Dark Matter and Cosmic Web Story
Page 20
This isolated location in the Strömberg diagram is an indirect argument against the stellar origin of the new population. The M/L value and the spatial distribution of the dark population differ greatly from respective properties of known stellar populations, thus it must have been formed much earlier than all known populations to form the gap in relations between various physical, kinematical and spatial structure parameters. The total mass of the new population exceeds the masses of known populations by an order of magnitude, thus we have a problem: How to transform at an early stage of the evolution of the Universe most of the gas into these invisible stars? It is known that star formation is a very inefficient process: in a star-forming gaseous nebula only about 1% of matter transforms to stars (as noted already in Chapter 3).
This topic was discussed in the 1975 Tallinn conference on dark matter. If the dark halo is of stellar origin, then these stars form an extended population around galaxies. From hydrodynamical considerations it follows that coronal stars must have much higher velocity dispersion than the stars belonging to the ordinary halo. No fast-moving stars as possible candidates for a stellar dark halo were found (Jaaniste & Saar, 1975). The publication story of the last paper is interesting. First the authors submitted the paper to “Astrophysics and Space Science”, but the editor Prof. Salomon Pikelner rejected the paper with justification: “you already have a paper on dark matter” (Chernin et al., 1976). His attitude to the dark matter subject was characteristic for the majority of the astronomical community: that this problem is not a topical one. He could not imagine that in years to come tens of thousands of papers will be written on this subject!
It is interesting to note that many astronomers did not understand the difference between the global and local dark matter even many years after their different nature was noticed and widely discussed. For instance, at the 1977 Tallinn IAU Symposium on Large Scale Structure of the Universe I had a short communication on the discrepancy of M/L-ratios (Einasto, 1978). In the discussion Jerry Ostriker argued that this is not a real discrepancy since a measurement is divided by an assumption. Actually I compared global measured M/L-ratios with local measured M/L-ratios using different data — global mass and light distribution from rotation curves and photometric profiles of galaxies, and local M/L-ratios found for star clusters, and in some cases for individual stellar populations. Jerry was supported by Beatrice Tinsley who argued that most stellar mass is contained in invisible dwarf stars (yes, but not so much). Both Jerry and Beatrice have apparently not studied properties of stellar populations in such detail, and did not appreciate the need for independent dynamical calibration of stellar population M/L-ratios, which is the central problem when we discuss the possible stellar nature of dark matter. In contrast, the Faber & Gallagher (1979) review of masses of galaxies is specifically addressed to the analysis of M/L-ratios of galaxies in general, and also of galactic populations.
Gaseous coronae of galaxies and clusters were discussed in the early 1970’s by Field (1972), Silk (1974), Komberg & Novikov (1975) and others. The conclusion was that gaseous coronae of galaxies and clusters cannot consist of neutral gas since the intergalactic hot gas would ionise the coronal gas. My own argument was that neutral gas would be seen through its 21-cm line, but such corona is not observed. The preliminary conclusion following this argumentation was that coronae can consist of hot gas, a suggestion made already by Kahn & Woltjer (1959) and Einasto (1974a). However, early observations by the UHURU X-ray satellite suggested that the mass of the hot gas is not sufficient to stabilise clusters of galaxies, see the discussion in the previous Chapter. I used these data as an indication that the presence of hot gas in clusters does not solve the mass discrepancy observed in clusters of galaxies (Einasto et al., 1974b). However, the morphological segregation of companion galaxies suggests that at least part of the coronal matter must be gaseous (Einasto et al., 1974c).
Modern data confirmed that a fraction of the coronal matter around galaxies and in groups and clusters of galaxies consists indeed of X-ray emitting hot gas, but the amount of this gas is not sufficient to explain the flat rotation curves of galaxies and the masses of clusters of galaxies.
