Evolution Of Large-scale Structure Research Articles

Life, the Universe, and Nothing: Life and Death in an Ever‐expanding Universe

Current evidence suggests that the cosmological constant is not zero, or that we live in an open universe. We examine the implications for the future under these assumptions, and find that they are striking. If the Universe is cosmological constant-dominated, our ability to probe the evolution of large scale structure will decrease with time ---presently observable distant sources will disappear on a time-scale comparable to the period of stellar burning. Moreover, while the Universe might expand forever, the integrated conscious lifetime of any civilization will be finite, although it can be astronomically long. We find that this latter result is far more general. In the absence of possible exotic and uncertain strong gravitational effects, the total information recoverable by any civilization over the entire history of our universe is finite, and assuming that consciousness has a physical computational basis, life cannot be eternal.

A Database ofCOBE‐normalized Cold Dark Matter Simulations

We have simulated the formation and evolution of large-scale structure in the universe, for 68 different COBE-normalized cosmological models. For each cosmological model, we have performed between one and three simulations, for a total of 160 simulations. This constitutes the largest database of cosmological simulations ever assembled, and the largest cosmological parameter space ever covered by such simulations. We are making this database available to the astronomical community. We provide instructions for accessing the database and for converting the data from computational units to physical units. The database includes tilted cold dark matter (TCDM) models, tilted open cold dark matter (TOCDM) models, and tilted Λ cold dark matter (TΛCDM) models. (For several simulations, the primordial exponent n of the power spectrum is near unity, hence these simulations can be considered as untilted.) The simulations cover a four-dimensional cosmological parameter phase space, the parameters being the present density parameter Ω0, cosmological constant λ0, and Hubble constant H0, and the rms density fluctuation σ8 at scale 8 h-1 Mpc. All simulations were performed using a P3M algorithm with 643 particles on a 1283 mesh, in a cubic volume of comoving size 128 Mpc. Each simulation starts at a redshift of 24 and is carried up to the present. More simulations will be added to the database in the future. We have performed a limited amount of data reduction and analysis of the final states of the simulations. We computed the rms density fluctuation, the two-point correlation function, the velocity moments, and the properties of clusters. Our results are the following: 1. The numerical value σ of the rms density fluctuation differs from the value σ obtained by integrating the power spectrum at early times and extrapolating linearly up the present. This results from the combined effects of discreteness in the numerical representation of the power spectrum, the presence of a Gaussian factor in the initial conditions, and late-time nonlinear evolution. The first of these three effects is negligible. The second and third are comparable, and can both modify the value of σ8 by up to 10%. Nonlinear effects, however, are important only for models with σ8 > 0.6, and can result in either an increase or a decrease in σ8. 2. The observed galaxy two-point correlation function is well reproduced (assuming an unbiased relation between galaxies and mass) by models with σ8 ~ 0.8, nearly independently of the values of the other parameters, Ω0, λ0, and H0. For models with σ8 > 0.8, the correlation function is too large and its slope is too steep. For models with σ8 0.9, it has a U shape, which is probably a numerical artifact caused by the finite number of particles used in the simulations. For all models, clusters have densities in the range 100-1000 times the mean background density, the spin parameters λ are in the range 0.008-0.2, with the median near 0.05, and about of the clusters are prolate. Rotationally supported disks do not form in these simulations.

Cosmic Concordance and Quintessence

We present a comprehensive study of the observational constraints on spatially flat cosmological models containing a mixture of matter and quintessence --- a time varying, spatially inhomogeneous component of the energy density of the universe with negative pressure. Our study also includes the limiting case of a cosmological constant. Low red shift constraints include the Hubble parameter, baryon fraction, cluster abundance, age of the universe, bulk velocity and shape of the mass power spectrum; intermediate red shift constraints are due to type 1a supernovae, gravitational lensing, the Ly-a forest, and the evolution of large scale structure; high red shift constraints are based on cosmic microwave background temperature anisotropy. Mindful of systematic errors, we adopt a conservative approach in applying these constraints. We determine that quintessence models in which the matter density parameter is $0.2 \ls \Omega_m \ls 0.5$ and the effective, density-averaged equation of state is $-1 \le w \ls -0.2$, are consistent with the most reliable, current low red shift and CMB observations at the $2\sigma$ level. Factoring in the constraint due to type 1a SNe, the range for the equation of state is reduced to $-1 \le w \ls -0.4$, where this range represents models consistent with each observational constraint at the 2$\sigma$ level or better (concordance analysis). A combined maximum likelihood analysis suggests a smaller range, $-1 \le w \ls -0.6$. We find that the best-fit and best-motivated quintessence models lie near $\Omega_m \approx 0.33$, $h \approx 0.65$, and spectral index $n_s=1$, with an effective equation of state $w \approx -0.65$ for ``tracker'' quintessence and $w=-1$ for ``creeper'' quintessence. (abstract shortened)

