Dark Matter and Cosmic Web Story

by Einasto, Jaan

  After the decoupling (recombination) the photons no longer interact with the baryonic matter and diffuse away. The pressure vanishes and the shell of baryonic matter is left at a fixed radius, called the sound horizon. The shell continues to attract matter and galaxies formed in a similar pattern in a shell surrounding a high-density peak. The number of peaks is large, thus actually there are many overlapping shells. It is difficult to see all the overlapping shells individually; they can be detected statistically by looking at the separations of a large number of galaxies.

  These acoustic waves in the tightly coupled baryon-photon fluid prior to the epoch of recombination will lead to the characteristic maxima and minima in the post-recombination matter power spectrum. The same mechanism is responsible for the peak structure in the CMB angular power spectrum (Sunyaev & Zeldovich, 1970; Peebles & Yu, 1970). The scale of these features reflects the size of the sound horizon, which is fully determined by the physical densities Ωbh2 and Ωmh2.

  Peaks in the CMB angular power spectrum have been discovered by the WMAP satellite, see the next Chapter. Recently similar features have been found also in the distribution of galaxies of the SDSS Luminous Red Giant (LRG) sample by Eisenstein et al. (2005), using the correlation function. The same sample was analysed by Hütsi (2006) using the power spectrum of LRG galaxies. Hütsi has found evidence for a full series of acoustic features down to the scales of k ~ 0.2 h Mpc−1. This corresponds up to the 7th peak in the CMB angular power spectrum. The acoustic scale derived, 105.4 ± 2.3 h−1 Mpc, agrees very well with the “concordance” model prediction and also with the one determined via the analysis of the spatial two-point correlation function by Eisenstein et al. (2005). Figure 7.30 shows the power spectrum of the SDSS LRG sample. This work is the first determination of the power spectrum of this sample. Acoustic features in the power spectrum are clearly visible, and are in good agreement with features expected from the model spectrum.

  Hütsi (2006) emphasised that the ability to observe baryonic features in the low redshift galaxy power spectrum demands a rather high baryonic to total matter density ratio. Blanchard et al. (2003) suggested an Einstein–de Sitter type model with zero cosmological constant, and emphasised that with such a model it is possible to fit a large body of observational data, if one adopts a low value for the Hubble parameter. In light of the results obtained by Hütsi (2006) these models are certainly disfavored due to the fact that the high dark matter density completely damps the baryonic features. And finally, purely baryonic models are also ruled out since for them the expected acoustic scale would be roughly two times larger than observed here. So the data seems to demand a weakly interacting nonrelativistic matter component.

  Fig. 7.30 Power spectrum of the SDSS LRG sample with bin width Δk ≈ 0.02 h Mpc−1. The upper solid line shows the best fitting model spectrum and the lower one corresponds to the linearly evolved matter power spectrum of the “concordance” cosmological model multiplied by the square of the bias parameter b = 1.95. Both of the spectra are convolved with a survey window. The dashed lines represent the “smoothed-out” versions of the above model spectra. The dotted line is the cubic spline fit to the data points. (Hütsi, 2006).

  The effect of baryonic acoustic oscillations (BAO) had previously been detected using correlation functions and power spectra of the galaxy distribution. Arnalte- Mur et al. (2012) presented a new method to detect the real-space structures associated with BAO. Baryon acoustic structures are spherical shells of relatively small density contrast, surrounding high density central regions. The authors designed a specific wavelet adapted to search for shells, and applied this method to detect shells surrounding high-density peaks of the SDSS density field. Peaks were found using the LRG sample of galaxies; to find shells around peaks the main galaxy sample of SDSS was used. To enhance shells they were stacked around high-density peaks.

  Fig. 7.31 Radial density profiles averaged over high-density centres. Density is expressed in units of the mean average density of the sample. The continuous black line with an error band shows the radial profile for the full sphere. The arrow signals the location of the maximum, rmax = 109.5 h−1 Mpc (Arnalte-Mur et al., 2012).