6.1.3 Nucleosynthesis constraints of baryonic matter
According to the Big Bang model, the Universe began in an extremely hot and dense state. For the first second it was so hot that atomic nuclei could not form — space was filled with a hot soup of protons, neutrons, electrons, photons and other shortlived particles. Occasionally a proton and a neutron collided and stuck together to form a nucleus of deuterium (a heavy isotope of hydrogen), but at such high temperatures they were broken immediately by high-energy photons. When the Universe cooled off, these high-energy photons became rare enough that it became possible for deuterium to survive. These deuterium nuclei could keep sticking to more protons and neutrons, forming nuclei of helium and other light elements. This process of element-formation is called “Big Bang nucleosynthesis”, suggested by Alpher et al. (1948). The denser the proton and neutron “gas” is at this time, the more light elements will be formed. As the Universe expands, the density of protons and neutrons decreases. Neutrons are unstable unless they are bound up inside a nucleus. After a few minutes the free neutrons will be gone and nucleosynthesis will stop. The relationship between the expansion rate of the Universe and the density of protons and neutrons (the baryonic matter density) determines how much of each of these light elements are formed in the early Universe.
According to nucleosynthesis data baryonic matter makes up 0.04 of the critical cosmological density, assuming a Hubble constant h ∼ 0.7. Only a small fraction, less than 10%, of the baryonic matter is condensed to visible stars, planets and other compact objects. Most of the baryonic matter is in the intergalactic matter (Warm-Hot-Intergalactic-Medium, Lymanα-forest), some of it concentrated also in hot X-ray coronae of galaxies and clusters.
After my talk in the Caucasus Winter School and publications of papers by Einasto et al. (1974b) and Ostriker et al. (1974), Zeldovich started to think about the discrepancy between new density estimates and the nucleosynthesis data. He writes (Zeldovich, 1975): “A major problem is now ripe for solution: the conflict between the mean density of matter in the Universe, and the interstellar deuterium abundance. This deuterium is believed to be primordial, having been produced by nucleosynthesis during the few seconds immediately after the singularity. ” As a solution he suggested a cosmological model with strong inhomogeneities in the baryon density near the singularity, where most of the volume has low density as suggested by nucleosynthesis data, and the rest has higher density as suggested by new density estimates. This model is rather speculative and Zeldovich soon abandoned it.
The results of early discussions on the nature of dark halos were inconclusive — no appropriate candidate was found. For many astronomers this was an argument against the presence of dark halos.
6.2 Non-baryonic dark matter
6.2.1 Cosmic microwave background radiation
According to the current understanding, the Universe began with a Big Bang and was initially very hot. It expanded rapidly and cooled, and at a certain epoch (about 300 hundred thousand years after the Big Bang) was cool enough for atoms to recombine. The effective temperature of this radiation drops as the Universe expands. Alpher et al. (1948) predicted that the radiation from this epoch should be still present. The authors predicted that the present temperature of this cosmic microwave background (CMB) radiation should be approximately 5 degrees Kelvin. The possibility of detecting this radiation was suggested by Doroshkevich & Novikov (1964).
The CMB radiation was actually detected by the American radio astronomers Penzias & Wilson (1965). They worked at Bell Labs in New Jersey with ultrasensitive cryogenic microwave receivers for radio astronomy observations. Their goal was to eliminate all possible terrestrial sources of noise. Their finding was, after eliminating all possible known sources, a certain noise remained. This noise was identified by Dicke et al. (1965)
as the CMB radiation. The temperature of the radiation is 2.7 K, and the spectrum peaks in the microwave range, corresponding to a 1.9 mm wavelength.
Fig. 6.2 The barrel diagram — principal models of the formation of structure in the Universe using different candidates of dark matter are shown as barrels. There are three main candidates for dark matter: neutrino, axion (and other cold particles), and the cosmological constant. The barrels are hooped together by two principal assumptions, Ω = 1, and a flat spectrum of initial perturbations. If these assumption do not work, there are some hoops in reserve: a non-flat spectrum and secondary ionization. Various observational tests are expressed as staves. The height of a stave indicates the degree of accordance of model with this particular test. One test is the beauty or internal harmony of the model. The level of the liquid in the barrel is equal to the height of the shortest stave, which determines the degree of acceptance of the model. If necessary, a cocktail from several liquids can be made, or some ferment as neutrino decay is added. Idea and artwork by L. Kofman (Einasto et al., 1987).
