The curvaton as a Bose-Einstein condensate of chiral pseudo Nambu-Goldstone bosons

Abstract

Explaining cosmic inflation by the effective dynamics of an $SU(N_C)$ pure gauge theory of scale $\La\sim 10^{-6} M_p$ is lacking an explanation of the (Gaussian) spatial curvature perturbations needed to seed the formation of large-scale structure after inflation. In this work it is demonstrated how fundamentally charged fermions of mass $m_F\sim 10^{-12} \La$, whose approximate chiral symmetry is spontaneously broken during inflation, can cure this shortcoming. The associated gas of weakly interacting pseudo Nambu-Goldstone bosons (PNGB) undergoes Bose-Einstein (BE) condensation well before the end of inflation. This causes the occurence of condensed light scalar fields effectively acting as a curvaton. Fermions may also be charged under a gauge group $G$ with a weak coupling $g$. Since fermions charged under $SU(N_C)$ are confined after inflation the decay of Nambu-Goldstone bosons, which reside in the condensates and in the PNGB radiation generated at reheating, is mainly into fermions {\sl solely} charged under $G$. The associated decay rate $\Gamma$ is estimated using PCAC and large $N_C$ counting. PNGB decay takes place during radiation domination after cosmological scales have entered the horizon. Neglecting the effects of the spontaneous breaking of G induced by the BE condensation, it is demonstrated that the observed spectrum of spatial curvature perturbations $P^{1/2}_\xi\sim 5\times 10^{-5}$ is compatible with $g\sim 10^{-3}$, a ratio $r_{\tiny{dec}}\sim 10^{-4}$ of curvaton to radiation energy at PNGB decay, and a curvaton effective equation of state $p_{\tiny{curv}}=-0.9 \rho_{\tiny{curv}}$.