  Radial density profiles averaged over high-density centres are shown in Fig. 7.31. The lines in different shades of gray mark the profiles of various subsamples; the black line shows the radial profile for the full sphere. The resulting profile has a high bump on short scales, corresponding to the central peak, and a clear maximum on the acoustic scale, rmax = 109.5 ± 3.9 h−1 Mpc. This study shows that BAO shells are real spatial structures, and that the BAO phenomenon can be studied in detail by examining those shells.

  Presently it is not clear whether there is a clear connection between the distribution of rich clusters and superclusters, discussed in the previous subsection, and the baryonic acoustic oscillations. The physics of these phenomena is different. The large-scale distribution of clusters and superclusters is probably given by the initial density perturbation field, generated during or after the inflation period of the evolution of the Universe. Baryonic acoustic oscillations are generated by sound waves in the epoch just before recombination. The connection between both effects is given by the fact that the BAO phenomenon is generated by the initial density perturbation field. To understand the possible relations between these two effects more detailed studies are needed.

  7.4 Tartu Observatory in the 1990’s

  7.4.1 Estonian path to independence

  In the Soviet period we often had guests who spent their summer vacation in the Observatory’s guesthouse. Among them were Yakov Zeldovich, Josif Shklovsky and several other prominent astronomers. During these visits we had a lot of time to discuss various topics on cosmology as well as Zeldovich’s experience in the secret work. We used our sauna also for receptions of guests. One of the most memorable sauna-visits was with Remo Ruffini and his students. It was winter, the pond next to sauna was covered with ice, and a hole was made in the ice to jump into the cold water from the sauna. For Italians this was a rather unexpected experience.

  Our sauna was in some aspects similar to ancient thermae in Rome — we used the sauna to discuss various topics from science to everyday life. So, in the late 1980’s an informal “sauna-club” formed which started to discuss problems related to the transition from a command-economy in the Soviet system to a free market-economy. At this time it was already clear that the Soviet system was near to a collapse, thus it was time to think about the transition. This club had its discussions confidentially, so its activity was not known to the whole community of the Observatory. The club developed, among other topics, the main principles to be used in privatisation of the economy.

  Elections for the last Estonian Supreme Soviet (Parliament in present sense) were held in March 1990. The elections were practically free and a number of intellectuals were elected to perform the reforms needed for the transition of the society. Among other intellectuals one representative was elected from the Tartu Observatory: Liia Hänni, a stellar astrophysicist and member of our secret sauna- club. Popular Front participated in elections and won the majority of seats. In contrast, the Estonian Citizens Committee declared that the Supreme Soviet is an organ of occupation and refused to participate in its activities.

  The Estonian Supreme Soviet formulated as its main goal the making of all necessary preparations to restore the independence of Estonia. As a first formal juridical act on March 30, 1990, it openly declared ‘a transition period’ to restore the Estonian Republic. In this way cooperation with the Soviet Government was still possible. In contrast, Lithuania declared its independence already on March 11, 1990, but a Soviet blockade followed, and no country recognised Lithuanian independence. On June 12, 1990, the Russian Federation declared its sovereignty, and elected Boris Yeltsin as the Russian President.

  In October 1990 the Estonian Supreme Soviet accepted the law of economic borders, and many checkpoints were organized at the main crossing points to check
documents. This was the first step in fixing the borders of the country. Similar laws were accepted in Latvia and Lithuania. At this time the Soviet Union had Special Purpose Militia Units, called OMON. Estonia did not form such units, but in Latvia and Lithuania they were formed. At the end of 1990 these units refused to obey the orders of Latvian and Lithuanian Interior Ministries. On January 20, 1991, the Riga OMON attacked Latvia’s Interior Ministry, killing six people. On June 14, 1991, the OMON attacked the Estonian crossing point between Latvia and Estonia, wounding three people. One of them was my relative Enn Vilbaste. His uncle Juhan Vilbaste was my close friend, an entomologist and a “Finnish Boy” during WWII.

