14801166
Inhomogeneous dusty universes and their deceleration
Abstract Exact results stemming directly from Einstein equations imply that inhomogeneous universes endowed with vanishing pressure density can only decelerate, unless the energy density of the universe becomes negative. Recent proposals seem to argue that inhomogeneous (but isotropic) space–times, filled only with incoherent matter, may turn into accelerated universes for sufficiently late times. To scrutinize these scenarios, fully inhomogeneous Einstein equations are discussed in the synchronous system. In a dust-dominated universe, the inhomogeneous generalization of the deceleration parameter is always positive semi-definite implying that no acceleration takes place.
14491801
Magnetogenesis and the dynamics of internal dimensions
The dynamical evolution of internal spacelike dimensions breaks the invariance of Maxwell's equations under Weyl rescaling of the (conformally flat) four-dimensional metric. Depending upon the number and upon the dynamics of internal dimensions large scale magnetic fields can be created. The requirements coming from magnetogenesis together with the other cosmological constraints are examined under the assumption that the internal dimensions either grow or shrink (in conformal time) prior to a radiation-dominated epoch. If the internal dimensions are growing, the magnitude of the generated magnetic fields can seed the galactic dynamo mechanism.
18910338
Hedgehogs in higher dimensional gravity with curvature self-interactions
Static solutions of the higher dimensional Einstein-Hilbert gravity supplemented by quadratic curvature self-interactions are discussed in the presence of hedgehog configurations along the transverse dimensions. The quadratic part of the action is parametrized in terms of the (ghost-free) Euler-Gauss-Bonnet curvature invariant. Spherically symmetric profiles of the transverse metric admit exponentially decaying warp factors both for positive and negative bulk cosmological constants.
15705016
Birefringence, CMB polarization, and magnetized B-mode
Even in the absence of a sizable tensor contribution, a B-mode polarization can be generated because of the competition between a pseudoscalar background and predecoupling magnetic fields. By investigating the dispersion relations of a magnetoactive plasma supplemented by a pseudoscalar interaction, the total B-mode polarization is shown to depend not only upon the plasma and Larmor frequencies but also on the pseudoscalar rotation rate. If the (angular) frequency channels of a given experiment are larger than the pseudoscalar rotation rate, the only possible source of (frequency-dependent) B-mode autocorrelations must be attributed to Faraday rotation. In the opposite case the pseudoscalar contribution dominates and the total rate becomes, in practice, frequency independent. The B-mode cross correlations can be used, under certain conditions, to break the degeneracy by disentangling the two birefringent contributions.
14486067
Primordial magnetic fields from string cosmology.
Sufficiently large seeds for generating the observed (inter)galactic magnetic fields emerge naturally in string cosmology from the amplification of electromagnetic vacuum fluctuations due to a dynamical dilaton background. The success of the mechanism depends crucially on two features of the so-called pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small coupling, and a sufficiently long intermediate stringy era preceding the standard radiation-dominated evolution.
118385267
Magnetic field contribution to the last electron?photon scattering
When the cosmic microwave photons scatter electrons just prior to the decoupling of matter and radiation, magnetic fields do contribute to the Stokes matrix as well as to the scalar, vector and tensor components of the transport equations for the brightness perturbations. The magnetized electron?photon scattering is hereby discussed in general terms by including, for the first time, the contribution of magnetic fields with arbitrary direction and in the presence of the scalar, vector and tensor modes of the geometry. The propagation of relic vectors and relic gravitons is discussed for a varying magnetic field orientation and for different photon directions. The source terms of the transport equations in the presence of the relativistic fluctuations of the geometry are also explicitly averaged over the magnetic field orientations and the problem of a consistent account of the small-scale and large-scale magnetic field is briefly outlined.
121279814
Fast flash-tube hodoscope chambers
Abstract A fast type of flash tube has been developed through appropriate choices of the glass, gas filling mixture and externally applied electric fields. Sensitive time in the microsecond region and recovery times of some 10 msec have been obtained, with no relevant long-lasting effects due to charge deposite on the tube walls, as those observed for ordinary flash tubes. Operation of fast hodoscope chambers, in experiments with particle accelerators, presents some advantages over other types of electrically pulsed chambers commonly in use, which are briefly discussed.