The structure of an acoustically forced, reacting two-phase jet

An experimental study of the structure of an acoustically forced, reacting two-phase jet was performed. The jet was acoustically forced to control the formation and evolution of large-scale structures in the near field of the jet. Phase-locked data acquisition techniques were used to correlate droplet statistics and dynamics with features of the large-scale structures. Phase Doppler interferometry was used to acquire droplet statistics. Planar imaging techniques were applied to document the distribution of droplets within the jet. The results show that the interaction between droplets and large-scale structures leads to a nonuniform distribution of droplets in the reacting jet. The combination of transport effects and droplet evaporation leads to the formation of droplet clusters. The group combustion behavior of the droplet field was evaluated by estimating the group combustion number from experimental data. External sheath burning is present in the early portion of the flame followed by a transition to external group combustion as clusters begin to be the dominant feature. Late in the cluster lifetime there is a shift to internal group combustion.

A Breakup Mechanism of High-Temperature Melt Jets. Observation in a Molten Tin Jet.

In order to clarify a breakup mechanism of high-temperature melt jets, a series of experiments was carried out by using a molten tin jet (13 mm in diameter) with initial temperatures from250°C to 950°C. The evolution of large-scale structures, which is caused by organized motion between jet and surrounding fluid, was clearly observed. Local swelling and cavity formation, collapse after swelling, and breakup with ejection and swelling were confirmed in jets with low, middle and high initial temperatures, respectively. These phenomena show that breakup is caused when a high pressure is produced by the fluid entrained inside the jet with enough termal energy due to organized motion.

Evolution and convection of large-scale structures in supersonic reattaching shear flows

Double-pulsed Mie scattering studies were performed to characterize the evolution of large-scale structures embedded within a planar supersonic base flow. Images were obtained at several streamwise stations along the shear layers, at reattachment, and in the near-wake regions. From these time-correlated images, the evolution characteristics of the large-scale structures were examined over a range of nondimensional time delays, as defined by local integral length and velocity scales. The double-pulsed images indicated that for short time delays (i.e., less than the representative eddy rollover time), the structures exhibited a simple translation in the streamwise direction. As the time delay was increased, rotation and elongation of the structures were observed in addition to the translation feature. Time delays that appreciably exceeded the local eddy rollover time generally resulted in a dramatic loss of structure identity. No eddy interactions, such as pairing, were observed at any of the imaging locations. Images obtained near reattachment provided evidence of shocklets moving in concert with the local eddies. In the initial portions of the shear layers, the mean convection velocity was measured to be significantly higher than the isentropic estimate, which is consistent with the results of previous convection velocity studies using mixing layers composed of supersonic/subsonic freestream combinations. The eddies decelerate through the recompression and reattachment regions, presumably due to the influence of the adverse pressure gradient. Downstream of reattachment, the large-scale structures accelerate as the wake develops.

Clustering at High Redshift: Precise Constraints from a Deep, Wide‐Area Survey

We present constraints on the evolution of large-scale structure from a catalog of 710,000 galaxies with IAB ? 24 derived from a KPNO 4 m CCD imaging survey of a contiguous 4? ? 4? region. The advantage of using large contiguous surveys for measuring clustering properties on even modest angular scales is substantial: the effects of cosmic scatter are strongly suppressed. We provide highly accurate measurements of the two-point angular correlation function, ?(?), as a function of magnitude on scales up to 15. The amplitude of ?(?) declines by a factor of ~10 over the range 16 ? I ? 20 but only by a factor of 2-3 over the range 20 20, our best-fit values shift toward lower r0 and more negative . A strong covariance between r0 and prevents us from rejecting > 0 even at faint magnitudes, but if > 1, we strongly reject r0 4 h-1 Mpc (comoving). The above expression for ?(r, z) and our data give a correlation length of r0(z = 0.5) ? 3.0 ? 0.4 h-1 Mpc, about a factor of 2 larger than the correlation length at z = 0.5 derived from the Canada-France Redshift Survey (CFRS). The small volume sampled by the CFRS and other deep redshift probes, however, makes these spatial surveys strongly susceptible to cosmic scatter and will tend to bias their derived correlation lengths toward the low end. Our results are consistent with redshift distributions in which ~30%-50% of the galaxies at I = 23 lie at z > 1. The best-fit power-law slope of the correlation function remains independent of I magnitude for I ? 22. At fainter limits, there is a suggestive trend toward flatter slopes that occurs at fluxes consistent with similar trends seen by Neuschaffer & Windhorst and Campos and coworkers. The galaxy counts span 11 magnitudes and provide an accurate calibration of the galaxy surface density. We find evidence for mild galaxy evolution: about 1 mag of brightening or a doubling of the density by I = 23 relative to an ?0 = 1 no-evolution model, about 0.5 mag of brightening or a factor of 1.5 increase in surface density relative to an open model. Our galaxy counts agree well with those from the Hubble Deep Field survey and, thus, argue against a significant inclusion of subgalactic components in the latter census for I < 24.

