CMB observations and the metal enrichment history of the universe
The main purpose of the work presented in this thesis is to investigate the\nphenomenon of resonant scattering of the Cosmic Microwave Background (CMB)\nphotons by atoms and molecules. The fine-structure transitions of the various\natoms and ions of Carbon, Nitrogen, Oxygen and other common metals have\nwavelengths in the far-infrared regions, which are particularly suitable for\nscattering the CMB photons at high redshifts ($2 \\lesssim z \\lesssim\n30$). Since the CMB photons are released at redshifts $z\\simeq 1100$, they must\ninteract with all the intervening matter before reaching us at $z=0$. \nTherefore scattering of these photons in the far-IR \nfine-structure lines of various\natoms and ions provide a plausible way to couple the radiation with the\nmatter at those redshifts and to study the enrichment and ionization\nhistory of the universe. Moreover, rotational transitions of diatomic\nmolecules like the CO have wavelengths extending into the sub-millimeter\nwavebands, and hence they can scatter the CMB photons at very low\nredshifts. Studying the very low density gas of nearby galaxies \nin CO lines can yield a\ndefinitive signature of resonant scattering of the CMB photons through a\ndecrement in the background intensity of the microwave sky. Observation of\nthis scattering signal from any object in the sky will tell us about its\nradial velocity in the CMB rest frame.\n\nIn this work we first derive the detailed formalism for the scattering\neffect in presence of the peculiar motion of the scatterer. Then we\ninvestigate the possibility to detect individual objects at different\nredshifts through scattering and try to find applications for this\neffect. Our main example is the possibility to find the peculiar motions of\nnearby galaxies in the CMB rest frame through observation of \nthe scattering signal, which\nwe explore in detail. Next we discuss the density limits in\nwhich scattering effect can dominate over \nthe line emission in individual objects. We\ndescribe three types of critical densities, and show that detection of\nsingle objects through scattering requires very low density, \nwhereas observation of the integrated scattering signal \ncoming from many unresolved objects in the sky will permit\nus to probe higher densities. We discuss this effect subsequently,\nas we compute the change in the angular fluctuations of the CMB sky\ntemperature through resonant scattering. We found that \nthe scattering signal gets strong enhancement \ndue to a non-zero correlation existing between the density perturbations at\nthe last scattering surface, where CMB anisotropies are \ngenerated, and at the epoch of\nscattering. This opens up a new way to study the ionization and enrichment\nhistory of the universe, and we investigate various enrichment \nscenarios and\nthe temperature fluctuations that might be caused by them. The resulting signal\nis already within the sensitivity limits of some upcoming space- and\nground-based CMB experiments, and we show upto what extent they \nshall be able to put\nconstraints on different enrichment histories. Finally we analyze the effect\nof line and dust emission in the same frequency range that we used\nfor the detection of scattering signal. These emissions are coming from very\nhigh density objects where active star formation is taking place, and due to\nthe compactness of their size as well as absence of any velocity\ndependence the emission signal is significantly suppressed\nat large angular scales, where scattering will be dominant. We present some\ndetailed analytic expressions for the scattering signal and also a method to\nsolve for the detailed statistical balance equations in a multi-level system\nin the appendix.