Numerical Simulations Of Structure Formation Research Articles

Reconstructing the cosmological density and velocity fields from redshifted galaxy distributions using V-net

The distribution of matter that is measured through galaxy redshift and peculiar velocity surveys can be harnessed to learn about the physics of dark matter, dark energy, and the nature of gravity. To improve our understanding of the matter of the Universe, we can reconstruct the full density and velocity fields from the galaxies that act as tracer particles. In this paper, we usethe simulated halos as proxies for the galaxies. We use a convolutional neural network, a V-net, trained on numerical simulations of structure formation to reconstruct the density and velocity fields. We find that, with detailed tuning of the loss function, the V-net could produce better fits to the density field in the high-density and low-density regions, and improved predictions for the probability distribution of the amplitudes of the velocities. However, the weights will reduce the precision of the estimated β parameter. We also find that the redshift-space distortions of the halo catalogue do not significantly contaminate the reconstructed real-space density and velocity field. We estimate the velocity field β parameter by comparing the peculiar velocities of halo catalogues to the reconstructed velocity fields, and find the estimated β values agree with the fiducial value at the 68% confidence level.

Including relativistic and primordial non-Gaussianity contributions in cosmological simulations by modifying the initial conditions

We present a method to implement relativistic corrections to the evolution of dark matter structures in Newtonian simulations of a ΛCDM universe via the initial conditions. We take the nonlinear correspondence between the Lagrangian (Newtonian) evolution of dark matter inhomogeneities and the synchronous-comoving (relativistic) matter density description, and use it to promote the relativistic constraint as the initial condition for numerical simulations of structure formation. In this case, the incorporation of Primordial non-Gaussianity (PNG) contributions as initial conditions is straightforward. We implement the relativistic, f NL and g NL contributions as initial conditions for the L-PICOLA code, and compute the power spectrum and bispectrum of the evolved matter field. We focus specifically on the case of largest values of non-Gaussianity allowed at 1-σ by Planck observations (f NL = −4.2 and g NL = −7000). As a checkup, we show consistency with the one-loop perturbative prescription and with a fully relativistic simulation (GRAMSES) on the adequate scales. Our results confirm that both relativistic and PNG features are most prominent at very large scales and for squeezed triangulations. We discuss future prospects to probe these two contributions in the bispectrum of the matter density distribution.

Mapping the inhomogeneous Universe with standard sirens: degeneracy between inhomogeneity and modified gravity theories

ABSTRACT The detection of gravitational waves (GWs) and an accompanying electromagnetic (E/M) counterpart have been suggested as a future probe for cosmology and theories of gravity. In this paper, we present calculations of the luminosity distance of sources taking into account inhomogeneities in the matter distribution that are predicted in numerical simulations of structure formation. In addition, we show that inhomogeneities resulting from clustering of matter can mimic certain classes of modified gravity theories, or other effects that dampen GW amplitudes, and deviations larger than $\delta \nu \sim \mathcal {O}(0.1)\ (99{{\ \rm per\ cent}}\ \rm {C.L.})$ to the extra friction term ν, from zero, would be necessary to distinguish them. For these, we assume mock GWs sources, with known redshift, based on binary population synthesis models, between redshifts z = 0 and z = 5. We show that future GW detectors, like Einstein Telescope or Cosmic Explorer, will be needed for strong constraints on the inhomogeneity parameters and breaking the degeneracy between modified gravity effects and matter anisotropies by measuring ν at $5 {{\ \rm per\ cent}}$ and $1 {{\ \rm per\ cent}}$ level with 100 and 350 events, respectively.