The inflationary Universe — From theory to observations

The inflationary Universe resolves some of the most outstanding issues of standard cosmology including the horizon and flatness problems and the origin of density fluctuations in the Universe. Inflationary models also predict the existence of a relic gravity wave background. Both gravity waves and density fluctuations induce fluctuations in the cosmic microwave background (CMB), the discovery of large angle anisotropies in the CMB having the scale invariant spectrum predicted by inflationary models has fuelled the hope that the inflationary scenario may indeed provide the correct description of the very early Universe. Upcoming large scale galaxy surveys (SDSS & 2dF) and CMB missions (MAP, Planck Surveyor) will further probe the inflationary scenario by throwing light on the origin and evolution of large scale structure in the Universe.

Constrained gravity models and structural evolution of the Ashanti Belt, southwest Ghana

The Palaeoproterozoic terrane of southwest Ghana comprises a sequence of folded sedimentary/volcaniclastic rocks which separate a series of northeast trending volcanic belts. While the stratigraphy and structure of the belts are similar, the Ashanti Belt is characterised by a more tectonised northwest margin along which most of the major epigenetic Au deposits in Ghana are located. Early models of the tectonic evolution of this belt, which generally invoked two phases of deformation, are inconsistent with more recent structural and isotopic evidence (Eisenlohr and Hirdes, 1992). Modelling of regional gravity data by Hastings (1982) acknowledged the tectonic significance of the belt margins but lacked the benefit of constraints provided by more detailed mapping. This paper re-examines existing gravity data over the Ashanti Belt and presents constrained cross-sectional models which honour the mapped geology along the length of the belt and are consistent with the concept of a single, continuous deformational event. A synoptic scenario for the large-scale structural evolution of the belt is proposed which has implications for the location and timing of Au mineralisation.

The CNOC2 Field Galaxy Redshipt Survey

Fundamental to our understanding of the universe is the evolution of structures, from galaxies to clusters of galaxies to large-scale sheets and filaments of galaxies and voids. The investigation of the evolution of large-scale structure not only provides us with the key test of theories of structure formation, but also allows us to measure fundamental cosmological parameters. The CNOC2 (Canadian Network for Observational Cosmology) Field Galaxy Redshift Survey is the first large redshift survey of faint galaxies carried out with the explicit goal of investigating the evolution of large scale structure. This survey also provides the largest redshift and photometric data set currently available for the study of galaxy population and evolution at the moderate redshift range between 0.1 and 0.6. In this paper we describe the scope and technique of the survey, its status, and some preliminary results.

The Virmos Project

Large and deep spectroscopic samples of galaxies are essential to study galaxies and large scale structure evolution out to look-back times ~ 10% the current age of the vmiverse. Keeping this scientific and observational goal in mind, we designed and are presently building two wide-field imaging spectrographs to be installed at the Nasmyth foci of the ESO-VLT Unit Telescopes 3 and 4.

Triadic resonant modes: dynamical model and truncation criterion

In the present paper we propose a three-dimensional theoretical model describing the local evolution of any two- or three-dimensional disturbance superimposed on a basic shear flow. A numerical investigation, using a truncated form of the evolution model, was performed to select resonant three-dimensional disturbance in a transitional cylinder wake. A truncation criterion of non-linear triadic resonance, which is a generalisation of the standard linear-resonance condition, allowed us to locate some triadic sets of linear eigenmodes. Numerical integration of the corresponding low-dimensional dynamical systems showed interesting results that are in qualitative agreement with those performed by Williamson and Prasad [J. Fluid Mech. 256 (1993) 269.] This seems to validate the truncation criterion which, together with the theoretical model, proved to be an appropriate tool for describing the local evolution of large-scale structures in shear flows.

