by Jaan Einasto
Discoveries which change the foundation of our worldview have often a different character. The discoveries of dark matter and the cosmic web seem to belong to this category. People often ask: Who discovered dark matter? Who discovered the cosmic web? As is characteristic in a paradigm shift, there is no single discoverer; the new concepts were developed step-by-step by many scientists, see also Kuhn (1970) and Tremaine (1987).
The timeline of the study of dark matter is shown in Table 9.1. Actually there are two dark matter problems — the local dark matter close to the plane of our Galaxy, and the global dark matter surrounding galaxies and clusters of galaxies. However, this difference was understood only later, thus we show in the Table the whole story.
The first essential milestone of the dark matter story is the discovery of the possible presence of dark matter in the Galactic disk by Oort (1932), and in the Coma cluster of galaxies by Zwicky (1933); earlier work was mostly ignored. In the following years new data on the presence of dark matter both in the Galactic disk and in systems of galaxies slowly accumulated. Kuzmin (1952b, 1955) and his students in Tartu Observatory showed that the amount of DM in the Galactic disk is small; in contrast Hill (1960); Oort (1960) and some other astronomers found evidence that up to half of the matter in the Solar vicinity may be dark. More accurate data showed that the amount of local dark matter is small (Gilmore et al., 1989).
Data on masses of galaxies and systems of galaxies also accumulated, both from rotation data on the periphery of galaxies, and from statistical dynamical data of groups and clusters of galaxies. For some reason, these studies did not capture the attention of the astronomical community. However, awareness of the presence of a controversy between the masses of galaxies and galaxy systems slowly increased.
The next important milestone in the dark matter story was the understanding that there are two different DM problems, the local and the global DM, which have different distribution and origin (Einasto, 1972a, 1974a). The local DM belongs to the flat disk of the Galaxy, thus it must dissipative to disperse the energy left over during its formation. In contrast, Einasto et al. (1974b) found that the global DM must form a new previously unknown population — corona or halo, which is essentially dissipationless, and is probably of non-stellar origin. They found that the size and the mass of coronas exceed the size and mass of known stellar populations in galaxies about tenfold. Einasto et al. (1974b) and Ostriker et al. (1974) suggested that the total cosmological density of galaxies including their coronae/halos is about 0.2 of the critical cosmological density, thus DM is the dominating population of the Universe.
Table 9.1 Dark matter timeline.
Year Description
1915 First estimates of local DM: Öpik (1915), Kapteyn (1922), Jeans (1922)
1932 Galactic model and Local DM: Oort (1932)
1933 DM in Coma cluster: Zwicky (1933)
1939 Hints of large M/L on the periphery of galaxies: Babcock (1939), Oort (1940)
1952 Galactic models and local DM: Kuzmin (1952b,a, 1954, 1955, 1956a,b)
1957 Large M/L on the periphery of M31: van de Hulst et al. (1957), Roberts (1966)
1959 Mass of the Local Group: Kahn Ö Woltjer (1959)
1961 Cluster Stability Conference: Neyman et al. (1961)
1962 Galaxy dynamic evolution: Eggen et al. (1962)
1965 Galactic models with populations: Einasto (1965, 1969b)
1965 Discovery of CMB: Penzias & Wilson (1965)
1968 Galaxy physical evolution: Tinsley (1968), Einasto (1972b)
1972 Cluster X-ray data on mass of hot gas: Forman et al. (1972); Gursky et al. (1972)
1972 Local and global DM different, global DM non-stellar: Einasto (1972a, 1974a)
1974 Parameters of DM coronas/halos: Einasto et al. (1974b), Ostriker et al. (1974)
1975 Flat rotation of 14 galaxies: Roberts (1975)
1975 Tallinn DM Conference: Doroshkevich et al. (1975)
1975 DM contradicts classical cosmological paradigm: Materne & Tammann (1976)
1977 M/L of galactic bulges low: Faber et al. (1977)
1978 Extended flat rotation curves: Bosma (1978), (Rubin et al., 1978)
1978 Two-stage galaxy formation model: White & Rees (1978)
1979 Gravitational lensing data on cluster masses
1981 Non-baryonic DM discussion in Tallinn and Vatican conferences
1984 Cold DM accepted: Blumenthal et al. (1984)
1989 Absence of large amounts of local DM accepted: Gilmore et al. (1989)
1992 CMB fluctuations detected: Smoot et al. (1992), Mather et al. (1994)
1998 Acceleration of the Universe discovered: Riess et al. (1998), Perlmutter et al. (1999)
2012 CDM particle annihilation detected?: Weniger (2012), Tempel et al. (2012b)
The possible nature of galactic coronae was discussed in the conference on dark matter in Tallinn 1975. No known candidate (stars, cold or hot gas, neutrinos) fit all known observational data (Doroshkevich et al., 1975). General problems of dark matter were discussed during the Third European Astronomy Meeting in Tbilisi 1975. Here Materne & Tammann (1976) showed that the presence of dark matter contradicts fundamental data of the classical cosmological paradigm. The DM problem was acknowledged as a problem and a crisis.
