Dark Matter and Cosmic Web Story

by Einasto, Jaan

  The second estimate is the determination of the Hubble constant using the gravitational lensing effect. Already Fritz Zwicky had suggested that a galaxy may act as a gravitational lens. Sjur Refsdal (1964) emphasised that this effect can be used to estimate the value of the Hubble constant. Suppose that we have at large distance an active object, such as a supernova or a quasar. If a galaxy is located near the line of sight toward the active object, the image of the distant object will be distorted or divided into two separated images, appearing on opposite sides of the galaxy. The path of the light from different images is different, and there is a difference in the arrival time of the light to the observer. As shown by Sjur, this time delay can be used to estimate the value of the Hubble constant, if the redshifts of both the distant object and the intervening galaxy are known.

  Quasars are ideal objects to measure the time delay of two images, because their luminosity is not constant, but changes irregularly. By shifting light-curves of both images of the quasar it is possible to determine the time delay due to the gravitational lens. This idea was applied for the double quasar 0957 + 561. Sjur with his colleagues in Hamburg (Bergedorf) Observatory got a preliminary value of the time delay of about 400 days, which corresponds to a Hubble constant of about 65 km s−1 Mpc−1.Another value of about 540 days was found by Press et al. (1992a,b). This result made Sjur anxious, because the corresponding value of the Hubble constant is about 50 km s−1 Mpc−1, outside the range allowed by other independent determinations in the early 1990’s.

  About this time one of the Sjur’s collaborators, Tom Schramm, visited Tartu Observatory, and had a seminar talk on gravitational lensing. One Tartu astronomer, Jaan Pelt, has a very good knowledge of mathematics and statistics. So a discussion started: could he help to solve the problem of which of these time delay value is correct? Sjur invited Jaan Pelt to Hamburg Observatory. Jaan with the help of Sjur and his team started a careful comparison of statistical methods applied by Bill Press and other investigators. Jaan developed his own method to analyse data, and compared step-by-step Bill’s analysis. Finally the joint Hamburg-Tartu team came to the conclusion that the correct value of the time delay should be close to 415 days. The authors made great efforts to prepare the paper (Pelt et al., 1994) in a suitable form, very clear but not aggressive against the Bill’s papers.

  This analysis caught the attention of the astronomical community. One young astronomer from Princeton, Tom Kundic, discovered a sharp drop in the light curve of component A of the double quasar 0957 + 561 in late 1994 (Kundic et al., 1995). The authors predicted that a similar feature should be observed in the light curve of component B either 415 or 520 days later, depending on the correct value of the time delay.

  The feature was detected — it was 415 days later than in image A (Kundic et al., 1997). This delay corresponds to a Hubble constant value 64 ± 14 km s−1 Mpc−1, in good agreement with other independent estimates. The Hamburg–Tartu team had arrived at the more accurate value of the time delay! Jaan with his Hamburg colleagues reanalysed new data (Pelt et al., 1996, 1998). Sjur recognised that Jaan had helped to save the prestige of the gravitational lensing method, first suggested by him.

  In the 1990’s I visited Hamburg Observatory several times, and always Sjur helped me to find a place to stay overnight in the Observatory. I have not studied gravitational lensing myself, so Sjur explained to me in detail their results obtained together with Jaan Pelt.

  Modern values of the Hubble constant and density parameters come from a combination of Hubble Space Telescope Key Program results, the CMB, and Sloan Digital Sky Survey observations. The mean values from this dataset are: H0 = 70.2± 1.4, Ωb = 0.0458±0.0016, ΩCDM = 0.229±0.015, ΩΛ = 0.725±0.016.

  On March 21, 2013 new results obtained with the Planck satellite were announced. The Planck data suggest a small revision to the previously accepted set of cosmological parameters. According to Planck Collaboration et al. (2013) cosmological parameters have the following values: H0 = 67.4 ± 1.4, Ωb = 0.0490 ± 0.0004, Ωm = 0.314 ± 0.020, ΩΛ = 0.686 ± 0.020. The reason for the discrepancy with previous results is not yet clear. The age of the Universe according to new Planck data is 13.813 ± 0.058 Gyr.

