accommodate a low inflation scale ?

The cosmological curvature perturbation may be generated when some ‘curvaton’ field, different from the inflaton, oscillates in a background of unperturbed radiation. In its simplest form the curvaton paradigm requires the Hubble parameter during inflation to be bigger than 10 GeV, but this bound may be evaded if the curvaton field (or an associated tachyon) is strongly coupled to a field which acquires a large value at the end of inflation. As a result the curvaton paradigm might be useful in improving the viability of low-scale inflation models, in which the supersymmetrybreaking mechanism is the same as the one which operates in the vacuum. Introduction In some of the most interesting inflation models, the inflationary potential comes from the same SUSY-breaking mechanism that operates in the vacuum, giving a Hubble parameter is of order the gravitino mass [1]. Inventing some of the terminology, these are; (i) non-hybrid modular inflation [3, 4], (ii) hybrid modular inflation [5, 6], (iii) μ-field inflation invoking either gauge-mediated [7], gravity-mediated [8] or gaugino-mediated [9] SUSY breaking, and (iv) locked inflation [10]. In gravity-mediated SUSY breaking the gravitino mass is of order TeV, and in the other schemes it is some orders of magnitude lower except for anomaly-mediated [11] where it is up to 100TeV. An inflationary Hubble parameter of order the gravitino mass is therefore low compared with the maximum value of order 10GeV allowed by the CMB anisotropy [12], and low compared with the value in other sensible-looking models of inflation [1]. One of the most important constraints on models of the very early Universe is the existence of a curvature perturbation ζ , known from observation to be present on cosmological scales a few Hubble times before such scales start to enter the horizon. At that epoch, the earliest one at which it can be directly observed, ζ is almost time-independent, with an almost scale-independent spectrum Pζ given by [13] P 1 2 ζ ≃ 5× 10. This curvature perturbation is supposed to be generated by some field which is light during inflation, because indeed inflation converts the vacuum fluctuation of every such field into an almost scale-invariant classical perturbation. The question is, which light field does the job?