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.

Mirror world, a parallel hidden sector with microphysics identical to ordinary particle physics, can have several interesting phenomenological and astrophysical implications and mirror matter can be a natural candidate for dark matter in the universe. If the ordinary and the mirror photons have a kinetic mixing due to the Lagrangian term (ǫ/2)F µν F ′µν , then mirror particles effectively acquire the electric charges ∼ ǫ with respect to the ordinary photon, so that they become a sort of particles historically coined as "millicharged" though nowadays they must be called more appropriately as "nanocharged". In this paper we revise the cosmological bounds on the kinetic mixing parameter and in the case of exact mirror parity set an upper limit ǫ < 3×10 −10 . Much weaker limit can be obtained in the case of asymmetric mirror sector, with an electroweak symmetry breaking scale larger than the ordinary electroweak scale.

Screening of Coulomb field of test charge in plasma with Bose condensate of electrically charged scalar field is considered. It is found that the screened potential contains several different terms: one decreases as a power of distance (in contrast to the usual exponential Debye screening), some other oscillate with an exponentially decreasing envelope. Similar phenomenon exists for fermions (Friedel oscillations), but fermionic and bosonic systems have quite different features. Several limiting cases and values of the parameters are considered and the resulting potentials are presented.

Bose-Einstein condensation of W bosons in the early universe is studied. It is shown that, in the broken phase of the standard electroweak theory, the condensed W bosons form a ferromagnetic state with aligned spins. In this case the primeval plasma may be spontaneously magnetized inside macroscopically large domains and form magnetic fields which may be the seeds for the observed today galactic and intergalactic fields. However, in a modified theory, e.g. in a theory with stronger quartic self interactions of gauge bosons e.g. due to a smaller value of the weak mixing angle, antiferromagnetic condensation is possible. In the latter case W bosons form scalar condensate with macroscopically large electric charge density i.e. with a large average value of the bilinear product of W-vector fields but with microscopically small average value of the field itself.

Mirror matter is a promising self-collisional dark matter candidate. Here we study the evolution of thermodynamical quantities in the early Universe for temperatures below ~100 MeV in presence of a hidden mirror sector with unbroken parity symmetry and with gravitational interactions only. This range of temperatures is interesting for primordial nucleosynthesis analyses, therefore we focus on the temporal evolution of number of degrees of freedom in both sectors. Numerically solving the equations, we obtain the interesting prediction that the effective number of extra-neutrino families raises for decreasing temperatures before and after Big Bang nucleosynthesis; this could help solving the discrepancy in this number computed at nucleosynthesis and cosmic microwave background formation epochs.Comment: 7 pages, 4 figures, 3 tables; changed values in Table I + minor change

The space-space component of the photon polarization operator is calculated in zero frequency limit for a medium with Bose-Einstein condensate (BEC) of electrically charged particles. It is found that the polarization operator tends to a finite value at vanishing photon 3-momentum, as it happens in superconducting media. It means that magnetic fields are exponentially screened in such a medium analogously to the Debye screening of electric charges. At non-zero temperature the screened magnetic field oscillates and contains a contribution which drops only as a power of distance. This phenomenon is unknown for superconductors, even in BEC phase and can be potentially observable.As is well known, electrically charged impurities in plasma are screened according to the Debye law [1]:where q is the charge of a test particle and m D is the screening (Debye) mass, which is expressed through the density of charge carriers in plasma. On the other hand, it is known that magnetic fields are not screened in plasma because of the absence of magnetic monopoles. The last statement has been proven in standard QED [3] and in scalar QED (SQED), when bosons are not condensed. We show below that, in the presence of Bose-Einstein condensate (BEC) of charged particles, magnetic fields in plasma are exponentially screened. Such screening is a manifestation of the well-known Meissner effect in superconductors. However, at non-zero temperature new phenomena of an oscillating exponential screening and a power law one are found here. The screening mass is of course different from the usual Debye one, which, in the relativistic case, is proportional to the temperature or the chemical potential. The charged condensate contributes to the photon polarization tensor by additive terms, whose amplitude is proportional to the amplitude of the condensate and is the same for electric and magnetic photons. The calculations are done in the framework of SQED with charged scalar boson condensate but it is evident that similar screening exists in the presence of charged vector condensate. Moreover, the result seems to be applicable to non-Abelian gauge theory as well. These problems will be studied elsewhere. The electric screening in plasma in the presence of charged Bose condensate was first studied in refs. [4] and recently in relativistic theory in refs. [5,6,7,8], see also [9] and references therein. It has been discovered that the photon polarization operator in plasma is singular at zero photon 3-momentum, having poles proportional to 1/k 2 [6,7] and 1/k [6,8], which lead to an oscillating and power law screening behavior. Such screening behavior is in good agreement with the earlier papers [4]. Magnetic, as well as electric, screening with charged BEC at zero temperature was 1

Screening in plasma with Bose-Einstein condensate is studied. Finite temperature effects are taken into account. It is shown that, due to condensate effects, the potential has several unusual features. It contains two oscillating terms, one of which is analogous to the fermionic Friedel oscillations in standard QED, and a power law decreasing term. In the T → 0 limit, only one of the oscillating terms survives. On the whole, any charge impurity is screened more efficiently than in ordinary plasma.