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.
118359419
Last scattering, relic gravitons, and the circular polarization of the CMB
The tensor contribution to the V-mode polarization induced by a magnetized plasma at last scattering vanishes exactly when the magnetic field is parallel to the direction of propagation of the gravitational wave. For propagation perpendicular to the magnetic field orientation the effect depends on the ratio between the Larmor frequency of the electrons and the observational frequency. If the source term of the V-mode equation is averaged over the directions of the magnetic field, the circular polarization evolves independently from all the other Stokes parameters.
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.
15441155
Gravitational wave constraints on post-inflationary phases stiffer than radiation
We point out that the existence of post-inflationary phases stiffer than radiation leads to the production of stochastic gravitational waves (GW) backgrounds whose logarithmic energy spectra (in critical units) are typically "blue" at high frequencies. The maximal spectral slope (for present frequencies larger than $10^{-16}$ Hz) is of order one and it is related to the maximal sound velocity of the stiff plasma governing the evolution of the geometry. The duration of the stiff phase is crucially determined by the backreaction of the GW leaving the horizon during the de Sitter phase and re-entering during the stiff phase. Therefore, the maximal (inflationary) curvature scale has to be fine-tuned to a value smaller than the limits set by the large scale measurements ($H_{dS}\laq 10^{-6}~M_{P}$) in order to a have a sufficiently long stiff phase reaching an energy scale of the order of the $1~{\rm TeV}$ and even lower if we want the stiff phase to touch the hadronic era (corresponding to $T_{had}\sim 140~{\rm MeV}$). By looking more positively at our exercise we see that, if an inflationary phase is followed by a stiff phase, there exist an appealing possibility of "graviton reheating" whose effective temperature can be generally quite low.
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.