The 2dF galaxy redshift survey measured the distances to hundreds of thousands of galaxies in the (relatively) nearby Universe. The large-scale structures of sheets, filaments, voids and galaxy superclusters are clearly visible, and give this region of space a honeycomb appearance.Credit: M. Colless (ANU) and the 2dF Galaxy Redshift Survey
In the local Universe, there are two large-scale structures of particular importance: the Great Wall and the Great Attractor. These structures influence the motions of galaxies in the Local Group, and are ultimately responsible for the fate of the Milky Way. Astronomers believe we will eventually merge with the Virgo cluster which will itself merge with the Centaurus and Shapley superclusters which make up the Great Attractor.
The Large-scale Structure Of The
A cold, dark matter simulation of the large-scale structure in the nearby Universe. This image is 800 million light years square. Red regions show high concentrations of galaxies, including galaxy superclusters and filaments, while blue areas show low galaxy concentrations in the voids.Credit: Chris Power, Swinburne UniversityFrom the point of view of cosmologists, large-scale structure is extremely important for two main reasons:
The cosmological model currently favoured is the cold dark matter (λ CDM) model. Supercomputer simulations (left) show that this model is able to reproduce the observed large-scale structure at the right time and on the correct scales to remarkable accuracy, provided we assume that the very early Universe did not have a perfectly uniform density. These tiny primordial density fluctuations are thought to arise from quantum mechanical effects, and are not only required to produce the large-scale structure, but also the observed Cosmic Microwave Background.
If this model is correct, it means that the largest structures in the Universe were initially seeded by events on scales smaller than the size of an atom in the fleeting instants immediately following the Big Bang.
Research over the past 25 years has led to the view that the rich tapestry of present-day cosmic structure arose during the first instants of creation, where weak ripples were imposed on the otherwise uniform and rapidly expanding primordial soup. Over 14 billion years of evolution, these ripples have been amplified to enormous proportions by gravitational forces, producing ever-growing concentrations of dark matter in which ordinary gases cool, condense and fragment to make galaxies. This process can be faithfully mimicked in large computer simulations, and tested by observations that probe the history of the Universe starting from just 400,000 years after the Big Bang.
Large-scale structure of the Universe refers to the patterns of galaxies and matter on scales much larger than individual galaxies and groupings of galaxies. These correlated structures can be seen up to billion of light years in length and are created and shaped by gravity.[1] On large scales Universe displays coherent structure, with galaxies residing in groups and clusters on scale of 1-3 Mpc/h, which lie at the intersections of long filament of galaxies that are> 10 Mpc/h in length. Vast regions of relatively empty space, known as voids, contain very few galaxies and span in the volume in between these structures.[2]
The density distribution arising at the nonlinear stage of gravitational instability is similar to intermittency phenomena in acoustic turbulence. Initially small-amplitude density fluctuations of Gaussian type transform into thin dense pancakes, filaments, and compact clumps of matter. It is perhaps surprising that the motion of self-gravitating matter in the expanding universe is like that of noninteracting matter moving by inertia. A similar process is the distribution of light reflected or refracted from rippled water. The similarity of gravitational instability to acoustic turbulence is highlighted by the fact that late nonlinear stages of density perturbation growth can be described by the Burgers equation, which is well known in the theory of turbulence. The phenomena discussed in this article are closely related to the problem of the formation of large-scale structure of the universe, which is also discussed.
