17,428 research outputs found
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
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
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
Broadband Spectrum Survey Measurements for Cognitive Radio Applications
It is well known that the existing spectrum licensing system results in a gross under-utilization of the frequency spectrum. Spectrum background measurements – spectrum surveys – provide useful data for spectrum regulation, planning or finding frequency niches for spectrum sharing. Dynamic spectrum sharing as a main goal of cognitive radio (CR) is the modern option on how to optimize usage of the frequency spectrum. A spectrum survey measurement system is introduced with results obtained from a variety of markedly different scenarios allowing us, unlike other studies, to focus on wideband and fast spectrum scans. The sensitivity of the receiver is no worse than -113 dBm in the whole band. The utilization of the frequency spectrum is analyzed to prove its under-utilization and to show spectrum sharing opportunities. This was shown to be true in the frequency band higher than 2.5 GHz. A comparison with other spectrum survey campaigns is provided
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