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| {{Cosmology}} | |||
| Stars are organised into galaxies which in turn appear to form clusters and superclusters, separated by voids. Prior to 1989 it was commonly assumed that the superclusters were the largest structures in existence, and that they were distributed more-or-less uniformly throughout the universe in every direction. However, in 1989, Margaret Geller and John Huchra discovered the 'Great Wall', a sheet of galaxies apparently more than 500 million light years long and 200 million wide, but only 15 million light years thick. | |||
| ] and ] examine the ] to understand the '''large-scale structure of the cosmos'''. So far, scientists have been able to discern and describe several different large-scale structures. ]s are organised into ], which in turn form ] and ]s that are separated by immense ]s. Prior to 1989, it was commonly assumed that superclusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, based on ] data, in ] ] and ] discovered the "]," a sheet of galaxies more than 500 million ]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 ]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 ] where ] is located and millions of others galaxies.]] | |||
| ⚫ | In more recent studies the universe appears as a collection of giant bubble-like ] separated by sheets and ] of galaxies, with the ]s appearing as occasional relatively dense nodes. | ||
| At the centre of the ] there is a gravitational anomaly, known as the ], which affects the motion of galaxies over a region hundreds of millions of light years across. | |||
| ⚫ | In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the |
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| ⚫ | These galaxies are all ]ed, in accordance with ], 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 ], 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 ] and ] ]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. | ||
| ⚫ | Another indicator of large-scale structure is the ']'. This is a collection of absorption lines which appear in the ]s of light from ]s, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly ]) gas. These sheets appear to be associated with the formation of new galaxies. | ||
| ⚫ | Finally, there have been ] of the ] of redshift. Although there have been numerous studies investigating this phenomenon, it is not widely regarded as valid, and remains the subject of considerable ]. | ||
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| Some caution is required in describing structures on a cosmic scale because things are not always as they appear to be. ] (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 ]), deflecting light rays that pass nearby. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. ] (gravitational shear) by the intervening universe in general also subtly changes the observed large-scale structure. ], 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 ] to measure distances to galaxies. For example, galaxies behind a galaxy cluster will be attracted to it, and so fall towards it, and so be slightly blueshifted (compared to how they would be if there were no cluster); on the near side, things are slightly redshifted. Thus, the environment of the cluster looks a bit squashed, if using redshifts to measure distance. An opposite effect works on the galaxies already within the cluster: the galaxies have some random motion around the cluster centre, and when these random motions are converted to redshifts, the cluster will appear elongated. This creates what is known as a '']'': the illusion of a long chain of galaxies pointed at the Earth. | |||
| There is much work in cosmology which attempts to model the large-scale structure of the universe. Using the ] 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 ]. Models which assume ] or ] do not provide a good fit with observations. The irregularities in the ] and ] 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 ]. | |||
| ⚫ | The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light years |
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| In the ] model of Hoyle, Burbidge and Narlikar, the process of structure formation has been studied in the context of a "]" which may be a viable alternative to the standard hot big bang model of structure formation. | |||
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| ⚫ | Another indicator of large |
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| ] | |||
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| Another indicator is the quasars themselves. A quasar ('quasi-stellar object') looks like a star but if estimates of their distance are correct then they are each many billions of times brighter than any star, and up to 1000 times brighter than ane entire giant galaxy. There is no quasar within 2 billion light years of earth, but they apparently cluster more thickly at greater distances, leading to speculation that they are a phenomenon of the early universe and that most of them have recently 'gone out', though in that case there ought to be some remnants of them to be discovered. | |||
| ⚫ | Finally, there |
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| 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 can result in images which appear to lie in a different direction from the true direction of the image source. This can result in optical illusions which obscure the actual processes which are taking place. | |||
| Currently accepted theories do not do a good job of explaining the structures that are observed. Instead, they make ad hoc assumptions about unobserved quantities of hypothetical dark matter, and other phenomena which are not observed on a local scale, in an attempt to arrive at a 'close fit' with observations. | |||
Revision as of 21:09, 10 April 2006
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Cosmology and Astronomy examine the universe to understand the large-scale structure of the cosmos. So far, scientists have been able to discern and describe several different large-scale structures. Stars are organised into galaxies, which in turn form clusters and superclusters that are separated by immense voids. Prior to 1989, it was commonly assumed that superclusters 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 years 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 redshifts.
In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the superclusters appearing as occasional relatively dense nodes.
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 redshifted, 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 constellations. 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.
Another indicator of large-scale structure is the 'Lyman alpha forest'. This is a collection of absorption lines which appear in the spectral lines of light from quasars, 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.
Finally, there have been occasional claims of the quantisation of redshift. Although there have been numerous studies investigating this phenomenon, it is not widely regarded as valid, and remains the subject of considerable controversy.
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 redshift to measure distances to galaxies. For example, galaxies behind a galaxy cluster will be attracted to it, and so fall towards it, and so be slightly blueshifted (compared to how they would be if there were no cluster); on the near side, things are slightly redshifted. Thus, the environment of the cluster looks a bit squashed, if using redshifts to measure distance. An opposite effect works on the galaxies already within the cluster: the galaxies have some random motion around the cluster centre, and when these random motions are converted to redshifts, the cluster will appear elongated. This creates what is known as a finger of God: the illusion of a long chain of galaxies pointed at the Earth.
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.
In the quasi-steady state model of Hoyle, Burbidge and Narlikar, the process of structure formation has been studied in the context of a "toy model" which may be a viable alternative to the standard hot big bang model of structure formation.
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