A collaborative experimental effort employing the minimally perturbed atmospheric surface-layer flow over the salt playa of western Utah has enabled us to map coherence in turbulent boundary layers at very high Reynolds numbers, \(Re_\tau\sim\mathcalO(10^6)\) . It is found that the large-scale coherence noted in the logarithmic region of laboratory-scale boundary layers are also present in the very high Reynolds number atmospheric surface layer (ASL). In the ASL these features tend to scale on outer variables (approaching the kilometre scale in the streamwise direction for the present study). The mean statistics and two-point correlation map show that the surface layer under neutrally buoyant conditions behaves similarly to the canonical boundary layer. Linear stochastic estimation of the three-dimensional correlation map indicates that the low momentum fluid in the streamwise direction is accompanied by counter-rotating roll modes across the span of the flow. Instantaneous flow fields confirm the inferences made from the linear stochastic estimations. It is further shown that vortical structures aligned in the streamwise direction are present in the surface layer, and bear attributes that resemble the hairpin vortex features found in laboratory flows. Ramp-like high shear zones that contribute significantly to the Reynolds shear-stress are also present in the ASL in a form nearly identical to that found in laboratory flows. Overall, the present findings serve to draw useful connections between the vast number of observations made in the laboratory and in the atmosphere.
These pie slice diagrams show the distances to all of the galaxies in a narrow strip of sky. The densest groups of points are the locations of clusters like Virgo, Coma, and Perseus. What you should notice is that the distribution of galaxies is not random. That is, the clusters appear to form clusters of clusters! The structure that you see in the pie slice diagrams is often described as being like soap bubbles. That is, the galaxies lie along the walls of the bubbles, and inside the bubbles are voids where very few galaxies are found. The voids are not completely empty. For example, the Hubble Deep Field image was taken in the center of a void. The poor groups like the Local Group lie in the voids.
Diagrams like the one above of the distribution of galaxies in the Universe seem to imply that the Universe isn't homogeneous and isotropic. In other words, the galaxies in one direction are not distributed in exactly the same way as the galaxies in another direction. However, if you look at the scale on the plot, the galaxies it contains only extend to a redshift of z
We present statistical analyses of the large-scale structure of 3 types of semantic networks: word associations, WordNet, and Roget's Thesaurus. We show that they have a small-world structure, characterized by sparse connectivity, short average path lengths between words, and strong local clustering. In addition, the distributions of the number of connections follow power laws that indicate a scale-free pattern of connectivity, with most nodes having relatively few connections joined together through a small number of hubs with many connections. These regularities have also been found in certain other complex natural networks, such as the World Wide Web, but they are not consistent with many conventional models of semantic organization, based on inheritance hierarchies, arbitrarily structured networks, or high-dimensional vector spaces. We propose that these structures reflect the mechanisms by which semantic networks grow. We describe a simple model for semantic growth, in which each new word or concept is connected to an existing network by differentiating the connectivity pattern of an existing node. This model generates appropriate small-world statistics and power-law connectivity distributions, and it also suggests one possible mechanistic basis for the effects of learning history variables (age of acquisition, usage frequency) on behavioral performance in semantic processing tasks.
Submesoscale baroclinic instabilities have been shown to restratify the surface mixed layer and to seasonally energize submesoscale turbulence in the upper ocean. But do these instabilities also affect the large-scale circulation and stratification of the upper thermocline? This question is addressed for the North Atlantic subtropical mode water region with a series of numerical simulations at varying horizontal grid spacings (16, 8, 4, and 2 km). These simulations are realistically forced and integrated long enough for the thermocline to adjust to the presence or absence of submesoscales. Linear stability analysis indicates that a 2 km grid spacing is sufficient to resolve the most unstable mode of the wintertime mixed-layer instability. As the resolution is increased, spectral slopes of horizontal kinetic energy flatten and vertical velocities increase in magnitude, consistent with previous regional and short-time simulations. The equilibrium stratification of the thermocline changes drastically as the grid spacing is refined from 16 to 8 km and mesoscale eddies are fully resolved. The thermocline stratification remains largely unchanged, however, between the 8, 4, and 2 km runs. This robustness is argued to arise from a mesoscale constraint on the buoyancy variance budget. Once mesoscale processes are resolved, the rate of mesoscale variance production is largely fixed. This constrains the variance destruction by submesoscale vertical buoyancy fluxes, which thus remain invariant across resolutions. The bulk impact of mixed-layer instabilities on upper-ocean stratification in the subtropical mode water region through an enhanced vertical buoyancy flux is therefore captured at 8 km grid spacing, even though the instabilities are severely under-resolved. 2ff7e9595c
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