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≲z≲30). Since the CMB photons are released at redshifts z≃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