  Using the Sloan Digital Sky Survey main galaxy sample, Tempel et al. (2011) derived for the mean luminosity density 0.01526 × 1010hL Mpc−3. Mass-to- luminosity ratios in visible regions depend slightly on the morphological type of galaxies, and are in the range of 4–6 in solar units, including the contribution from dark matter. M/L of baryonic matter in visible galactic regions is about 3. From these data it follows that the mean density of baryonic matter in galaxies is about 0.5 % of the critical density.

  8.4.4 New cosmology paradigm is ready: What next?

  During the last 50 years the astronomy community has been witness to several paradigm changes. The most important paradigm changes are:

  • Cosmic microwave background (CMB) radiation was detected which confirmed the paradigm of hot big bang cosmology.

  • Most of the matter in the Universe is dark and consists of weakly interacting non-baryonic particles; the density of dark matter is about 0.25 of the critical cosmological density.

  • The early evolution of the Universe includes a period of very rapid expansion or inflation which made the Universe homogeneous and its density equal to the critical density. The latest version of inflation theory suggests an initial chaotic stage within a multiverse.

  • The Universe has structure in the form of a cosmic web. The seeds of the cosmic web were created at the very early stages of the evolution and give us information on the properties of Universe at this epoch.

  • Most of the matter/energy content is the dark energy, about 0.7 of the critical density. Dark energy causes an accelerating expansion of the Universe.

  Can we say that the modern cosmological paradigm is now complete? Probably not. The modern paradigm explains almost all observational facts, but there are still a few important problems open. As suggested by Peebles (2002), the situation now has some similarity with the situation in physics at the eve of the 20th century. A hundred years ago almost all experimental data were explained by classical 19th century physics. But there were some clouds on the horizon — the Michelson experiment which suggested the isotropy of the velocity of light, and difficulties in the theory of gases. To explain these clouds the whole of modern 20th century physics evolved: the theory of relativity, quantum physics, and much more.

  As Peebles suggests, cosmology has now also several clouds on the horizon: What is the nature of dark matter and dark energy? Which physical processes created our Universe such as it is? Or in other words, what is the physics of the multiverse?

  These questions concern both the particle physics and the cosmology of the very early Universe. In these domains scientists deal with energies which are outside the possibilities of present-day accelerators. So far the only information from these early events comes from astronomical data, in particular, indirectly from the structure of the cosmic web on large scales. Recent reviews on astrophysical probes of dark matter are given by Arneodo (2013) and Profumo (2013).

  To measure the effects of dark energy, among other projects, the Dark Energy Survey (DES) has been initiated. The Survey aims to probe the dynamics of the expansion of the Universe and the growth of large scale structure. It is a collaboration of research institutes and universities from the United States, Brazil, the United Kingdom, Germany and Spain. The Survey will use the 4-meter Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. The main instrument is the DECam camera, which covers a field of view of diameter 2.2 degrees, and has 62 2048 × 4096 pixel CCDs, and g, r, i, z, y filters similar to the SDSS survey. Starting in September 2012 and continuing five years, the Survey covers a 5000 square degree field in the Southern sky, and has a depth of 24th magnitude in the i-band. It is expected that the Survey will get a sample of 300 million galaxies with photometric redshifts up to redshift z ∼ 1.4. The Survey’s princi
pal aims are to measure BAO, to detect about 170,000 galaxy clusters, to investigate the large-scale distribution of galaxies at different redshifts, and to detect supernovas at high redshifts.

  Such large surveys have some similarity with high-energy physics experiments, where very large teams of researchers, instrument designers and programmers are involved. Such large collectives have working cultures different from traditional astronomical cultures. White (2007) discussed in detail the differences between traditional astronomical and high-energy physics cultures. He emphasises that in doing such large projects one must not forget the major goals of astronomy: the understanding of the Universe as a complex system.