In the following years experimenters found new evidence in favour of the presence of dark matter around galaxies, and in groups and clusters of galaxies. The work by Rubin and Bosma on galaxy rotation curves, X-ray studies of clusters, as well as investigation of gravitational lensing in clusters belong to this type of study.
The third important milestone in the dark matter story was the understanding that it is non-baryonic. The nature of DM and its role in the evolution of the Universe were discussed in 1981 in conferences in Tallinn and the Vatican. The dark matter concept was generally accepted when it was understood that its particles form a cold non-baryonic medium — Cold Dark Matter (Blumenthal et al., 1984). The close collaboration between cosmologists and particle physicists started — this was the birth of astro-particle physics.
The fourth essential milestone was the detection of CMB fluctuations by the COBE satellite (Smoot et al., 1992), its black-body spectrum (Mather et al., 1994), and the detection of the acceleration of the Universe due to dark energy (Riess et al., 1998; Perlmutter et al., 1999). These observations together with direct data from the distribution of galaxies allowed the density of all matter/energy components to be calculated with high accuracy; for dark matter these data yield ΩDM = 0.229. Also new data show that only a small fraction of dark matter is hot (neutrinos) or warm; mostly it should be cold.
So far physical experiments have not been able to determine the properties of dark matter particles. Some help comes from special astronomical observations of the results of the annihilation of DM particles. To such observations belong Fermi satellite Large Area Telescope observations of a double gamma-ray spectral line at 130 GeV (Weniger, 2012; Tempel et al., 2012b). However, confirmation by independent observations is needed to be sure that DM particle annihilation is actually observed.
The development of the understanding of the structure of the Universe was initially completely independent of the development of its matter content. The timeline of cosmic web study is presented in Table 9.2. Here I would like to emphasise the following milestones in the development.
The first milestone is the determination of distances to spiral nebulae, which demonstrated that there exist stellar systems outside the Milky Way. In other words, it was discovered that the Milky Way is not the whole Universe. This observation was quickly followed by the another very important discovery — the whole Universe is expanding and thus had a beginning.
There followed a gradual buildup of the classical paradigm of modern cosmology. This included the observations of redshifts of distant galaxies and the determination of the expansion speed — the Hubble constant. Also the distribution of galaxies was investigated, using galaxy counts on photographic plates. These efforts
culminated with the Palomar Sky Survey. This survey was the basis behind the first relatively deep catalogues of galaxies and clusters of galaxies. The same survey was also used to detect second order clusters of galaxies or superclusters. Also the mean density of matter in the Universe was determined using catalogues of galaxies in the nearby Universe, where distances of galaxies could be measured.
Table 9.2 Cosmic web timeline.
