294 research outputs found
The integrated Sachs-Wolfe effect in the AvERA cosmology
The recent AvERA cosmological simulation of R\'acz et al. (2017) has a
-like expansion history and removes the tension between
local and Planck (cosmic microwave background) Hubble constants. We contrast
the AvERA prediction of the integrated Sachs--Wolfe (ISW) effect with that of
. The linear ISW effect is proportional to the derivative
of the growth function, thus it is sensitive to small differences in the
expansion histories of the respective models. We create simulated ISW maps
tracing the path of light-rays through the Millennium XXL cosmological
simulation, and perform theoretical calculations of the ISW power spectrum.
AvERA predicts a significantly higher ISW effect than ,
times larger depending on the index of the spherical power
spectrum, which could be utilized to definitively differentiate the models. We
also show that AvERA predicts an opposite-sign ISW effect in the redshift range
, in clear contrast with . Finally,
we compare our ISW predictions with previous observations. While at present
these cannot distinguish between the two models due to large error bars, and
lack of internal consistency suggesting systematics, ISW probes from future
surveys will tightly constrain the models.Comment: 9 pages, 8 figures. Submitted to MNRA
Refined position angle measurements for galaxies of the SDSS Stripe 82 co-added dataset
Position angle measurements of Sloan Digital Sky Survey (SDSS) galaxies, as
measured by the surface brightness profile fitting code of the SDSS photometric
pipeline (Lupton 2001), are known to be strongly biased, especially in the case
of almost face-on and highly inclined galaxies. To address this issue we
developed a reliable algorithm which determines position angles by means of
isophote fitting. In this paper we present our algorithm and a catalogue of
position angles for 26397 SDSS galaxies taken from the deep co-added Stripe 82
(equatorial stripe) images.Comment: 4 pages, 4 figures. Data are published on-line at
http://www.vo.elte.hu/galmorp
Galaxy shape measurement with convolutional neural networks
We present our results from training and evaluating a convolutional neural
network (CNN) to predict galaxy shapes from wide-field survey images of the
first data release of the Dark Energy Survey (DES DR1). We use conventional
shape measurements as ground truth from an overlapping, deeper survey with less
sky coverage, the Canada-France Hawaii Telescope Lensing Survey (CFHTLenS). We
demonstrate that CNN predictions from single band DES images reproduce the
results of CFHTLenS at bright magnitudes and show higher correlation with
CFHTLenS at fainter magnitudes than maximum likelihood model fitting estimates
in the DES Y1 im3shape catalogue. Prediction of shape parameters with a CNN is
also extremely fast, it takes only 0.2 milliseconds per galaxy, improving more
than 4 orders of magnitudes over forward model fitting. The CNN can also
accurately predict shapes when using multiple images of the same galaxy, even
in different color bands, with no additional computational overhead. The CNN is
again more precise for faint objects, and the advantage of the CNN is more
pronounced for blue galaxies than red ones when compared to the DES Y1
metacalibration catalogue, which fits a single Gaussian profile using riz band
images. We demonstrate that CNN shape predictions within the metacalibration
self-calibrating framework yield shear estimates with negligible multiplicative
bias, , and no significant PSF leakage. Our proposed setup is
applicable to current and next generation weak lensing surveys where higher
quality ground truth shapes can be measured in dedicated deep fields
StePS: A Multi-GPU Cosmological N-body Code for Compactified Simulations
We present the multi-GPU realization of the StePS (Stereographically
Projected Cosmological Simulations) algorithm with MPI-OpenMP-CUDA hybrid
parallelization and nearly ideal scale-out to multiple compute nodes. Our new
zoom-in cosmological direct N-body simulation method simulates the infinite
universe with unprecedented dynamic range for a given amount of memory and, in
contrast to traditional periodic simulations, its fundamental geometry and
topology match observations. By using a spherical geometry instead of periodic
boundary conditions, and gradually decreasing the mass resolution with radius,
our code is capable of running simulations with a few gigaparsecs in diameter
and with a mass resolution of in the center in four days
on three compute nodes with four GTX 1080Ti GPUs in each. The code can also be
used to run extremely fast simulations with reasonable resolution for fitting
cosmological parameters. These simulations are useful for prediction needs of
large surveys. The StePS code is publicly available for the research community
An improved cosmological parameter inference scheme motivated by deep learning
Dark matter cannot be observed directly, but its weak gravitational lensing
slightly distorts the apparent shapes of background galaxies, making weak
lensing one of the most promising probes of cosmology. Several observational
