inflation. In many realistic examples of particle physics having more than one light scalar fields is quite natural, especially in the case of MSSM [91]. However, as we shall argue below that within MSSM there are only handful of such good candidates for a curvaton [134– 136] $^{104}$. 

In the original curvaton paradigm it was assumed that the curvaton is responsible for generating the entire curvature perturbations, and the perturbations arising from the inflaton component are subdominant [126, 129–131]. However, there are variants where both inflaton and curvaton can contribute to the curvature perturbations, see for instance [831, 832]. Irrespective of their origins the curvaton must possess the following properties: 

• Lightness of the curvaton: 

During inflation the Hubble expansion rate is: H$_{inf}$ ≫ m$_{ϕ}$ , where m$_{ϕ}$ is the effective mass of the curvaton. Since it does not cost anything in energy, therefore the quantum fluctuations are free to accumulate along a curvaton direction and form a condensate with a large VEV, ϕ$_{0}$ . During inflation, V ( φ ) ≫ V ( ϕ ), where φ is the inflaton here. After inflation, H ∝ t − $^{1}$, and the curvaton stays at a relatively large VEV due to large Hubble friction term until H ≃ m$_{ϕ}$ , when the curvaton starts oscillating around the origin with an initial amplitude ∼ ϕ$_{0}$ . From then on | ϕ | is redshifted by the Hubble expansion ∝ H ( t ) for matter dominated and ∼ H 3 / $^{4}$( t ) for radiation dominated Universe. The energy of the oscillating flat direction may eventually start to dominate over the inflaton decay products. 

• Longevity of the curvaton and its dominance: 

Furthermore, the curvaton must not evaporate due to thermal interactions from the plasma already created by the inflaton decay products [136, 833]. The curvaton and the inflaton decay products can be weakly coupled, i.e. via non-renormalizable in- teractions. In such a case the curvaton can survive long enough and its oscillations can possibly dominate the energy density while decaying. It has been shown that 