The discovery of the CMB radiation is a strong test of the Big Bang model of the Universe. CMB radiation contains a lot of information on the structure of the Universe at early times. CMB observations give the strongest argument against the Steady State theory of the formation of the Universe. The CMB radiation has a black-body spectrum with deviations of the order of 3 × 10−5 — this is the most exact black body known (Mather et al., 1990, 1994). This accuracy can only be explained if the Universe was hot at early times as assumed in the Big Bang model.
6.2.2 Fluctuations of the CMB radiation
The detection of the CMB radiation was a very important observation which cast doubts on baryonic matter as the dark matter candidate. Initially the Universe was very hot, the gas was ionised, and all density and temperature fluctuations of the primordial soup were damped by very intense radiation. But as the Universe expanded, the gas cooled, and at a certain epoch called recombination the gas became neutral. From this time on, density fluctuations in the gas had a chance to grow by gravitational instability. Matter is attracted to the regions where the density is higher, and it flows away from low-density regions. But gravitational clustering is a very slow process. Calculations showed that density fluctuations are of the same order as temperature fluctuations. Thus astronomers started to search for temperature fluctuations in the CMB radiation. None were found. As the accuracy of measurement increased, lower and lower upper limits for the amplitude of CMB fluctuations were obtained.
Let me recall one moment in our search for understanding of cosmic evolution in the late 1970’s. At the Tallinn symposium Parijskij (1978) made a report on his search for temperature fluctuations of CMB radiation with RATAN-600, the largest and most sensitive radio telescope at the time. No temperature fluctuations were found. The upper limit was about 10−4 of the mean temperature. After the talk Zeldovich discussed these results with Parijskij and expressed his opinion that something must be wrong in his observations. Theoretical calculations show that at the epoch of recombination the density (and temperature) fluctuations must have an amplitude of the order of 10−3, otherwise structure cannot form, since the gravitational instability that is responsible for the growth of the amplitude of fluctuations works very slowly in an expanding Universe — the amplitude of fluctuations grows linearly with the expansion factor a = 1/(1 + z), and the redshift of CMB radiation is zCMB ≈ 1000.
What was actually wrong was our understanding of the nature of dark matter.
Fluctuations of the CMB were finally measured by the COBE satellite by Smoot et al. (1992) and Bennett et al. (1996). Four-year COBE measurements yielded for the amplitude of temperature fluctuations 15.3 ± 3 μK, and for the power law spectral index n = 1.2 ± 0.3. Modern WMAP and Planck satellite data give even more accurate values of these parameters. Seven-year observations with WMAP satellite yield for the spectral index of fluctuations a value n = 0.968 ± 0.012 Komatsu et al. (2011). The angular power spectrum of CMB temperature fluctuations has the first maximum at wavenumber l = 200, which suggests that the total matter/energy density of the Universe is equal to the critical density (Bennett et al., 2003).
6.2.3 Neutrinos as dark matter candidates
Already in the 1970’s suggestions were made that some sort of non-baryonic elementary particles, such as massive neutrinos, may serve as candidates for dark matter particles. There were several reasons to search for non-baryonic particles as a dark matter candidate. First of all, no baryonic matter candidate fit the observational data. Second, the total amount of matter is of the order of 0.2-0.3 in units of the critical cosmological density, while the nucleosynthesis constraints suggest that the amount of baryonic matter cannot be higher than about 0.04 of the critical density.
