2,047 research outputs found
Precision Cosmology? Not Just Yet
The recent announcement from the Wilkinson Microwave Anisotropy Probe (WMAP)
satellite experiment combined with other recent advances in observational
cosmology verifies key components of the standard cosmological model. However,
we argue that there remain some significant open issues regarding the basic
history and composition of the Universe and uncertainties in some of the most
important parameters.Comment: 2 pages, 2 figures. Online journal version
http://www.sciencemag.org/cgi/content/full/299/5612/153
Using Wii technology to explore real spaces via virtual environments for people who are blind
Purpose - Virtual environments (VEs) that represent real spaces (RSs) give people who are blind the opportunity to build a cognitive map in advance that they will be able to use when arriving at the RS. Design - In this research study Nintendo Wii based technology was used for exploring VEs via the Wiici application. The Wiimote allows the user to interact with VEs by simulating walking and scanning the space. Finding - By getting haptic and auditory feedback the user learned to explore new spaces. We examined the participants' abilities to explore new simple and complex places, construct a cognitive map, and perform orientation tasks in the RS. Originality â To our knowledge, this finding presents the first virtual environment for people who are blind that allow the participants to scan the environment and by this to construct map model spatial representations
Measuring the Mach number of the Universe via the Sunyaev-Zeldovich effect
We introduce a new statistic to measure more accurately the cosmic sound
speed of clusters of galaxies at different redshifts. This statistic is
evaluated by cross-correlating cosmic microwave background (CMB) fluctuations
caused by the Sunyaev-Zel'dovich effect from observed clusters of galaxies with
their redshifts. When clusters are distributed in redshift bins of narrow
width, one could measure the mean squared cluster peculiar velocity with an
error \sigma_{C_S^2}\lsim (300{\rm km/s})^2. This can be done around z>0.3 with
clusters of flux above 200 mJy which will be detected by PLANCK, coupled with
high resolution microwave images to eliminate the cosmological part of the CMB
fluctuations. The latter can be achieved with observations by the planned ALMA
array or the NSF South Pole telescope and other surveys. By measuring the
cosmic sound speed and the bulk flow in, e.g., 4 spheres of ~ 100h^{-1}Mpc at
z=0.3, we could have a direct measurement of the matter density
0.21<\Omega_m<0.47 at 95 % confidence level.Comment: Ap.J.Letters, submitte
A buyer's guide to the Hubble Constant
Since the expansion of the universe was first established by Edwin Hubble and Georges LemaĂźtre about a century ago, the Hubble constant H0 which measures its rate has been of great interest to astronomers. Besides being interesting in its own right, few properties of the universe can be deduced without it. In the last decade, a significant gap has emerged between different methods of measuring it, some anchored in the nearby universe, others at cosmological distances. The SH0ES team has found H0=73.2±1.3kmsâ1Mpcâ1 locally, whereas the value found for the early universe by the Planck Collaboration is H0=67.4±0.5kmsâ1Mpcâ1 from measurements of the cosmic microwave background. Is this gap a sign that the well-established ÎCDM cosmological model is somehow incomplete? Or are there unknown systematics? And more practically, how should humble astronomers pick between competing claims if they need to assume a value for a certain purpose? In this article, we review results and what changes to the cosmological model could be needed to accommodate them all. For astronomers in a hurry, we provide a buyerâs guide to the results, and make recommendations
Combining cosmological datasets: hyperparameters and Bayesian evidence
A method is presented for performing joint analyses of cosmological datasets,
in which the weight assigned to each dataset is determined directly by it own
statistical properties. The weights are considered in a Bayesian context as a
set of hyperparameters, which are then marginalised over in order to recover
the posterior distribution as a function only of the cosmological parameters of
interest. In the case of a Gaussian likelihood function, this marginalisation
may be performed analytically. Calculation of the Bayesian evidence for the
data, with and without the introduction of hyperparameters, enables a direct
determination of whether the data warrant the introduction of weights into the
analysis; this generalises the standard likelihood ratio approach to model
comparison. The method is illustrated by application to the classic toy problem
of fitting a straight line to a set of data. A cosmological illustration of the
technique is also presented, in which the latest measurements of the cosmic
microwave background power spectrum are used to infer constraints on
cosmological parameters.Comment: 12 pages, 6 figures, submitted to MNRA
Bayesian `Hyper-Parameters' Approach to Joint Estimation: The Hubble Constant from CMB Measurements
Recently several studies have jointly analysed data from different
cosmological probes with the motivation of estimating cosmological parameters.