  In January 1991 the Soviet army together with OMON units tried to suppress the independence movements in Lithuania, Latvia and Estonia. The first actions were in Vilnius. Lithuanians defended the TV-tower and many civilians were killed. In Latvia the Soviet army attacked demonstrators in Riga. To avoid this in Estonia, our leaders phoned Yeltsin and invited him for negotiations at Tallinn. In this meeting Yeltsin signed a “Treaty on the Basis of Interstate Relations Between the Russian Federal Socialist Republic and the republic of Estonia”; similar treaties were signed also with representatives of Latvia and Lithuania. Yeltsin also made an appeal to the Soviet army condemning attacks against civilians. This appeal prevented Soviet military actions in Estonia. A small remark — all these documents were typed in Lippmaa’s office.

  A union-wide referendum was performed on March 17, 1991 which restricted the rights of the Soviet republics. Estonia and Latvia boycotted this referendum and organised their own preventive referendums; Lithuania had already declared its independence. The Estonian referendum was held on March 3. The overwhelming majority supported the restoration of an independent Estonian state.

  This development made conservative forces in the Soviet Union very anxious and they formed the State Committee of Emergency, which organised the August Coup. In the early morning of August 19 over the state radio and television a broadcast was transmitted stating the Emergency. I was on this day in Tallinn to buy air tickets to attend a summer school in Erice, Italy, and heard of the Coup while standing in the queue for tickets.

  The tickets purchased, I went to Toompea (Dome Hill) where our government and Estonian Supreme Soviet had their offices. I searched for Liia Hänni, our representative in the Soviet. I found her in a small office. To my question of what the Soviet is doing, Liia said that they were discussing the next law project. I replied that all other items are of secondary importance, and now the only major issue was to declare Estonian independence, as it might be impossible later. Liia answered that she has the same opinion, but the Estonian Committee is strongly opposing this, because in their view the Supreme Soviet is an illegal organ of occupation.

  In my opinion the actual reason was that both leading forces, the Popular Front and the Estonian Committee, wanted sole credit for the declaration of independence. The Estonian Committee did not understand that neither the Soviet Union nor any foreign country would recognise an independence declaration made by an informal organisation which did not have actual control of the country. To overcome this rivalry the chairman of the Supreme Soviet Arnold Rüütel argued that a compromise between the Popular Front and the Estonian Committee was needed. In further discussions with Liia we both found that the major issue is to find the compromise.

  The next day two other astronomers visited Tallinn and also spoke to Liia Hänni urging just the same. To achieve the compromise, on the next day, August 20, a mass meeting was organised by Popular Front in the central square of Tallinn (Vabaduse plats — the Square of Freedom). Soviet tanks were already in Tallinn to take over the TV tower and other important buildings, so it was high time for action. The people’s message in the mass meeting to our leaders was clear — “Take it now!”

  Two scientists, Liia Hänni and Marju Lauristin (both were members of Supreme Soviet as well as Estonian Congress), played a major role in achieving a compromise between the Estonian Congress and the Supreme Soviet. The idea of the compromise was to declare the Estonian Republic as a continuation of the prewar Republic, and to form the Estonian Constitutional Assembly to prepare a new constitution of Estonia. It should consist of equal numbers of members from the Estonian Congress and the Supreme Soviet. That very evening the compromise was achieved, and the Supreme Soviet proclaimed the restoration of the independent state of Estonia.

  The real events of the Coup were in Moscow. Already on August 19 citizens of Moscow began to gather around the White House, the Parliament of the Russian Federation. A tank battalion was sent to guard the White House, and the chief of the tank battalion declared its loyalty to the Russian Federation. The president of the Russian FederationYeltsin climbed one of the tanks and addressed the meeting, declaring the Coup as illegal. On August 22 the Coup leaders understood that the Coup had failed. The real power was in the hands of the President of the Russian Federation.

