Growth Of Large-scale Structure Research Articles

Freezing out early dark energy

A phenomenological model of dark energy that tracks the baryonic and cold dark matter at early times but resembles a cosmological constant at late times is explored. In the transition between these two regimes, the dark energy density drops rapidly as if it were a relic species that freezes out, during which time the equation of state peaks at +1. Such an adjustment in the dark energy density, as it shifts from scaling to potential-domination, could be the signature of a trigger mechanism that helps explain the late-time cosmic acceleration. We show that the non-negligible dark energy density at early times, and the subsequent peak in the equation of state at the transition, leave an imprint on the cosmic microwave background anisotropy pattern and the rate of growth of large scale structure. The model introduces two new parameters, consisting of the present-day equation of state and the redshift of the freeze-out transition. A Monte Carlo Markov Chain analysis of a ten-dimensional parameter space is performed to compare the model with pre-Planck cosmic microwave background, large scale structure and supernova data and measurements of the Hubble constant. We find that the transition described by this model could have taken place as late as a redshift z~400. We explore the capability of future cosmic microwave background and weak lensing experiments to put tighter constraints on this model. The viability of this model may suggest new directions in dark-energy model building that address the coincidence problem.

EVOLUTION IN THE H I GAS CONTENT OF GALAXY GROUPS: PRE-PROCESSING AND MASS ASSEMBLY IN THE CURRENT EPOCH

We present an analysis of the neutral hydrogen (HI) content and distribution of galaxies in groups as a function of their parent dark matter halo mass. The Arecibo Legacy Fast ALFA survey alpha.40 data release allows us, for the first time, to study the HI properties of over 740 galaxy groups in the volume of sky common to the SDSS and ALFALFA surveys. We assigned ALFALFA HI detections a group membership based on an existing magnitude/volume-limited SDSS DR7 group/cluster catalog. Additionally, we assigned group "proximity" membership to HI detected objects whose optical counterpart falls below the limiting optical magnitude--thereby not contributing substantially to the estimate of the group stellar mass, but significantly to the total group HI mass. We find that only 25% of the HI detected galaxies reside in groups or clusters, in contrast to approximately half of all optically detected galaxies. Further, we plot the relative positions of optical and HI detections in groups as a function of parent dark matter halo mass to reveal strong evidence that HI is being processed in galaxies as a result of the group environment: as optical membership increases, groups become increasingly deficient of HI rich galaxies at their center and the HI distribution of galaxies in the most massive groups starts to resemble the distribution observed in comparatively more extreme cluster environments. We find that the lowest HI mass objects lose their gas first as they are processed in the group environment, and it is evident that the infall of gas rich objects is important to the continuing growth of large scale structure at the present epoch, replenishing the neutral gas supply of groups. Finally, we compare our results to those of cosmological simulations and find that current models cannot simultaneously predict the HI selected halo occupation distribution for both low and high mass halos.

Nonlinear structure formation in the cubic Galileon gravity model

We model the linear and nonlinear growth of large scale structure in the Cubic Galileon gravity model, by running a suite of N-body cosmological simulations using the ECOSMOG code. Our simulations include the Vainshtein screening effect, which reconciles the Cubic Galileon model with local tests of gravity. In the linear regime, the amplitude of the matter power spectrum increases by ∼ 20% with respect to the standard ΛCDM model today. The modified expansion rate accounts for ∼ 15% of this enhancement, while the fifth force is responsible for only ∼ 5%. This is because the effective unscreened gravitational strength deviates from standard gravity only at late times, even though it can be twice as large today. In the nonlinear regime (k≳0.1h Mpc−1), the fifth force leads to only a modest increase (≲8%) in the clustering power on all scales due to the very efficient operation of the Vainshtein mechanism. Such a strong effect is typically not seen in other models with the same screening mechanism. The screening also results in the fifth force increasing the number density of halos by less than 10%, on all mass scales. Our results show that the screening does not ruin the validity of linear theory on large scales which anticipates very strong constraints from galaxy clustering data. We also show that, whilst the model gives an excellent match to CMB data on small angular scales (l≳50), the predicted integrated Sachs-Wolfe effect is in tension with Planck/WMAP results.

