165,474 research outputs found
Robustness of Cosmological Simulations I: Large Scale Structure
The gravitationally-driven evolution of cold dark matter dominates the
formation of structure in the Universe over a wide range of length scales.
While the longest scales can be treated by perturbation theory, a fully
quantitative understanding of nonlinear effects requires the application of
large-scale particle simulation methods. Additionally, precision predictions
for next-generation observations, such as weak gravitational lensing, can only
be obtained from numerical simulations. In this paper, we compare results from
several N-body codes using test problems and a diverse set of diagnostics,
focusing on a medium resolution regime appropriate for studying many
observationally relevant aspects of structure formation. Our conclusions are
that -- despite the use of different algorithms and error-control methodologies
-- overall, the codes yield consistent results. The agreement over a wide range
of scales for the cosmological tests is test-dependent. In the best cases, it
is at the 5% level or better, however, for other cases it can be significantly
larger than 10%. These include the halo mass function at low masses and the
mass power spectrum at small scales. While there exist explanations for most of
the discrepancies, our results point to the need for significant improvement in
N-body errors and their understanding to match the precision of near-future
observations. The simulation results, including halo catalogs, and initial
conditions used, are publicly available.Comment: 32 pages, 53 figures, data from the simulations is available at
http://t8web.lanl.gov/people/heitmann/arxiv, accepted for publication in
ApJS, several minor revisions, reference added, main conclusions unchange
Subsonic turbulence in smoothed particle hydrodynamics and moving-mesh simulations
Highly supersonic, compressible turbulence is thought to be of tantamount
importance for star formation processes in the interstellar medium. Likewise,
cosmic structure formation is expected to give rise to subsonic turbulence in
the intergalactic medium, which may substantially modify the thermodynamic
structure of gas in virialized dark matter halos and affect small-scale mixing
processes in the gas. Numerical simulations have played a key role in
characterizing the properties of astrophysical turbulence, but thus far
systematic code comparisons have been restricted to the supersonic regime,
leaving it unclear whether subsonic turbulence is faithfully represented by the
numerical techniques commonly employed in astrophysics. Here we focus on
comparing the accuracy of smoothed particle hydrodynamics (SPH) and our new
moving-mesh technique AREPO in simulations of driven subsonic turbulence. To
make contact with previous results, we also analyze simulations of transsonic
and highly supersonic turbulence. We find that the widely employed standard
formulation of SPH yields problematic results in the subsonic regime. Instead
of building up a Kolmogorov-like turbulent cascade, large-scale eddies are
quickly damped close to the driving scale and decay into small-scale velocity
noise. Reduced viscosity settings improve the situation, but the shape of the
dissipation range differs compared with expectations for a Kolmogorov cascade.
In contrast, our moving-mesh technique does yield power-law scaling laws for
the power spectra of velocity, vorticity and density, consistent with
expectations for fully developed isotropic turbulence. We show that large
errors in SPH's gradient estimate and the associated subsonic velocity noise
are ultimately responsible for producing inaccurate results in the subsonic
regime. In contrast, SPH's performance is much better for supersonic
turbulence. [Abridged]Comment: 22 pages, 20 figures, accepted in MNRAS. Includes a rebuttal to
arXiv:1111.1255 of D. Price and significant revisions to address referee
comments. Conclusions of original submission unchange
Toward an accurate mass function for precision cosmology
Cosmological surveys aim to use the evolution of the abundance of galaxy
clusters to accurately constrain the cosmological model. In the context of
LCDM, we show that it is possible to achieve the required percent level
accuracy in the halo mass function with gravity-only cosmological simulations,
and we provide simulation start and run parameter guidelines for doing so. Some
previous works have had sufficient statistical precision, but lacked robust
verification of absolute accuracy. Convergence tests of the mass function with,
for example, simulation start redshift can exhibit false convergence of the
mass function due to counteracting errors, potentially misleading one to infer
overly optimistic estimations of simulation accuracy. Percent level accuracy is
possible if initial condition particle mapping uses second order Lagrangian
Perturbation Theory, and if the start epoch is between 10 and 50 expansion
factors before the epoch of halo formation of interest. The mass function for
halos with fewer than ~1000 particles is highly sensitive to simulation
parameters and start redshift, implying a practical minimum mass resolution
limit due to mass discreteness. The narrow range in converged start redshift
suggests that it is not presently possible for a single simulation to capture
accurately the cluster mass function while also starting early enough to model
accurately the numbers of reionisation era galaxies, whose baryon feedback
processes may affect later cluster properties. Ultimately, to fully exploit
current and future cosmological surveys will require accurate modeling of
baryon physics and observable properties, a formidable challenge for which
accurate gravity-only simulations are just an initial step.Comment: revised in response to referee suggestions, MNRAS accepte
Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations
We explore some of the effects that discreteness and two-body scattering may
have on N-body simulations with ``realistic'' cosmological initial conditions.