Numerical Simulation of Structure Formation of Surface-Modified Nanoparticles during Solvent Evaporation

Numerical Simulation of Structure Formation of Surface-Modified Nanoparticles during Solvent Evaporation

Strong Gravitational Lensing by Wave Dark Matter Halos

Abstract Wave dark matter (WaveDM) has recently gained attention as a viable candidate to account for the dark matter content of the universe. In this paper we explore the extent to which, and under what conditions, dark matter halos in this model are able to reproduce strong-lensing systems. First, we explore analytically the lensing properties of the model, finding that a pure WaveDM density profile, the soliton profile, produces a weaker lensing effect than similar cored profiles. Then, we analyze models with a soliton embedded within a Navarro, Frenk, and White (NFW) profile, as has been found in numerical simulations of structure formation. We use a benchmark model with a boson mass of m a = 10−22 eV, for which we see that there is a bimodality in the contribution of the external NFW part of the profile, and some of the free parameters associated with it are not well constrained. We find that for configurations with boson masses 10−23 to 10−22 eV, a range of masses preferred by dwarf galaxy kinematics, the soliton profile alone can fit the data, but its size is incompatible with the luminous extent of the lens galaxies. Likewise, boson masses of the order of 10−21 eV, which would be consistent with Lyα constraints and consist of more compact soliton configurations, necessarily require the NFW part in order to reproduce the observed Einstein radii. We then conclude that lens systems impose a conservative lower bound m a > 10−24 eV and that the NFW envelope around the soliton must be present to satisfy the observational requirements.

Structure Formation of Magnetic Particles under Magnetic Fields toward Anisotropic Materials

We performed numerical simulation of structure formation of magnetic particles under magnetic field toward anisotropic material development. In this study, we examined the unsteady process of the structure formation and its feature using magnetic particles with diameter of micrometers. This study evaluated the process of structure forming with various particle diameters and particle volume concentrations using the Non-dimensional Boundary Area (NBA). The structure formed by the magnetic particles does not depend on the particle diameter but depends on the particle volume concentration. In the case of particle volume concentration ϕ = 5 vol%, the bundle of chain-like cluster is not made and each chain-like cluster by magnetic particles is almost single chain. However, in the case of more than ϕ = 10 vol%, the bundle structure formed by contacts of multiple chain-like clusters is proceeding. On the process of structure formation by magnetic particles, firstly, the single particle rotates by itself with orienting the magnetic moment to the direction of the applied magnetic field. After then, particles connect with each other and the chain-like cluster is formed.

Head-on collision of multistate ultralight BEC dark matter configurations

Density profiles of ultralight Bose-Condensate dark matter inferred from numerical simulations of structure formation, ruled by the Gross-Pitaevskii-Poisson (GPP) system of equations, have a core-tail structure. Multi-state equilibrium configurations of the GPP system on the other hand, have a similar core-tail density profile. We now submit these multi-state configurations to highly dynamical scenarios and show their potential as providers of appropriate density profiles of structures. What we do is to present the simulation of head-on collisions between two equilibrium configurations of the GPP system of equations, including the collision of ground state with multi-state configurations. We study the regimes of solitonic and merger behavior, and show generic properties of the dynamics of the system, including the relaxation process and attractor density profiles. We show the merger of multi-state configurations have the potential to produce core-tail density profiles, with the core dominated by the ground state and a halo dominated by an additional states.

Realistic estimation for the detectability of dark matter subhalos using Fermi-LAT catalogs

Numerical simulations of structure formation have recorded a remarkable progress in the recent years, in particular due to the inclusion of baryonic physics evolving with the dark matter component. We generate Monte Carlo realizations of the dark matter sub-halo population based on the results of the recent hydrodynamical simulation suite of Milky Way-sized galaxies. We then simulate the gamma-ray sky for both the setup of the 3FGL and 2FHL Fermi Large Area Telescope (LAT) catalogs, including the contribution from the annihilation of dark matter in the sub-halos. We find that the flux sensitivity threshold strongly depends on the particle dark matter mass, and more mildly also on its annihilation channel and the observation latitude. The results differ for the 3FGL and 2FHL catalogs, given their different energy thresholds. We also predict that the number of dark matter sub-halos among the unassociated sources is very small. A null number of detectable sub-halos in the Fermi-LAT 3FGL catalog would imply upper limits on the dark matter annihilation cross section into $b\bar{b}$ of $2 \cdot 10^{-26}$ ($5 \cdot 10^{-25}$) cm$^3$/s with $M_{\rm DM}$= 50 (1000) GeV. We find less than one extended sub-halo in the Fermi-LAT 3FGL catalog. As a matter of fact, the differences in the spatial and mass distribution of sub-halos between hydrodynamic and dark matter-only runs do not have significant impact on the gamma-ray dark matter phenomenology.

