Dark matter. An introduction (Q2879080)

\noindent Until the measurements of the famous astronomers Jan Hendrik Oort in 1932 and the investigations of the Coma cluster by Fritz Zwicky in 1933, the mass of the universe, expected from its luminosity, was thought to comprise the whole matter of the universe. But, soon, it became clear that the luminous mass contributes only to about 4 per cent of the whole matter. Consequently, there must be some dark matter, i.e. nonluminous neutral particles, which cannot be seen but exist in consistency with its gravitational effects. According to the present knowledge, there might be dark relic particles which were frozen-out at a special temperature during the development of the universe. That means, once the dark particles were in chemical and thermal equilibrium, but with the expansion of the universe, ``they failed to interact with each other, they decoupled from the universe and remained frozen with a relic density'' (ibid.\ p. 6). Massive dark particles with temperatures larger than the temperature of the universe at the freeze-out time would move nonrelativistically. They are called cold dark matter (CDM), in comparison with light dark particles at freezing-out time, which would move relativistically, and which are called hot dark matter (HDM). All the dark matter seems to contribute to about 27 per cent (ibid, p.\ 5) of the matter of the universe. Apparently, most of the dark matter does not follow the present Standard Model of particle physics. The masses of dark matter candidates are suggested to be of the order of 1--10 GeV.NEWLINENEWLINE\noindent Thus the recent research in the field of dark matter encompasses three main areas of physics, cosmology, particle physics, and astrophysics. Therefore, after a brief discussion on relativity focusing on Galilean and Lorentz transformations as well as on the electromagnetic theory, the present work concentrates on the basics of particle physics and cosmology. After that, it is systematically discussed why one knows about the existence of dark matter, which types of dark matter exist, and which methods to detect it are developed. The book shows, that the study of dark matter will certainly help to better understand the dynamics of the universe. It concludes that ``dark matter physics also has the potential to probe new unknown fundamental physics and perhaps new unknown symmetries of Nature that might predict new particles in Nature as yet unknown to us with which the dark matter may perhaps be constituted'' (ibid.\ p.\ 10).NEWLINENEWLINE\noindent The chapter on basics of particle astrophysics of the present work deals with the elementary, structureless particles of Nature described within the Standard Model, the spin-1/2 fermions (leptons and quarks) and spin-1 bosons (gauge bosons). Also the spin-0 (scalar) boson, the Higgs particle, required in the Standard Model to explain the masses of the fundamental particles, is introduced. The four fundamental types of interactions elementary particles may undergo are discussed, i.e.\ the electromagnetic interaction, the weak, the strong, and the gravitational one. Especially the Klein-Gordon equation as the equation of motion of spin-0 particles and the Dirac equation as the equation of motion of the spin-1/2 particles are mentioned. Discrete and continuous symmetries in Nature are presented. In doing so the group theory is confined to groups that will be required to understand the symmetry properties of elementary particles and fundamental interactions. Quantum electrodynamics is discussed by the invariance of a Lagrangian under local U(1) transformation, that means under abelian gauge invariance. Quantum chromodynamics that leads to the theory of strong interaction, and the theory of weak interaction are explained within the theory of non-abelian gauge invariance introducing Lagrangians invariant under local SU(3) and SU(2) transformations, respectively. Finally, in addition to the SU(2) invariance, its superposition with two invariances under U(1) transformation is considered for accommodating both electromagnetic and left-handed weak interactions.NEWLINENEWLINE\noindent The physics of the universe as a whole is presented in the chapter on the basics of cosmology. Here length scales of a billion light-years are taken into account, where the main interaction is given by the gravitation. Thus, in principle, the dynamics of the universe is described by the Einstein equations of general relativity. In doing so, the author focuses especially on the main features of an homogeneous and isotropic cosmology describable by the simple Friedmann-Robertson-Walker metric. The universe's expansion is taken into account by Hubble's law on the growth of the recession velocity of galaxies from a point with growing distance from the point. In the chapter, some of the mathematical equations related to cosmological parameters and cosmological measurements are explicitely deduced, so for the luminosity distance, the deceleration parameter, and the bolometric magnitude.NEWLINENEWLINE\noindent Recently, the estimation of the energy budget of the universe is made by measuring the anisotropies in the cosmic microwave background radiation (CMBR) also dealt with in the cosmology chapter. CMBR are the photons which were not scattered by free electrons, as the temperature of the universe decreased up to a point where electrons and ions intensively formed atoms. The wavelengths of these photons then grew together with the expansion of the universe. So, at present, these wavelengths are of the microwave order. But the CMBR anisotropies give, up to now, enormous information on the mass distribution in the universe