Large Scale Structure of the Cosmos

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.

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 is drawing in galaxies over a region hundreds of millions of light years across. These galaxies are all redshifted, in accordance with the 'Hubble flow', as if 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.

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 is evidence of quantisation of redshift. There have been numerous studies investigating this phenomenon, but it is not universally accepted as real, and is 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 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.