Fundamental contributions to the discovery of dark matter and the cosmic web.

The discoveries of dark matter and of the cosmic web have changed the cosmological paradigm. Both discoveries were made gradually during decades of work by astronomers from many countries.

Dark matter. At Tartu Observatory in Estonia, a high level in studies of galactic models and stellar dynamics was achieved already by the early works of Ernst Öpik and Grigori Kuzmin. Starting from 1960s, Jaan Einasto extended the methods of studying matter distribution in galaxies using all kinds of available photometric data about a galaxy as a whole as well as its basic individual stellar populations: disc, bulge and halo. He combined these data with spectroscopic measurements of the motions of the stars and gas inside the galaxy (most importantly – the rotation curves), thus determining the mass-to-luminosity ratio (M/L) of individual galactic populations. For an accurate analytical description of density distribution in galaxies, Einasto introduced a simple generalised exponential function, now in wide-spread use as the “Einasto profile”. Calculations showed that the nearly distance-independent rotation velocities measured for the outer regions of galaxies cannot be reconciled with realistic stellar populations. In the periphery of galaxies, the local M/L value exceeds 1000, but no known stellar population can have such a high value. To bring the model into agreement with observational data, a new component of unknown, essentially non-stellar nature was assumed to be present for the first time [1]. Data on local galaxies suggested that the population has to be very massive and extended and that it was not concentrated on the plane of the galaxy. In order to avoid confusion with the stellar metal-poor halo, the new hypothetical population was called the “corona”. Galactic models with the corona component yielded good agreement between the rotation and photometric data using realistic stellar populations for the first time.

In 1970s, the rotation curves of galaxies did not reach far enough to determine the total mass and extent of coronas. Instead, the team lead by Einasto used dwarf satellite galaxies as test bodies of the outer gravitational potential of a galaxy [2]. It was found that all giant galaxies have massive coronas of some unknown origin (“dark matter”), and that the total mass of galaxies exceeds the mass of the known populations by at least an order of magnitude. Thus dark matter has to be the gravitationally dominant component in the whole Universe.

The cosmic web. Because of the relatively low speed of random motions of galaxies, the principal driving forces of structure formation in the Universe are carved in the distribution of galaxies at very large scales. Any regularities in the large-scale distribution of galaxies should thus reflect specific conditions in the Universe at the time of galaxy formation. In order to conduct an observational test on the various existing structure formation scenarios, Einasto and his team collected all the available spectroscopic and photometric distance estimates for nearby galaxy clusters, groups, pairs, Markarian galaxies and other single galaxies over the whole Northern celestial hemisphere. This collection of data allowed the team to reconstruct the true spatial distribution of galaxies and their systems in the nearby Universe. Wedge diagrams showed that galactic systems and single galaxies form elongated chain-like structures (“filaments”), resulting in a connected three-dimensional network – “the cosmic web”. The space between clusters, superclusters and filaments is essentially devoid of galaxies, causing the large-scale distribution of matter to appear “cellular”.

In order to promote their work, Jaan Einasto and his colleagues organised an IAU Symposium in Tallinn in 1977. The title of the Symposium, “the Large Scale Structure of the Universe”, coined the term for the topic. At the symposium, the results on the cosmic web were presented to the public for the first time [3]; a more thorough study was published later [4].

Because of its overwhelming gravitational contribution, dark matter must have an impact on the formation and evolution of the large-scale structure of the Universe. From among the multitude of dark matter candidates, numerical simulations of structure formation should reveal the most likely ones. By conducting such simulations, Einasto and colleagues showed that a neutrino-dominated Universe lacks the observed fine filamentary details [5]. However, simulations with the hypothetical slow (and thus “cold”) and massive particles called “axions” were in good agreement with the observations [6], laying grounds for the Cold Dark Matter cosmology.

With the pioneering studies of Jaan Einasto and his team, Cold Dark Matter and the Cosmic Web have become an essential part of the general cosmological paradigm by now.

References

[1] Einasto, J. 1974, Galactic models and stellar orbits (Invited Lecture), in: Stars and the Milky Way system; Proceedings of the First European Astronomical Meeting, Athens, September 4 - 9, 1972, ed. L.N. Mavridis; Springer, Vol. 2, 291

[2] Einasto, J., Kaasik, A., & Saar, E. 1974, Dynamic evidence on massive coronas of galaxies, Nature, 250, 309

[3] Jõeveer, M. & Einasto, J. 1978, Has the universe the cell structure, in: The Large Scale Structure of the Universe; IAU Symposium no 79, Tallinn, September 12 – 16, 1977, eds. Longair, M.S., Einasto, J.; Dordrecht, D. Reidel Publishing Co., 241