during the last scattering. This fact use the satellite experiments WMAP (Wilkinson Microwave Anisotropy Probe) and PLANCK. CMBR and the nucleosynthesis of the light elements H, D, \(^3\)He, \(^4\)He, and \(^7\)Li within the first minute after the Big Bang result into the above mentioned recent estimate of the dark matter content of 27 per cent.NEWLINENEWLINE\noindent The existence of dark matter is primarily known because of its gravitational effects. Several astrophysical observations and estimations resulted into this conclusions. The Chapters 5 and 6 of the book thus focus on the discrepancy between luminous and gravitational mass in the universe. It is dealt with the study of the rotation curves of spiral galaxies, the observation of dark matter in galaxy clusters, gravitational lensing, the pass of the two bullet clusters through each other without their distorsion, and the Lyman alpha forest. Chapter 6 is devoted to the galactic halo of dark matter, especially of that of the Milky Way. Models of density and velocity distributions of dark matter are presented, which are useful for recent dark matter detection experiments.NEWLINENEWLINE\noindent The luminous mass is more or less understood. It means, on the microscopic level, the fundamental particles such as quarks, leptons, the vector gauge bosons and the scalar Higgs boson, and, on the large scale, galaxies and clusters of galaxies, stars, planets, and interstellar dust. But the exact nature of dark matter is not yet known. Nevertheless, one can classify the dark matter on the basis of its possible production (thermal or non-thermal), or according to the particle nature of their constituents (barionic or non-barionic matter), or depending on the mass and speed (CDM, HDM) of the dark matter particles, which is done in Chapter 7. Further it is explained, which role dark matter plays in the formation of the structure of the universe, of its galaxies and galaxy clusters.NEWLINENEWLINE\noindent Within Standard Model, only the neutral neutrinos might contribute to dark matter (Section 13.1). But their relic density is very small in comparison with the obtained total relic density. Although neutrinos have no mass within the frame of the Standard Model, experiments showed that a small mass of about 1 eV might exist. A so-called sterile neutrino with energy between HDM and CDM might very little contribute to the total dark matter.NEWLINENEWLINE\noindent Calculating the relic density of the cold dark matter from the CMBR anisotropy observations, one finds for the product of the annihilation cross-section of dark matter particles multiplied by their relative velocity values of \(10^{-32}\) m\(^3\)s\(^{-1}\). This corresponds to a very weak interaction, and consequently CDM is often termed WIMP (weakly interacting massive particles). The author concludes that the most probable dark matter candidate should be a non-barionic CDM particle which does not agree with the Standard Model characteristics. Thus, Chapter 8 gives a very brief introduction to the theories of supersymmetry (symmetry between fermions and bosons) and extra-dimensions (unifying gravity and gauge interactions) with the dark matter candidates neutralino or Kaluza-Klein dark matter, respectively. Scalar singlet and inert doublet dark matter as well as axions are introduced within the frame of extensions of the Standard Model. Chapter 9 considers the calculation of the relic densities of thermally produced dark matter candidates.NEWLINENEWLINE\noindent Recently, dark matter physics became more and more experimental. A number of experiments is operational or will be soon operational. Therefore, the author attempted to give a detailed account of various experimental techniques to search for dark matter. The main actual experiments are clearly presented. The author distinguishes between direct experiments considered in Chapters 10 and 11 and indirect experiments dealt with in Chapter 12. ``The direct detection of dark matter is attempted following the principle that, if a dark matter particle hits a nucleus of a detecting material, it suffers elastic scattering, as a result of which the target nucleus undergoes a recoil'' (p.\ 8) (recoil energy \(\sim\) a few keV). Here also the annual modulation of the dark matter signal is used for observation. The presented variety of direct experiments like CDMS, CRESST, DAMA, CoGENT, XENON, PiCASSO, and DRIFT illustrate the present dark matter hunting using direct experiments.NEWLINENEWLINE\noindent The motion of dark matter can also be influenced by heavy cosmic objects like the solar core or the galactic Milky Way center, dwarf galaxies and galaxy clusters, galactic halos, and extra-galactic sources. Particularly, decay and annihilation processes of dark matter particles may occur. Fermion-antifermion pairs and photons may be generated, which are detected in indirect experiments (e.g.\ ICE-CUBE, ANTARES, HESS, VERITUS, FERMI-LAT, PAMELA, AMS).NEWLINENEWLINE\noindent The final Chapter 13 of the book considers the possible contribution of sterile neutrinos to the total dark matter, strange quark nuggets as candidates of dark matter MACHOS (massive astrophysical compact halo objects), and inelastic dark matter (e.g.\ the supersymmetric neutralino or the sneutrino).NEWLINENEWLINE\noindent The present textbook is completed with a list of 191 well-chosen references. Altogether, the present work is intended for young researchers pursuing a research career in dark matter in particular, or astroparticle physics or cosmology in general. But the book contains also discussions which are of interest to a more advanced reader interested in astrophysical aspects of dark matter.