554 research outputs found
L-PICOLA: A parallel code for fast dark matter simulation
Robust measurements based on current large-scale structure surveys require
precise knowledge of statistical and systematic errors. This can be obtained
from large numbers of realistic mock galaxy catalogues that mimic the observed
distribution of galaxies within the survey volume. To this end we present a
fast, distributed-memory, planar-parallel code, L-PICOLA, which can be used to
generate and evolve a set of initial conditions into a dark matter field much
faster than a full non-linear N-Body simulation. Additionally, L-PICOLA has the
ability to include primordial non-Gaussianity in the simulation and simulate
the past lightcone at run-time, with optional replication of the simulation
volume. Through comparisons to fully non-linear N-Body simulations we find that
our code can reproduce the power spectrum and reduced bispectrum of dark
matter to within 2% and 5% respectively on all scales of interest to
measurements of Baryon Acoustic Oscillations and Redshift Space Distortions,
but 3 orders of magnitude faster. The accuracy, speed and scalability of this
code, alongside the additional features we have implemented, make it extremely
useful for both current and next generation large-scale structure surveys.
L-PICOLA is publicly available at https://cullanhowlett.github.io/l-picolaComment: 22 Pages, 20 Figures. Accepted for publication in Astronomy and
Computin
Maximal compression of the redshift space galaxy power spectrum and bispectrum
We explore two methods of compressing the redshift space galaxy power
spectrum and bispectrum with respect to a chosen set of cosmological
parameters. Both methods involve reducing the dimension of the original
data-vector ( e.g. 1000 elements ) to the number of cosmological parameters
considered ( e.g. seven ) using the Karhunen-Lo\`eve algorithm. In the first
case, we run MCMC sampling on the compressed data-vector in order to recover
the one-dimensional (1D) and two-dimensional (2D) posterior distributions. The
second option, approximately 2000 times faster, works by orthogonalising the
parameter space through diagonalisation of the Fisher information matrix before
the compression, obtaining the posterior distributions without the need of MCMC
sampling. Using these methods for future spectroscopic redshift surveys like
DESI, EUCLID and PFS would drastically reduce the number of simulations needed
to compute accurate covariance matrices with minimal loss of constraining
power. We consider a redshift bin of a DESI-like experiment. Using the power
spectrum combined with the bispectrum as a data-vector, both compression
methods on average recover the 68% credible regions to within 0.7% and 2% of
those resulting from standard MCMC sampling respectively. These confidence
intervals are also smaller than the ones obtained using only the power spectrum
by (81%, 80%, 82%) respectively for the bias parameter b_1, the growth rate f
and the scalar amplitude parameter A_s.Comment: 27 pages, 8 figures, 1 table, Accepted 2018 January 28. Received 2018
January 25; in original form 2017 September 11. Added clarifications in the
text on the bias modelling and compression limits following referee's
comments. Removed tetraspectrum term from the pk-bk cross covariance +
correction in the appendi
The Clustering of the SDSS DR7 Main Galaxy Sample I: A 4 per cent Distance Measure at z=0.15
We create a sample of spectroscopically identified galaxies with
from the Sloan Digital Sky Survey (SDSS) Data Release 7, covering 6813 deg.
