29,439 research outputs found
Speed of gravity and gravitomagnetism
A v_J/c correction to the Shapiro time delay seems verified by a 2002 Jovian
observation by VLBI. In this Essay, this correction is interpreted as an effect
of the aberration of light in an optically refractive medium which supplies an
analog of Jupiter's gravity field rather than as a measurement of the speed of
gravity, as it was first proposed by other authors. The variation of the index
of refraction is induced by the Lorentz invariance of the weak gravitational
field equations for Jupiter in a uniform translational slow motion with
velocity v_J=13.5 km/s. The correction on time delay and deflection is due not
to the Kerr (or Lense-Thirring) stationary gravitomagnetic field of Jupiter,
but to its Schwarzschild gravitostatic field when measured from the barycenter
of the solar system.Comment: 6 pags, final published version, Honorable Mention in the 2004 Essay
Competition of the Gravity Research Foundation, GR
Conductances in normal and normal-superconductor structures
We study theoretically electronic transport through a normal metal--
superconductor (NS) interface and show that more than one conductance may be
defined, depending on the pair of chemical potentials whose difference one
chooses to relate linearly to the current. We argue that the situation is
analogous to that found for purely normal transport, where different
conductance formulae can be invoked. We revisit the problem of the "right"
conductance formula in a simple language, and analyze its extension to the case
of mesoscopic superconductivity. The well-known result that the standard
conductance of a NS interface becomes 2 (in units of ) in the
transmissive limit, is viewed here in a different light. We show that it is not
directly related to the presence of Andreev reflection, but rather to a
particular choice of chemical potentials. This value of 2 is measurable because
only one single-contact resistance is involved in a typical experimental setup,
in contrast with the purely normal case, where two of them intervene. We
introduce an alternative NS conductance that diverges in the transmissive limit
due to the inability of Andreev reflection to generate a voltage drop. We
illustrate numerically how different choices of chemical potential can yield
widely differing I--V curves for a given NS interface.Comment: Minor changes have been introduced and several references have been
added, 12 pages, submitted to special issue of ``Superlattices and
Microstructures
Measuring the transition to homogeneity with photometric redshift surveys
We study the possibility of detecting the transition to homogeneity using
photometric redshift catalogs. Our method is based on measuring the fractality
of the projected galaxy distribution, using angular distances, and relies only
on observable quantites. It thus provides a way to test the Cosmological
Principle in a model-independent unbiased way. We have tested our method on
different synthetic inhomogeneous catalogs, and shown that it is capable of
discriminating some fractal models with relatively large fractal dimensions, in
spite of the loss of information due to the radial projection. We have also
studied the influence of the redshift bin width, photometric redshift errors,
bias, non-linear clustering, and surveyed area, on the angular homogeneity
index H2 ({\theta}) in a {\Lambda}CDM cosmology. The level to which an upcoming
galaxy survey will be able to constrain the transition to homogeneity will
depend mainly on the total surveyed area and the compactness of the surveyed
region. In particular, a Dark Energy Survey (DES)-like survey should be able to
easily discriminate certain fractal models with fractal dimensions as large as
D2 = 2.95. We believe that this method will have relevant applications for
upcoming large photometric redshift surveys, such as DES or the Large Synoptic
Survey Telescope (LSST).Comment: 14 pages, 14 figure
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