Superclusters

Large Scale Structures
The Virgo Supercluster
Between the Superclusters
Formation of Superclusters
Density Fluctuations
Power Spectrum of Cosmic Structure
References
Index

Large Scale Structures^1,2

The reality of superclusters did not sink in until the 1980s, when new telescopes and sensors could produce three-dimensional maps of the universe. Superclusters are typically seen as long and thin strands of clusters and galaxies, intracluster gases and, presumably, "dark matter" on a two dimensional surface. The strands are interspersed by large voids nearly empty of matter. Figure 03-01a shows the Coma supercluster in a slice of the universe with our position at the vertex. This map extends to a depth of 300 million

		light years (3x10²⁶ cm). It confirmed that the galaxies in the universe are arranged in sheets and walls surrounding large nearly-empty voids. A new study in 2006 has found that spiral galaxies line up like beads on a string, with their spin axes aligned with the filaments that outline voids (Figure 03-01b). The finding supports current galaxy-formation theory, which derives a galaxy's rotation from the uneven distribution of the visible and dark matter from which it coalesces. It predicts a galaxy's axis should be
Figure 03-01a The Coma Supercluster [view large image]	Figure 03-01b Spin of Galaxies [view large image]	more-or-less perpendicular to the line between the galaxy and the center of the void.

Figure 03-02a is a three dimensional map of the universe inside a sphere with a radius of 500 million light years. Each point represents a group of galaxies. The solid lines indicate which quadrant the points are located on the upper hemisphere, while the dotted lines indicate the locations on the lower hemisphere. The Milky Way (a member of the Virgo supercluster) is at the center of the sphere. It shows most of the major galaxy superclusters that surround the Virgo supercluster. These superclusters

		are not isolated in space but together with many other smaller concentrations of galaxies, they form parts of extensive walls of galaxies surrounding large voids. Three of the biggest walls as well as several of the largest voids are marked on the map. The Virgo along with the Hydra and other superclusters are streaming at a speed of 6x10⁷ cm/sec toward the "Great Attractor" (Figure 03-02b, 2-D plot on galactic plane with Milky Way at the center, arrows show the directions and magnitude of the motion), which is a gigantic unseen mass located near the A3627 (Norma) cluster
Figure 03-02a Large Scale Structures [view large image]	Figure 03-02b Attractor A3627 [view large image]	(Figure 03-02c) in the Centaurus Wall near the galactic plane. In comparison, the speed of cosmic expansion is about 7x10⁸ cm/sec at a distance of 100 megapc.

		Figure 03-02d (X-ray: blue; H-alpha: red; Optical: white) is the image of the galaxy ESO 137-001 with a tail that has been created as it plunges toward the center of A3627, shedding material and forming stars behind it. It is estimated that the observed mass of cluster A3627 is not able to account for the huge gravitational attraction exerted on the other clusters. There seems to be something more massive hidden by the dust and gas on the galactic plane represented by the pale blue ribbon of the Milky Way in Figure 03-02c.
Figure 03-02c Attractor Location [view large image]	Figure 03-02d Attraction to A3627

In 2008, using the cluster catalog and WMAP's data, bulk cluster motions of nearly 10⁸ cm/sec has been identified toward a

		patch of sky near A3627 in the direction between the constellations of Centaurus and Vela (the pink area in Figure 03-02e, also see Figure 03-02c). The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase. It is suggested that such motion (now called darkflow) is caused by the gravitational attraction of matter that lies beyond the
Figure 03-02eUnobser -vable Attractor	Figure 03-02f Darkflow [view large image]	observable universe (Figure 03-02f). Insert in Figure 03-02e shows one of such clusters 1E 0657-56.

By analyzing the absorption lines of magnesium and iron atoms from quasars more than ten billion years away, astronomers in 2010 found that the fine structure constant is smaller (by 1/10⁶) on one side of the universe and bigger on the other side with an axis close to the direction of the Darkflow. It is also aligned with a dipole in the abundance of deuterium in the early universe, and another dipole for the intensity of light emitted by supernovae. It is estimated that the chance of being a genuine effect is about 99.9937%, but a scientific discovery traditionally has to be at 99.99937%. If such effect is real, then special relativity has to be revised, and life may not be possible in some parts or epochs of the universe. It also implies that there may be more dimensions as predicted by the superstring theory.

Figure 03-02g
Variation of Alpha

The fine structure constant

= e²/c

= 1/137 is a dimension-less number, which is the embodiment of the constants from the electromagnetic interaction e, special relativity c, and quantum theory

. It is related to other fundamental constants such as :

Classical radius of the electron _c = 2.82x10^-13 cm, where _c = (/mc) = 0.363x10^-10 cm is the reduced Compton wavelength, which is actually the de Broglie wavelength for the electron moving with moment p = 2mc.
Radius of the Bohr orbit in hydrogen atom = _c/ = 5.3x10^-9 cm.
Ground state energy of the hydrogen atom = -²mc²/2 = -2.17x10^-11 erg = -13.6 ev.
It appears in the Compton scattering cross section as d()/d = (_c)²(1 + cos²)/2.
In general a factor of ()ⁿ, where n = 1, 2, 3, ... , appears in the S-matrix of all the quantum electromagnetic processes. It is related to the probability of the occurrence of each one. This is actually the consequence of adopting the perturbation expansion for QED.

