1,111 research outputs found
Planck-scale modifications to Electrodynamics characterized by a space-like symmetry-breaking vector
In the study of Planck-scale ("quantum-gravity induced") violations of
Lorentz symmetry, an important role was played by the deformed-electrodynamics
model introduced by Myers and Pospelov. Its reliance on conventional effective
quantum field theory, and its description of symmetry-violation effects simply
in terms of a four-vector with nonzero component only in the time-direction,
rendered it an ideal target for experimentalists and a natural concept-testing
ground for many theorists. At this point however the experimental limits on the
single Myers-Pospelov parameter, after improving steadily over these past few
years, are "super-Planckian", {\it i.e.} they take the model out of actual
interest from a conventional quantum-gravity perspective. In light of this we
here argue that it may be appropriate to move on to the next level of
complexity, still with vectorial symmetry violation but adopting a generic
four-vector. We also offer a preliminary characterization of the phenomenology
of this more general framework, sufficient to expose a rather significant
increase in complexity with respect to the original Myers-Pospelov setup. Most
of these novel features are linked to the presence of spatial anisotropy, which
is particularly pronounced when the symmetry-breaking vector is space-like, and
they are such that they reduce the bound-setting power of certain types of
observations in astrophysics
Constraining Fundamental Physics with Future CMB Experiments
The Planck experiment will soon provide a very accurate measurement of Cosmic
Microwave Background anisotropies. This will let cosmologists determine most of
the cosmological parameters with unprecedented accuracy. Future experiments
will improve and complement the Planck data with better angular resolution and
better polarization sensitivity. This unexplored region of the CMB power
spectrum contains information on many parameters of interest, including
neutrino mass, the number of relativistic particles at recombination, the
primordial Helium abundance and the injection of additional ionizing photons by
dark matter self-annihilation. We review the imprint of each parameter on the
CMB and forecast the constraints achievable by future experiments by performing
a Monte Carlo analysis on synthetic realizations of simulated data. We find
that next generation satellite missions such as CMBPol could provide valuable
constraints with a precision close to that expected in current and near future
laboratory experiments. Finally, we discuss the implications of this
intersection between cosmology and fundamental physics.Comment: 11 pages, 14 figure
Delayed Recombination and Cosmic Parameters
Current cosmological constraints from Cosmic Microwave Background (CMB)
anisotropies are typically derived assuming a standard recombination scheme,
however additional resonance and ionizing radiation sources can delay
recombination, altering the cosmic ionization history and the cosmological
inferences drawn from CMB data. We show that for recent observations of CMB
anisotropy, from the Wilkinson Microwave Anisotropy Probe satellite mission
5-year survey (WMAP5) and from the ACBAR experiment, additional resonance
radiation is nearly degenerate with variations in the spectral index, n_s, and
has a marked effect on uncertainties in constraints on the Hubble constant, age
of the universe, curvature and the upper bound on the neutrino mass. When a
modified recombination scheme is considered, the redshift of recombination is
constrained to z_*=1078\pm11, with uncertainties in the measurement weaker by
one order of magnitude than those obtained under the assumption of standard
recombination while constraints on the shift parameter are shifted by 1-sigma
to R=1.734\pm0.028. Although delayed recombination limits the precision of
parameter estimation from the WMAP satellite, we demonstrate that this should
not be the case for future, smaller angular scales measurements, such as those
by the Planck satellite mission.Comment: 9 pages, 9 figure
Determining the Neutrino Mass Hierarchy with Cosmology
The combination of current large scale structure and cosmic microwave
background (CMB) anisotropies data can place strong constraints on the sum of
the neutrino masses. Here we show that future cosmic shear experiments, in
combination with CMB constraints, can provide the statistical accuracy required
to answer questions about differences in the mass of individual neutrino
species. Allowing for the possibility that masses are non-degenerate we combine
Fisher matrix forecasts for a weak lensing survey like Euclid with those for
the forthcoming Planck experiment. Under the assumption that neutrino mass
splitting is described by a normal hierarchy we find that the combination
Planck and Euclid will possibly reach enough sensitivity to put a constraint on
the mass of a single species. Using a Bayesian evidence calculation we find
that such future experiments could provide strong evidence for either a normal
or an inverted neutrino hierachy. Finally we show that if a particular neutrino
hierachy is assumed then this could bias cosmological parameter constraints,
for example the dark energy equation of state parameter, by > 1\sigma, and the
sum of masses by 2.3\sigma.Comment: 9 pages, 6 figures, 3 table
Laue Lens Development for Hard X-rays (>60 keV)
Results of reflectivity measurements of mosaic crystal samples of Cu (111)
are reported. These tests were performed in the context of a feasibility study
of a hard X-ray focusing telescope for space astronomy with energy passband
from 60 to 600 keV. The technique envisaged is that of using mosaic crystals in
transmission configuration that diffract X-rays for Bragg diffraction (Laue
lens). The Laue lens assumed has a spherical shape with focal length . It is
made of flat mosaic crystal tiles suitably positioned in the lens. The samples
were grown and worked for this project at the Institute Laue-Langevin (ILL) in
Grenoble (France), while the reflectivity tests were performed at the X-ray
facility of the Physics Department of the University of Ferrara.Comment: 6 pages, 12 figures, accepted for publication in IEEE Transactions on
Nuclear Scienc
Robot Programming by Demonstration: Trajectory Learning Enhanced by sEMG-Based User Hand Stiffness Estimation