Why Nonuniform Density Suppresses the Precessing Vortex Core

Linear stability analysis is applied to a swirl-stabilized combustor flow with the aim to understand how the flame shape and associated density field affects the manifestation of self-excited flow instabilities. In isothermal swirling jets, self-excited flow oscillations typically manifest in a precessing vortex core and synchronized growth of large-scale spiral-shaped vortical structures. Recent theoretical studies relate these dynamics to a hydrodynamic global instability. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density inhomogeneities created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a perfectly premixed steam-diluted detached flame featuring a strong precessing vortex core. The second represents a perfectly premixed dry flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change in instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows for relating the flame position and the resulting density field to the emergence of a precessing vortex core.

DBI Galileons in the Einstein frame: Local gravity and cosmology

It is shown that a disformally coupled theory in which the gravitational sector has the Einstein-Hilbert form is equivalent to a quartic DBI Galileon Lagrangian, possessing non-linear higher derivative interactions, and hence allowing for the Vainshtein effect. This Einstein Frame description considerably simplifies the dynamical equations and highlights the role of the different terms. The study of highly dense, non-relativistic environments within this description unravels the existence of a disformal screening mechanism, while the study of static vacuum configurations reveals the existence of a Vainshtein radius, at which the asymptotic solution breaks down. Disformal couplings to matter also allow the construction of Dark Energy models, which behave differently than conformally coupled ones and introduce new effects on the growth of Large Scale Structure over cosmological scales, on which the scalar force is not screened. We consider a simple Disformally Coupled Dark Matter model in detail, in which standard model particles follow geodesics of the gravitational metric and only Dark Matter is affected by the disformal scalar field. This particular model is not compatible with observations in the linearly perturbed regime. Nonetheless, disformally coupled theories offer enough freedom to construct realistic cosmological scenarios, which can be distinguished from the standard model through characteristic signatures.

Distant galaxy clusters in the XMM Large Scale Structure survey

(Abridged) Distant galaxy clusters provide important tests of the growth of large scale structure in addition to highlighting the process of galaxy evolution in a consistently defined environment at large look back time. We present a sample of 22 distant (z>0.8) galaxy clusters and cluster candidates selected from the 9 deg2 footprint of the overlapping X-ray Multi Mirror (XMM) Large Scale Structure (LSS), CFHTLS Wide and Spitzer SWIRE surveys. Clusters are selected as extended X-ray sources with an accompanying overdensity of galaxies displaying optical to mid-infrared photometry consistent with z>0.8. Nine clusters have confirmed spectroscopic redshifts in the interval 0.8<z<1.2, four of which are presented here for the first time. A further 11 candidate clusters have between 8 and 10 band photometric redshifts in the interval 0.8<z<2.2, while the remaining two candidates do not have information in sufficient wavebands to generate a reliable photometric redshift. All of the candidate clusters reported in this paper are presented for the first time. Those confirmed and candidate clusters with available near infrared photometry display evidence for a red sequence galaxy population, determined either individually or via a stacking analysis, whose colour is consistent with the expectation of an old, coeval stellar population observed at the cluster redshift. We further note that the sample displays a large range of red fraction values indicating that the clusters may be at different stages of red sequence assembly. We compare the observed X-ray emission to the flux expected from a suite of model clusters and find that the sample displays an effective mass limit M200 ~ 1e14 Msolar with all clusters displaying masses consistent with M200 < 5e14 Msolar. This XMM distant cluster study represents a complete sample of X-ray selected z>0.8 clusters.

Large-scale growth evolution in the Szekeres inhomogeneous cosmological models with comparison to growth data

We use the Szekeres inhomogeneous cosmological models to study the growth of large-scale structure in the universe including nonzero spatial curvature and a cosmological constant. In particular, we use the Goode and Wainwright formulation of the solution, as in this form the models can be considered to represent exact nonlinear perturbations of an averaged background. We identify a density contrast in both classes I and II of the models, for which we derive growth evolution equations. By including $\ensuremath{\Lambda}$, the time evolution of the density contrast as well as kinematic quantities of interest can be tracked through the matter- and $\ensuremath{\Lambda}$-dominated cosmic eras up to the present and into the future. In class I, we consider a localized cosmic structure representing an overdensity neighboring a central void, surrounded by an almost Friedmann-Lema\^{\i}tre-Robertson-Walker background, while for class II, the exact perturbations exist globally. In various models of class I and class II, the growth rate is found to be stronger in the matter-dominated era than that of the standard lambda-cold dark matter ($\ensuremath{\Lambda}\mathrm{CDM}$) cosmology, and it is suppressed at later times due to the presence of the cosmological constant. We find that there are Szekeres models able to provide a growth history similar to that of $\ensuremath{\Lambda}\mathrm{CDM}$ while requiring less matter content and nonzero spatial curvature, which speaks to the importance of including the effects of large-scale inhomogeneities in analyzing the growth of large-scale structure. Using data for the growth factor $f$ from redshift space distortions and the Lyman-$\ensuremath{\alpha}$ forest, we obtain best fit parameters for class II models and compare their ability to match observations with $\ensuremath{\Lambda}\mathrm{CDM}$. We find that there is negligible difference between best fit Szekeres models with no priors and those for $\ensuremath{\Lambda}\mathrm{CDM}$, both including and excluding Lyman-$\ensuremath{\alpha}$ data. We also find that the standard growth index $\ensuremath{\gamma}$ parametrization cannot be applied in a simple way to the growth in Szekeres models, so a direct comparison of the function $f$ to the data is performed. We conclude that the Szekeres models can provide an exact framework for the analysis of large-scale growth data that includes inhomogeneities and allows for different interpretations of observations.