Here we generalise this procedure to take into account the relative weights of
various probes. This is done by including in the joint \chi^2 function a set of
`Hyper-Parameters', which are dealt with using Bayesian considerations. The
resulting algorithm (in the case of uniform priors on the log of the
Hyper-Parameters) is very simple: instead of minimising \sum \chi_j^2 (where
\chi_j^2 is per data set j) we propose to minimise \sum N_j \ln (\chi_j^2)
(where N_j is the number of data points per data set j). We illustrate the
method by estimating the Hubble constant H_0 from different sets of recent CMB
experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang).Comment: submitted to MNRAS, 6 pages, Latex, with 3 figures embedde
The X-ray Cluster Dipole
We estimate the dipole of the whole sky X-ray flux-limited sample of
Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster
dipole. The X-ray cluster dipole is well aligned () with the
CMB dipole, while it follows closely the radial profile of its optical cluster
counterpart although its amplitude is per cent lower. In view of
the fact that the the XBACs sample is not affected by the volume incompleteness
and the projection effects that are known to exist at some level in the optical
parent Abell/ACO cluster catalogue, our present results confirm the previous
optical cluster dipole analysis that there are significant contributions to the
Local Group motion from large distances (Mpc). In order to
assess the expected contribution to the X-ray cluster dipole from a purely
X-ray selected sample we compare the dipoles of the XBACs and the Brightest
Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting
dipoles are in mutual good aggreement with an indication that the XBACs sample
slightly underestimates the full X-ray dipole (by per cent) while the
Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster
dipole. Using linear perturbation theory to relate the X-ray cluster dipole to
the Local group peculiar velocity we estimate the density parameter to be
.Comment: 16 pages, latex, + 4 ps figures, submitted to Ap
Cosmic Mach Number as a Function of Overdensity and Galaxy Age
We carry out an extensive study of the cosmic Mach number (\mach) on scales
of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We
particularly put emphasis on the environmental dependence of \mach on
overdensity, galaxy mass, and galaxy age. We start by discussing the difference
in the resulting \mach according to different definitions of \mach and
different methods of calculation. The simulated Mach numbers are slightly lower
than the linear theory predictions even when a non-linear power spectrum was
used in the calculation, reflecting the non-linear evolution in the simulation.
We find that the observed \mach is higher than the simulated mean by more than
2-standard deviations, which suggests either that the Local Group is in a
relatively low-density region or that the true value of \Omega_m is ~ 0.2,
significantly lower than the simulated value of 0.37. We show from our
simulation that the Mach number is a weakly decreasing function of overdensity.
We also investigate the correlations between galaxy age, overdensity and \mach
for two different samples of galaxies --- DWARFs and GIANTs. Older systems
cluster in higher density regions with lower \mach, while younger ones tend to
reside in lower density regions with larger \mach, as expected from the
hierarchical structure formation scenario. However, for DWARFs, the correlation
is weakened by the fact that some of the oldest DWARFs are left over in
low-density regions during the structure formation history. For giant systems,
one expects blue-selected samples to have higher \mach than red-selected ones.
We briefly comment on the effect of the warm dark matter on the expected Mach
number.Comment: 43 pages, including 15 figures. Accepted version in ApJ. Included
correlation function of different samples of galaxies, and the cumulative
number fraction distribution as a fcn. of overdensity. Reorganized figures
and added some reference
Spherical Collapse and the Halo Model in Braneworld Gravity
We present a detailed study of the collapse of a spherical perturbation in
DGP braneworld gravity for the purpose of modeling simulation results for the
halo mass function, bias and matter power spectrum. The presence of evolving
modifications to the gravitational force in form of the scalar brane-bending
mode lead to qualitative differences to the collapse in ordinary gravity. In
particular, differences in the energetics of the collapse necessitate a new,
generalized method for defining the virial radius which does not rely on strict
energy conservation. These differences and techniques apply to smooth dark
energy models with w unequal -1 as well. We also discuss the impact of the
exterior of the perturbation on collapse quantities due to the lack of a
Birkhoff theorem in DGP. The resulting predictions for the mass function, halo
bias and power spectrum are in good overall agreement with DGP N-body
simulations on both the self-accelerating and normal branch. In particular, the
impact of the Vainshtein mechanism as measured in the full simulations is
matched well. The model and techniques introduced here can serve as practical
tools for placing consistent constraints on braneworld models using
observations of large scale structure.Comment: 20 pages, 16 figures; v2: minor addition to appendix; matches
published version; v3: typos in Eqs. (20), (23) correcte
Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts
The mean sky-averaged Comptonization parameter, y, describing the scattering
of the CMB by hot gas in clusters of galaxies is calculated in an array of flat
and open cosmological and dark matter models. The models are globally
normalized to fit cluster X-ray data, and intracluster gas is assumed to have
evolved in a manner consistent with current observations. We predict values of
y lower than the COBE/FIRAS upper limit. The corresponding values of the
overall optical thickness to Compton scattering are < 10^{-4} for relevant
parameter values. Of more practical importance are number counts of clusters
across which a net flux (with respect to the CMB) higher than some limiting
value can be detected. Such number counts are specifically predicted for the
COBRAS/SAMBA and BOOMERANG missions.Comment: 23 pages, Latex, 11 PostScript figures, 5 PostScript tables, to
appear in Ap
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