  8.5 Remembering contacts with colleagues

  8.5.1 Encounters with astronomers from other centres

  My contacts with astronomers from other scientific centres started when I visited the Sternberg Institute in Moscow in 1948. These visits became more regular as Prof. Pavel Parenago from the Sternberg Institute was at the time the leader in galactic studies. With Sternberg Institute astronomers I maintained good contacts over the years. In my first visit to the Institute an older professor asked me to send his best greetings to Ernst Karlovich, this is the style in which Russian people call each other. He did not know that Ernst Öpik had left Estonia in 1944. I have some joint publications with my Sternberg colleagues. For my 80th birthday I got from Sternberg astronomers many books, one with the signatures of several tens of people — my old friends.

  The next step was to visit the Leningrad University Astronomical Observatory and the Pulkovo Observatory. This resulted in good contacts with Prof. Kirill Ogorodnikov and his stellar dynamics group — Tatheus Agekian and younger colleagues. Our stellar astrophysics group collaborates very closely with the theoretical astrophysics group lead by Prof. V. V. Sobolev. Our previous collaborator Sergei Kutuzov, a student of Grigori Kuzmin, is presently working in the St. Petersburg Astronomical Observatory as Professor and head of the Department of Space Technologies and Applied Astrodynamics. With Sergei we developed the methods of models of galaxies using various data on galactic populations. He was born near Leningrad, but during WWII the town he lived in was occupied by the German army, and the family was evacuated to Tartu, so he got his education at Tartu University. The last time we met was in Pulkovo at a conference organised by St. Petersburg astronomers in honour of Grigori Kuzmin’s 90th birthday.

  Pulkovo Observatory is actually a younger sister of Tartu University Observatory. It was planned by Friedrich Georg Wilhelm Struve and, when finished, Struve moved with all his staff to Pulkovo. From Pulkovo the development of modern astronomy in the Russian Empire spread to other centres. With Prof. Aleksandr Mikhailov, director of the Pulkovo Observatory, I had many communications. Once during a visit to Pulkovo he invited me to his apartment in the Observatory building. We had a very interesting discussion on the history of astronomy, and he showed me his collection of historical cameras. For each camera he had a story: who built it and what role the camera had in the history of photography. In April 2011 Tartu University Observatory celebrated its 200th anniversary with a small conference. One of the main speakers was Prof. Viktor Abalakin, a former director of Pulkovo, who gave a very interesting overview on the history of both observatories.

  My next visit was to the Abastumani Observatory in Georgia, where I spent my observational practicum in 1951. The supervisor of the practicum was Prof. Evgeny Kharadze, the director of the Observatory and the President of the Georgian Academy of Sciences. With him I became good friends. After the death of Pavel Parenago he was elected the Chairman of the Committee of Galactic Studies of the Astronomical Council of the USSR Academy of Sciences. After some years he suggested me as the Chairman of the Committee. Quite often we met either in Abastumani or in his office in the Academy in Tbilisi, discussing issues of Galactic studies.

  With the Byurakan Observatory and Viktor Ambartsumian our contacts started even earlier. Aksel Kipper invited Ambartsumian to Tartu in 1948 to discuss plans of the building of a new observatory outside the town. At this time I was a student and happened to be present in these discussions. When the new observatory in Tõravere was officially opened in 1964, Ambartsumian was one of our main guests. For many years he was member of the Science Council of our Observatory. My own visits to Byurakan started in the early 1950’s; the last visit was in September 2012 to receive the Viktor Ambartsumian Prize. We made jointly observations of galaxies in the late 1970’s, and discussed the development of astronomy in the 1980’s.

  With foreign astronomers I had first contact in 1958, when the IAU General Assembly was held in Moscow. At this time I had no interesting results to discuss with other astronomers. But I met most influential astronomers whose work I had studied: Jan Henrik Oort, Fritz Zwicky, Harlow Shapley, Bengt Strömgren, Bertil Lindblad and many others. Very important was the participation of Grigori Kuzmin. He met in Moscow George Contopolous, a Greek astronomer, who had independently found the third integral of motions in stellar dynamics. They started an exchange of letters. Kuzmin sent him reprints of all his papers, and Contopolous acquainted Kuzmin’s results to other astronomers in his publications and talks.