Background-Noise Effects on Modelling Transitional Shear Flows.

Presented in this article is an experimental and numerical study aimed at analyzing the wake dynamics downstream of a cylinder in an unsteady flow. The experimental study was carried out by measuring the mean velocity field with a laser Doppler anemometer, visualizing the fluid field by means of laser induced fluorescence, and performing a statistical analysis of the instantaneous velocity signals using Fourier and wavelet transforms. The visualized images were processed to determine the mean-statistical characteristics of large-scale structures in the wake. Experimental results provided the essential information for the numerical investigation of the temporal evolution of large-scale structures. A theoretical model based on the theory of dynamical systems with few degrees of freedom was adopted. The effects induced by the unsteady nature of the incoming flow were simulated. Experimental and numerical results were in excellent agreement, underlining the validity of the methods of analysis and simulation proposed.

Large-scale structure evolution in supersonic interacting shear layers

An experimental investigation is made of the evolution of the large-scale structures in a flow consisting of two planar turbulent shear layers formed by a Mach-3 planar jet bounded by a Mach-5 freestream. The flow is characterized by two planar shear layers at a convective Mach number of 0.28 that are independent in the near field but that interact farther downstream. Measurements were made using fast-response hot-wire probes and planar laser scattering from a condensed alcohol fog. The hot-wire data were used to calculate power spectra and cross correlations from which large-scale-structure length scales and orientation were inferred. The images reveal roller-like structures, but they do not appear as dominant or as coherent compared with low-Mach-number shear layers at a similar Reynolds number. The hot-wire data confirm the relatively unorganized and three-dimensional nature of the independent shear layers. In the far field, the visualizations reveal that the shear layers interact to form a more organized structure, similar to vortex shedding in incompressible turhulent wakes. These organized structures result in a distinct peak in the power spectrum and larger spanwise coherence lengths than for the independent shear layer.

Galaxy Formation in a CDM + Lambda Universe. I. Properties of Gas and Galaxies

view Abstract Citations (42) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galaxy Formation in a CDM + Lambda Universe. I. Properties of Gas and Galaxies Gnedin, Nickolay Y. Abstract A new approach to cosmological hydrodynamics, called "softened Lagrangian hydrodynamics" is now supplemented with the dark matter evolution, radiative processes including cooling, heating, and ionization, and evolution of radiation background and a galaxy-formation algorithm. The code is therefore able to follow the evolution of a representative piece of the universe with allowance for all physical processes that play dominant roles in the evolution of large-scale structure on scales where the galaxy formation is thought to occur. A series of simulations of a CDM + Λ model were performed with 64 h-1 Mpc box size, 643 baryonic quasi-Lagrangian cells, an equal number of dark matter particles and radiation spectrum represented on 150 points from 1 eV to 200 keV. The softening length of simulations is 100 h-1 kpc. Three production runs were performed: run A without radiative processes and galaxy formation, run B with radiative cooling included, and run C with the complete description of radiative processes and galaxy formation included. The present paper discusses in detail physical properties of gas and galaxies (spatial distributions discussed subsequently). Comparison of cosmic gas properties between these three runs shows that radiative heating by UV emission from galaxies plays a dominant role in heating and ionizing voids, whereas the supernovae thermal energy injection into the intergalactic medium is the dominant mechanism in regulating gravitational collapse, and shock transformation of kinetic to thermal energy is most important in clusters of galaxies. Galaxy formation seems to proceed from collapsing of cold dense gas, cooled by exciting hydrogen lines at 104 K. However, hydrogen quickly becomes ionized and heats up to 105 K, where helium becomes the dominant cooling agent and galaxy formation proceeds further with cooling by exciting helium lines. The local geometry of a gas element that is gravitationally unstable and cools faster than it collapses is neither spherical nor filamentary but more pancake-like and gravitationally unstable but hot and unable to cool gas is typically in filamentary regions. The places where galaxies are born are remarkably similar in their physical properties (densities, temperatures, local geometry) at z = 3 and z = 0. However, the galaxy age correlates with the density of environment where galaxies are found: old galaxies are primarily found in dense clusters, while young galaxies live on caustics. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176623 Bibcode: 1996ApJ...456....1G Keywords: COSMOLOGY: THEORY; COSMOLOGY: DARK MATTER; GALAXIES: FORMATION; HYDRODYNAMICS; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; METHODS: NUMERICAL full text sources ADS | Related Materials (1) Part 2: 1996ApJ...456...34G

