Pseudoscalar N-flation and axial coupling revisited

We revisit the dynamics of the axial coupling between many N-flatons and an Abelian gauge field, with special attention to its statistically anisotropic signal. The anisotropic power spectrum of curvature perturbations associated to the large wavelength modes of the gauge vector field is generally undetectable, since the anisotropy is confined to small scales. If the gauge field is the electromagnetic field, provided that the number of fields participating in the exponential expansion is large, it could be possible to generate sizable large scale magnetic fields. However, its spectrum is blue, and appreciable power on large scales implies an overly strong field on smaller scales, incompatibly with observations. Furthermore, the anisotropy is also markedly enhanced, and might be at odds with the isotropic observed sky. These aspects further demand that the scale of inflation is kept to a minimum.Comment: 14 pages - v2 with minor changes in the conclusions, v3 to match published versio

The anisotropy of a three- and a one-form

We calculate the anisotropic signal associated with the coupling of a three-form with an Abelian vector gauge field. In the simplest examples of three-form inflation the amplification of the vector fluctuations is exponential; this makes it almost certain that a large anisotropy will develop, severely constraining the viability of the coupling

Three-magnetic fields

A completely new mechanism to generate the observed amount of large-scale cosmological magnetic fields is introduced in the context of three-form inflation. The amplification of the fields occurs via fourth order dynamics of the vector perturbations and avoids the backreaction problem that plagues most previously introduced mechanisms.Comment: 4 pages, 2 figures -- v2 as published (title changed in the published version to "Cosmic magnetization in three-form inflation"

Doubly-boosted vector cosmologies from disformal metrics

A systematic dynamical system approach is applied to study the cosmology of anisotropic Bianchi I universes in which a vector field is assumed to operate on a disformal frame. This study yields a number of new fixed points, among which anisotropic scaling solutions. Within the simplifying assumption of (nearly) constant-slope potentials these are either not stable attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, some solutions do have an appeal for cosmological applications in that isotropy is retained due to rapid oscillations of the vector field.Comment: 11 pages, 4 figures, prepared during the NORDITA Extended Theories of Gravity program. arXiv admin note: substantial text overlap with arXiv:1407.344

New limits on extragalactic magnetic fields from rotation measures

We take advantage of the wealth of rotation measures data contained in the NRAO VLA Sky Survey catalogue to derive new, statistically robust, upper limits on the strength of extragalactic magnetic fields. We simulate the extragalactic magnetic field contribution to the rotation measures for a given field strength and correlation length, by assuming that the electron density follows the distribution of Lyman-$\alpha$ clouds. Based on the observation that rotation measures from distant radio sources do not exhibit any trend with redshift, while the extragalactic contribution instead grows with distance, we constrain fields with Jeans' length coherence length to be below 1.7~nG at the $2\sigma$ level, and fields coherent across the entire observable Universe below 0.65~nG. These limits do not depend on the particular origin of these cosmological fields.Comment: 5 pages, 3 figures -- v2 to match PRL versio