Testing Lorentz invariance of dark matter

We study the possibility to constrain deviations from Lorentz invariance in darkmatter (DM) with cosmological observations. Breaking of Lorentz invariance generically introduces new lightgravitational degrees of freedom, which we represent through a dynamical timelike vector field.If DM does not obey Lorentz invariance, it couples to thisvector field. We find that this coupling affects the inertial mass ofsmall DM halos which no longer satisfy the equivalence principle. For large enough lumps ofDM we identify a (chameleon) mechanism that restores the inertial mass to itsstandard value. As a consequence,the dynamics of gravitational clustering are modified. Two prominent effects are a scale dependent enhancement in the growth of large scale structure and a scale dependent biasbetween DM and baryon density perturbations. The comparison with the measured linear matter power spectrum in principle allows to bound the departure fromLorentz invariance of DM at the per cent level.

THE FATE OF DWARF GALAXIES IN CLUSTERS AND THE ORIGIN OF INTRACLUSTER STARS. II. COSMOLOGICAL SIMULATIONS

We combine a N-body simulation algorithm with a subgrid treatment of galaxy formation, mergers, and tidal destruction, and an observed conditional luminosity function Phi(L|M), to study the origin and evolution of galactic and extragalactic light inside a cosmological volume of size (100 Mpc)^3, in a concordance LCDM model. This algorithm simulates the growth of large-scale structures and the formation of clusters, the evolution of the galaxy population in clusters, the destruction of galaxies by mergers and tides, and the evolution of the intracluster light. We find that destruction of galaxies by mergers dominates over destruction by tides by about an order of magnitude at all redshifts. However, tidal destruction is sufficient to produce intracluster light fractions f_ICL that are sufficiently high to match observations. The bulk of the intracluster light (60%) is provided by intermediate galaxies of total masses 10^11 Msun-10^12 Msun and stellar masses 6x10^8 Msun-3x10^10 Msun that were tidally destroyed by even more massive galaxies. The contribution of low-mass galaxies to the intracluster light is small and the contribution of dwarf galaxies is negligible, even though, by numbers, most galaxies that are tidally destroyed are dwarfs. Tracking clusters back in time, we find that their values of f_ICL tend to increase over time, but can experience sudden changes that are sometimes non-monotonic. These changes occur during major mergers involving clusters of comparable masses but very different intracluster luminosities. Most of the tidal destruction events take place in the central regions of clusters. As a result, the intracluster light is more centrally concentrated than the galactic light. Our results support tidal destruction of intermediate-mass galaxies as a plausible scenario for the origin of the intracluster light.

Modelling self-similar appearance of galaxy clusters in X-rays

The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. A large effort is therefore undertaken to compile global galaxy cluster properties in particular obtained through X-ray observations and to study their scaling relations. However, the scaling schemes used in the literature differ. The present paper aims to clarify this situation by providing a thorough review of the scaling laws within the standard model of large-scale structure growth and to discus various steps of practical approximations. We derive the scaling laws for X-ray observables and cluster mass within the pure gravitational structure growth scenario. Using N-body simulations we test the recent formation approximation used in earlier analytic approaches which involves a redshift dependent overdensity parameter.We find this approximation less precise than the use of a fiducial radius based on a fixed overdensity with respect to critical density. Inspired by the comparison of the predicted scaling relations with observations we propose a first order modification of the scaling scheme to include the observed effects of hydrodynamics in structure formation. This modification involves a cluster mass dependent gas mass fraction. We also discuss the observational results of the reshift evolution of the most important scaling relations and find that also a redshift dependence of the gas mass to total mass relation has to be invoked within our modification scheme. We find that the current observational data are within their uncertainties consistent with the proposed modified scaling laws.