Nevertheless, many prominent physicists have ascribed a mystical meaning to this number.

Figure 03-03 presents a different view of the large scale structure, which covers a region of sky about 100^o by 50^o around the South Galactic Pole. The APM³ (Automatic Plate Measuring) Galaxy Survey contains positions, magnitudes, sizes and shapes for about 3 million galaxies. The picture shows the galaxy distribution as a density map on the sky. Each pixel covers a small patch of sky 0.1^o on a side, and is shaded according to the number of galaxies within the area: where there are more galaxies, the pixels are brighter. Galaxy clusters, containing hundreds of galaxies closely packed together, are seen as small bright patches. The larger elongated bright areas are superclusters and filaments.

Figure 03-03 APM Galaxy Survey
[view large image]

These surround darker voids where there are fewer galaxies. The colours are coded according to the apparent magnitude of the galaxies in each pixel: fainter galaxies are shown as red, intermediate are shown as green and bright are show as blue. The

more distant galaxies tend to be fainter, and also show less clustering, and so the maps has a generally uniform reddish background. The more nearby galaxies tend to be bright, and are more clustered, so the more prominent clusters of galaxies in the map tend to show up as blue. The small empty patches in the map are regions that have been excluded around bright stars, nearby dwarf galaxies, and globular clusters. Figure 03-04 shows an all sky distribution produced by the Two Micron All Sky Survey⁴ (2MASS) with more than one million galaxies and similar color codes (as the APM). The Milky Way is at the center of the map.

Figure 03-04 Galaxy Survey, 2MASS [view large image]

The 2dF Galaxy Redshift Survey⁵ (completed in 2002) provides yet another view of the large-scale structure as shown in Figure 03-05. It used the 3.9 meter Anglo-Australian Telescope to obtain spectra for nearly a quarter million galaxies up to redshift of 0.20. The pattern is remarkably similar to the computer simulation assuming the WMAP dark matter and dark energy composition of 30% and 70% respectively.

Figure 03-05 2dF Galaxy Survey
[view large image]

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The Virgo Supercluster⁶

The Milky Way is a member of the Local Group of galaxies, which in turn is a part of the Virgo supercluster (see Figure 03-06a). It is centered on the Virgo cluster and extends some 150 million light years across. The Virgo cluster itself contains thousands of galaxies including M87, which is known to surround a gigantic black hole. Virgo's gravity affects the movement of its neighbors, including the Local Group. The supercluster is the last outpost before a space traveler would enter a nearly galaxy-free region called a cosmic void. Actually, even the supercluster has a mass equaling some thousand trillion suns, virtually all its volume is empty in such a vast space. The Local Group of galaxies extends some 4 million light years across. Most galaxies in the group are considered dwarfs, but the two largest - the Milky Way and the Andromeda galaxy - are giant spirals moving toward each other at a speed of about 100 km/sec. All the galaxies of the Local Group are traveling together through space - indicating a common origin.

Figure 03-06a Virgo Supercluster [view large image]

Figure 03-06b is the NASA/ESA vision of the galactic encounter between the Andromeda galaxy and the Milky Way ending with the formation of a giant elliptical galaxy some 7 billion years in the future. As the Sun evolves toward its dying phase, The Earth would become inhabitable about 1.2 billion years from now, and it will melt under the expanding Sun in another 6.5 billion years.


Figure 03-06b Galactic Encounter [view large image]

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Between the Superclusters

The space between superclusters is not absolutely void. Invisible clouds of hydrogen have been detected by their effect on the spectra of distant quasars. As shown in Figure 03-07, when light from the quasars travels through the hydrogen clouds, each cloud imprints an absorption line of Lyman-alpha onto the continuum spectrum. As the clouds are expanding with different rate respect to the quasar, a series of red-shifted lines called Lyman-alpha forest is formed. These features encode information about the distribution and density of cold gas along the line of sight to the quasars.

Figure 03-07 Lyman-alpha Forest [view large image]

Incidentally, such pattern constitutes another evidence for cosmic expansion.

Figure 03-08a shows two quasar spectra. One is the relatively nearby quasar 3C273 at redshift z = 0.158 while Q1422 is a high redshift object at z = 3.62. The intervening neutral hydrogen clouds would absorb that part of the continuum spectrum from the light source at

₀/(1+z'), where

₀ = 1216Å for the Lyman alpha spectral line and 0z'z is the relative red shift between the source and the clouds. Since there are more clouds in between the high redshift object and us, the Q1422 spectrum displays much more numerous absorption lines earning the name of a "forest". The data also show that there are small number of very big clumps of hydrogen in the distant Universe: the galaxies, but small chunks related to the dwarf galaxies are very much more common.

