Plane-Symmetric Inhomogeneous Bulk Viscous Cosmological Models with Variable Λ\Lambda

A plane-symmetric non-static cosmological model representing a bulk viscous fluid distribution has been obtained which is inhomogeneous and anisotropic and a particular case of which is gravitationally radiative. Without assuming any {\it adhoc} law, we obtain a cosmological constant as a decreasing function of time. The physical and geometric features of the models are also discussed.Comment: 11 page

On the T-dependence of the magnetic penetration depth in unconventional superconductors at low temperatures: can it be linear?

We present a thermodynamics argument against a strictly linear temperature dependence of the magnetic penetration depth, which applies to superconductors with arbitrary pairing symmetry at low temperatures.Comment: 5 pages, expanded version of cond-mat/971102

Bianchi Type I Magnetofluid Cosmological Models with Variable Cosmological Constant Revisited

The behaviour of magnetic field in anisotropic Bianchi type I cosmological model for bulk viscous distribution is investigated. The distribution consists of an electrically neutral viscous fluid with an infinite electrical conductivity. It is assumed that the component $\sigma^{1}_{1}$ of shear tensor $\sigma^{j}_{i}$ is proportional to expansion ($\theta$) and the coefficient of bulk viscosity is assumed to be a power function of mass density. Some physical and geometrical aspects of the models are also discussed in presence and also in absence of the magnetic field.Comment: 13 page

Ellipsoidal Universe Can Solve The CMB Quadrupole Problem

The recent three-year WMAP data have confirmed the anomaly concerning the low quadrupole amplitude compared to the best-fit \Lambda CDM prediction. We show that, allowing the large-scale spatial geometry of our universe to be plane-symmetric with eccentricity at decoupling or order 10^{-2}, the quadrupole amplitude can be drastically reduced without affecting higher multipoles of the angular power spectrum of the temperature anisotropy.Comment: 4 pages, 2 figures, minor changes, reference added, to appear in Phys. Rev. Let

Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy

We consider the effect of inhomogeneous neutrino degeneracy on Big Bang nucleosynthesis for the case where the distribution of neutrino chemical potentials is given by a Gaussian. The chemical potential fluctuations are taken to be isocurvature, so that only inhomogeneities in the electron chemical potential are relevant. Then the final element abundances are a function only of the baryon-photon ratio $\eta$, the effective number of additional neutrinos $\Delta N_\nu$, the mean electron neutrino degeneracy parameter $\bar \xi$, and the rms fluctuation of the degeneracy parameter, $\sigma_\xi$. We find that for fixed $\eta$, $\Delta N_\nu$, and $\bar \xi$, the abundances of helium-4, deuterium, and lithium-7 are, in general, increasing functions of $\sigma_\xi$. Hence, the effect of adding a Gaussian distribution for the electron neutrino degeneracy parameter is to decrease the allowed range for $\eta$. We show that this result can be generalized to a wide variety of distributions for $\xi$.Comment: 9 pages, 3 figures, added discussion of neutrino oscillations, altered presentation of figure

The spherical collapse model in time varying vacuum cosmologies

We investigate the virialization of cosmic structures in the framework of flat FLRW cosmological models, in which the vacuum energy density evolves with time. In particular, our analysis focuses on the study of spherical matter perturbations, as they decouple from the background expansion, "turn around" and finally collapse. We generalize the spherical collapse model in the case when the vacuum energy is a running function of the Hubble rate, $\Lambda=\Lambda(H)$. A particularly well motivated model of this type is the so-called quantum field vacuum, in which $\Lambda(H)$ is a quadratic function, $\Lambda(H)=n_0+n_2\,H^2$, with $n_0\neq 0$. This model was previously studied by our team using the latest high quality cosmological data to constrain its free parameters, as well as the predicted cluster formation rate. It turns out that the corresponding Hubble expansion history resembles that of the traditional $\Lambda$CDM cosmology. We use this $\Lambda(t)$CDM framework to illustrate the fact that the properties of the spherical collapse model (virial density, collapse factor, etc.) depend on the choice of the considered vacuum energy (homogeneous or clustered). In particular, if the distribution of the vacuum energy is clustered, then, under specific conditions, we can produce more concentrated structures with respect to the homogeneous vacuum energy case.Comment: 14 pages, 4 figures, minor changes, accepted for publication in Phys. Rev.

