37,491 research outputs found
Measuring Space-Time Geometry over the Ages
Theorists are often told to express things in the "observational plane". One
can do this for space-time geometry, considering "visual" observations of
matter in our universe by a single observer over time, with no assumptions
about isometries, initial conditions, nor any particular relation between
matter and geometry, such as Einstein's equations. Using observables as
coordinates naturally leads to a parametrization of space-time geometry in
terms of other observables, which in turn prescribes an observational program
to measure the geometry. Under the assumption of vorticity-free matter flow we
describe this observational program, which includes measurements of
gravitational lensing, proper motion, and redshift drift. Only 15% of the
curvature information can be extracted without long time baseline observations,
and this increases to 35% with observations that will take decades. The rest
would likely require centuries of observations. The formalism developed is
exact, non-perturbative, and more general than the usual cosmological analysis.Comment: Originally written for the Gravity Research Foundation 2012 Awards
for Essays on Gravitation and received Honorable Mentio
X-ray Localization of the Globular Cluster G1 with XMM-Newton
We present an accurate X-ray position of the massive globular cluster G1 by
using XMM-Newton and the Hubble Space Telescope (HST). The X-ray emission of G1
has been detected recently with XMM-Newton. There are two possibilities for the
origin of the X-ray emission. It can be either due to accretion of the central
intermediate-mass black hole, or by ordinary low-mass X-ray binaries. The
precise location of the X-ray emission might distinguish between these two
scenarios. By refining the astrometry of the XMM-Newton and HST data, we
reduced the XMM-Newton error circle to 1.5". Despite the smaller error circle,
the precision is not sufficient to distinguish an intermediate-mass black hole
and luminous low-mass X-ray binaries. This result, however, suggests that
future Chandra observations may reveal the origin of the X-ray emission.Comment: 4 pages, 2 figures; accepted for publication in Ap
Non-invasive vibrational mode spectroscopy of ion Coulomb crystals through resonant collective coupling to an optical cavity field
We report on a novel non-invasive method to determine the normal mode
frequencies of ion Coulomb crystals in traps based on the resonance enhanced
collective coupling between the electronic states of the ions and an optical
cavity field at the single photon level. Excitations of the normal modes are
observed through a Doppler broadening of the resonance. An excellent agreement
with the predictions of a zero-temperature uniformly charged liquid plasma
model is found. The technique opens up for investigations of the heating and
damping of cold plasma modes, as well as the coupling between them.Comment: 4 pages, 4 figure
Bose-Einstein condensation in complex networks
The evolution of many complex systems, including the world wide web, business
and citation networks is encoded in the dynamic web describing the interactions
between the system's constituents. Despite their irreversible and
non-equilibrium nature these networks follow Bose statistics and can undergo
Bose-Einstein condensation. Addressing the dynamical properties of these
non-equilibrium systems within the framework of equilibrium quantum gases
predicts that the 'first-mover-advantage', 'fit-get-rich' and
'winner-takes-all' phenomena observed in competitive systems are
thermodynamically distinct phases of the underlying evolving networks
Statistical Dynamics of Religions and Adherents
Religiosity is one of the most important sociological aspects of populations.
All religions may evolve in their beliefs and adapt to the society
developments. A religion is a social variable, like a language or wealth, to be
studied like any other organizational parameter.
Several questions can be raised, as considered in this study: e.g. (i) from a
``macroscopic'' point of view : How many religions exist at a given time? (ii)
from a ``microscopic'' view point: How many adherents belong to one religion?
Does the number of adherents increase or not, and how? No need to say that if
quantitative answers and mathematical laws are found, agent based models can be
imagined to describe such non-equilibrium processes.
It is found that empirical laws can be deduced and related to preferential
attachment processes, like on evolving network; we propose two different
algorithmic models reproducing as well the data. Moreover, a population
growth-death equation is shown to be a plausible modeling of evolution dynamics
in a continuous time framework. Differences with language dynamic competition
is emphasized.Comment: submitted to EP
Estimating age of spotted and spinner dolphins (Stenella attenuata and Stenella longirostris) from teeth
This paper is an account of preparation and examination techniques and criteria used to estimate age in decalcified and stained tooth thin sections from spinner and spotted dolphins. A dentinal growth layer group (GLG),
composed of two thin light and two thicker dark-stained layers, is deposited annually. The GLG component layers are variably visible, but the "ideal" pattern and successive thinning of dentinal GLGs are used as a guide to determine GLG limits. Age-specific thicknesses of dentinal GLGs found in Hawaiian spinner dolphin teeth seem to be applicable to teeth of spotted dolphins and can be used as an aid in locating GLG boundaries. Cementa1 GLGs are composed of a dark-stained and alightly stained layer and usually are
deposited at a rate of one per year, but may be deposited every other year or two or three times per year. Two slightly different methods of counting dentinal GLGs are presented, along with guidelines for determining whether
dentinal or cementa1 GLG counts provide the best estimate of age for a specimen. (PDF contains 23 pages.
Search for variable gamma-ray emission from the Galactic plane in the Fermi data
High-energy gamma-ray emission from the Galactic plane above ~100 MeV is
composed of three main contributions: diffuse emission from cosmic ray
interactions in the interstellar medium, emission from extended sources, such
as supernova remnants and pulsar wind nebulae, and emission from isolated
compact source populations. The diffuse emission and emission from the extended
sources provide the dominant contribution to the flux almost everywhere in the
inner Galaxy, preventing the detection of isolated compact sources. In spite of
this difficulty, compact sources in the Galactic plane can be singled out based
on the variability properties of their gamma-ray emission. Our aim is to find
sources in the Fermi data that show long-term variability. We performed a
systematic study of the emission variability from the Galactic plane, by
constructing the variability maps. We find that emission from several
directions along the Galactic plane is significantly variable on a time scale
of months. These directions include, in addition to known variable Galactic
sources and background blazars, the Galactic ridge region at positive Galactic
longitudes and several regions containing young pulsars. We argue that
variability on the time scale of months may be common to pulsars, originating
from the inner parts of pulsar wind nebulae, similarly to what is observed in
the Crab pulsar.Comment: 4 pages, 4 figures, accepted to Astronomy & Astrophysic
Structure and shaping processes within the extended atmospheres of AGB stars
We present recent studies using the near-infrared instrument AMBER of the VLT
Interferometer (VLTI) to investigate the structure and shaping processes within
the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER
observations of the Mira variable S Ori have revealed wavelength-dependent
apparent angular sizes. These data were successfully compared to dynamic model
atmospheres, which predict wavelength-dependent radii because of geometrically
extended molecular layers. Most recently, AMBER closure phase measurements of
several AGB stars have also revealed wavelength-dependent deviations from 0/180
deg., indicating deviations from point symmetry. The variation of closure phase
with wavelength indicates a complex non-spherical stratification of the
extended atmosphere, and may reveal whether observed asymmetries are located
near the photosphere or in the outer molecular layers. Concurrent observations
of SiO masers located within the extended molecular layers provide us with
additional information on the morphology, conditions, and kinematics of this
shell. These observations promise to provide us with new important insights
into the shaping processes at work during the AGB phase. With improved imaging
capabilities at the VLTI, we expect to extend the successful story of imaging
studies of planetary nebulae to the photosphere and extended outer atmosphere
of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F.
Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for
Astrophysics, Manchester, UK, 201
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