63,018 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
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
The Two-Nucleon 1S0 Amplitude Zero in Chiral Effective Field Theory
We present a new rearrangement of short-range interactions in the
nucleon-nucleon channel within Chiral Effective Field Theory. This is intended
to reproduce the amplitude zero (scattering momentum 340 MeV) at
leading order, and it includes subleading corrections perturbatively in a way
that is consistent with renormalization-group invariance. Systematic
improvement is shown at next-to-leading order, and we obtain results that fit
empirical phase shifts remarkably well all the way up to the pion-production
threshold. An approach in which pions have been integrated out is included,
which allows us to derive analytic results that also fit phenomenology
surprisingly well.Comment: 34 pages, 7 figure
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