Dark Matter Self Interactions and its Impact on Large Scale Structures
  The LambdaCDM model of cosmology, though very successful at large scales, has
some discrepancy with observations at the galactic and sub-galactic scales.
These include the core-cusp problem, missing satellites problem etc. Spergel
and Steingardt (2000) proposed that if dark matter undergoes feeble self
interactions with each other, then such problems can be averted. In this
thesis, a two-component Feebly Interacting Massive Particle (FIMP) dark matter
model involving two singlet scalar fields capable of self-interactions has been
proposed and its impact on large scale structure formation has been studied
through cosmological simulations. The proposed model involves simple extensions
of the Standard Model with two singlet scalar fields formed non-thermally
through the decay of heavier particles in the very early universe. These
particles acquire their relic abundance through a freeze-in mechanism. The
coupled Boltzmann equation of the FIMP-FIMP model was solved and the relic
densities for different values of the coupling parameters were obtained and
matched with PLANCK results. The impact of dark matter self interactions was
studied through cosmological simulations using a modified version of the
parallel TreePM code GADGET-2 and the halo mass function and halo catalog for
different dark matter self interaction cross sections were obtained. Lastly,
the newly developed Effective Theory of Structure Formation (ETHOS) framework
which is a new and innovative paradigm in the study of the cosmological effects
of different dark matter models was studied and using the public code,
ETHOS-CAMB the signatures of dark acoustic oscillations in the matter power
spectrum for a particular dark matter model was obtained.