On the projected mass distribution around galaxy clusters

Gravitational lensing allows to quantify the angular distribution of the convergence field around clusters of galaxies to constrain their connectivity to the cosmic web. We describe in this paper the corresponding theory in Lagrangian space where analytical results can be obtained by identifying clusters to peaks in the initial field. We derive the three-point Gaussian statistics of a two-dimensional field and its first and second derivatives. The formalism allows us to study the statistics of the field in a shell around a central peak, in particular its multipolar decomposition. The peak condition is shown to significantly remove power from the dipolar contribution and to modify the monopole and quadrupole. As expected, higher order multipoles are not significantly modified by the constraint. Analytical predictions are successfully checked against measurements in Gaussian random fields. The effect of substructures and radial weighting is shown to be small and does not change the qualitative picture. The non-linear evolution is shown to induce a non-linear bias of all multipoles proportional to the cluster mass.We predict the Gaussian and weakly non-Gaussian statistics of multipolar moments of a two-dimensional field around a peak as a proxy for the azimuthal distribution of the convergence field around a cluster of galaxies. A quantitative estimate of this multipolar decomposition of the convergence field around clusters in numerical simulations of structure formation and in observations will be presented in two forthcoming papers.

COLA with scale-dependent growth: applications to screened modified gravity models

We present a general parallelized and easy-to-use code to perform numerical simulations of structure formation using the COLA (COmoving Lagrangian Acceleration) method for cosmological models that exhibit scale-dependent growth at the level of first and second order Lagrangian perturbation theory. For modified gravity theories we also include screening using a fast approximate method that covers all the main examples of screening mechanisms in the literature. We test the code by comparing it to full simulations of two popular modified gravity models, namely f(R) gravity and nDGP, and find good agreement in the modified gravity boost-factors relative to ΛCDM even when using a fairly small number of COLA time steps.

Sussing merger trees: the impact of halo merger trees on galaxy properties in a semi-analytic model

A halo merger tree forms the essential backbone of a semi-analytic model for galaxy formation and evolution. Recent studies have pointed out that extracting merger trees from numerical simulations of structure formation is non-trivial; different tree building algorithms can give differing merger histories. These differences should be carefully understood before merger trees are used as input for models of galaxy formation. We investigate the impact of different halo merger trees on a semi-analytic model. We find that the z=0 galaxy properties in our model show differences between trees when using a common parameter set. The star formation history of the Universe and the properties of satellite galaxies can show marked differences between trees with different construction methods. Independently calibrating the semi-analytic model for each tree can reduce the discrepancies between the z=0 global galaxy properties, at the cost of increasing the differences in the evolutionary histories of galaxies. Furthermore, the underlying physics implied can vary, resulting in key quantities such as the supernova feedback efficiency differing by factors of 2. Such a change alters the regimes where star formation is primarily suppressed by supernovae. Therefore, halo merger trees extracted from a common halo catalogue using different, but reliable, algorithms can result in a difference in the semi-analytic model. Given the uncertainties in galaxy formation physics, however, these differences may not necessarily be viewed as significant.

Massive stars. A chemical signature of first-generation very massive stars.

Numerical simulations of structure formation in the early universe predict the formation of some fraction of stars with several hundred solar masses. No clear evidence of supernovae from such very massive stars has, however, yet been found in the chemical compositions of Milky Way stars. We report on an analysis of a very metal-poor star SDSS J001820.5-093939.2, which possesses elemental-abundance ratios that differ significantly from any previously known star. This star exhibits low [α-element Fe] ratios and large contrasts between the abundances of odd and even element pairs, such as scandium/titanium and cobalt/nickel. Such features have been predicted by nucleosynthesis models for supernovae of stars more than 140 times as massive as the Sun, suggesting that the mass distribution of first-generation stars might extend to 100 solar masses or larger.

