1,891 research outputs found
Improved Ways to Compare Simulations to Data
Theoretical models for structure formation with Gaussian initial fluctuations
have been worked out in considerable detail and compared with observations on
various scales. It is on nonlinear scales \lsim 10 \ h^{-1}\ {\rm Mpc} that
the greatest differences exist between models that have been
normalized to agree on the largest scales with the COBE data; here especially
there is a need for better statistical tests which are simultaneously {\it
robust}, {\it discriminatory}, and {\it interpretable}. The era at which galaxy
and cluster formation occurs is also a critical test of some models. Needs for
the future include faster and cleverer codes, better control of cosmic variance
in simulations, better understanding of processes leading to galaxy formation,
better ways of comparing observational data with models, and better access to
observational and simulation data.Comment: 9 pages, self-extracting uuencoded postscript, encoded with uufile
The Universe at Extreme Scale: Multi-Petaflop Sky Simulation on the BG/Q
Remarkable observational advances have established a compelling
cross-validated model of the Universe. Yet, two key pillars of this model --
dark matter and dark energy -- remain mysterious. Sky surveys that map billions
of galaxies to explore the `Dark Universe', demand a corresponding
extreme-scale simulation capability; the HACC (Hybrid/Hardware Accelerated
Cosmology Code) framework has been designed to deliver this level of
performance now, and into the future. With its novel algorithmic structure,
HACC allows flexible tuning across diverse architectures, including accelerated
and multi-core systems.
On the IBM BG/Q, HACC attains unprecedented scalable performance -- currently
13.94 PFlops at 69.2% of peak and 90% parallel efficiency on 1,572,864 cores
with an equal number of MPI ranks, and a concurrency of 6.3 million. This level
of performance was achieved at extreme problem sizes, including a benchmark run
with more than 3.6 trillion particles, significantly larger than any
cosmological simulation yet performed.Comment: 11 pages, 11 figures, final version of paper for talk presented at
SC1
Separate Universe Simulations with IllustrisTNG: baryonic effects on power spectrum responses and higher-order statistics
We measure power spectrum response functions in the presence of baryonic
physical processes using separate universe simulations with the IllustrisTNG
galaxy formation model. The response functions describe how the small-scale
power spectrum reacts to long-wavelength perturbations and they can be
efficiently measured with the separate universe technique by absorbing the
effects of the long modes into a modified cosmology. Specifically, we focus on
the total first-order matter power spectrum response to an isotropic density
fluctuation , which is fully determined by the logarithmic derivative
of the nonlinear matter power spectrum and the
growth-only response function . We find that is not
affected by the baryonic physical processes in the simulations at redshifts and on all scales probed (, i.e. length scales
). In practice, this implies that the power spectrum
fully specifies the baryonic dependence of its response function. Assuming an
idealized lensing survey setup, we evaluate numerically the baryonic impact on
the squeezed-lensing bispectrum and the lensing super-sample power spectrum
covariance, which are given in terms of responses. Our results show that these
higher-order lensing statistics can display varying levels of sensitivity to
baryonic effects compared to the power spectrum, with the squeezed-bispectrum
being the least sensitive. We also show that ignoring baryonic effects on
lensing covariances slightly overestimates the error budget (and is therefore
conservative from the point of view of parameter error bars) and likely has
negligible impact on parameter biases in inference analyses.Comment: 15 pages, 6 figures, 1 table; comments welcomed! v2 matches version
published in MNRA
Properties of Cosmological Filaments extracted from Eulerian Simulations
Using a new parallel algorithm implemented within the VisIt framework, we
analysed large cosmological grid simulations to study the properties of baryons
in filaments. The procedure allows us to build large catalogues with up to
filaments per simulated volume and to investigate the
properties of cosmic filaments for very large volumes at high resolution (up to
simulated with cells). We determined scaling
relations for the mass, volume, length and temperature of filaments and
compared them to those of galaxy clusters. The longest filaments have a total
length of about with a mass of several . We
also investigated the effects of different gas physics. Radiative cooling
significantly modifies the thermal properties of the warm-hot-intergalactic
medium of filaments, mainly by lowering their mean temperature via line
cooling. On the other hand, powerful feedback from active galactic nuclei in
surrounding halos can heat up the gas in filaments. The impact of
shock-accelerated cosmic rays from diffusive shock acceleration on filaments is
small and the ratio of between cosmic ray and gas pressure within filaments is
of the order of percent.Comment: 27 pages, 24 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Ultra-High Energy Cosmic Rays in a Structured and Magnetized Universe
We simulate propagation of cosmic ray nucleons above 10^{19} eV in scenarios
where both the source distribution and magnetic fields within about 50 Mpc from
us are obtained from an unconstrained large scale structure simulation. We find
that consistency of predicted sky distributions with current data above 4 x
10^{19} eV requires magnetic fields of ~0.1 microGauss in our immediate
environment, and a nearby source density of ~10^{-4}-10^{-3} Mpc^{-3}. Radio
galaxies could provide the required sources, but only if both high and
low-luminosity radio galaxies are very efficient cosmic ray accelerators.
Moreover, at ~10^{19} eV an additional isotropic flux component, presumably of
cosmological origin, should dominate over the local flux component by about a
factor three in order to explain the observed isotropy. This argues against the
scenario in which local astrophysical sources of cosmic rays above ~10^{19} eV
reside in strongly magnetized (B~0.1 microGauss) and structured intergalactic
medium. Finally we discuss how future large scale full-sky detectors such as
the Pierre Auger project will allow to put much more stringent constraints on
source and magnetic field distributions.Comment: 11 revtex pages, 10 postscript figures included, final version to
appear in PR
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