This thesis collects three works we have done previously.
First, we consider a class of the inflaton scalar potential which does not depend on the inflaton field value during inflation. In particular, we consider supernatural inflation, its hilltop version, A-term inflation, and supersymmetric (SUSY) D- and F-term hybrid inflation on the brane. We show that the parameter space can be broadened, the inflation scale generally can be lowered, and still possible to have the spectral index n_s = 0.96.
Second, we consider the effect of two right-handed sneutrino curvaton decays and investigate the parameter space. We compare the difference of the result between single- and two-curvaton cases. We find that one Yukawa coupling of the right- handed sneutrinos can be as large as λ ~ O(0.1) while the other one is much smaller. This is consistent with the idea that only one generation of the neutrino mass may be much smaller than others. When the curvatons decay, we assume both of them subdominate the energy density of the Universe. We find that, unlike a single curvaton case, here a small or negative, as well as a large fNL can be generated.
Third, we investigate the idea that the decay of a curvaton is kinematically blocked and show that the coupling constant of curvaton decay can be as large as O(1). We also find in this case, the lower bound of the Hubble parameter at horizon exit from Big Bang nucleosynthesis (BBN) is H* ~7.2 × 10^(-9)Mpl ~10^(10) GeV . Similar to conventional curvaton scenario, the nonlinear parameter can be as large as f_(NL) = 100.

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