CMB observations and the metal enrichment history of the universe

The main purpose of the work presented in this thesis is to
investigate the phenomenon of resonant scattering of the Cosmic
Microwave Background (CMB) photons by atoms and molecules. The
fine-structure transitions of the various atoms and ions of Carbon,
Nitrogen, Oxygen and other common metals have wavelengths in the
far-infrared regions, which are particularly suitable for
scattering the CMB photons at high redshifts ($2 \lesssim z
\lesssim 30$). Since the CMB photons are released at redshifts
$z\simeq 1100$, they must interact with all the intervening matter
before reaching us at $z=0$. Therefore scattering of these photons
in the far-IR fine-structure lines of various atoms and ions
provide a plausible way to couple the radiation with the matter at
those redshifts and to study the enrichment and ionization history
of the universe. Moreover, rotational transitions of diatomic
molecules like the CO have wavelengths extending into the
sub-millimeter wavebands, and hence they can scatter the CMB
photons at very low redshifts. Studying the very low density gas of
nearby galaxies in CO lines can yield a definitive signature of
resonant scattering of the CMB photons through a decrement in the
background intensity of the microwave sky. Observation of this
scattering signal from any object in the sky will tell us about its
radial velocity in the CMB rest frame. In this work we first derive
the detailed formalism for the scattering effect in presence of the
peculiar motion of the scatterer. Then we investigate the
possibility to detect individual objects at different redshifts
through scattering and try to find applications for this effect.
Our main example is the possibility to find the peculiar motions of
nearby galaxies in the CMB rest frame through observation of the
scattering signal, which we explore in detail. Next we discuss the
density limits in which scattering effect can dominate over the
line emission in individual objects. We describe three types of
critical densities, and show that detection of single objects
through scattering requires very low density, whereas observation
of the integrated scattering signal coming from many unresolved
objects in the sky will permit us to probe higher densities. We
discuss this effect subsequently, as we compute the change in the
angular fluctuations of the CMB sky temperature through resonant
scattering. We found that the scattering signal gets strong
enhancement due to a non-zero correlation existing between the
density perturbations at the last scattering surface, where CMB
anisotropies are generated, and at the epoch of scattering. This
opens up a new way to study the ionization and enrichment history
of the universe, and we investigate various enrichment scenarios
and the temperature fluctuations that might be caused by them. The
resulting signal is already within the sensitivity limits of some
upcoming space- and ground-based CMB experiments, and we show upto
what extent they shall be able to put constraints on different
enrichment histories. Finally we analyze the effect of line and
dust emission in the same frequency range that we used for the
detection of scattering signal. These emissions are coming from
very high density objects where active star formation is taking
place, and due to the compactness of their size as well as absence
of any velocity dependence the emission signal is significantly
suppressed at large angular scales, where scattering will be
dominant. We present some detailed analytic expressions for the
scattering signal and also a method to solve for the detailed
statistical balance equations in a multi-level system in the
appendix.