Model-independent tests of cosmic gravity

Gravitation governs the expansion and fate of the universe, and the growth of large-scale structure within it, but has not been tested in detail on these cosmic scales. The observed acceleration of the expansion may provide signs of gravitational laws beyond general relativity (GR). Since the form of any such extension is not clear, from either theory or data, we adopt a model-independent approach to parametrizing deviations to the Einstein framework. We explore the phase space dynamics of two key post-GR functions and derive a classification scheme, and an absolute criterion on accuracy necessary for distinguishing classes of gravity models. Future surveys will be able to constrain the post-GR functions' amplitudes and forms to the required precision, and hence reveal new aspects of gravitation.

Constraints on gravity on cosmic scales with upcoming large-scale structure surveys

We consider how upcoming, prospective large-scale structure surveys, measuring galaxy weak lensing and position in tandem with the cosmic microwave background temperature anisotropies, constrain cosmic scale modifications to general relativity. In such theories, both the homogeneous expansion history and the growth of large-scale structure can have signatures of the modification. We consider an equation of state figure of merit parameter, and introduce an analogous figure of merit parameter for modified gravity, to quantify the relative constraints. We discuss how assumptions about the presence of astrophysical and instrumental systematics such as galaxy bias, intrinsic alignments, weak lensing shear calibration uncertainties and photometric redshift offsets can impact the prospective dark energy constraints.

A comparison of cosmological codes: properties of thermal gas and shock waves in large-scale structures

Cosmological hydrodynamical simulations are a valuable tool for understanding the growth of large-scale structure and the observables connected with this. Yet, comparably little attention has been given to validation studies of the properties of shocks and of the resulting thermal gas between different numerical methods – something of immediate importance as gravitational shocks are responsible for generating most of the entropy of the large-scale structure in the Universe. Here, we present results for the statistics of thermal gas and the shock wave properties for a large volume simulated with three different cosmological numerical codes: the Eulerian total variations diminishing (TVD) code, the Eulerian piecewise parabolic method based code enzo and the Lagrangian smoothed particle hydrodynamics (SPH) code gadget. Starting from a shared set of initial conditions, we present convergence tests for a cosmological volume of side-length 100 Mpc h−1, studying in detail the morphological and statistical properties of the thermal gas as a function of mass and spatial resolution in all codes. By applying shock-finding methods to each code, we measure the statistics of shock waves and the related cosmic ray acceleration efficiencies, within the sample of simulations and for the results of the different approaches. We discuss the regimes of uncertainties and disagreement among codes, with a particular focus on the results at the scale of galaxy clusters. Even if the bulk of thermal and shock properties is reasonably in agreement among the three codes, yet some significant differences exist (especially between Eulerian methods and SPH). In particular, we report (a) differences of huge factors (∼10–100) in the values of average gas density, temperature, entropy, Mach number and shock thermal energy flux in the most rarefied regions of the simulations (ρ/ρcr < 1) between grid and SPH methods; (b) the hint of an entropy core inside clusters simulated in grid codes; (c) significantly different phase diagrams of shocked cells in grid codes compared to SPH and (d) sizable differences in the morphologies of accretion shocks between grid and SPH methods.

Cosmological large-scale structures beyond linear theory in modified gravity

We consider the effect of modified gravity on the growth of large-scale structures at second order in perturbation theory. We show that modified gravity models changing the linear growth rate of fluctuations are also bound to change, although mildly, the mode coupling amplitude in the density and reduced velocity fields. We present explicit formulae which describe this effect.We then focus on models of modified gravity involving a scalar field coupled to matter, in particular chameleons and dilatons, where it is shown that there exists a transition scale around which the existence of an extra scalar degree of freedom induces significant changes in thecoupling properties of the cosmic fields. We obtain the amplitude of this effect for realistic dilaton models at the tree-orderlevel for the bispectrum, finding them to be comparable in amplitude to those obtained in the DGP and f(R) models.

