Curvature perturbations and magnetogenesis from the electroweak gauge bosons

Observations provide strong evidence that the Universe underwent a phase of inflation in its early history. One of the most important consequences of inflation is the generation of the curvature perturbation, which is necessary for structure formation and is observed through the CMB anisotropy [1]. Quantum fluctuations of suitable fields give rise to a flat superhorizon spectrum of perturbations through the process of particle production [2]. Under certain circumstances these perturbations can create the curvature perturbation of the Universe. So far only scalar fields have been employed for this task. In recent works, however, it has been shown that Abelian gauge fields can also work [3, 4, 5]. Indeed, in Ref. [3] it was shown that, if a vector field obtains a flat superhorizon spectrum of perturbations during inflation, it can act as a curvaton field [6] provided, at some point after inflation, its mass-square becomes positive and bigger than the Hubble scale. In this case the vector field condensate oscillates coherently, behaving as pressureless isotropic matter [3]. Thus, it can dominate the radiation background without introducing significant anisotropy, imposing thereby its own curvature perturbation according to the curvaton mechanism [6]. In Ref. [4] the mechanism was shown to work when the phase transition, which renders the vector field massive occurs near electroweak unification. Hence, it is intriguing to explore whether the massive gauge fields of the standard model can play the role of vector curvaton.