17,490 research outputs found
Spacetimes with vector distortion: Inflation from generalised Weyl geometry
Spacetime with general linear vector distortion is introduced. Thus, the
torsion and the nonmetricity of the affine connection are assumed to be
proportional to a vector field (and not its derivatives). The resulting
two-parameter family of non-Riemannian geometries generalises the conformal
Weyl geometry and some other interesting special cases. Taking into account the
leading order quadratic curvature correction to the Einstein-Hilbert action
results uniquely in the one-parameter extension of the Starobinsky inflation
known as the alpha-attractor. The most general quadratic curvature action
introduces, in addition to the canonical vector kinetic term, novel ghost-free
vector-tensor interactions.Comment: 5 pages, 1 table, 0 figures. V2: Added the general quadratic theor
An AER-Based Actuator Interface for Controlling an Anthropomorphic Robotic Hand
Bio-Inspired and Neuro-Inspired systems or circuits are a
relatively novel approaches to solve real problems by mimicking the biology
in its efficient solutions. Robotic also tries to mimic the biology and
more particularly the human body structure and efficiency of the muscles,
bones, articulations, etc. Address-Event-Representation (AER) is
a communication protocol for transferring asynchronous events between
VLSI chips, originally developed for neuro-inspired processing systems
(for example, image processing). Such systems may consist of a complicated
hierarchical structure with many chips that transmit data among
them in real time, while performing some processing (for example, convolutions).
The information transmitted is a sequence of spikes coded using
high speed digital buses. These multi-layer and multi-chip AER systems
perform actually not only image processing, but also audio processing,
filtering, learning, locomotion, etc. This paper present an AER interface
for controlling an anthropomorphic robotic hand with a neuro-inspired
system.Unión Europea IST-2001-34124 (CAVIAR)Ministerio de Ciencia y Tecnología TIC-2003-08164-C03-0
Cosmic magnetic fields and dark energy in extended electromagnetism
We discuss an extended version of electromagnetism in which the usual gauge
fixing term is promoted into a physical contribution that introduces a new
scalar state in the theory. This new state can be generated from vacuum quantum
fluctuations during an inflationary era and, on super-Hubble scales, gives rise
to an effective cosmological constant. The value of such a cosmological
constant coincides with the one inferred from observations as long as inflation
took place at the electroweak scale. On the other hand, the new state also
generates an effective electric charge density on sub-Hubble scales that
produces both vorticity and magnetic fields with coherent lengths as large as
the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2010, Granada, Spain, 6-10 September 201
The canonical frame of purified gravity
In the recently introduced gauge theory of translations, dubbed Coincident
General Relativity, gravity is described with neither torsion nor curvature in
the spacetime affine geometry. The action of the theory enjoys an enhanced
symmetry and avoids the second derivatives that appear in the conventional
Einstein-Hilbert action. While it implies the equivalent classical dynamics,
the improved action principle can make a difference in considerations of
energetics, thermodynamics, and quantum theory. This essay reports on possible
progress in those three aspects of gravity theory. In the so-called purified
gravity, 1) energy-momentum is described locally by a conserved, symmetric
tensor, 2) the Euclidean path integral is convergent without the addition of
boundary or regulating terms and 3) it is possible to identify a canonical
frame for quantisation.Comment: 7 pages, no figure
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
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