2,346,013 research outputs found
The Coyote Universe III: Simulation Suite and Precision Emulator for the Nonlinear Matter Power Spectrum
Many of the most exciting questions in astrophysics and cosmology, including
the majority of observational probes of dark energy, rely on an understanding
of the nonlinear regime of structure formation. In order to fully exploit the
information available from this regime and to extract cosmological constraints,
accurate theoretical predictions are needed. Currently such predictions can
only be obtained from costly, precision numerical simulations. This paper is
the third in a series aimed at constructing an accurate calibration of the
nonlinear mass power spectrum on Mpc scales for a wide range of currently
viable cosmological models, including dark energy. The first two papers
addressed the numerical challenges, and the scheme by which an interpolator was
built from a carefully chosen set of cosmological models. In this paper we
introduce the "Coyote Univers"' simulation suite which comprises nearly 1,000
N-body simulations at different force and mass resolutions, spanning 38 wCDM
cosmologies. This large simulation suite enables us to construct a prediction
scheme, or emulator, for the nonlinear matter power spectrum accurate at the
percent level out to k~1 h/Mpc. We describe the construction of the emulator,
explain the tests performed to ensure its accuracy, and discuss how the central
ideas may be extended to a wider range of cosmological models and applications.
A power spectrum emulator code is released publicly as part of this paper.Comment: 10 pages, 10 figures, minor changes to address referee report,
version v1.1 of the power spectrum emulator code can be downloaded at
http://www.hep.anl.gov/cosmology/CosmicEmu/emu.html, includes now fortran
wrapper and choice of any redshift between z=0 and z=1 (note: webpage now
maintained at Argonne National Laboratory
Cosmological Dynamics of a Dirac-Born-Infeld field
We analyze the dynamics of a Dirac-Born-Infeld (DBI) field in a cosmological
set-up which includes a perfect fluid. Introducing convenient dynamical
variables, we show the evolution equations form an autonomous system when the
potential and the brane tension of the DBI field are arbitrary power-law or
exponential functions of the DBI field. In particular we find scaling solutions
can exist when powers of the field in the potential and warp-factor satisfy
specific relations. A new class of fixed-point solutions are obtained
corresponding to points which initially appear singular in the evolution
equations, but on closer inspection are actually well defined. In all cases, we
perform a phase-space analysis and obtain the late-time attractor structure of
the system. Of particular note when considering cosmological perturbations in
DBI inflation is a fixed-point solution where the Lorentz factor is a finite
large constant and the equation of state parameter of the DBI field is .
Since in this case the speed of sound becomes constant, the solution can
be thought to serve as a good background to perturb about.Comment: 24 pages, 7 figures, minor corrections, references adde
On the equilibrium morphology of systems drawn from spherical collapse experiments
We present a purely theoretical study of the morphological evolution of
self-gravitating systems formed through the dissipationless collapse of N-point
sources. We explore the effects of resolution in mass and length on the growth
of triaxial structures formed by an instability triggered by an excess of
radial orbits. We point out that as resolution increases, the equilibria shift,
from mildly prolate, to oblate. A number of particles N ~= 100000 or larger is
required for convergence of axial aspect ratios. An upper bound for the
softening, e ~ 1/256, is also identified. We then study the properties of a set
of equilibria formed from scale-free cold initial mass distributions, ro ~ r^-g
with 0 <= g <= 2. Oblateness is enhanced for initially more peaked structures
(larger values of g). We map the run of density in space and find no evidence
for a power-law inner structure when g <= 3/2 down to a mass fraction <~0.1 per
cent of the total. However, when 3/2 < g <= 2, the mass profile in equilibrium
is well matched by a power law of index ~g out to a mass fraction ~ 10 per
cent. We interpret this in terms of less-effective violent relaxation for more
peaked profiles when more phase mixing takes place at the centre. We map out
the velocity field of the equilibria and note that at small radii the velocity
coarse-grained distribution function (DF) is Maxwellian to a very good
approximation.Comment: 16 page
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