  On August 24 many thousands of Moscow citizens took part in the funerals of three men killed during the Coup. Rüütel was in Moscow to meet Yeltsin. After the funerals Yeltsin invited Rüütel to his office, and Yeltsin signed the document of recognition of Estonia as an independent state. In the document an appeal to the Soviet Union and foreign countries was included inviting them to recognise Estonian independence. In the following days almost all European countries followed this appeal. But some countries waited until the State Council of USSR recognised Estonian independence on September 6, 1991. On December 21, 1991, representatives of all Soviet republics except the three Baltic states and Georgia declared the end of the Soviet Union.

  After the declaration of independence the economical situation quickly worsened, because almost all imports were made on the basis of international prices, but the value of the ruble was falling very rapidly. In June 1992 the Estonian currency, kroon, was introduced, and in September 1992 the new Estonian Parliament was elected. Our currency was pegged to the Deutsche Mark, the most stable currency in Europe, which gave confidence to our economy. On January 1, 2011, the Euro replaced our kroon as the official currency.

  After the introduction of the kroon the economic situation started to improve. The new government formed after the election of the Parliament was dominated by the nationalist party who started radical economic reforms. The minister for Privatisation was Liia Hänni, the minister of Social Affairs was Marju Lauristin, both members of the Moderate Party, which was later renamed as the Social Democratic Party.

  But the Russian Army was still there. It took several years of difficult negotiations to withdraw all Russian troops. Here our allies in the West helped: the U.S. Senate threatened in July 1994 to halt all aid to Russia if the forces were not withdrawn by the end of August. Final withdrawal was really completed on August 31, 1994.

  For us, only now has World War II ended.

  7.4.2 Science reform

  In the first year of restored independence the value of the currency, ruble, fell very rapidly, and the prices for imported goods became international. Thus the budget of the Observatory was sufficient only to buy fuel for central heating, and a bit was left over for salaries. This led to a drastic decrease in the number of staff—almost the whole technical staff left and formed new private enterprises. Thus in the early 1990’s on Observatory territory there were about ten private enterprises ranging from a computer company and typography to a center of education. A number of scientists also left, mostly young men and women. Most of them organised their own enterprises and went to work there, some left to join politics like Liia Hänni, and some found a better place abroad. Among them were our best young people in the cosmology department, Lev Kofman and Dimitry Pogosyan.

  During the Soviet period most scientific studies were made in institutes of the Estonian Academy of Sciences. In the early 1990’s the science and university system was reformed. Most former institutes of the Academy were included into universities — Tartu University (the only classical university in Est
onia), Tallinn Technical University, Estonian University of Life Sciences, or Tallinn University (former Tallinn Pedagogical University). In most cases these institutes dissolved as scientific units as scientists were distributed between many subunits within universities. To avoid such a situation we succeeded in keeping Tartu Observatory as an independent institution which collaborates closely with Tartu University. So most astronomers teach courses in the University or serve students supervisors.

  In the early 1990’s the Soros Foundation helped us to get some equipment; most urgently we needed modern personal computers. In the mid 1990’s in Estonia a grant system for science was introduced. Grants are given on the basis of scientific excellence, and in most cases foreign referees are used to estimate the scientific level of the grant. Also collaboration with astronomers from other countries was easier.

  The general level of science financing was in the 1990’s still rather low. So a lot of young men and women entering universities chose soft sciences such as financing, law, project managers etc instead of physics, mathematics or astronomy. Only in the early 2000’s did the financial situation improve, and now we have a number of young investigators in our staff.

  7.4.3 Participation in international organisations

  Soon after the restoration of Estonian independence and its recognition by Western countries I got a letter from George Contopoulos. He was the Editor-in-Chief of the European Journal “Astronomy and Astrophysics” (A&A). This is a journal for all European astronomers, created by merging of all previous European astronomical journals, and coordinated by the European Southern Observatory. Contopoulos suggested that Tartu Observatory should make an official appeal to the Journal and ask for membership, first as associate member-state, and later as a full member- state. He asked me to write the letter promptly, so that he could make necessary arrangements during his period as Editor-in-Chief. The director of the Observatory wrote the letter, and soon I got an invitation to participate in the next annual Meeting of the Board of Directors of the Journal.