Nonlinear growth of structure in cosmological simulations

Upcoming cosmological observations (South Pole Telescope, Atacama Cosmology Telescope, Baryon Oscillation Spectroscopic Survey, and Planck) will allow for accurately. probing structures and their growth, some into highly nonlinear regimes. These observations, in combination with already very accurate measurements of the expansion rate of the universe, will not only constrain cosmological parameters to a percent level, but will also answer what is the theory . of gravity on the largest scales. In order to obtain theoretical predictions for different measurables (like the distribution of masses, spatial· correlations), large numerical simulations have to be carried out. In this context, their main goal is to quantify how are such measurables affected by a change of cosmological parameters. The promised high accuracy of observations make the simulation task very demanding, as the theoretical predictions have to be at least as accurate as the observations. In this thesis, we study the formation and evolution of dark matter halos in ACDM models.o~er a wide range of cosmologicalepochs, from redshift z=20 to the present. First, we focus on the halo mass function, likely a key probe of cosmological growth of structure. By performing a large suite (60 simulations) of nested-box N-body simulations with careful convergence and error controls, we determine the mass function and its evolution with excellent statistical and systematic errors, reaching a few percent over most of the considered redshift and mass range. Our results are consistent with a 'universal' form for the mass function, and are in a good agreement with the Warren et al. analytic fit. Next, we. study the· structure of halos and ratification of different halo mass definitions. This analysis is important for connecting structure formation theory with observations, and also impacts the widely used approaches of assigning visible galaxies to dark matter halos - the halo occupancy distribution models. We find that the vast majority of halos (80-85%) appear as isolated objects, allowing for an accurate mapping between the two main mass definitions (friends-of-friends and spherical overdensity). Based on results from Monte Carfo realizations of .ideal Navarro-Frenk-White halos and N-body simulations we provide a mass mapping formula. Furthermore, investigation of non-isolated, bridged halos, reveals that the fraction of these halos and their satellite mass distribution is cosmology dependent, and can be expressed in a cosmology universal form. Third, we turn to the spatial distribution of. halos, which serves as a 'biased' mass tracer. While this bias is scale dependent, at large distances it asymptotes to a constant value. We show that commonly used, heuristic approach to relatingthe mass function to the bias (peak-background split) clearly fails at the accuracy we are interested in (less or equal to 10%). Using our large set of simulations we provide universal formula for halo bias as a function· of mass. This formula fit well not only our data, put the current state of the art simulation data (Millenium simulation). Finally, we present the results of a comparison between 10 different cosmology codes. These include virtually all major codes used today, and more importantly, they completely cover the range of numerical algorithms used in cosmological N-body simulations. For the mass function, the matter power spectrum, arid halo profiles - the most important statistics for this thesis - codes agree at less then 10% over wide dynamic ranges. This robustness gives us additional confidence in our numerical results.Ope

Report from the Tri-Agency Cosmological Simulation Task Force

The Tri-Agency Cosmological Simulations (TACS) Task Force was formed when Program Managers from the Department of Energy (DOE), the National Aeronautics and Space Administration (NASA), and the National Science Foundation (NSF) expressed an interest in receiving input into the cosmological simulations landscape related to the upcoming DOE/NSF Vera Rubin Observatory (Rubin), NASA/ESA's Euclid, and NASA's Wide Field Infrared Survey Telescope (WFIRST). The Co-Chairs of TACS, Katrin Heitmann and Alina Kiessling, invited community scientists from the USA and Europe who are each subject matter experts and are also members of one or more of the surveys to contribute. The following report represents the input from TACS that was delivered to the Agencies in December 2018.Comment: 36 pages, 3 figures. Delivered to NASA, NSF, and DOE in Dec 201

The Universe at Extreme Scale: Multi-Petaflop Sky Simulation on the BG/Q

Remarkable observational advances have established a compelling cross-validated model of the Universe. Yet, two key pillars of this model -- dark matter and dark energy -- remain mysterious. Sky surveys that map billions of galaxies to explore the `Dark Universe', demand a corresponding extreme-scale simulation capability; the HACC (Hybrid/Hardware Accelerated Cosmology Code) framework has been designed to deliver this level of performance now, and into the future. With its novel algorithmic structure, HACC allows flexible tuning across diverse architectures, including accelerated and multi-core systems. On the IBM BG/Q, HACC attains unprecedented scalable performance -- currently 13.94 PFlops at 69.2% of peak and 90% parallel efficiency on 1,572,864 cores with an equal number of MPI ranks, and a concurrency of 6.3 million. This level of performance was achieved at extreme problem sizes, including a benchmark run with more than 3.6 trillion particles, significantly larger than any cosmological simulation yet performed.Comment: 11 pages, 11 figures, final version of paper for talk presented at SC1

