1,131 research outputs found
Numerical Simulations of the Dark Universe: State of the Art and the Next Decade
We present a review of the current state of the art of cosmological dark
matter simulations, with particular emphasis on the implications for dark
matter detection efforts and studies of dark energy. This review is intended
both for particle physicists, who may find the cosmological simulation
literature opaque or confusing, and for astro-physicists, who may not be
familiar with the role of simulations for observational and experimental probes
of dark matter and dark energy. Our work is complementary to the contribution
by M. Baldi in this issue, which focuses on the treatment of dark energy and
cosmic acceleration in dedicated N-body simulations. Truly massive dark
matter-only simulations are being conducted on national supercomputing centers,
employing from several billion to over half a trillion particles to simulate
the formation and evolution of cosmologically representative volumes (cosmic
scale) or to zoom in on individual halos (cluster and galactic scale). These
simulations cost millions of core-hours, require tens to hundreds of terabytes
of memory, and use up to petabytes of disk storage. The field is quite
internationally diverse, with top simulations having been run in China, France,
Germany, Korea, Spain, and the USA. Predictions from such simulations touch on
almost every aspect of dark matter and dark energy studies, and we give a
comprehensive overview of this connection. We also discuss the limitations of
the cold and collisionless DM-only approach, and describe in some detail
efforts to include different particle physics as well as baryonic physics in
cosmological galaxy formation simulations, including a discussion of recent
results highlighting how the distribution of dark matter in halos may be
altered. We end with an outlook for the next decade, presenting our view of how
the field can be expected to progress. (abridged)Comment: 54 pages, 4 figures, 3 tables; invited contribution to the special
issue "The next decade in Dark Matter and Dark Energy" of the new Open Access
journal "Physics of the Dark Universe". Replaced with accepted versio
A Constrained Transport Scheme for MHD on Unstructured Static and Moving Meshes
Magnetic fields play an important role in many astrophysical systems and a
detailed understanding of their impact on the gas dynamics requires robust
numerical simulations. Here we present a new method to evolve the ideal
magnetohydrodynamic (MHD) equations on unstructured static and moving meshes
that preserves the magnetic field divergence-free constraint to machine
precision. The method overcomes the major problems of using a cleaning scheme
on the magnetic fields instead, which is non-conservative, not fully Galilean
invariant, does not eliminate divergence errors completely, and may produce
incorrect jumps across shocks. Our new method is a generalization of the
constrained transport (CT) algorithm used to enforce the condition on fixed Cartesian grids. Preserving at the discretized level is necessary to maintain the
orthogonality between the Lorentz force and . The possibility of
performing CT on a moving mesh provides several advantages over static mesh
methods due to the quasi-Lagrangian nature of the former (i.e., the mesh
generating points move with the flow), such as making the simulation
automatically adaptive and significantly reducing advection errors. Our method
preserves magnetic fields and fluid quantities in pure advection exactly.Comment: 13 pages, 9 figures, accepted to MNRAS. Animations available at
http://www.cfa.harvard.edu/~pmocz/research.htm
Sloshing of Galaxy Cluster Core Plasma in the Presence of Self-Interacting Dark Matter
The "sloshing" of the cold gas in the cores of relaxed clusters of galaxies
is a widespread phenomenon, evidenced by the presence of spiral-shaped "cold
fronts" in X-ray observations of these systems. In simulations, these flows of
cold gas readily form by interactions of the cluster core with small
subclusters, due to a separation of the cold gas from the dark matter (DM), due
to their markedly different collisionalities. In this work, we use numerical
simulations to investigate the effects of increasing the DM collisionality on
sloshing cold fronts in a cool-core cluster. For clusters in isolation, the
formation of a flat DM core via self-interactions results in modest adiabatic
expansion and cooling of the core gas. In merger simulations, cold fronts form
in the same manner as in previous simulations, but the flattened potential in
the core region enables the gas to expand to larger radii in the initial
stages. Upon infall, the subcluster's DM mass decreases via collisions,
reducing its influence on the core. Thus, the sloshing gas moves slower,
inhibiting the growth of fluid instabilities relative to simulations where the
DM cross section is zero. This also inhibits turbulent mixing and the increase
in entropy that would otherwise result. For values of the cross section
