We present a general formalism that provides a systematic computation of the linear and non-linear perturbations for an arbitrary number of cosmological fluids in the early Universe going through various transitions, in particular the decay of some species (such as a curvaton or a modulus). Using this formalism, we revisit the question of isocurvature non-Gaussianities in the mixed inflaton-curvaton scenario and show that one can obtain significant non-Gaussianities dominated by the isocurvature mode while satisfying the present constraints on the isocurvature contribution in the observed power spectrum. We also study two-curvaton scenarios, taking into account the production of dark matter, and investigate in which cases significant non-Gaussianities can be produced.upper bounds on primordial non-Gaussianities have started to be used to constrain early Universe scenarios.Although the simplest early Universe models are based on inflationary models with a single scalar field, many models involve additional scalar fields, which can play a dynamical role during inflation or simply be spectactor fields (see e.g. [1] for introductory lectures). The existence of several degrees of freedom opens up the possibility of isocurvature perturbations, i.e. perturbations in the particle density ratio between two fluids, for example cold dark matter (CDM) isocurvature perturbations (between CDM and radiation) or baryon isocurvature perturbations (between baryons and radiation). Since primordial isocurvature perturbations leave distinctive features in the CMB anisotropies, they can be in principle disentangled from the usual adiabatic mode. The present upper bound on the isocurvature contribution to the power spectrum provides a stringent constraint. This is the case for the curvaton scenario [2] where large residual isocurvature perturbations (for CDM or baryons) can be generated, depending on how and when CDM or baryons are produced [3,4] (see also [5,6] for more detailed scenarios). The same constraints apply to moduli that are light during inflation, and thus acquire super-Hubble fluctuations, as discussed recently in [7].Another potentially useful information on primordial perturbations is the amplitude and shape of their non-Gaussianity. So far, the current CMB data seem to favour a non-zero amount of so-called local non-Gaussianity [8], but Planck data will be needed to confirm or infirm this trend. Several models can generate local non-Gaussianity (see e.g. [9] for a recent review): multiple field inflation (during inflation or at the end of inflation: see e.g. [10]), modulated reheating [11,12], curvaton, modulated trapping [13], etc. It is thus interesting to combine the constraints on isocurvature modes and non-Gaussianity to explore the early Universe physics, as has been done recently in various scenarios [14,15,16,17,18,19,20,21].The purpose of the present work is to give a unified treatment of linear and nonlinear perturbations, which enables to compute their evolution through one or several cosmological transition...
Electrodynamics of charged scalar bosons and spin 1/2 fermions is studied at non-zero temperature, chemical potentials, and possible Bose condensate of the charged scalars. Debye screening length, plasma frequency, and the photon dispersion relation are calculated. It is found that in presence of the condensate the time-time component of the photon polarization operator in the first order in electric charge squared acquires infrared singular parts proportional to inverse powers of the spatial photon momentum k.
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