Large-scale structure of the universe

The panoramic view of the entire sky of the nearby infrared reveals the distribution of galaxies beyond the Milky Way. The image has been obtained from the 2MASS catalogue, with more than 1.5 million galaxies and the Point Source Catalog (PSC), with about 500 million stars in the Milky Way. The galaxies are encoded by their "red-up" color obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS expeditions (and several observations compiled by NASA's extragalactic databases) or photometrically deducted from the K band (2.2 um). The blues are the closest sources (z ≤1), the greens are at moderate distances (0.01 quot z quot 0.04) and the red ones are the most distant ones that the 2MASS can solve (0.04 quot quot " ". The map is projected with the same area as the galactic system (the Milky Way in the middle). Figures by Thomas Jarret (IPAC).

In physical cosmology, the term large-scale structure refers to the characterization of the observable distributions of matter and light on the largest scales (typically on the order of billions of light-years). Sky-watching expeditions and mapping of various wavelength bands of electromagnetic radiation (particularly 21 cm emissions) have provided much insight into the content and character of the structure of the universe. The organization of the structure seems to follow a hierarchical model with the organization at the upper scale of superclusters and filaments. Above this, there appears to be no continuing structure, a phenomenon that has been known as the End of Greatness, and which is clear proof of the cosmological principle.

Characterization of structures

The organization of structures arguably begins at the stellar level, although many cosmologists rarely approach astrophysics on this scale. The stars are organized into galaxies, which form clusters and superclusters that are separated by the immense void. Until 1989, it was commonly assumed that viralized galaxy clusters were the largest structures in existence and that they were distributed more or less evenly across the universe in every direction. However, based on data from redshift expeditions, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a group of galaxies more than 500 million light-years away and 200 million years across, but only 15 million light-years deep. The existence of this structure escaped notice for too long because it requires locating the position of galaxies in three dimensions, which involves combining location information about galaxies with redshift distance information.

In April 2003, another large-scale structure, the Sloan Great Wall, was discovered. However, it is not technically a 'structure', since the objects in it are not gravitationally related to each other but only appear that way, caused by the distance measurements that were used. One of the largest voids in space is the Capricorn void, with an estimated diameter of 230 million light-years. However, in August 2007 the existence of a new supervoid was confirmed in the constellation Eridanus, which is almost a thousand thousand million years ago. Originally, it had been discovered in 2004 and was known as the WMAP Cold Spot.

In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies in which the supercluster resembles occasional relatively dense nodes.

Astrocartography of our neighborhood

At the center of the Virgo Supercluster is a gravitational anomaly, known as the Great Attractor, which affects the motion of galaxies over a region of hundreds of millions of light-years. All of these galaxies are redshifted, according to Hubble's law, indicating that they are receding from us and from each other, but the variations in their redshift are enough to reveal the existence of an equivalent concentration of mass. to tens of thousands of galaxies. The Great Attractor, discovered in 1986, is located at a distance of between 150 million and 250 million light years (250 million is the most recent estimate), in the direction of the constellations of Hydra and Centaurus. In your neighborhood there is a preponderance of large old galaxies, many of which are colliding with their neighbors 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 that appear in the spectral lines of light from quasars, which are interpreted as indicating the existence of large thin sheets of intergalactic gas (mainly hydrogen). These sheets seem to be associated with the formation of new galaxies.

Some caution is needed when describing structures on a cosmic scale because things are not always as they appear to be. Gravitational light bending (gravitational lensing) can result in images that appear to originate from a different direction from their actual source. This is caused by background objects (such as galaxies) curving space around themselves (as predicted by general relativity), deflecting light rays that pass by. It's quite useful, strong gravitational lensing can sometimes amplify distant galaxies, making them easier to detect. Weak lensing (gravitationally sheared) by the intervention of the universe in general also subtly changes the observed large-scale structure. In 2004, measurements of this subtle shearing show it to hold considerable promise as a test of cosmological models.

The large-scale structure of the universe also appears different if only redshift is used to measure distances to galaxies. For example, galaxies behind a galaxy cluster will be pulled towards it and fall into it and so will be slightly blueshifted (compared to how they would be if there were no cluster), on the near side, things would be slightly redshifted. Thus, the cluster's surroundings would appear a bit squashed if redshift were used to measure distance. An opposite effect already occurs in galaxies within a cluster: galaxies have some random motion around the center of the cluster, 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 pointing from Earth.

Modelling

There is much work in physical cosmology that attempts to model the large-scale structure of the universe. Using the Big Bang model and assuming the type of matter that makes up the universe, the distribution of matter can be predicted and by comparison with observations go back to support or refute certain cosmological theories. Currently, observations indicate that much of the universe must be composed of cold dark matter. Models assuming hot dark matter or baryonic dark matter do not fit the observations well enough. Irregularities in the microwave background radiation and large redshifts from supernovae provide alternative insights to constrain the same models, and there is a growing consensus that these joint observations are providing proof that we live in an accelerating universe..