Figure 03-08a Lyman-alpha Forest Examples [view large image]

Huge blob of Lyman alpha cloud (hundred times larger than the galaxies) has been found in the 2000's. They are seen in an era about 11 billion years ago when galaxies collided and merged with bursts of star formation. The Lyman alpha emission could be either from cooling gas or a central source such as a black hole. Detection of polarized emission suggests that photons from a central source are able to escape because the medium is highly ionized or its wavelength is shifted by random motions of the atoms, otherwise it would be repeatedly absorbed trapping it inside the cloud. Such kind of scattering would form concentric ring patterns as observed and shown in Figure 03-08b.

Figure 03-08b Lyman-alpha Blob Polarization [view large image]

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Formation of Superclusters⁷

Recently in 1999, the X-ray telescope on the spacecraft Rosat has detected intergalactic wind blowing through superculsters. The observations showed the direction of the winds by the way they bent jets of electrified gases (plasma) emitted from the cores of galaxies (the same effect as bending smoke from a chimney). The wind directions were lined up with the galactic clusters within superclusters, i.e., along the supercluster axis. It seems to feed matter, including stars, galaxies and gas swept up and transported by the winds, into the growing galactic clusters.

Formation of superclusters may be the next stage in a process that is shaping and forming fundamental units in the universe. It is believed that the process began after the Big Bang, when matter in the universe expanded out rapidly. Some matter clumped together to form stars. Then gravity took over and the stars formed galaxies, then groups, then clusters and, now, superclusters. The supercluster formation occurring now is at an early stage. These objects may be at the critical point of overcoming the random motion and are now collapsing under its own gravitation into an increasingly dense superstructure. Figure 03-09a is a computer simulation of the growth of large scale structure as matter is accreted along the filaments. Each square represents a step in the evolution of the universe. The sequence commences at redshift 10.0, less than 500 million years after the Big Bang, and terminates at redshift 0 corresponding to the current epoch. However, superclusters are recognizable by observation only up to a distance of about 8x10⁹ lys at z ~ 1; and we cannot see the Virgo supercluster (within ~ 200 million lys) in which we are living (Figure 03-09b).

Figure 03-09a Com

puter Simulation of Large Scale Structure Evolution [view large image]


Figure 03-09b Cosmic Timeline [view large image]

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Density Fluctuations⁸

The inflation theory attributes the origin of large scale structures to quantum fluctuation (of the scalar field), which occurred near the beginning of Big Bang. The fluctuation is subsequently enlarged by the inflation and served as a blue-print for the large scale structures such as the superclusters. Figure 03-10 depicts the supercluster formation from quantum fluctuations. The dot at the top shows the actual size, just at the end of inflation. An enlargement (about 300X) of a small section of the universe at this time is shown in the middle. Eventually, after about 14 billion years, the imprint has accumulated enough matter and form the Coma supercluster today. In gravitational terms, the superclusters are merely slight irregularities on a basically smooth universe. It requires only one part in 100,000 of its rest-mass energy to pull the structure apart. This is in agreement with the observed fluctuation level in CMBR and is referred to as scale invariance..

There is a problem with the formation of superclusters. Theory associates a characteristic time for the gravitational settling near the center of a clump. For a density fluctuation of 1.7%, it is of the order of 1 billion years; it would be 13 billion years for 0.3% fluctuation, etc. However, CMBR measurements imply a fluctuation of only 0.001%, which requires

Figure 03-10 Supercluster Formation [view large image]

a settling time 1000 times longer than the age of the universe. The inconsistency can be resolved only if there is "dark matter" to enhance the fluctuation.

Since dark matter interacts with normal matter only through gravity, the pressure that kept the normal gas from collapsing coundn't act on it. Particles of dark matter enjoyed an unimpeded assembly into large structures (in the form of primordial fluctuation) long before the normal gas could begin to get organized. By the time normal matter decoupled from the photons, the dark matter had already grown into a primitive web-like network. As soon as the normal matter lost its support from the photon pressure, the gravity from the pre-existing dark matter structures quickly pulled normal gas into the web. In this way, normal matter was given a gravitational "head start" by dark matter.

Recently in early 2004, several new measurements⁹ of galaxies and clusters in the early universe indicate that the structures involving galaxies and clusters are larger than expected with the new standard "dark-energy" cosmology. The controversy centers on the inability of a dark-energy dominated universe to create such large structures within such a short time (1/5 of the present age). More researches are required to validate such observations. The next step is to map an area of sky ten times larger, to get a better idea of the large-scale structure. Several such surveys are currently under way. Figure 03-11 is a computer-generated illustration of a universe that shows a string of galaxies of the size measured - 300 million light years.