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Two models of galaxy formation were being investigated simultaneously on the 1970’s. The bottom-up model was championed by Peebles, and the top-down model by Zeldovich. At first, dark matter was not part of either model, but this effort to explain the origin of galaxies eventually stalled for both models the because the temperature fluctuations in the cosmic background radiation are too small to accommodate galaxy formation from baryons alone. At first massive neutrinos were introduced as dark matter, and when this failed to word, cold dark matter (CDM) was introduced. CDM forms early halos, and then baryons eventually fall into these halos. The first CDM computer models of galaxy formation were introduced by Melott and Shandarin and later developed by the “Gang of Four” (White, Davis, Efstathiou and Frenk). Eventually, the top-down and bottom-up models gracefully merged, and the concept of “biasing” became part of the final model.
The standard model of cosmology called LCDM has its origins in the work of great scientists including Einstein, Friedmann, Slipher, Hubble, Lemaitre, and Gamow. Lemaitre’s 1930s “Cosmic Egg” or “Primeval Nucleus” was the basis for the Big Bang model. In its new variant called LCDM, “L” represents the cosmological constant Lambda and “CDM” represents Cold Dark Matter. These two components, L and CDM, account for 95 percent of the mass–energy content of the Universe. Edwin Hubble correctly showed in the 1930s that galaxies are distributed on the largest scales in a homogeneous and isotropic way, but on a more local scale of 300 million light-years Hubble failed to recognize significant inhomogeneities. Hubble and Humason validated the velocity–distance relation for galaxies and galaxy clusters demonstrating the expansion of the Universe. They did not call out how significant the velocity–distance relationship would become in our effort to determine the 3D structure in the galaxy distribution.
An overview is presented of the breakthroughs that led to the discovery of cosmic voids and supercluster structure in the galaxy distribution and of those who did the work. The first step was the introduction of the image intensified camera to observatories in Arizona and its early use in the spectroscopy of galaxies. After a sufficient number of galaxy redshifts were collected, 3D maps of the local Universe were created. These maps revealed the dramatic structure including cosmic voids. Next, theoretical models were proposed to explain the observed structure. This step included a face-off between bottom-up evolutionary models in the west and top-down models from the USSR. As the models matured, it was recognized that normal matter (baryons) were insufficient to explain the observed structure in the galaxy distribution and that dark matter was a necessary new constituent. In recent years, cosmic voids have become a tool for precision cosmology.
Astronomers in the past clearly recognized the irregular nature of the galaxy distribution in the nearby Universe. Both Herschel (ca. 1800) and Shapley (ca. 1932) detected and described these effects. They both named specific regions in the sky that are crowded with galaxies and other regions that are significantly deficient in galaxies. However, the scientific views of Hubble published in 1936 overshadowed these early results, and based on his beliefs (with no significant substantiating evidence) Hubble asserted that the Universe (both locally and at great distances) is isotropic and homogeneous. Hubble’s 1936 analysis used counts of faint galaxies to show that the Universe – with galaxies as designated “markers” in space – extends in depth to the greatest limits he was able to obtain at Mt. Wilson Observatory. In the 1930s, Holmberg and others set the foundation for hierarchical structure formation to explain the origin of groups and clusters of galaxies.
Two challenges have been made regarding the Gregory and Thompson 1978 discovery priority of cosmic voids and the extended structure (called “bridges”) that connect one rich cluster with its nearest neighbor(s). The primary challenge is by the Center for Astrophysics group called CfA2 headed by Geller and her late collaborator Huchra. A less significant challenge is by Chincarini, one of the Arizona redshift survey members. These issues are discussed point by point starting with the CfA2 challenge. Table 8.1 summarizes the Arizona work as of 1984–1985 (just before the CfA2 survey began). This table as well as the extensive “timeline” table (Table 8.2) demonstrate that the CfA2 survey was a latecomer in the pioneering period and represents nothing more than an incremental step forward. The Chincarini challenge is based on data that belonged to our Arizona consortium (a subgroup headed by Tarenghi) and was published by Chincarini without permission.
Jaan Einasto at first investigated the structure of nearby galaxies and helped to deduce that they are dominated by dark matter. Joeveer at first studied the distribution and dynamics of stars in our Milky Way galaxy. In a joint effort in the mid-1970s, they investigated the galaxy distribution using catalogued data and began to see evidence for large-scale inhomogeneities. A careful review of their investigation reveals shortcomings. The Tartu Observatory 1.5-m telescope was built and commissioned in this era, but it was not equipped with a spectrograph capable of detecting galaxy redshifts. The greatest advantage held by the Estonians came from their early knowledge of computer simulations by Shandarin based on the Zeldovich approximation. At IAU Symposium No. 79 organized by the Estonian astronomers, the first open discussion was held of cosmic voids. Also participating in the meeting was Brent Tully, an expert on the structure of the Local supercluster.
Two new wide-field photographic survey telescopes were placed into operation soon after World War II, and two new nearly-all-sky galaxy surveys emerged: the Lick Observatory Shane and Wirtanen survey and the National Geographic Palomar Observatory Sky Survey. These made it possible for the first time to study the galaxy distribution in 2D as projected onto the sky. Both Shane and Abell found evidence for galaxy superclusters, but Zwicky remained steadfast in saying that superclusters do not exist. Starting in 1953, Gerard de Vaucouleurs studied the properties of the Local supercluster showing that only 10 percent of the local volume of space is occupied by groups of galaxies. Table 4.1 lists all known galaxy superclusters from this early era. The subject of cosmic voids did not arise in a formal sense, but Neyman and Scott devised a model of the galaxy distribution showing that all galaxies might all belong to groups or clusters of galaxies. Still, some cosmologists remained holdouts for homogeneity.
Tifft and Gregory began to collect Coma cluster redshifts at Steward Observatory’s 90-inch telescope in the mid-1970s when Chincarini and Rood were doing similar work at the Kitt Peak 84-inch telescope. Tifft branched into non-cosmological redshift work while Gregory and Thompson began to collaborate. For our redshift survey work, we adopted a new strategy of mapping the galaxy distribution between two rich clusters – Coma and A1367. Another collaborative effort to study the Hercules supercluster was started by Tarenghi that involved Tifft, Chincarini, Rood, and Thompson. The Gregory and Thompson work was completed first and was submitted for publication in 1977 immediately before IAU Symposium No. 79. Chincarini took preliminary Hercules redshift data and published them on his own in Nature. A new team – Kirshner, Oemler, Schechter, and Shectman – discovered the Bootes void in 1981. Meanwhile, the first Center for Astrophysics team (CfA1) published a shallow all-sky redshift survey in 1982, and in 1986 the CfA2 team published their “Slice of the Universe” redshift map.