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

Astrophysical turbulence modeling

The role of turbulence in various astrophysical settings is reviewed. Among the differences to laboratory and atmospheric turbulence we highlight the ubiquitous presence of magnetic fields that are generally produced and maintained by dynamo action. The extreme temperature and density contrasts and stratifications are emphasized in connection with turbulence in the interstellar medium and in stars with outer convection zones, respectively. In many cases turbulence plays an essential role in facilitating enhanced transport of mass, momentum, energy, and magnetic fields in terms of the corresponding coarse-grained mean fields. Those transport properties are usually strongly modified by anisotropies and often completely new effects emerge in such a description that have no correspondence in terms of the original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic

The Inertial Range of Turbulence in the Inner Heliosheath and in the Local Interstellar Medium

The governing mechanisms of magnetic field annihilation in the outer heliosphere is an intriguing topic. It is currently believed that the turbulent fluctuations pervade the inner heliosheath (IHS) and the Local Interstellar Medium (LISM). Turbulence, magnetic reconnection, or their reciprocal link may be responsible for magnetic energy conversion in the IHS.   As 1-day averaged data are typically used, the present literature mainly concerns large-scale analysis and does not describe inertial-cascade dynamics of turbulence in the IHS. Moreover, lack of spectral analysis make IHS dynamics remain critically understudied. Our group showed that 48-s MAG data from the Voyager mission are appropriate for a power spectral analysis over a frequency range of five decades, from 5e-8 Hz to 1e-2 Hz [Gallana et al., JGR 121 (2016)]. Special spectral estimation techniques are used to deal with the large amount of missing data (70%). We provide the first clear evidence of an inertial-cascade range of turbulence (spectral index is between -2 and -1.5). A spectral break at about 1e-5 Hz is found to separate the inertial range from the enegy-injection range (1/f energy decay). Instrumental noise bounds our investigation to frequencies lower than 5e-4 Hz. By considering several consecutive periods after 2009 at both V1 and V2, we show that the extension and the spectral energy decay of these two regimes may be indicators of IHS regions governed by different physical processes. We describe fluctuations’ regimes in terms of spectral energy density, anisotropy, compressibility, and statistical analysis of intermittency.   In the LISM, it was theorized that pristine interstellar turbulence may coexist with waves from the IHS, however this is still a debated topic. We observe that the fluctuating magnetic energy cascades as a power law with spectral index in the range [-1.35, -1.65] in the whole range of frequencies unaffected by noise. No spectral break is observed, nor decaying turbulence

Doctor of Philosophy

dissertationRecent trends in high performance computing present larger and more diverse computers using multicore nodes possibly with accelerators and/or coprocessors and reduced memory. These changes pose formidable challenges for applications code to attain scalability. Software frameworks that execute machine-independent applications code using a runtime system that shields users from architectural complexities oer a portable solution for easy programming. The Uintah framework, for example, solves a broad class of large-scale problems on structured adaptive grids using fluid-flow solvers coupled with particle-based solids methods. However, the original Uintah code had limited scalability as tasks were run in a predefined order based solely on static analysis of the task graph and used only message passing interface (MPI) for parallelism. By using a new hybrid multithread and MPI runtime system, this research has made it possible for Uintah to scale to 700K central processing unit (CPU) cores when solving challenging fluid-structure interaction problems. Those problems often involve moving objects with adaptive mesh refinement and thus with highly variable and unpredictable work patterns. This research has also demonstrated an ability to run capability jobs on the heterogeneous systems with Nvidia graphics processing unit (GPU) accelerators or Intel Xeon Phi coprocessors. The new runtime system for Uintah executes directed acyclic graphs of computational tasks with a scalable asynchronous and dynamic runtime system for multicore CPUs and/or accelerators/coprocessors on a node. Uintah's clear separation between application and runtime code has led to scalability increases without significant changes to application code. This research concludes that the adaptive directed acyclic graph (DAG)-based approach provides a very powerful abstraction for solving challenging multiscale multiphysics engineering problems. Excellent scalability with regard to the different processors and communications performance are achieved on some of the largest and most powerful computers available today

