31,592 research outputs found
Gravitational lensing and modified Newtonian dynamics
Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.Comment: PASA (OzLens edition), in press; 5 pages, 1 figur
Gravitational lensing and modified Newtonian dynamics
Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.Comment: PASA (OzLens edition), in press; 5 pages, 1 figur
Transfer-matrix renormalization group study of the spin ladders with cyclic four-spin interactions
The temperature dependence of the specific heat and spin susceptibility of
the spin ladders with cyclic four-spin interactions in the rung-singlet phase
is explored by making use of the transfer-matrix renormalization group method.
The values of spin gap are extracted from the specific heat and susceptibility,
respectively. It is found that for different relative strength between
interchain and intrachain interactions, the spin gap is approximately linear
with the cyclic four-spin interaction in the region far away from the critical
point. Furthermore, we show that the dispersion for the one-triplet magnon
branch can be obtained by numerically fitting on the partition function.Comment: 7 pages, 7 figures, 1 tabl
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Impedance Analysis of Bunch Length Measurements at the ATF Damping Ring
We present energy spread and bunch length measurements at the Accelerator
Test Facility (ATF) at KEK, as functions of current, for different ring rf
voltages, and with the beam both on and off the coupling resonance. We fit the
on-coupling bunch shapes to those of an impedance model consisting of a
resistor and an inductor connected in series. We find that the fits are
reasonably good, but that the resulting impedance is unexpectedly large.Comment: 9 pages, 5 figures, presented at 10th International Symposium on
Applied Electromagnetics and Mechanics (ISEM2001
- ā¦