature allow fast simplification generating high quality approximations. This work also proposes two new metrics that are an adequate alternative for certain applications, and also generate great quality approximations. Other aspects of triangle mesh simplification are also studied, including their impact in the quality of the approximations. For the investigation of the different options of the simplification method, without the interference of distinct implementations, a program for triangle mesh simplification was implemented, allowing that the change in specific simplification options can be properly verified. Different works from the literature are also discussed, that employ the simplification operations for the creation of a multi-resolution representation, from which it is possible to extract a triangle mesh with a required level of detail, without the necessity of applying a simplification method
Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization
A number of radio interferometers are currently being planned or constructed
to observe 21 cm emission from reionization. Not only will such measurements
provide a detailed view of that epoch, but, since the 21 cm emission also
traces the distribution of matter in the Universe, this signal can be used to
constrain cosmological parameters at 6 < z < 20. The sensitivity of an
interferometer to the cosmological information in the signal may depend on how
precisely the angular dependence of the 21 cm 3-D power spectrum can be
measured. Utilizing an analytic model for reionization, we quantify all the
effects that break the spherical symmetry of the 3-D 21 cm power spectrum and
produce physically motivated predictions for this power spectrum. We find that
upcoming observatories will be sensitive to the 21 cm signal over a wide range
of scales, from larger than 100 to as small as 1 comoving Mpc. We consider
three methods to measure cosmological parameters from the signal: (1) direct
fitting of the density power spectrum to the signal, (2) using only the
velocity field fluctuations in the signal, (3) looking at the signal at large
enough scales such that all fluctuations trace the density field. With the
foremost method, the first generation of 21 cm observations should moderately
improve existing constraints on cosmological parameters for certain
low-redshift reionization scenarios, and a two year observation with the second
generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m
h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially
ionized by z = 12 or if spin temperature fluctuations are important, we show
that it will be difficult to place competitive constraints on cosmological
parameters with any of the considered methods.Comment: 20 pages, 12 figures, accepted by Ap
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