The only known non-baryonic particle was the neutrino, thus it was natural that first neutrinos were considered as dark matter particle candidates. Szalay & Marx (1976) considered neutrinos as dark matter candidates using as argument the total density of matter and the density of known baryonic matter. A similar suggestion was made by Rees (1977). An experimental study by Lubimov et al. (1980) suggested that electronic neutrinos might have finite rest mass on the order of mv ≈ 30 eV. Bisnovatyi-Kogan & Novikov (1980) used this estimate to show that if this mass estimate is correct, the total mass density due to neutrinos is close to the critical density, and neutrinos could be dark matter particles. Chernin (1981) showed that, if dark matter is non-baryonic, then this helps to explain the paradox of small temperature fluctuations of the cosmic microwave background radiation. Density perturbations of non-baryonic dark matter already start growing during the radiation-dominated era, whereas the growth of baryonic matter is damped by radiation. If non-baryonic dark matter dominates dynamically, the total density perturbation can have an amplitude of the order 10−3 at the recombination epoch, which is needed for the formation of the observed structure of the Universe.
This problem was discussed at a conference in Tallinn in April 1981. Here all prominent Soviet cosmologists and particle physicists participated. The central problem was the nature of the dark matter. In the conference banquet Zeldovich gave an enthusiastic speech: “Observers work hard in sleepless nights to collect data; theorists interpret observations, are often in error, correct their errors and try again; and there are only very rare moments of clarification. Today it is one of such rare moments when we have a holy feeling of understanding secrets of the Nature. ” Non-baryonic dark matter is needed to start structure formation early enough. The non-baryonic nature of dark matter explains the role of dark matter in the evolution of the Universe, as well as the discrepancy between the total cosmological density of matter and the density of baryonic matter, as found from the nucleosynthesis constraint.
The non-baryonic dark matter and the relationship between particle physics and cosmology were discussed in detail during the “Study Week on Cosmology and Fundamental Physics” in the Vatican, September 28-October 2,1981 (Brueck et al., 1982). The concept of non-baryonic dark matter was generally accepted for the same reasons as discussed in Tallinn in April. Joe Silk (1982) discussed in detail fundamental tests of galaxy formation theories, based on adiabatic and isothermal scenarios. He also came to the conclusion that dark matter must be non-baryonic for the same reasons as mentioned above. In addition to neutrinos he considered photinos as one of the possible candidate for the dark matter, see the next section. Joe concludes his analysis as follows: “It seems that the large-scale structure of the Universe is intimately related to its microscopic structure on elementary particle scales. This is perhaps not surprising if one recalls that it is the initial seed of fluctuations at the Planck epoch that are likely to determine the asymptotic growth of irregularities in the expanding Universe.”
Fig. 6.3 The modified Uroborus. There are links between the microworld of particles (left), and the macroworld of cosmos (right) (Primack, 1984).
These two conferences mark probably the birth of astropa
rticle physics. Cos-mologists and particle physicists understood that properties of the micro-world and macro-world are intimately related. The relationship between the micro- and macro-world can be expressed by the modified Uroborus-symbol, as shown in Fig. 6.3.
Uroborus is an ancient symbol depicting a serpent or dragon eating its own tail. It symbolizes the recycling and renewal of the Universe. Primack (1984) and Rees (2000) noticed that the Uroborus symbol can be used to illustrate the links between the micro-world, shown in the left part of the Figure, and the macro-world in the right part. Let us put the human at the center (lowest part) of the serpent near the scale 1 cm. There are left-right connections across it: medium small to medium large, even smaller to even larger. Properties of atoms determine properties of objects on the Earth, properties of nuclei of atoms determine properties of the Sun and stars. Dark matter holds together galaxies and systems of galaxies. The inflation model suggests that properties of the whole Universe depend on interactions on the grand unification scale (Primack, 1984).
Now, finally, the presence of dark matter was accepted by leading theorists. The search of dark matter can be illustrated with the words of Sherlock Holmes “When you have eliminated the impossible, whatever remains, however improbable, must be the truth” (cited by Binney & Tremaine (1987)).
This was, however, not the end of the story. The neutrino-dominated or hot dark matter generates almost no fine structure of the Universe, as shall be discussed in the next Chapter. Thus some other solution had to be found.