| Large-scale Structure Of The Cosmos |
Website Links For Structure |
Information AboutLarge-scale Structure Of The Cosmos |
| CATEGORIES ABOUT LARGE-SCALE STRUCTURE OF THE COSMOS | |
| large-scale structure of the cosmoslarge-scale structure of the cosmos | |
| nature | |
| physical cosmology | |
| extragalactic astronomy | |
|
In Physical Cosmology , the term large-scale structure refers to the characterization of Observable Distribution s of Matter and Light on the largest scales (typically on the order of Billion s of Light Year s. Sky Surveys and Mapping s of the various Wavelength bands of Electromagnetic Radiation (in particular 21-cm Emission ) have yielded much information on the content and character of the Universe 's structure. The organization of structure appears to follow as a Hierarchical model with organization up to the Scale of Supercluster s and Filaments . Larger than this, there seems to be no continued structure, a phenomenon which has been referred to as the End Of Greatness . CHARACTERIZATION OF STRUCTURE The organization of structure arguably begins at the stellar level, though most cosmologists rarely address Astrophysics on that scale. Star s are organised into Galaxies , which in turn form Clusters and Supercluster s that are separated by immense Void s. Prior to 1989, it was commonly assumed that Virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, based on Redshift Survey data, in 1989 Margaret Geller and John Huchra discovered the " Great Wall ," a sheet of galaxies more than 500 million Light Year s long and 200 million wide, but only 15 million light years thick. The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from Redshift s. . The galaxies pictured above are part of a cluster of galaxies called ACO 3627 (or the Norma cluster) near the center of the Great Attractor. The Great Attractor is a diffuse mass concentration 250 million light-years away but so large it affects the motion of the Milky Way Galaxy where Earth is located and millions of others galaxies.]] In more recent studies the universe appears as a collection of giant bubble-like Voids separated by sheets and Filaments of galaxies, with the Supercluster s appearing as occasional relatively dense nodes. ASTROCARTOGRAPHY OF OUR NEIGHBORHOOD At the centre of the Local Supercluster there is a gravitational anomaly, known as the Great Attractor , which affects the motion of galaxies over a region hundreds of millions of light years across. These galaxies are all Redshift ed, in accordance with Hubble's Law , indicating that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies. The Great Attractor, discovered in 1986 , lies at a distance of between 150 million and 250 million light years (250 million is the most recent estimate), in the direction of the Hydra and Centaurus Constellation s. In its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, and/or radiating large amounts of radio waves. OBSERVATIONS Another indicator of large-scale structure is the ' Lyman Alpha Forest '. This is a collection of absorption lines which appear in the Spectral Line s of light from Quasar s, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly Hydrogen ) gas. These sheets appear to be associated with the formation of new galaxies. Some caution is required in describing structures on a cosmic scale because things are not always as they appear to be. Bending Of Light By Gravitation (gravitational lensing) can result in images which appear to originate in a different direction from their real source. This is caused by foreground objects (such as galaxies) curving the space around themselves (as predicted by General Relativity ), deflecting light rays that pass nearby. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak Lensing (gravitational shear) by the intervening universe in general also subtly changes the observed large-scale structure. As Of 2004 , measurements of this subtle shear show considerable promise as a test of cosmological models. The large-scale structure of the Universe also looks different if one only uses '': the illusion of a long chain of galaxies pointed at the Earth. MODELING There is much work in cosmology which attempts to model the large-scale structure of the universe. Using the Big Bang model and assumptions about the type of matter that makes up the universe, it is possible to predict the expected distribution of matter, and by comparison with observation work backward to support and refute certain cosmological theories. Currently, observations indicate that most of the universe must consist of Cold Dark Matter . Models which assume Hot Dark Matter or Baryonic Dark Matter do not provide a good fit with observations. The irregularities in the Cosmic Microwave Background Radiation and High Redshift Supernovae give complementary approaches to constraining the same models, and there is a growing consensus that these approaches together are giving evidence that we live in an Accelerating Universe . |
|
|