Consistency check ofΛCDMphenomenology

The standard model of cosmology LCDM assumes general relativity, flat space, and the presence of a positive cosmological constant. We relax these assumptions allowing spatial curvature, time-dependent effective dark energy equation of state, as well as modifications of the Poisson equation for the lensing potential, and modifications of the growth of linear matter density perturbations in alternate combinations. Using six parameters characterizing these relations, we check LCDM for consistency utilizing cosmic microwave background anisotropies, cross correlations thereof with high-redshift galaxies through the integrated Sachs-Wolfe effect, the Hubble constant, supernovae and baryon acoustic oscillation distances, as well as the relation between weak gravitational lensing and galaxy flows. In all scenarios, we find consistency of the concordance model at the 95% confidence level. However, we emphasize that constraining supplementary background parameters and parametrizations of the growth of large-scale structure separately may lead to a priori exclusion of viable departures from the concordance model.

Constraints on scalar-tensor theories of gravity from observations

In spite of their original discrepancy, both dark energy and modified theory of gravity can be parameterized by the effective equation of state (EOS) ω for the expansion history of the Universe. A useful model independent approach to the EOS of them can be given by so-called Chevallier-Polarski-Linder (CPL) parametrization where two parameters of it (ω0 and ωa) can be constrained by the geometrical observations which suffer from degeneracies between models. The linear growth of large scale structure is usually used to remove these degeneracies. This growth can be described by the growth index parameter γ and it can be parameterized by γ0+γa(1−a) in general. We use the scalar-tensor theories of gravity (STG) and show that the discernment between models is possible only when γa is not negligible. We show that the linear density perturbation of the matter component as a function of redshift severely constrains the viable subclasses of STG in terms of ω and γ. From this method, we can rule out or prove the viable STG in future observations. When we use Z(ϕ) = 1, F shows the convex shape of evolution in a viable STG model. The viable STG models with Z(ϕ) = 1 are not distinguishable from dark energy models when we strongly limit the solar system constraint.

THE CLUSTERING OF MASSIVE GALAXIES ATz∼ 0.5 FROM THE FIRST SEMESTER OF BOSS DATA

We calculate the real- and redshift-space clustering of massive galaxies at z~0.5 using the first semester of data by the Baryon Oscillation Spectroscopic Survey (BOSS). We study the correlation functions of a sample of 44,000 massive galaxies in the redshift range 0.4<z<0.7. We present a halo-occupation distribution modeling of the clustering results and discuss the implications for the manner in which massive galaxies at z~0.5 occupy dark matter halos. The majority of our galaxies are central galaxies living in halos of mass 10^{13}Msun/h, but 10% are satellites living in halos 10 times more massive. These results are broadly in agreement with earlier investigations of massive galaxies at z~0.5. The inferred large-scale bias (b~2) and relatively high number density (nbar=3e-4 h^3 Mpc^{-3}) imply that BOSS galaxies are excellent tracers of large-scale structure, suggesting BOSS will enable a wide range of investigations on the distance scale, the growth of large-scale structure, massive galaxy evolution and other topics.

Scattering of dark matter and dark energy

We demonstrate how the two dominant constituents of the Universe, dark energy and dark matter, could possess a large scattering cross-section without considerably impacting observations. Unlike models involving energy exchange between the two fluids, the background cosmology remains unaltered, leaving fewer observational signatures. Following a brief review of the scattering cross-sections between cosmologically significant particles, we explore the implications of an elastic interaction between dark matter and dark energy. The growth of large scale structure is suppressed, yet this effect is found to be weak due to the persistently low dark energy density. Thus we conclude that the dark matter-dark energy cross section may exceed the Thomson cross-section by several orders of magnitude.

Testing general relativity with current cosmological data

Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.

Inhomogeneity and the foundations of concordance cosmology

The apparent accelerating expansion of the universe is forcing us to examine the foundational aspects of the standard model of cosmology—in particular, the fact that dark energy is a direct consequence of the homogeneity assumption. We discuss the foundations of the assumption of spatial homogeneity, in the case when the Copernican principle is adopted. We present results that show how (almost) homogeneity follows from (almost) isotropy of various observables. The analysis requires fully nonlinear field equations—i.e. it is not possible to use second- or higher-order perturbation theory, since one cannot assume a homogeneous and isotropic background. Then we consider what happens if the Copernican principle is abandoned in our Hubble volume. The simplest models are inhomogeneous but spherically symmetric universes which do not require dark energy to fit the distance modulus. Key problems in these models are to compute the CMB anisotropies and the features of large-scale structure. We review how to construct perturbation theory on a non-homogeneous cosmological background, and discuss the complexities that arise in using this to determine the growth of large-scale structure.