Year Description
1922 Determination of distances of external galaxies: Öpik (1922a), Hubble (1925, 1926)
1929 Discovery of the expansion of the Universe: Hubble (1929a)
1935 Survey of Northern sky galaxies: Shapley (1935, 1937)
1949 Palomar Sky Survey
1953 Virgo supercluster: de Vaucouleurs (1953)
1958 Cluster catalogue: Abell (1958); Abell et al. (1989)
1967 Lick Galaxy Survey: Shane & Wirtanen (1967)
1968 Catalogue of galaxies and clusters: Zwicky et al. (1968)
1970 Galaxy clustering scenario: Peebles & Yu (1970)
1970 Pancake scenario of structure formation: Zeldovich (1970)
1972 Determination of the Hubble constant: Sandage (1972); Sandage & Tammann (1975, 1976)
1973 Statistical analysis of galaxy catalogues: Peebles (1973, 1974a)
1973 Simulation of pancake model: Doroshkevich & Shandarin (1973)
1976 Discovery of cosmic voids: Chincarini & Rood (1976); Gregory & Thompson (1978);
Kirshner et al. (1981)
1977 Discovery of cosmic web: Jõeveer et al. (1977, 1978); Einasto et al. (1980a,b)
1977 Threshold galaxy formation: Jõeveer et al. (1977); Einasto et al. (1980a)
1980 Inflation theory: Starobinsky (1980); Guth (1981); Linde (1982)
1982 Quantitative comparison of models with observations: Zeldovich et al. (1982)
1983 Simulations of CDM models: Melott et al. (1983); Davis et al. (1985); White et al. (1987)
1984 Galaxy biasing: Kaiser (1984)
1986 Simulations of ΛCDM models: Einasto et al. (1986a); Gramann (1987, 1988); Efstathiou
et al. (1990)
1986 Topology of cosmic web: Gott et al. (1986); Einasto et al. (1986a)
1986 Second CfA galaxy redshift survey: de Lapparent et al. (1986)
1994 Supercluster catalogues: Einasto et al. (1994b, 1997b, 2001); Liivamagi et al. (2012)
2000 Sloan Digital Sky Survey: York et al. (2000)
2001 Two-degree Field Galaxy redshift survey: Colless et al. (2001)
2005 Detection of baryon acoustic oscillations: Eisenstein et al. (2005); Hiitsi (2006)
The third milestone in the understanding of the structure of the Universe is related to the introduction of new sensitive detectors, which allowed the determination of redshifts of faint galaxies. This was the fundamental data needed to study the 3-dimensional distribution of galaxies, and to discover the cosmic web with filaments of galaxies and clusters of galaxies, and voids between them.
The next important milestone was the development of the theory of the formation and evolution of the cosmic web. It was followed by the development of the inflation model of the early evolution of the Universe. It became evident that the seeds of the cosmic web were formed already in the very early Universe, perhaps during the inflation. The structure of the Universe evolves very slowly, thus the present structure contains imprints of the very early Universe. Since the dominant population of the Universe is dark matter, it is possible to get information on properties of DM particles from the structure of the cosmic web. This development emphasises that there exists a deep physical link between dark matter and the cosmic web. This caused the birth of the astro-particle physics. Since then the development of studies of dark matter and the cosmic web has occurred hand-in- hand. From the late 1970’s to the early 1980’s there was an intensive discussion between supporters of the classical and the new cosmological paradigms.
The rapid development of the new cosmological paradigm occurred in the 1970’s and 1980’s, at the peak of the Cold War between the Western and the Eastern Worlds. The East-West conflict influenced the development in several ways. First, this conflict made contact between scientists of both Worlds more difficult. Second, there was a clear tendency to ignore some developments in the other World.
Most experimental studies on dark matter and the cosmic web were made by Western astronomers. The interpretation of observational data and the buildup of the modern theory of formation and evolution of the Universe was made in parallel both in the West and East, as seen in both timelines. The new paradigm wins when its theoretical foundation is established. In the case of the dark matter this was done by Blumenthal et al. (1984) with the non-baryonic cold dark matter hypothesis. The connection between cosmology and particle physics was discussed in the Tallinn conference and in the Vatican Study Week, both in 1981. The new cosmological paradigm, which included all major new elements — dark matter, cosmic web and inflation — was discussed probably for first time in the lectures by Primack (1984).
Word on the development of the new cosmology paradigm spread more rapidly in the East: the first dark matter conference was held in Tallinn in 1975; the first official IAU dark matter conference was held only ten years later. The first popular discussions of the dark matter problem were given in “Priroda” and “Zemlya i Vselennaya” (the Russian counterparts of “Scientific American” and “Sky & Telescope”) by Einasto et al. (1975c); Einasto (1975); Einasto & Jõeveer (1978) and in the respective journal in Estonian. In USA the first popular discussions of dark matter were given by Bok (1981) and Rubin (1983). The cosmic web was discussed in a popular book in Estonian by Einasto & Jõeveer (1979), in Russian by Einasto & Jaaniste (1982), and in English by Gregory & Thompson (1982) and Geller & Huchra (1989).
To conclude we can say that the story of dark matter and the cosmic web is not over yet — we still do not know of what non-baryonic particles dark matter is made of, and the nature of dark energy is completely unknown.
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