studies have measured the effect, and there are currently running, and planned
efforts to provide even larger, and higher resolution weak lensing maps. Due to
nonlinearities on small scales, the traditional analysis with two-point
statistics does not fully capture all the underlying information. Multiple
inference methods were proposed to extract more details based on higher order
statistics, peak statistics, Minkowski functionals and recently convolutional
neural networks (CNN). Here we present an improved convolutional neural network
that gives significantly better estimates of and
cosmological parameters from simulated convergence maps than the state of art
methods and also is free of systematic bias. We show that the network exploits
information in the gradients around peaks, and with this insight, we construct
a new, easy-to-understand, and robust peak counting algorithm based on the
'steepness' of peaks, instead of their heights. The proposed scheme is even
more accurate than the neural network on high-resolution noiseless maps. With
shape noise and lower resolution its relative advantage deteriorates, but it
remains more accurate than peak counting
Do the rich get richer? An empirical analysis of the BitCoin transaction network
The possibility to analyze everyday monetary transactions is limited by the
scarcity of available data, as this kind of information is usually considered
highly sensitive. Present econophysics models are usually employed on presumed
random networks of interacting agents, and only macroscopic properties (e.g.
the resulting wealth distribution) are compared to real-world data. In this
paper, we analyze BitCoin, which is a novel digital currency system, where the
complete list of transactions is publicly available. Using this dataset, we
reconstruct the network of transactions, and extract the time and amount of
each payment. We analyze the structure of the transaction network by measuring
network characteristics over time, such as the degree distribution, degree
correlations and clustering. We find that linear preferential attachment drives
the growth of the network. We also study the dynamics taking place on the
transaction network, i.e. the flow of money. We measure temporal patterns and
the wealth accumulation. Investigating the microscopic statistics of money
movement, we find that sublinear preferential attachment governs the evolution
of the wealth distribution. We report a scaling relation between the degree and
wealth associated to individual nodes.Comment: Project website: http://www.vo.elte.hu/bitcoin/; updated after
publicatio
Photo-Met: a non-parametric method for estimating stellar metallicity from photometric observations
Getting spectra at good signal-to-noise ratios takes orders of magnitudes
more time than photometric observations. Building on the technique developed
for photometric redshift estimation of galaxies, we develop and demonstrate a
non-parametric photometric method for estimating the chemical composition of
galactic stars. We investigate the efficiency of our method using
spectroscopically determined stellar metallicities from SDSS DR7. The technique
is generic in the sense that it is not restricted to certain stellar types or
stellar parameter ranges and makes it possible to obtain metallicities and
error estimates for a much larger sample than spectroscopic surveys would
allow. We find that our method performs well, especially for brighter stars and
higher metallicities and, in contrast to many other techniques, we are able to
reliably estimate the error of the predicted metallicities.Comment: 5 pages, 4 figures, accepted for publication in A
Measuring the dimension of partially embedded networks
Scaling phenomena have been intensively studied during the past decade in the
context of complex networks. As part of these works, recently novel methods
have appeared to measure the dimension of abstract and spatially embedded
networks. In this paper we propose a new dimension measurement method for
networks, which does not require global knowledge on the embedding of the
nodes, instead it exploits link-wise information (link lengths, link delays or
other physical quantities). Our method can be regarded as a generalization of
the spectral dimension, that grasps the network's large-scale structure through
local observations made by a random walker while traversing the links. We apply
the presented method to synthetic and real-world networks, including road maps,
the Internet infrastructure and the Gowalla geosocial network. We analyze the
theoretically and empirically designated case when the length distribution of
the links has the form P(r) ~ 1/r. We show that while previous dimension
concepts are not applicable in this case, the new dimension measure still
exhibits scaling with two distinct scaling regimes. Our observations suggest
that the link length distribution is not sufficient in itself to entirely
control the dimensionality of complex networks, and we show that the proposed
measure provides information that complements other known measures
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