13,183 research outputs found
Interacting Constituents in Cosmology
Universe evolution, as described by Friedmann's equations, is determined by
source terms fixed by the choice of pressure energy-density equations
of state . The usual approach in Cosmology considers equations of
state accounting only for kinematic terms, ignoring the contribution from the
interactions between the particles constituting the source fluid. In this work
the importance of these neglected terms is emphasized. A systematic method,
based on the Statistical Mechanics of real fluids, is proposed to include them.
A toy-model is presented which shows how such interaction terms can engender
significant cosmological effects.Comment: 24 pages, 6 figures. It includes results presented in "Cosmic
Acceleration from Elementary Interactions" [arXiv:gr-qc/0512135]. Citations
added in v.
AX-GADGET: a new code for cosmological simulations of Fuzzy Dark Matter and Axion models
We present a new module of the parallel N-Body code P-GADGET3 for
cosmological simulations of light bosonic non-thermal dark matter, often
referred as Fuzzy Dark Matter (FDM). The dynamics of the FDM features a highly
non-linear Quantum Potential (QP) that suppresses the growth of structures at
small scales. Most of the previous attempts of FDM simulations either evolved
suppressed initial conditions, completely neglecting the dynamical effects of
QP throughout cosmic evolution, or resorted to numerically challenging
full-wave solvers. The code provides an interesting alternative, following the
FDM evolution without impairing the overall performance. This is done by
computing the QP acceleration through the Smoothed Particle Hydrodynamics (SPH)
routines, with improved schemes to ensure precise and stable derivatives. As an
extension of the P-GADGET3 code, it inherits all the additional physics modules
implemented up to date, opening a wide range of possibilities to constrain FDM
models and explore its degeneracies with other physical phenomena. Simulations
are compared with analytical predictions and results of other codes, validating
the QP as a crucial player in structure formation at small scales.Comment: 18 page
Topics on Hydrodynamic Model of Nucleus-Nucleus Collisions
A survey is given on the applications of hydrodynamic model of
nucleus-nucleus collisons, focusing especially on i) the resolution of
hydrodynamic equations for arbitrary configurations, by using the
smoothed-particle hydrodynamic approach; ii) effects of the event-by-event
fluctuation of the initial conditions on the observables; iii) decoupling
criteria; iv) analytical solutions; and others.Comment: 30 pages, 29 figures; corrected typo
Gravitational collisions and the quark-gluon plasma
This thesis addresses the thermalisation of heavy-ion collisions within the
context of the AdS/CFT duality. The first part clarifies the numerical set-up
and studies the relaxation of far-from-equilibrium modes in homogeneous
systems. Less trivially we then study colliding shock waves and uncover a
transparent regime where the strongly coupled shocks initially pass right
through each other. Furthermore, in this regime the later plasma relaxation is
insensitive to the longitudinal profile of the shock, implying in particular a
universal rapidity shape at strong coupling and high collision energies.
Lastly, we study radial expansion in a boost-invariant set-up, allowing us to
find good agreement with head-on collisions performed at the LHC accelerator.
As a secondary goal of this thesis, a special effort is made to clearly
expose numerical computations by providing commented Mathematica notebooks for
most calculations presented. Furthermore, we provide interpolating functions of
the geometries computed, which can be of use in other projects.Comment: PhD thesis, 100 pages, 80 figures.
http://dspace.library.uu.nl/handle/1874/294809 , Mathematica notebooks can be
found at sites.google.com/site/wilkevanderschee/phd-thesi
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Inertial Coupling Method for particles in an incompressible fluctuating fluid
We develop an inertial coupling method for modeling the dynamics of
point-like 'blob' particles immersed in an incompressible fluid, generalizing
previous work for compressible fluids. The coupling consistently includes
excess (positive or negative) inertia of the particles relative to the
displaced fluid, and accounts for thermal fluctuations in the fluid momentum
equation. The coupling between the fluid and the blob is based on a no-slip
constraint equating the particle velocity with the local average of the fluid
velocity, and conserves momentum and energy. We demonstrate that the
formulation obeys a fluctuation-dissipation balance, owing to the
non-dissipative nature of the no-slip coupling. We develop a spatio-temporal
discretization that preserves, as best as possible, these properties of the
continuum formulation. In the spatial discretization, the local averaging and
spreading operations are accomplished using compact kernels commonly used in
immersed boundary methods. We find that the special properties of these kernels
make the discrete blob a particle with surprisingly physically-consistent
volume, mass, and hydrodynamic properties. We develop a second-order
semi-implicit temporal integrator that maintains discrete
fluctuation-dissipation balance, and is not limited in stability by viscosity.
Furthermore, the temporal scheme requires only constant-coefficient Poisson and
Helmholtz linear solvers, enabling a very efficient and simple FFT-based
implementation on GPUs. We numerically investigate the performance of the
method on several standard test problems...Comment: Contains a number of corrections and an additional Figure 7 (and
associated discussion) relative to published versio
- …