Neutrino Oscillations and the Early Universe

The observational and theoretical status of neutrino oscillations in connection with solar and atmospheric neutrino anomalies is presented in brief. The effect of neutrino oscillations on the early Universe evolution is discussed in detail. A short review is given of the standard Big Bang Nucleosynthesis and the influence of resonant and nonresonant neutrino oscillations on active neutrinos and on primordial nucleosynthesis of He-4. BBN cosmological constraints on neutrino oscillation parameters are discussed.Comment: 21 p., 6 figures, a review based on raview talk at NCYA Conference and a presentation at CAPP200

Breaking Of Conformal Invariance And Electromagnetic Field Generation In The Universe

It is shown that the breaking of the conformal invariance in quantum electrodynamics due to the trace anomaly results in the generation of long wave electromagnetic fields during inflationary stage of the universe evolution. If the coefficient of the logarithmic charge renormalization is large (due to a large number of charged particles species), these primordial electromagnetic fields can be strong enough to create the observed galactic magnetic fields.Comment: 6 pages, UM - TH - 93 - 0

Production of a sterile species via active-sterile mixing: an exactly solvable model

The production of a sterile species via active-sterile mixing in a thermal medium is studied in an exactly solvable model. The \emph{exact} time evolution of the sterile distribution function is determined by the dispersion relations and damping rates $\Gamma_{1,2}$ for the quasiparticle modes. These depend on \wtg = \Gamma_{aa}/2\Delta E, with $\Gamma_{aa}$ the interaction rate of the active species in absence of mixing and $\Delta E$ the oscillation frequency in the medium without damping. \wtg \ll1,\wtg \gg 1 describe the weak and strong damping limits respectively. For \wtg\ll1, \Gamma_1 = \Gamma_{aa}\cos^2\tm ; \Gamma_{2}=\Gamma_{aa}\sin^2\tm where \tm is the mixing angle in the medium and the sterile distribution function \emph{does not} obey a simple rate equation. For \wtg \gg 1, $\Gamma_1= \Gamma_{aa}$ and \Gamma_2 = \Gamma_{aa} \sin^22\tm/4\wtg^2, is the sterile production rate. In this regime sterile production is suppressed and the oscillation frequency \emph{vanishes} at an MSW resonance, with a breakdown of adiabaticity. These are consequences of quantum Zeno suppression. For active neutrinos with standard model interactions the strong damping limit is \emph{only} available near an MSW resonance \emph{if} $\sin\theta \lesssim \alpha_w$ with $\theta$ the vacuum mixing angle. The full set of quantum kinetic equations for sterile production for arbitrary \wtg are obtained from the quantum master equation. Cosmological resonant sterile neutrino production is quantum Zeno suppressed relieving potential uncertainties associated with the QCD phase transition.Comment: To appear in Phys. Rev.

Production of a sterile species: quantum kinetics

Production of a sterile species is studied within an effective model of active-sterile neutrino mixing in a medium in thermal equilibrium. The quantum kinetic equations for the distribution functions and coherences are obtained from two independent methods: the effective action and the quantum master equation. The decoherence time scale for active-sterile oscillations is $\tau_{dec} = 2/\Gamma_{aa}$, but the evolution of the distribution functions is determined by the two different time scales associated with the damping rates of the quasiparticle modes in the medium: \Gamma_1=\Gamma_{aa}\cos^2\tm ; \Gamma_2=\Gamma_{aa}\sin^2\tm where $\Gamma_{aa}$ is the interaction rate of the active species in absence of mixing and \tm the mixing angle in the medium. These two time scales are widely different away from MSW resonances and preclude the kinetic description of active-sterile production in terms of a simple rate equation. We give the complete set of quantum kinetic equations for the active and sterile populations and coherences and discuss in detail the various approximations. A generalization of the active-sterile transition probability \emph{in a medium} is provided via the quantum master equation. We derive explicitly the usual quantum kinetic equations in terms of the ``polarization vector'' and show their equivalence to those obtained from the quantum master equation and effective action.Comment: To appear in Phys. Rev.