Numerical Models of AGN Jet Feedback

Active Galactic Nuclei (AGNs) power the most luminous sources in the Universe, and generate very energetic large-scale radio jets. These processes are important in galaxy formation models and numerical simulations, which require energy from AGNs to prevent central gas from overcooling. In this thesis we use hydrodynamical simulations to explore the impact of jets on the intra-galactic medium of individual halos, with particular focus on massive galaxies. The simulations are performed with the FLASH code, and we took advantage of the Adaptive Mesh Refinement scheme to deal with the complex, multiscale physics of AGNs on scales ranging from Megaparsec down to a few tens of parsecs. In the first part of this thesis we describe in detail the first few millions years of AGN jets, identifying precise evolutionary stages and testing our findings against theoretical models. We discuss gas circulation within the “cocoon” carved by the jet as a possible self-regulation mechanism for jet activities. In the second part, we extend the analysis to cosmologically relevant timescales, and carry on a detailed thermodynamyc analysis of the jet-gas system, including mechanical work, global energy transfer and volume fraction of the heated gas. Finally, we present a few extensions of our model such as multiple jet events, and take a few steps towards direct comparison with X-ray observations

The Multiscale Microphysics of Galactic Outflows

Observations indicate spiral galaxies ubiquitously launch multiphase outflows, which help to explain observations of self-regulating star formation in the disk and metallicities of the circumgalactic, intergalactic, and intracluster media much higher than primordial abundances. These outflows are composed of hot $10^{6-7}$,K X-ray emitting gas, cool atomic $sim10^4$,K gas, cold molecular gas, and cosmic rays. The observed rapidly outflowing cool and cold gas phases are theoretically puzzling, as such gas should be incorporated into the hot phase before it can be entrained. The presence of cold gas in the ram pressure stripped tails of galaxies in cluster outskirts is similarly surprising, as the intracluster medium is even hotter $sim$10$^8$,K. In this dissertation, I study the physics responsible for launching galactic outflows at multiple scales. Performing simulations on the scale of interstellar medium patches, I find that cosmic-ray transport plays a crucial role in their ability to launch outflows. Temperature-dependent transport of cosmic rays helps launch fast outflows and generate large-scale radio halos. Studying the microscopic scale of individual cold clouds in a thermally driven, transonic outflow, I find molecular material can survive the entrainment process for clouds larger than a critical radius. At the macroscopic scale of global spiral disks in cluster environments, I find cosmic rays modify the response of the interstellar medium to the ram pressure of the intracluster medium wind. Specifically, I find that cosmic rays protect cold, tenuous gas that is otherwise stripped in purely thermal models. Moreover, the influence of cosmic rays on the star formation rate and the accretion of material towards the galactic center, powering the activity of galactic nuclei, may provide new constraints on cosmic-ray transport and calorimetry. These results imply that thermal and cosmic-ray feedback play a crucial role in understanding multiphase galactic outflows.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169903/1/rjfarber_1.pd

Eulerian and Lagrangian Statistics from high resolution Numerical Simulations of weakly compressible turbulence

We report a detailed study of Eulerian and Lagrangian statistics from high resolution Direct Numerical Simulations of isotropic weakly compressible turbulence. Reynolds number at the Taylor microscale is estimated to be around 600. Eulerian and Lagrangian statistics is evaluated over a huge data set, made by $1856^3$ spatial collocation points and by 16 million particles, followed for about one large-scale eddy turn over time. We present data for Eulerian and Lagrangian Structure functions up to ten order. We analyse the local scaling properties in the inertial range and in the viscous range. Eulerian results show a good superposition with previous data. Lagrangian statistics is different from existing experimental and numerical results, for moments of sixth order and higher. We interpret this in terms of a possible contamination from viscous scale affecting the estimate of the scaling properties in previous studies. We show that a simple bridge relation based on Multifractal theory is able to connect scaling properties of both Eulerian and Lagrangian observables, provided that the small differences between intermittency of transverse and longitudinal Eulerian structure functions are properly considered.Comment: 20 pages, 10 figure