Late time anisotropy as an imprint of cosmological backreaction

Backreaction effects of the large scale structure on the background dynamics have been claimed to lead to a renormalization of the background dynamics that may account for the late time acceleration of the cosmic expansion. This article emphasizes that generically the averaged flow is locally anisotropic, a property that can be related to observation. Focusing on perturbation theory, the spatially averaged shear, that characterizes the anisotropy of the flow, is computed. It is shown that this shear arising from backreaction differs from a homogeneous shear: its time evolution is different and its amplitude is completely determined by the cosmological parameters and the matter power spectrum. It ranges within (2-37)% at a redshift of order 0.5 so that the isotropy of the Hubble flow may allow to constrain the backreaction approach to dark energy.Comment: 14 pages, 7 figures. Typos corrected. Article published in Phys. Rev. D 86, 063528 (2012

Comment on "Origin of cosmic magnetic fields"

We argue that the result presented in "Origin of cosmic magnetic fields" by L. Campanelli [arXiv:1304.6534] is unphysical.Comment: 1 page, 1 figure. Typos fixed, minor corrections, a comment added, version published in PR

An exact Jacobi map in the geodesic light-cone gauge

The remarkable properties of the recently proposed geodesic light-cone (GLC) gauge allow to explicitly solve the geodetic-deviation equation, and thus to derive an exact expression for the Jacobi map J^A_B(s,o) connecting a generic source s to a geodesic observer o in a generic space time. In this gauge J^A_B factorizes into the product of a local quantity at s times one at o, implying similarly factorized expressions for the area and luminosity distance. In any other coordinate system J^A_B is simply given by expressing the GLC quantities in terms of the corresponding ones in the new coordinates. This is explicitly done, at first and second order, respectively, for the synchronous and Poisson gauge-fixing of a perturbed, spatially-flat cosmological background, and the consistency of the two outcomes is checked. Our results slightly amend previous calculations of the luminosity-redshift relation and suggest a possible non-perturbative way for computing the effects of inhomogeneities on observations based on light-like signals.Comment: 26 pages, no figures. Inconsequential modification of an equation, comments and references added. Version accepted for publication in JCA

A new approach to the propagation of light-like signals in perturbed cosmological backgrounds

We present a new method to compute the deflection of light rays in a perturbed FLRW geometry. We exploit the properties of the Geodesic Light Cone (GLC) gauge where null rays propagate at constant angular coordinates irrespectively of the given (inhomogeneous and/or anisotropic) geometry. The gravitational deflection of null geodesics can then be obtained, in any other gauge, simply by expressing the angular coordinates of the given gauge in terms of the GLC angular coordinates. We apply this method to the standard Poisson gauge, including scalar perturbations, and give the full result for the deflection effect in terms of the direction of observation and observed redshift up to second order, and up to third order for the leading lensing terms. We also compare our results with those presently available in the literature and, in particular, we provide a new non trivial check of a previous result on the luminosity-redshft relation up to second order in cosmological perturbation theory.Comment: 37 pages, no figures. Typos corrected, comments and references added. Version accepted for publication in JCA

Observation angles, Fermi coordinates, and the Geodesic-Light-Cone gauge

We show that the angular directions locally measured by a static geodesic observer in a generic cosmological background and expressed in the system of Fermi Normal Coordinates always coincide with those expressed in the Geodesic-Light-Cone (GLC) gauge, up to a local transformation which exploits the residual gauge freedom of the GLC coordinates. This is not the case for other gauges - like, for instance, the synchronous and longitudinal gauge - commonly used in the context of observational cosmology. We also make an explicit proposal for the GLC gauge-fixing condition that ensures a full identification of its angles with the observational ones.Comment: 14 pages, version accepted for publication on JCA

The ECB and the cost of independence: uncovering a new ‘doom-loop’ in Europe’s Economic and Monetary Union

The European Central Bank has frequently used unconventional monetary policy approaches, such as large scale bond purchases, since the financial crisis. Armando Marozzi presents evidence of a new ‘doom-loop’ in Europe’s Economic and Monetary Union arising from these measures. He shows that once the ECB adopts unconventional monetary policy approaches, it tends to take stances that promote conservative fiscal policies among member states. This fiscal conservatism in turn produces lower GDP and inflation, increasing the need for further large scale bond purchases