A new framework for numerical simulations of structure formation

The diversity of structures in the Universe (from the smallest galaxies to the largest superclusters) has formed under the pull of gravity from the tiny primordial perturbations that we see imprinted in the cosmic microwave background. A quantitative description of this process would require description of motion of zillions of dark matter particles. This impossible task is usually circumvented by coarse-graining the problem: one either considers a Newtonian dynamics of "particles" with macroscopically large masses or approximates the dark matter distribution with a continuous density field. There is no closed system of equations for the evolution of the matter density field alone and instead it should still be discretized at each timestep. In this work we describe a method of solving the full 6-dimensional Vlasov-Poisson equation via a system of auxiliary Schroedinger-like equations. The complexity of the problem gets shifted into the choice of the number and shape of the initial wavefunctions that should only be specified at the beginning of the computation (we stress that these wavefunctions have nothing to do with quantum nature of the actual dark matter particles). We discuss different prescriptions to generate the initial wave functions from the initial conditions and demonstrate the validity of the technique on two simple test cases. This new simulation algorithm can in principle be used on an arbitrary distribution function, enabling the simulation of warm and hot dark matter structure formation scenarios.

The structural elements of the cosmic web

AbstractIn 1970 Zel'dovich published a far-reaching paper presenting a simple equation describing the nonlinear growth of primordial density inhomogeneities. The equation was remarkably successful in explaining the large scale structure in the Universe that we observe: a Universe in which the structure appears to be delineated by filaments and clusters of galaxies surrounding huge void regions. In order to concretise this impression it is necessary to define these structural elements through formal techniques with which we can compare the Zel'dovich model and N-body simulations with the observational data.We present an overview of recent efforts to identify voids, filaments and clusters in both the observed galaxy distribution and in numerical simulations of structure formation. We focus, in particular, on methods that involve no fine-tuning of parameters and that handle scale dependence automatically. It is important that these techniques should result in finding structures that relate directly to the dynamical mechanism of structure formation.

Weighing the light gravitino mass with weak lensing surveys

We explore the discovery potential of light gravitino mass m_{3/2} by combining future cosmology surveys and collider experiments. The former probe the imprint of light gravitinos in the cosmic matter density field, whereas the latter search signatures of a supersymmetry breaking mechanism. Free-streaming of light gravitinos suppresses the density fluctuations at galactic and sub-galactic length scales, where weak gravitational lensing can be used as a powerful probe. We perform numerical simulations of structure formation to quantify the effect. We then run realistic ray-tracing simulations of gravitational lensing to measure the cosmic shear in models with light gravitino. We forecast the possible reach of future wide-field surveys by Fisher analysis; the light gravitino mass can be determined with an accuracy of m_{3/2}=4\pm 1 eV by a combination of the Hyper Suprime Cam survey and cosmic microwave background anisotropy data obtained by Planck satellite. The corresponding accuracy to be obtained by the future Large Synoptic Survey Telescope is \delta m_{3/2}=0.6 eV. Data from experiments at Large Hadron Collider at 14 TeV will provide constraint at m_{3/2} \simeq 5 eV in the minimal framework of gauge-mediated supersymmetry breaking (GMSB) model. We conclude that a large class of the GMSB model can be tested by combining the cosmological observations and the collider experiments.

Towards understanding the structure of voids in the cosmic web

Our goal is to see how density waves of different scale combine to form voids between galaxy systems of various scale. We perform numerical simulations of structure formation in cubes of size 100 and 256 Mpc/h, with resolutions 256^3 and 512^3 particles and cells. To understand the role of density perturbations of various scale we cut power spectra at scales from 8 to 128 Mpc/h, using in all series identical initial random realisations. We find that small haloes and short filaments form all over the simulation box, if perturbations only up to scale 8 Mpc/h are present. The phenomenon of large multi-scale voids in the cosmic web requires the presence of an extended spectrum of primordial density perturbations. The void phenomenon is due to the action of two processes: the synchronisation of density perturbations of medium and large scales, and the suppression of galaxy formation in low-density regions by the combined action of negative sections of medium- and large-scale density perturbations, so that their densities are less than the mean density, and thus during the evolution their densities decrease.

