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The Large-scale Structure Of The 2021


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




The Large-scale Structure Of The


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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.


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.


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