[4] Einasto, J., Jõeveer, M., & Saar, E. 1980, Structure of superclusters and supercluster formation, MNRAS, 193, 353

[5] Zeldovich, Y. B., Einasto, J., & Shandarin, S. F. 1982, Giant voids in the universe, Nature, 300, 407

[6] Melott, A. L., Einasto, J., Saar, E., et al. 1983, Cluster analysis of the nonlinear evolution of large-scale structure in an axion/gravitino/photino-dominated universe, Physical Review Letters, 51, 935

In 1964 Igor Novikov and A. Doroshkevich published a paper where they predicted that the intensity of the Cosmic Microwave Background Radiation (CMB, relict of the Hot Universe at the beginning of the Big Bang) is much higher than the intensity of all other sources of radiation in the Universe, in the centimeter and the millimeter bands. In this publication, Novikov and Doroshkevich predicted that this radiation can be discovered, and they also indicated what instrument should be used to do so. Indeed, CMB was discovered in 1965 using the radio telescope mentioned in the paper.

This prediction must surely rank as one of the major predictions of modern cosmology.

In 1965 Novikov (with Doroshkevich and Zeldovich) demonstrated that the collapse of a non–symmetrical, (possibly rotating) body of arbitrary shape, produces a black hole, which very rapidly becomes axially symmetrical. During the formation of the black hole any deviation from such a symmetry must be carried away by the gravitational waves. Novikov, Doroshkevich and Zeldovich proved that the gravitational field of an uncharged black hole is completely determined by just two parameters: its mass and its angular momentum. This seminal work had enormous influence on experts within the field.

In 1966 Novikov and Ya. Zeldovich predicted that black holes (and neutron stars) could act as extremely powerful sources of X–ray radiation because of the physical processes in their vicinity. Such emission occurs when the matter from an ordinary star located quite close to a black hole (in a binary system) is accelerated in the gravitational field of the black hole. This X–ray emission makes black hole visible. A few years after this prediction, first black holes of stellar mass (components of the binary stellar systems) were discovered because of their X–ray emission.

In 1964 (just after quasars were discovered) Novikov and Zeldovich conjectured that the main engine of a quasar is a super massive black hole located in the center, with gas accretion onto it. On the basis of observational data on 3C273, they gave estimates of such a central mass; for that they hypothesized that the radiation pressure balances out gravity. It turned out that the central mass is approximately a hundred millions solar masses. Many ideas of this work remain popular today.

In 1972 Novikov (with K. Thorne) worked out the theory of relativistic gas accretion onto a black hole. This theory forms a basis of the astrophysics of black holes. Since then, Novikov worked out a modern theory of disk accretion.

In 1964 Novikov was the first to point out that general relativity permits the existence of white holes; and later (1975–1976) Novikov with Starobinsky and Zeldovich discovered and elucidated the instability of white holes against quantum effects, which presumably (along with a classical instability discovered by Douh Eardley), has prevented any white holes that formed in the Big Bang from surviving into the present era.

Some of Novikov's important research projects are devoted to the internal structure of black holes. The Kerr metric, which describes a spinning black hole, possesses a "Cauchy Horizon" in its interiors, through which, in principle, one could pass and enter another universe. Novikov (with Starobinsky) gave the first evidence that (in accordance with an early conjecture by Penrose) this Cauchy Horizon is unstable against both classical and quantum perturbations. Such instability leads to formation of a true singularity (1980).

Novikov and Zeldovich in 1966 worked out a theory of origin of primordial black holes.

In 1967 Novikov worked out one of the theories of the origin of galaxies from the small primordial perturbations in the expanding Universe. In the subsequent years he actively worked on the development of this theory.

During recent years Igor Novikov obtained interesting results in the numerical approach to the theory of collisions of black holes.

Over many years, Novikov's research on anisotropy of CMB in various cosmological models (with application to the observations from space and ground based telescopes) have also had much impact.

I.Novikov is deeply involved in the ESA PLANCK Mission project, which is a project devoted to measurements of the CMB anisotropy. He became a member of the Plank Scientific Evaluation Committee in 1998. The task of this Committee was to evaluate and select instruments and data processing centers for the PLANCK mission. At present I.Novikov is one of the coordinators of the project, on which he works very actively.

Igor Novikov is known as an outstanding organizer and scientific leader of research groups both in Moscow and in Copenhagen.

In 1994 he founded the Theoretical Astrophysics Center in Copenhagen, of which he was the Director until 2004.

I.Novikov has taught and supervised dozens and dozens of students of all levels. He is a member of many boards of international astrophysical and physical journals. Igor Novikov also taught the general public about cosmology and astrophysics. His numerous popular lectures and books, which have been translated into 13 languages, have had a great impact on cosmology matters around the world.