Galaxies are chosen to sample the highest mass haloes, with an effective bias
of 1.5, allowing us to construct 1000 mock galaxy catalogs (described in Paper
II), which we use to estimate statistical errors and test our methods. We use
an estimate of the gravitational potential to "reconstruct" the linear density
fluctuations, enhancing the Baryon Acoustic Oscillation (BAO) signal in the
measured correlation function and power spectrum. Fitting to these
measurements, we determine Mpc; this is a better than 4 per cent distance measurement. This "fills
the gap" in BAO distance ladder between previously measured local and higher
redshift measurements, and affords significant improvement in constraining the
properties of dark energy. Combining our measurement with other BAO
measurements from BOSS and 6dFGS galaxy samples provides a 15 per cent
improvement in the determination of the equation of state of dark energy and
the value of the Hubble parameter at (). Our measurement is fully
consistent with the Planck results and the CDM concordance cosmology,
but increases the tension between PlanckBAO determinations and direct
measurements.Comment: Accepted by MNRAS, distance likelihood is available in source file
Large-scale Bias and Efficient Generation of Initial Conditions for Non-Local Primordial Non-Gaussianity
We study the scale-dependence of halo bias in generic (non-local) primordial
non-Gaussian (PNG) initial conditions of the type motivated by inflation,
parametrized by an arbitrary quadratic kernel. We first show how to generate
non-local PNG initial conditions with minimal overhead compared to local PNG
models for a general class of primordial bispectra that can be written as
linear combinations of separable templates. We run cosmological simulations for
the local, and non-local equilateral and orthogonal models and present results
on the scale-dependence of halo bias. We also derive a general formula for the
Fourier-space bias using the peak-background split (PBS) in the context of the
excursion set approach to halos and discuss the difference and similarities
with the known corresponding result from local bias models. Our PBS bias
formula generalizes previous results in the literature to include non-Markovian
effects and non-universality of the mass function and are in better agreement
with measurements in numerical simulations than previous results for a variety
of halo masses, redshifts and halo definitions. We also derive for the first
time quadratic bias results for arbitrary non-local PNG, and show that
non-linear bias loops give small corrections at large-scales. The resulting
well-behaved perturbation theory paves the way to constrain non-local PNG from
measurements of the power spectrum and bispectrum in galaxy redshift surveys.Comment: 43 pages, 10 figures. v2: references added. 2LPT parallel code for
generating non-local PNG initial conditions available at
http://cosmo.nyu.edu/roman/2LP
GEOMAX: beyond linear compression for 3pt galaxy clustering statistics
We present the GEOMAX algorithm and its Python implementation for a two-step
compression of bispectrum measurements. The first step groups bispectra by the
geometric properties of their arguments; the second step then maximises the
Fisher information with respect to a chosen set of model parameters in each
group. The algorithm only requires the derivatives of the data vector with
respect to the parameters and a small number of mock data, producing an
effective, non-linear compression. By applying GEOMAX to bispectrum monopole
measurements from BOSS DR12 CMASS redshift-space galaxy clustering data, we
reduce the credible intervals for the inferred parameters
by
with respect to standard MCMC on the full data vector. We run the analysis and
comparison between compression methods over one hundred galaxy mocks to test
the statistical significance of the improvements. On average GEOMAX performs
better than geometrical or maximal linear compression alone and is
consistent with being lossless. Given its flexibility, the GEOMAX approach has
the potential to optimally exploit three-point statistics of various
cosmological probes like weak lensing or line-intensity maps from current and
future cosmological data-sets such as DESI, Euclid, PFS and SKA.Comment: 17 pages, 9 figures, accepted version by MNRA
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: mock galaxy catalogues for the low-redshift sample
We present one thousand mock galaxy catalogues for the analysis of the Low
Redshift Sample (LOWZ, effective redshift z ~ 10.32) of the Baryon Oscillation
Spectroscopic Survey Data Releases 10 and 11. These mocks have been created
following the PTHalos method of Manera13 et al. (2013) revised to include new
developments. The main improvement is the introduction of a redshift dependence
in the Halo Occupation Distribution in order to account for the change of the
galaxy number density with redshift. These mock catalogues are used in the
analyses of the LOWZ galaxy clustering by the BOSS collaboration.Comment: 10 pages, 8 figure
COLA with scale-dependent growth: applications to screened modified gravity models
We present a general parallelized and easy-to-use code to perform numerical simulations of structure formation using the COLA (COmoving Lagrangian Acceleration) method for cosmological models that exhibit scale-dependent growth at the level of first and second order Lagrangian perturbation theory. For modified gravity theories we also include screening using a fast approximate method that covers all the main examples of screening mechanisms in the literature. We test the code by comparing it to full simulations of two popular modified gravity models, namely f(R) gravity and nDGP, and find good agreement in the modified gravity boost-factors relative to ΛCDM even when using a fairly small number of COLA time steps
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