Dark Matter Halos from the Inside Out

The balance of evidence indicates that individual galaxies and groups or clusters of galaxies are embedded in enormous distributions of cold, weakly interacting dark matter. These dark matter “halos” provide the scaffolding for all luminous structures in the universe, and their properties comprise an essential part of the current cosmological model. I review the internal properties of dark matter halos, focussing on the simple universal trends predicted by numerical simulations of structure formation. Simulations indicate that halos should all have roughly the same spherically averaged density profile and kinematic structure and predict simple distributions of shape, formation history, and substructure in density and kinematics, over an enormous range of halo mass and for all common variants of the concordance cosmology. I describe observational progress towards testing these predictions by measuring masses, shapes, profiles, and substructure in real halos using baryonic tracers or gravitational lensing. An important property of simulated halos (possibly the most important property) is their dynamical “age”, or degree of internal relaxation. I review recent gravitational lensing studies of galaxy clusters which will measure substructure and relaxation in a large sample of individual cluster halos, producing quantitative measures of age that are well matched to theoretical predictions.

ANALYSIS OF METHODS FOR DETECTING THE PROXIMITY EFFECT IN QUASAR SPECTRA

Using numerical simulations of structure formation, we investigate multiple methods of determining the strength of the proximity effect in the HI Lyalpha forest. We analyze three high resolution (~10kpc) redshift snapshots (z=4,3,2.25) of a Hydro-Particle-Mesh simulation to obtain realistic absorption spectra of the HI Lyalpha forest. We begin our analysis investigating the intrinsic biases thought to arise in the widely adopted standard technique of combining multiple lines of sight when searching for the proximity effect. We confirm the existence of this biases. We then concentrate on the analysis of the proximity effect along individual lines of sight. We construct the proximity effect strength distribution (PESD) and confirm that the PESD inferred from a simple averaging technique accurately recovers the input strength of the proximity effect at all redshifts. Moreover, the PESD closely follows the behaviors found in observed samples of quasar spectra. However, the PESD obtained from our new simulated sight lines presents some differences to that of simple Monte Carlo simulations. After developing three new theoretical methods of recovering the strength of the proximity effect on individual lines of sight, we compare their accuracy to the PESD from the simple averaging technique. All our new approaches are based on the maximization of the likelihood function, albeit invoking some modifications. The new techniques presented here fail to recover the input proximity effect in an un-biased way. Thus, employing complex 3D simulations, we provide strong evidence in favor of the proximity effect strength distribution obtained from the simple averaging technique, as method of estimating the UV background intensity, free of any biases.

The Tully-Fisher Relation in Numerical Simulations of Structure Formation

AbstractThe Tully-Fisher Relation (TFR) is of fundamental importance for galaxy formation as it provides information about the relation between the baryonic content of galaxies and the depth of their dark halos potential wells. In recent years, it has been possible to study this relation at different redshifts. However, there are still controversies about its origin and evolution. In this work, we try to address the origin of the Tully-Fisher Relation by employing cosmological hydrodynamical simulations.

Cosmological simulations of structure formation and the Vlasov equation

In cosmology numerical simulations of structure formation are now of central importance, as they are the sole instrument for providing detailed predictions of current cosmological models for a whole class of important constraining observations. These simulations are essentially molecular dynamics simulations of N (≫1, now up to of order several billion) particles interacting through their self-gravity. While their aim is to produce the Vlasov limit, which describes the underlying (“cold dark matter”) models, the degree to which they actually do produce this limit is currently understood, at best, only very qualitatively, and there is an acknowledged need for “a theory of discreteness errors”. In this talk I will describe, for non-cosmologists, both the simulations and the underlying theoretical models, and will then focus on the issue of discreteness, describing some recent progress in addressing this question quantitatively.