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Recent Wilkinson Microwave Anisotropy Probe (WMAP) data confirm the Cosmic Microwave Background (CMB) quadrupole anomaly. We further elaborate our previous proposal that the quadrupole power can be naturally suppressed in axis-symmetric universes. In particular, we discuss in greater detail the CMB quadrupole anisotropy and considerably improve our analysis. As a result, we obtain tighter constraints on the direction of the axis of symmetry as well as on the eccentricity at decoupling. We find that the quadrupole amplitude can be brought in accordance with observations with an eccentricity at decoupling of about 0.64 × 10 −2 . Moreover, our determination of the direction of the symmetry axis is in reasonable agreement with recent statistical analyses of cleaned CMB temperature fluctuation maps obtained by means of improved internal linear combination methods as Galactic foreground subtraction technique.

We analyze the magnitude-redshift data of type Ia supernovae included in the Union and Union2 compilations in the framework of an anisotropic Bianchi type I cosmological model and in the presence of a dark energy fluid with anisotropic equation of state. We find that the amount of deviation from isotropy of the equation of state of dark energy, the skewness δ, and the present level of anisotropy of the large-scale geometry of the Universe, the actual shear Σ0, are constrained in the ranges −0.16 δ 0.12 and −0.012 Σ0 0.012 (1σ C.L.) by Union2 data. Supernova data are then compatible with a standard isotropic universe (δ = Σ0 = 0), but a large level of anisotropy, both in the geometry of the Universe and in the equation of state of dark energy, is allowed.

We study the evolution of magnetic fields in freely decaying magnetohydrodynamic turbulence. By quasi-linearizing the Navier-Stokes equation, we solve analytically the induction equation in quasinormal approximation. We find that, if the magnetic field is not helical, the magnetic energy and correlation length evolve in time respectively as EB ∝ t −2(1+p)/(3+p) and ξB ∝ t 2/(3+p) , where p is the index of initial power-law spectrum. In the helical case, the magnetic helicity is an almost conserved quantity and forces the magnetic energy and correlation length to scale as EB ∝ (log t) 1/3 t −2/3 and ξB ∝ (log t) −1/3 t 2/3 .PACS numbers: 52.30.Cv, 98.62.EnThe origin of presently-observed large scale magnetic fields throughout the universe is still unclear [1]. Essentially, there are two possible classes of mechanisms to produce cosmic fields depending on when they are generated: Astrophysical mechanisms acting during or after large-scale structure formation, and mechanisms acting in the primordial universe. Magnetic fields created in the early universe (except those generated during inflation), usually suffer from a "small-scale problem", that is their comoving correlation length is much smaller then the characteristic scale of the observed cosmic fields. However, if magnetohydrodynamic (MHD) turbulence operates during their evolution, an enhancement of correlation length can occur, especially if the magnetic field is helical. As pointed out by Banerjee and Jedamzik [2], the evolution of a magnetic field in the early universe goes through different phases depending on the particular conditions of the primordial plasma. In this paper, we are interested in the case of magnetic fields evolving in the turbulent primordial universe well before recombination epoch and when kinematic dissipative effects are due to diffusing particles. Therefore, we are concerned with the so-called phase of "turbulent MHD". In other phases, such as "viscous MHD" and "MHD with ambipolar diffusion" described in Ref.[2], the dynamics of the magnetic field is very different from that studied here. The problem of determining the evolution properties of magnetic fields in MHD turbulence has been deeply and widely discussed in the literature using different methods and approximations. A direct integration of the full set of MHD equations would allow us to deeply understand the dynamics of freely decaying MHD turbulence. However, MHD equations are quite difficult to handle due to their high non-linearity and it has not been yet brought in a definitive verdict for the evolution laws of magnetic energy and correlation length (for recent numerical studies of freely decaying magnetohydrodynamic turbulence see, e.g., Ref. [2,3,4]).The turbulent MHD equations for incompressible fluids, in the case of non-expanding universe, are [5]:and ∇·v = ∇·B = 0. Here, v is the velocity of bulk fluid motion, B the magnetic field, J = ∇ × B the magnetic current, ν the kinematic viscosity, η the resistivity. The thermal pressure of the fluid, p, is not an independen...

The recent three-year WMAP data have confirmed the anomaly concerning the low quadrupole amplitude compared to the best-fit ΛCDM 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.

We consider the generation of gravitational waves by primordial helical inverse cascade magnetohydrodynamic (MHD) turbulence produced by bubble collisions at the electroweak phase transition. We extend the previous study [1] by considering both currently discussed models of MHD turbulence. For popular electroweak phase transition parameter values, the generated gravitational wave spectrum is only weakly dependent on the MHD turbulence model. Compared to the unmagnetized electroweak phase transition case, the spectrum of MHD-turbulence-generated gravitational waves peaks at lower frequency with larger amplitude and can be detected by the proposed Laser Interferometer Space Antenna.

Measurements of the Faraday rotation of the cosmic microwave background radiation (CMBR) polarization could provide evidence for the existence of primordial magnetic fields. The Faraday rotation could also allow the study of some properties of these fields. In this paper, we calculate the angular dependence of the Faraday rotation correlator for different assumptions about the spectral index and correlation length of the magnetic field. We show that the helical part of the magnetic field does not make any contribution to the correlator. We stress the importance of the angular resolution of the detector in the Faraday rotation measure, showing that it could severely reduce the effect, even for a relatively large magnetic field correlation length.

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