  My first visit to a foreign country was in 1959 to Hungary. At this time I was the head of the Tartu University Satellite Tracking Station. The Astronomical Council coordinated the actions of all these stations, including stations located in ‘Socialist Countries’. My trip was a few years after the ‘Hungarian Revolution’, thus quite soon we started to discuss these events with Hungarian astronomers. I made no secret of the attitude of the Estonian public of these events, and on the repression of the Revolution by the Soviet army. Hungarian astronomers told me a lot of details that I did not know before.

  As an inspector of satellite stations I visited in 1962 the German Democratic Republic. Here I visited all the main astronomical institutions, including the Tautenburg Observatory, Jena University, Potsdam Institute of Astrophysics, and many other places of interest. Here astronomers told me the story of the 1953 uprising in East Germany, violently suppressed by Soviet tanks. I also saw the Berlin Wall, built just a year before my visit.

  In 1967 the General Assembly of the IAU took place in Prague, and I was happy to be included in the Soviet delegation. Here I had a chance to discuss with Margaret Burbidge the structure of galaxies. I had interesting discussions with Luboŝ Perek, the Czech astronomer who had studied the modelling of galaxies. Soon he became the President of the IAU Commission 33 on the Structure and Dynamics of the Galaxy, and the Secretary General of IAU.

  In Prague I met for the first time Alar Toomre, the MIT astronomer of Estonian origin. He is an admirer of Grigori Kuzmin’s work, in fact he continued the study where Kuzmin has stopped, by using new possibilities opened up by fast-improving computers to simulate the evolution of galaxies. Together with his brother Juri he conducted the first computer simulations of galaxy mergers. They found that galaxies like the Antennae Galaxies (NGC 4038/4039) are actually colliding galaxies; the computer simulation reproduces very well the structure of the object. This process of the collision evolution is known as the Toomre sequence.

  After the General Assembly the Soviet delegation had a tour through Czechoslovakia. We visited Karlovy Vary and other places of interest in the Western part of the country. In hotels I avoided speaking Russian; for me it was difficult to speak in the language of the occupying country. The administrator of one hotel asked me why I do not speak Russian as I am one of the Soviet delegates. What could I answer? Next year, after the Czech Spring and its suppression, I remembered this incident. Then I would be ready to answer his question. But I got from several Czech astronomers new-year cards. In the card the spectrum of a Nova was seen, taken in the night 20–21 August, the night when Soviet tanks entered Prague. The explanation was: “Nova has reached a stadium of nebulous stage, the direction of the further evolution is unknown”. Everything was clear!

  During the IAU General Assembly in 1970 in Brighton I had
a chance to talk to Luboŝ Perek again. I thanked him for the card, sent a few years ago, and explained why I was not able to thank him by mail earlier — all my mail was carefully controlled by ‘competent organs’. Our astronomy community followed the ‘Czech events’ very carefully. Estonia had our own ‘Spring’ in the mid 1960’s. After the suppression of the Czech Spring it was clear that we also had ahead years of suppression and stagnation. Recently one colleague of mine brought back to my memory that after the ‘Czech events’ I said “Empires are not forever, perhaps next time we succeed”.

  My first report in an international conference was in 1969 in Basel at an IAU Symposium on the structure of the Galaxy. Next year there was the IAU General Assembly in Brighton. I also had a short report on galactic models in the Commission 33 Meeting. This time I was there together with Prof. Aksel Kipper, and we had a lot of time to discuss science, in particular our local problems in the Observatory. There were internal tensions in the Observatory, it was a fight between different groups. Kipper told me how to finish such fights: “you have to separate both sides so that they cannot disturb each other, and do not accuse or justify any of the sides”. Indeed, when we got back home Kipper acted according to these rules, and after some time the atmosphere in the Observatory was again normal.

  In 19721 got an invitation from George Contopolous to give an invited review lecture on galactic models at the First European Astronomy Meeting, to be held in Athens in September. I had calculated models for most Local Group galaxies and the giant elliptical galaxy M87 in the Virgo cluster, so I had a lot of stuff to speak on. So far the explanation of flat rotation curves of galaxies was difficult, since no stellar population could have properties needed to explain the rotation. I discussed the problem with Enn Saar, and he suggested abandoning the idea that only normal stellar populations exist in galaxies. As explained earlier, this assumption means that a population with unknown properties must be present in galaxies.