Trajectory learning is one of the key components of robot Programming by Demonstration approaches, which in many cases, especially in industrial practice, aim at defining complex manipulation patterns. In order to enhance these methods, which are generally based on a physical interaction between the user and the robot, guided along the desired path, an additional input channel is considered in this article. The hand stiffness, that the operator continuously modulates during the demonstration, is estimated from the forearm surface electromyography and translated into a request for a higher or lower accuracy level. Then, a constrained optimization problem is built (and solved) in the framework of smoothing B-splines to obtain a minimum curvature trajectory approximating, in this manner, the taught path within the precision imposed by the user. Experimental tests in different applicative scenarios, involving both position and orientation, prove the benefits of the proposed approach in terms of the intuitiveness of the programming procedure for the human operator and characteristics of the final motion
Robot Programming by Demonstration: Trajectory Learning Enhanced by sEMG-Based User Hand Stiffness Estimation
Trajectory learning is one of the key components of robot Programming by Demonstration approaches, which in many cases, especially in industrial practice, aim at defining complex manipulation patterns. In order to enhance these methods, which are generally based on a physical interaction between the user and the robot, guided along the desired path, an additional input channel is considered in this article. The hand stiffness, that the operator continuously modulates during the demonstration, is estimated from the forearm surface electromyography and translated into a request for a higher or lower accuracy level. Then, a constrained optimization problem is built (and solved) in the framework of smoothing B-splines to obtain a minimum curvature trajectory approximating, in this manner, the taught path within the precision imposed by the user. Experimental tests in different applicative scenarios, involving both position and orientation, prove the benefits of the proposed approach in terms of the intuitiveness of the programming procedure for the human operator and characteristics of the final motion
Combined joint-cartesian mapping for simultaneous shape and precision teleoperation of anthropomorphic robotic hands
There are many applications involving robotic hands in which teleoperation-based approaches are preferred to autonomous solutions. The main reason is that cognitive skills of human operators are desirable in some task scenarios, in order to overcome limitations of robotic hands abilities in dealing with unstructured environments and/or unpredetermined requirements. In particular, in this work we focus on the use of anthropomorphic grasping devices and, specifically, on their teleoperation based on movements of the human operator's hand (the master hand.) Indeed, the mapping of human hand configurations to an anthropomorphic robotic hand (the slave device) is still an open problem, because of the presence of dissimilar kinematics between master and slave that produce shape and/or Cartesian errors - as addressed within our study. In this work, we propose a novel algorithm that combines joint and Cartesian mappings in order to enhance the preservation of both finger shapes and fingertip positions during the teleoperation of the robotic hand. In particular, a transition between the joint and Cartesian mappings is realized on the basis of the distance between the fingertip of the master hands' thumb and the opposite fingers, in which the mapping of the thumb fingertip is specifically addressed. The result of the testing of the algorithm with a ROS-based simulator of a commercially available robotic hand is reported, showing the effectiveness of the proposed mapping
The trispectrum of 21-cm background anisotropies as a probe of primordial non-Gaussianity
The 21-cm anisotropies from the neutral hydrogen distribution prior to the
era of reionization is a sensitive probe of primordial non-Gaussianity. Unlike
the case with cosmic microwave background, 21-cm anisotropies provide
multi-redshift information with frequency selection and is not damped at
arcminute angular scales. We discuss the angular trispectrum of the 21-cm
background anisotropies and discuss how the trispectrum signal generated by the
primordial non-Gaussianity can be measured with the three-to-one correlator and
the corresponding angular power spectrum. We also discuss the separation of
primordial non-Gaussian information in the trispectrum with that generated by
the subsequent non-linear gravitational evolution of the density field. While
with the angular bispectrum of 21-cm anisotropies one can limit the second
order corrections to the primordial fluctuations below f_NL< 1, using the
trispectrum information we suggest that the third order coupling term, f_2 or
g_NL, can be constrained to be arounde 10 with future 21-cm observations over
the redshift interval of 50 to 100.Comment: 12 pages, PRD submitte
Molecular signals from primordial clouds at high redshift
The possibility to detect cosmological signals from the post-recombination
Universe is one of the main aims of modern cosmology. In a previous paper we
emphasized the role that elastic resonant scattering through LiH molecules can
have in dumping primary CBR anisotropies and raising secondary signals. Here we
extend our analysis to all the evolutionary stages of a primordial cloud,
starting with the linear phase, through the turn-around and to the non linear
collapse. We have done calculations for proto-clouds in a CDM scenario and,
more generally, for a set of clouds with various masses and various turn-around
redshifts, in this case without referring to any particular structure formation
scenario. We found that the first phase of collapse, for
is the best one for simultaneous detection of
the first two LiH rotational lines. The observational frequency falls between
30 and 250 GHz and the line width is between
and . As far as we know this is the most favourable process to detect
primordial clouds before they start star formation processes.Comment: 26 pages, uuencoded compressed postscript, 7 figures included.
Accepted for publication in Ap.
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