Compressibility effects and mixing enhancement in turbulent free shear flows

Computational simulations have been performed to study compressible, spatially developing turbulent free shear layers with various velocity regimes—subsonic/subsonic, supersonic/subsonic, and supersonic/supersonic— for convective Mach numbers in the range of 0.14-1.28. The numerical code used the finite volume technique and a modified Godunov's scheme. The computed results for the supersonic/subsonic case are first compared with experimental axial mean-velocity profiles, vorticity thickness, and turbulence parameters. Mixing layers with various velocity regimes are then calculated to investigate compressibility effects on the evolution of large-scale structures through the flow visualization of the vorticity field, growth rate of the vorticity thickness, and vorticity dynamics analysis. Various forcing frequencies are applied at the inflow boundary to examine mixing enhancement for free shear layers with higher convective Mach numbers. It is found for the first time that the growth rate of supersonic/supersonic free shear layers increases markedly when the forced layers move up and down with time instead of forming vortex roll-up and pairing.

Deficit of distant X-ray-emitting galaxy clusters and implications for cluster evolution

CLUSTERS of galaxies are the largest gravitationally bound systems in the Universe and therefore provide important constraints on the formation and evolution of large-scale structure. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution, and the intracluster gas is compressed and shock-heated by the gravitational collapse1. If the gas is in hydrostatic equilibrium the resulting X-ray emission is related in a simple way to the evolving gravitational potential. But if processes such as radiative cooling or pre-collapse heating of the gas are also important, the X-ray evolution will be strongly influenced by the thermal history of the gas. Here we present the results of a faint flux-limited sample of X-ray selected clusters observed by Rosat. Very few distant clusters have been identified, and their redshift distribution seems to be inconsistent with simple models based on the evolution of the gravitational potential. Our results thus suggest that radiative cooling or non-gravitational heating of intracluster gas must be important in the evolution of clusters.

The characteristics and evolution of large-scale structures in compressible mixing layers

Single- and double-pulsed visualizations were employed to study the characteristics and evolution of large-scale structures in compressible mixing layers with convective Mach numbers (Mc) of 0.51 and 0.86. Instantaneous images and spatial correlations based on large ensembles of images show that large-scale structures which span the entire mixing layer thickness are a more dominant feature at Mc=0.51 than at Mc=0.86. Double-pulsed images in the Mc=0.51 developing region show the structures’ formation to proceed as the roll-up of a wavy mixing region, in agreement with the Kelvin–Helmholtz picture of structure formation derived from studies of incompressible mixing layers. However, little indication of roll-up was present for Mc=0.86. In the Mc=0.51 fully developed region, the large-scale structures evolve similarly to those of incompressible mixing layers, even undergoing pairing processes. Similar structure evolution processes were not encountered in Mc=0.86; instead, it was more difficult to even track large-scale motions between the initial and delayed images. Convective velocities derived from space-time correlations of ensembles of double-pulsed images are in good agreement with the theoretical convective velocity at the center of the mixing layer, but are higher than the theoretical value toward the high-speed side of the mixing layer and lower toward the low-speed side.

Structure formation with decaying neutrinos.

We consider the effects of a massive, unstable neutrino on the evolution of large-scale structure and anisotropies in the cosmic microwave background. A comparison with large-scale structure data allows us to rule out a wide range of masses and lifetimes for such neutrinos. We also define a range of masses and lifetimes which delay matter-radiation equality and improve the agreement with the data of cold dark matter models with critical density.

Constraints on Self-interacting Dark Matter

We consider the growth of density perturbations in the presence of self--interacting dark matter, SIDM, proposed by Carlson, Machacek and Hall (1992). We determine the range of values for the coupling constant $\lambda$ and the particle mass $m^\prime$, for which the power spectrum lies in the ``allowed" range based on constraints from the IRAS galaxy survey and damped Lyman--$\alpha $ systems. Our results show that no combination of parameters can meet both limits. We consider constraints on the $\2-2$ scatterings which keep the SIDM particles in pressure equilibrium, and we show that if such interactions maintain pressure equilibrium down to the present, they will be strong enough to disrupt galaxy mergers and may lead to stripping of galaxy halos as galaxies move through the dark matter background of these particles. Hence, we also investigate the evolution of large-scale structure in the SIDM model when the particles drop out of pressure equilibrium at some higher redshift. The resulting free-streaming leads to an additional suppression of small-scale perturbations, but it does not significantly affect our results.