724 research outputs found
Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: II. Forecasts
In the first paper of this series, we showed that the CMB quadrupole at high
redshifts results in a small circular polarization of the emitted 21 cm
radiation. In this paper we forecast the sensitivity of future radio
experiments to measure the CMB quadrupole during the era of first cosmic light
(). The tomographic measurement of 21 cm circular polarization allows
us to construct a 3D remote quadrupole field. Measuring the -mode component
of this remote quadrupole field can be used to put bounds on the
tensor-to-scalar ratio . We make Fisher forecasts for a future Fast Fourier
Transform Telescope (FFTT), consisting of an array of dipole antennas in a
compact grid configuration, as a function of array size and observation time.
We find that a FFTT with a side length of 100 km can achieve after ten years of observation and with a sky coverage
. The forecasts are dependent on the evolution of the
Lyman- flux in the pre-reionization era, that remains observationally
unconstrained. Finally, we calculate the typical order of magnitudes for
circular polarization foregrounds and comment on their mitigation strategies.
We conclude that detection of primordial gravitational waves with 21 cm
observations is in principle possible, so long as the primordial magnetic field
amplitude is small, but would require a very futuristic experiment with
corresponding advances in calibration and foreground suppression techniques.Comment: 19 pages, matches PRD accepted versio
Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: I. Formalism
We propose a new method to measure the tensor-to-scalar ratio using the
circular polarization of the 21 cm radiation from the pre-reionization epoch.
Our method relies on the splitting of the hyperfine level of neutral
hydrogen due to the quadrupole moment of the CMB. We show that unlike the
Zeeman effect, where have opposite energy shifts, the CMB
quadrupole shifts together relative to . This splitting
leads to a small circular polarization of the emitted 21 cm radiation. In this
paper (Paper I in a series on this effect), we present calculations on the
microphysics behind this effect, accounting for all processes that affect the
hyperfine transition. We conclude with an analytic formula for the circular
polarization from the Dark Ages as a function of pre-reionization parameters
and the value of the remote quadrupole of the CMB. We also calculate the
splitting of the hyperfine level due to other anisotropic radiation
sources and show that they are not dominant. In a companion paper (Paper II) we
make forecasts for measuring the tensor-to-scalar ratio using future radio
arrays.Comment: 20 pages, significantly expanded description of the effect; matches
PRD accepted versio
Elastic anisotropy measures for heterogeneous materials
Heterogeneous materials exhibit anisotropy to varying extent that is
influenced by factors such as individual phase properties and microstructural
configuration. A review of the existing anisotropy measures proposed in the
context of single crystals reveal that they do not account for the material and
microstructural descriptors influencing the extent of anisotropy in
heterogeneous materials. To overcome this limitation, existing anisotropy
indices have been re-interpreted by considering the effective elastic
properties of heterogeneous materials obtained by appropriate effective
property estimates. Anisotropy quantification has been demonstrated considering
two phase composite materials highlighting the role of constituent volume
fractions, secondary phase shape and elastic contrast in influencing the extent
of anisotropy. The proposed approach can be generalized to consider other
physical fields, multiphysics and non-linearity
New probe of magnetic fields in the prereionization epoch. I. Formalism
We propose a method of measuring extremely weak magnetic fields in the
intergalactic medium prior to and during the epoch of cosmic reionization. The
method utilizes the Larmor precession of spin-polarized neutral hydrogen in the
triplet state of the hyperfine transition. This precession leads to a
systematic change in the brightness temperature fluctuations of the 21-cm line
from the high-redshift universe, and thus the statistics of these fluctuations
encode information about the magnetic field the atoms are immersed in. The
method is most suited to probing fields that are coherent on large scales; in
this paper, we consider a homogenous magnetic field over the scale of the 21-cm
fluctuations. Due to the long lifetime of the triplet state of the 21-cm
transition, this technique is naturally sensitive to extremely weak field
strengths, of order G at a reference redshift of (or
G if scaled to the present day). Therefore, this might open up the
possibility of probing primordial magnetic fields just prior to reionization.
If the magnetic fields are much stronger, it is still possible to use this
method to infer their direction, and place a lower limit on their strength. In
this paper (Paper I in a series on this effect), we perform detailed
calculations of the microphysics behind this effect, and take into account all
the processes that affect the hyperfine transition, including radiative decays,
collisions, and optical pumping by Lyman- photons. We conclude with an
analytic formula for the brightness temperature of linear-regime fluctuations
in the presence of a magnetic field, and discuss its limiting behavior for weak
and strong fields.Comment: 26 pages, 4 figures, updated to match published versio
Remodeling of Fibrous Extracellular Matrices by Contractile Cells: Predictions from Discrete Fiber Network Simulations
Contractile forces exerted on the surrounding extracellular matrix (ECM) lead
to the alignment and stretching of constituent fibers within the vicinity of
cells. As a consequence, the matrix reorganizes to form thick bundles of
aligned fibers that enable force transmission over distances larger than the
size of the cells. Contractile force-mediated remodeling of ECM fibers has
bearing on a number of physiologic and pathophysiologic phenomena. In this
work, we present a computational model to capture cell-mediated remodeling
within fibrous matrices using finite element based discrete fiber network
simulations. The model is shown to accurately capture collagen alignment,
heterogeneous deformations, and long-range force transmission observed
experimentally. The zone of mechanical influence surrounding a single
contractile cell and the interaction between two cells are predicted from the
strain-induced alignment of fibers. Through parametric studies, the effect of
cell contractility and cell shape anisotropy on matrix remodeling and force
transmission are quantified and summarized in a phase diagram. For highly
contractile and elongated cells, we find a sensing distance that is ten times
the cell size, in agreement with experimental observations.Comment: Accepted for publication in the Biophysical Journa
Detection of DNA and Poly-L-Lysine using CVD Graphene-channel FET Biosensors
A graphene channel field-effect biosensor is demonstrated for detecting the
binding of double-stranded DNA and poly-l-lysine. Sensors consist of CVD
graphene transferred using a clean, etchant-free transfer method. The presence
of DNA and poly-l-lysine are detected by the conductance change of the graphene
transistor. A readily measured shift in the Dirac Voltage (the voltage at which
the graphenes resistance peaks) is observed after the graphene channel is
exposed to solutions containing DNA or poly-l-lysine. The Dirac voltage shift
is attributed to the binding/unbinding of charged molecules on the graphene
surface. The polarity of the response changes to positive direction with
poly-l-lysine and negative direction with DNA. This response results in
detection limits of 8 pM for 48.5 kbp DNA and 11 pM for poly-l-lysine. The
biosensors are easy to fabricate, reusable and are promising as sensors of a
wide variety of charged biomolecule
Low-power photothermal self-oscillation of bimetallic nanowires
We investigate the nonlinear mechanics of a bimetallic, optically absorbing
SiN-Nb nanowire in the presence of incident laser light and a reflecting Si
mirror. Situated in a standing wave of optical intensity and subject to
photothermal forces, the nanowire undergoes self-induced oscillations at low
incident light thresholds of due to engineered strong
temperature-position (-) coupling. Along with inducing self-oscillation,
laser light causes large changes to the mechanical resonant frequency
and equilibrium position that cannot be neglected. We present
experimental results and a theoretical model for the motion under laser
illumination. In the model, we solve the governing nonlinear differential
equations by perturbative means to show that self-oscillation amplitude is set
by the competing effects of direct - coupling and parametric
excitation due to - coupling. We then study the linearized
equations of motion to show that the optimal thermal time constant for
photothermal feedback is rather than the widely reported
. Lastly, we demonstrate photothermal quality factor ()
enhancement of driven motion as a means to counteract air damping.
Understanding photothermal effects on micromechanical devices, as well as
nonlinear aspects of optics-based motion detection, can enable new device
applications as oscillators or other electronic elements with smaller device
footprints and less stringent ambient vacuum requirements.Comment: New references adde
Efficient Dynamic Group Signature Scheme with Verifier Local Revocation and Time-Bound Keys using Lattices
Revocation is an important feature of group signature schemes. Verifier Local Revocation (VLR) is a popular revocation mechanism which involves only verifiers in the revocation process. In VLR, a revocation list is maintained to store the information about revoked users. The verification cost of VLR based schemes islinearly proportional to the size of recvocation list. In many applications, the size of revocation list grows with time, which makes the verification process expensive. In this paper, we propose a lattice based dynamic group signature using VLR and time bound keys to reduce the size of revocation list to speed up the verification process. In the proposed scheme, an expiration date is fixed for signing key of each group member, and verifiers can find out (at constantcost) if a signature is generated using an expired key. Hence revocation information of members who are revoked before signing key expiry date (premature revocation) are kept in revocation list, and other members are part of natural revocation. This leads to a significant saving on the revocation check by assuming natural revocation accounts for large fraction of the total revocation. This scheme also takes care of non-forgeability of signing key expiry date
Correlation and prediction of dynamic human isolated joint strength from lean body mass
A relationship between a person's lean body mass and the amount of maximum torque that can be produced with each isolated joint of the upper extremity was investigated. The maximum dynamic isolated joint torque (upper extremity) on 14 subjects was collected using a dynamometer multi-joint testing unit. These data were reduced to a table of coefficients of second degree polynomials, computed using a least squares regression method. All the coefficients were then organized into look-up tables, a compact and convenient storage/retrieval mechanism for the data set. Data from each joint, direction and velocity, were normalized with respect to that joint's average and merged into files (one for each curve for a particular joint). Regression was performed on each one of these files to derive a table of normalized population curve coefficients for each joint axis, direction, and velocity. In addition, a regression table which included all upper extremity joints was built which related average torque to lean body mass for an individual. These two tables are the basis of the regression model which allows the prediction of dynamic isolated joint torques from an individual's lean body mass
Proactively Investigating “Limits of use” with Medical Device Manufacturers – An Example Case: Robotic Surgery
AbstractThis paper presents an exploration of strategies undertaken by a consortium of clinical teams and manufacturers to proactively prepare and support use of new categories of medical devices. Potential avenues of exploration include leveraging other domains, such as the US Space program, providing innovative methods of training and research and development in known areas of concern such as systems integration. By a commitment to creating a safer system, we are committing to providing safer care to patients. We believe incorporating the ‘lessons learned’ from the largest possible set of real-world scenarios, into the design and use of medical devices, will accelerate success
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