Measuring the thermal and ionization state of the low-zz IGM using likelihood free inference

We present a new approach to measure the power-law temperature density relationship $T=T_0 (\rho / \bar{\rho})^{\gamma -1}$ and the UV background photoionization rate $\Gamma_{\rm HI}$ of the IGM based on the Voigt profile decomposition of the Ly$\alpha$ forest into a set of discrete absorption lines with Doppler parameter $b$ and the neutral hydrogen column density $N_{\rm HI}$. Previous work demonstrated that the shape of the $b$-$N_{\rm HI}$ distribution is sensitive to the IGM thermal parameters $T_0$ and $\gamma$, whereas our new inference algorithm also takes into account the normalization of the distribution, i.e. the line-density d$N$/d$z$, and we demonstrate that precise constraints can also be obtained on $\Gamma_{\rm HI}$. We use density-estimation likelihood-free inference (DELFI) to emulate the dependence of the $b$-$N_{\rm HI}$ distribution on IGM parameters trained on an ensemble of 624 Nyx hydrodynamical simulations at $z = 0.1$, which we combine with a Gaussian process emulator of the normalization. To demonstrate the efficacy of this approach, we generate hundreds of realizations of realistic mock HST/COS datasets, each comprising 34 quasar sightlines, and forward model the noise and resolution to match the real data. We use this large ensemble of mocks to extensively test our inference and empirically demonstrate that our posterior distributions are robust. Our analysis shows that by applying our new approach to existing Ly$\alpha$ forest spectra at $z\simeq 0.1$, one can measure the thermal and ionization state of the IGM with very high precision ($\sigma_{\log T_0} \sim 0.08$ dex, $\sigma_\gamma \sim 0.06$, and $\sigma_{\log \Gamma_{\rm HI}} \sim 0.07$ dex).Comment: 20 pages, 17 figures, accepted for publication in MNRA

Particle mesh simulations of the Lyman-alpha forest and the signature of Baryon Acoustic Oscillations in the intergalactic medium

We present a set of ultra-large particle-mesh simulations of the LyA forest targeted at understanding the imprint of baryon acoustic oscillations (BAO) in the inter-galactic medium. We use 9 dark matter only simulations which can, for the first time, simultaneously resolve the Jeans scale of the intergalactic gas while covering the large volumes required to adequately sample the acoustic feature. Mock absorption spectra are generated using the fluctuating Gunn-Peterson approximation which have approximately correct flux probability density functions (PDFs) and small-scale power spectra. On larger scales there is clear evidence in the redshift space correlation function for an acoustic feature, which matches a linear theory template with constant bias. These spectra, which we make publicly available, can be used to test pipelines, plan future experiments and model various physical effects. As an illustration we discuss the basic properties of the acoustic signal in the forest, the scaling of errors with noise and source number density, modified statistics to treat mean flux evolution and misestimation, and non-gravitational sources such as fluctuations in the photo-ionizing background and temperature fluctuations due to HeII reionization.Comment: 11 pages, 10 figures, minor changes to address referee repor

Nonlinear Growth of Structure in Cosmological Simulations

157 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.Finally, we present the results of a comparison between 10 different cosmology codes. These include virtually all major codes used today, and more importantly, they completely cover the range of numerical algorithms used in cosmological N-body simulations. For the mass function, the matter power spectrum, and halo profiles --- the most important statistics for this thesis --- codes agree at less then 10% over wide dynamic ranges. This robustness gives us additional confidence in our numerical results.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Nonlinear Growth of Structure in Cosmological Simulations

157 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.Finally, we present the results of a comparison between 10 different cosmology codes. These include virtually all major codes used today, and more importantly, they completely cover the range of numerical algorithms used in cosmological N-body simulations. For the mass function, the matter power spectrum, and halo profiles --- the most important statistics for this thesis --- codes agree at less then 10% over wide dynamic ranges. This robustness gives us additional confidence in our numerical results.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD