Bounds on neutrino masses from baryogenesis in thermal and non-thermal   scenarios
  Baryogenesis via leptogenesis is an attractive scenario that links the physics of right handed neutrino sector with the low energy neutrino data. In the light of current neutrino oscillation data we studied the bound on the mass scale ($M_1$) of lightest right handed neutrino ($N_1$) as well as their mass hierarchy ($M_2/M_1$ and $M_3/M_1$) from the leptogenesis constraint which we discussed in two different parts of the thesis. In part-I, we studied the canonical leptogenesis in a {\it thermal} scenario and hence the bound on $M_1$. It is shown that the mass scale of $N_1$ must satisfy the constraint $M_1\geq 10^8$ GeV in order to produce the adequate lepton asymmetry. We then led to propose a model where this bound can be relaxed to TeV scale. In part-II, we studied the formation and evolution of topological defects which are non-thermal objects and are expected to be formed during the early Universe phase transitions. In particular, we considered $B-L$ cosmic strings and their effect on leptogenesis and hence the bound on the mass scale of $N_1$. It is shown that for the $B-L$ symmetry breaking scale $\eta_{B-L}>10^{11}$GeV, in certain parameter space, the cosmic strings make a more dominant contribution than thermal leptogenesis. 
Narendra Sahu