111 research outputs found
Simulating the universe on an intercontinental grid of supercomputers
Understanding the universe is hampered by the elusiveness of its most common
constituent, cold dark matter. Almost impossible to observe, dark matter can be
studied effectively by means of simulation and there is probably no other
research field where simulation has led to so much progress in the last decade.
Cosmological N-body simulations are an essential tool for evolving density
perturbations in the nonlinear regime. Simulating the formation of large-scale
structures in the universe, however, is still a challenge due to the enormous
dynamic range in spatial and temporal coordinates, and due to the enormous
computer resources required. The dynamic range is generally dealt with by the
hybridization of numerical techniques. We deal with the computational
requirements by connecting two supercomputers via an optical network and make
them operate as a single machine. This is challenging, if only for the fact
that the supercomputers of our choice are separated by half the planet, as one
is located in Amsterdam and the other is in Tokyo. The co-scheduling of the two
computers and the 'gridification' of the code enables us to achieve a 90%
efficiency for this distributed intercontinental supercomputer.Comment: Accepted for publication in IEEE Compute
Simulating the Universe with MICE: The abundance of massive clusters
We introduce a new set of large N-body runs, the MICE simulations, that
provide a unique combination of very large cosmological volumes with good mass
resolution. They follow the gravitational evolution of ~ 8.5 billion particles
(2048^3) in volumes covering up to 450 (Gpc/h)^3. Our main goal is to
accurately model and calibrate basic cosmological probes that will be used by
upcoming astronomical surveys. Here we take advantage of the very large volumes
of MICE to make a robust sampling of the high-mass tail of the halo mass
function (MF). We discuss and avoid possible systematic effects in our study,
and do a detailed analysis of different error estimators. We find that
available fits to the local abundance of halos (Warren et al. (2006)) match
well the abundance in MICE up to M ~ 10^{14}\Msun, but significantly deviate
for larger masses, underestimating the mass function by 10% (30%) at M = 3.16 x
10^{14}\Msun (10^{15}\Msun). Similarly, the widely used Sheth & Tormen (1999)
fit, if extrapolated to high redshift assuming universality, leads to an
underestimation of the cluster abundance by 30%, 20% and 15% at z=0, 0.5, 1 for
M ~ [7 - 2.5 - 0.8] x 10^{14}\Msun respectively ().
We provide a re-calibration of the halo MF valid over 5 orders of magnitude in
mass, 10^{10} < M/(\Msun) < 10^{15}, that accurately describes its redshift
evolution up to z=1. We explore the impact of this re-calibration on the
determination of dark-energy, and conclude that using available fits may
systematically bias the estimate of w by as much as 50% for medium-depth (z <=
1) surveys. MICE halo catalogues are publicly available at
http://www.ice.cat/miceComment: 16 pages, 11 figures. Data publicly available at
http://www.ice.cat/mice. New version adds discussion on halo definition (SO
vs FoF) and minor modifications. Accepted for publication in MNRA
Simulating the universe(s) Ill: observables for the full bubble collision spacetime
This is the third paper in a series establishing a quantitative relation between inflationary scalar field potential landscapes and the relic perturbations left by the collision between bubbles produced during eternal inflation. We introduce a new method for computing cosmological observables from numerical relativity simulations of bubble collisions in one space and one time dimension. This method tiles comoving hypersurfaces with locally-perturbed Friedmann-Robertson-Walker coordinate patches. The method extends previous work, which was limited to the spacetime region just inside the future light cone of the collision, and allows us to explore the full bubble-collision spacetime. We validate our new methods against previous work, and present a full set of predictions for the comoving curvature perturbation and local negative spatial curvature produced by identical and non-identical bubble collisions, in single scalar field models of eternal inflation. In both collision types, there is a non-zero contribution to the spatial curvature and cosmic microwave background quadrupole. Some collisions between non-identical bubbles excite wall modes, giving extra structure to the predicted temperature anisotropies. We comment on the implications of our results for future observational searches. For non-identical bubble collisions, we also find that the surfaces of constant field can readjust in the presence of a collision to produce spatially infinite sections that become nearly homogeneous deep into the region affected by the collision. Contrary to previous assumptions, this is true even in the bubble into which the domain wall is accelerating
Interactions of Satellite Galaxies in Cosmological Dark Matter Halos
We present a statistical analysis of the interactions between satellite
galaxies in cosmological dark matter halos taken from fully self-consistent
high-resolution simulations of galaxy clusters. We show that the number
distribution of satellite encounters has a tail that extends to as many as 3-4
encounters per orbit. On average 30% of the substructure population had at
least one encounter (per orbit) with another satellite galaxy. However, this
result depends on the age of the dark matter host halo with a clear trend for
more interactions in younger systems. We also report a correlation between the
number of encounters and the distance of the satellites to the centre of the
cluster: satellite galaxies closer to the centre experience more interactions.
However, this can be simply explained by the radial distribution of the
substructure population and merely reflects the fact that the density of
satellites is higher in those regions.
In order to find substructure galaxies we applied (and present) a new
technique based upon the N-body code MLAPM. This new halo finder MHF
(MLAPM's-Halo-Finder) acts with exactly the same accuracy as the N-body code
itself and is therefore free of any bias and spurious mismatch between
simulation data and halo finding precision related to numerical effects.Comment: 6 pages, 4 figures, accepted by PASA (refereed contribution to the
5th Galactic Chemodynamics workshop, July 2003
MPWide: a light-weight library for efficient message passing over wide area networks
We present MPWide, a light weight communication library which allows
efficient message passing over a distributed network. MPWide has been designed
to connect application running on distributed (super)computing resources, and
to maximize the communication performance on wide area networks for those
without administrative privileges. It can be used to provide message-passing
between application, move files, and make very fast connections in
client-server environments. MPWide has already been applied to enable
distributed cosmological simulations across up to four supercomputers on two
continents, and to couple two different bloodflow simulations to form a
multiscale simulation.Comment: accepted by the Journal Of Open Research Software, 13 pages, 4
figures, 1 tabl
Testing eternal inflation with the kinetic Sunyaev Zel'dovich effect
Perhaps the most controversial idea in modern cosmology is that our
observable universe is contained within one bubble among many, all inhabiting
the eternally inflating multiverse. One of the few way to test this idea is to
look for evidence of the relic inhomogeneities left by the collisions between
other bubbles and our own. Such relic inhomogeneities induces a coherent bulk
flow over gigaparsec scales. Therefore, bubble collisions leave unique imprints
in the cosmic microwave background (CMB) through the kinetic Sunyaev Zel'dovich
(kSZ) effect, temperature anisotropies induced by the scattering of photons
from coherently moving free electrons in the diffuse intergalactic medium. The
kSZ signature produced by bubble collisions has a unique directional dependence
and is tightly correlated with the galaxy distribution; it can therefore be
distinguished from other contributions to the CMB anisotropies. An important
advantage of the kSZ signature is that it peaks on arcminute angular scales,
where the limiting factors in making a detection are instrumental noise and
foreground subtraction. This is in contrast to the collision signature in the
primary CMB, which peaks on angular scales much larger than one degree, and
whose detection is therefore limited by cosmic variance. In this paper, we
examine the prospects for probing the inhomogeneities left by bubble collisions
using the kSZ effect. We provide a forecast for detection using
cross-correlations between CMB and galaxy surveys, finding that the
detectability using the kSZ effect can be competitive with constraints from CMB
temperature and polarization data.Comment: 33 pages, 17 figures. Minor clarifications added in version 2,
conclusions are unchange
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