We use an identical subset of particles from the initial conditions for a
Particle-Mesh (PM) calculation as the initial conditions for a variety
PM and Tree code runs. We investigate the effect of mass resolution (the
mean interparticle separation) since most ``high resolution'' codes only have
high resolution in gravitational force. The phase-insensitive two--point
statistics, such as the power spectrum (autocorrelation) are somewhat affected
by these variations, but phase-sensitive statistics show greater differences.
Results converge at the mean interparticle separation scale of the lowest
mass-resolution code. As more particles are added, but the force resolution is
held constant, the PM and the Tree runs agree more and more strongly with
each other and with the PM run which had the same initial conditions. This
shows high particle density is necessary for correct time evolution, since many
different results cannot all be correct. However, they do not so converge to a
PM run which continued the fluctuations to small scales. Our results show that
ignoring them is a major source of error on comoving scales of the missing
wavelengths. This can be resolved by putting in a high particle density. Since
the codes never agree well on scales below the mean comoving interparticle
separation, we find little justification for quantitative predictions on this
scale. Some measures vary by 50%, but others can be off by a factor of three or
more. Our results suggest possible problems with the density of galaxy halos,
formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure
changed; expanded discussion, more information on code parameters. Latex, 44
pages, including 19 figures. Higher resolution versions of Figures 10-15
available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod
The impact of numerical viscosity in SPH simulations of galaxy clusters
A SPH code employing a time-dependent artificial viscosity scheme is used to
construct a large set of N-body/SPH cluster simulations for studying the impact
of artificial viscosity on the thermodynamics of the ICM and its velocity field
statistical properties. Spectral properties of the gas velocity field are
investigated by measuring for the simulated clusters the velocity power
spectrum E(k). The longitudinal component E_c(k) exhibits over a limited range
a Kolgomorov-like scaling k^{-5/3}, whilst the solenoidal power spectrum
component E_s(k) is strongly influenced by numerical resolution effects. The
dependence of the spectra E(k) on dissipative effects is found to be
significant at length scales 100-300Kpc, with viscous damping of the velocities
being less pronounced in those runs with the lowest artificial viscosity. The
turbulent energy density radial profile E_{turb}(r) is strongly affected by the
numerical viscosity scheme adopted in the simulations, with the
turbulent-to-total energy density ratios being higher in the runs with the
lowest artificial viscosity settings and lying in the range between a few
percent and ~10%. These values are in accord with the corresponding ratios
extracted from previous cluster simulations realized using mesh-based codes. At
large cluster radii, the mass correction terms to the hydrostatic equilibrium
equation are little affected by the numerical viscosity of the simulations,
showing that the X-ray mass bias is already estimated well in standard SPH
simulations. Finally, simulations in which the gas can cool radiatively are
characterized by the presence in the cluster inner regions of high levels of
turbulence, generated by the interaction of the compact cool gas core with the
ambient medium.Comment: 32 pages, 22 figures, accepted for publication in A&
The Coyote Universe I: Precision Determination of the Nonlinear Matter Power Spectrum
Near-future cosmological observations targeted at investigations of dark
energy pose stringent requirements on the accuracy of theoretical predictions
for the clustering of matter. Currently, N-body simulations comprise the only
viable approach to this problem. In this paper we demonstrate that N-body
simulations can indeed be sufficiently controlled to fulfill these requirements
for the needs of ongoing and near-future weak lensing surveys. By performing a
large suite of cosmological simulation comparison and convergence tests we show
that results for the nonlinear matter power spectrum can be obtained at 1%
accuracy out to k~1 h/Mpc. The key components of these high accuracy
simulations are: precise initial conditions, very large simulation volumes,
sufficient mass resolution, and accurate time stepping. This paper is the first
in a series of three, with the final aim to provide a high-accuracy prediction
scheme for the nonlinear matter power spectrum.Comment: 18 pages, 22 figures, minor changes to address referee repor
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