, subclusters do not survive as self-gravitating structures for
more than two core passages. Additionally, separations between the peaks in the
X-ray emissivity and thermal Sunyaev-Zeldovich effect signals during sloshing
may place constraints on DM self-interactions.Comment: 20 pages, 14 figures, submitted to Ap
The large-scale properties of simulated cosmological magnetic fields
We perform uniformly sampled large-scale cosmological simulations including
magnetic fields with the moving mesh code AREPO. We run two sets of MHD
simulations: one including adiabatic gas physics only; the other featuring the
fiducial feedback model of the Illustris simulation. In the adiabatic case, the
magnetic field amplification follows the scaling derived
from `flux-freezing' arguments, with the seed field strength providing an
overall normalization factor. At high baryon overdensities the amplification is
enhanced by shear flows and turbulence. Feedback physics and the inclusion of
radiative cooling change this picture dramatically. In haloes, gas collapses to
much larger densities and the magnetic field is amplified strongly and to the
same maximum intensity irrespective of the initial seed field of which any
memory is lost. At lower densities a dependence on the seed field strength and
orientation, which in principle can be used to constrain models of cosmic
magnetogenesis, is still present. Inside the most massive haloes magnetic
fields reach values of , in agreement with galaxy
cluster observations. The topology of the field is tangled and gives rise to
rotation measure signals in reasonable agreement with the observations.
However, the rotation measure signal declines too rapidly towards larger radii
as compared to observational data.Comment: 23 pages, 19 figures, 1 table. Accepted for publication in MNRAS.
Edited to match published versio
Dark matter halo's and self similarity
This papers explores the self similar solutions of the Vlasov-Poisson system
and their relation to the gravitational collapse of dynamically cold systems.
Analytic solutions are derived for power law potential in one dimension, and
extensions of these solutions in three dimensions are proposed. Next the self
similarity of the collapse of cold dynamical systems is investigated
numerically. The fold system in phase space is consistent with analytic self
similar solutions, the solutions present all the proper self-similar scalings.
An additional point is the appearance of an law at the center of
the system for initial conditions with power law index larger than . It
is found that the first appearance of the law corresponds to the
formation of a singularity very close to the center. Finally the general
properties of self similar multi dimensional solutions near equilibrium are
investigated. Smooth and continuous self similar solutions have power law
behavior at equilibrium. However cold initial conditions result in
discontinuous phase space solutions, and the smoothed phase space density
looses its auto similar properties. This problem is easily solved by observing
that the probability distribution of the phase space density is identical
except for scaling parameters to the probability distribution of the smoothed
phase space density . As a consequence inherit the self similar
properties of . This particular property is at the origin of the universal
power law observed in numerical simulation for . The self
similar properties of implies that other quantities should have also an
universal power law behavior with predictable exponents. This hypothesis is
tested using a numerical model of the phase space density of cold dark matter
halo's, an excellent agreement is obtained.Comment: Final versio
Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement
We calculate how the relic density of dark matter particles is altered when
their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa
interaction between the annihilating particles. Maintaining a dark matter
abundance consistent with current observational bounds requires the
normalization of the s-wave annihilation cross section to be decreased compared
to a model without enhancement. The level of suppression depends on the
specific parameters of the particle model, with the kinetic decoupling
temperature having the most effect. We find that the cross section can be
reduced by as much as an order of magnitude for extreme cases. We also compute
the mu-type distortion of the CMB energy spectrum caused by energy injection
from such Sommerfeld-enhanced annihilation. Our results indicate that in the
vicinity of resonances, associated with bound states, distortions can be large
enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the
COBE/FIRAS experiment.Comment: 10 pages, 6 figures, accepted for publication in Physical Review D.
Corrections to eqs. 9,10,14 and 16. Figures updated accordingly. No major
changes to previous results. Website with online tools for Sommerfeld-related
calculations can be found at
http://www.mpa-garching.mpg.de/~vogelsma/sommerfeld
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