1,930 research outputs found
The GALILEO GALILEI small-satellite mission with FEEP thrusters (GG)
The Equivalence Principle, formulated by Einstein generalizing Galileo’s and Newton’s work, is a fundamental principle of modern physics. As such it should be tested as accurately as possible. Its most direct consequence, namely the Universality of Free Fall, can be tested in space, in a low Earth orbit, the crucial advantage being that the driving signal is about three orders of magnitude stronger
than on Earth. GALILEO GALILEI (GG) is a small space mission designed for such a high-accuracy test. At the time of print, GG has been selected by ASI (Agenzia Spaziale Italiana) as a candidate for the next small Italian mission. Ground tests of the proposed apparatus now indicate that an accuracy of 1 part in 1017 is within the reach of this small mission
Limitations to testing the equivalence principle with satellite laser ranging
Abstract We consider the possibility of testing the equivalence principle (EP) in the gravitational field of the Earth from the orbits of LAGEOS and LAGEOS II satellites, which are very accurately tracked from ground by laser ranging. The orbital elements that are affected by an EP violation and can be used to measure the corresponding dimensionless parameter ? are semimajor axis and argument of pericenter. We show that the best result is obtained from the semimajor axis, and it is limited-with all available ranging data to LAGEOS and LAGEOS II-to ? = 2 × 10-9, more than 3 orders of magnitude worse than experimental results provided by torsion balances. The experiment is limited because of the non uniformity of the gravitational field of the Earth and the error in the measurement of semimajor axis, precisely in the same way as they limit the measurement of the product GM of the Earth. A better use of the pericenter of LAGEOS II can be made if the data are analyzed searching for a new Yukawa-like interaction with a distance scale of one Earth radius. It is found that the pericenter of LAGEOS II is 3 orders of magnitude more sensitive to a composition dependent new interaction with this particular scale than it is to a composition dependent effect expressed by the ? parameter only. Nevertheless, the result is still a factor 500 worse than EP tests with torsion balances in the gravitational field of the Earth (i.e. at comparable distance), though a detailed data analysis has yet to be performed. While EP tests with satellite laser ranging are not competitive, laser ranging to the Moon has been able to provide a test of the EP almost 1 order of magnitude better than torsion balances. We show that this is due to the much greater distance of the test masses (the Earth and the Moon) from the primary body (the Sun) and the correspondingly smaller gradients of its gravity field. We therefore consider a similar new experiment involving the orbit of LAGEOS: testing LAGEOS and the Earth for an EP violation in the gravitational field of the Sun. We show that this test may be of interest, though it is a factor 300 less sensitive than in the case of the Moon due to the fact that LAGEOS is closer to the Earth than theMoon and consequently its orbit is less affected by the Sun. The limitations we have pointed out for laser ranging can be overcome by flying in low Earth orbit a spacecraft carrying concentric test masses of different composition with the capability, already demonstrated in ground laboratories, to accurately sense in situ any differential effects between them
Limitations to testing the equivalence principle with satellite laser ranging
Abstract We consider the possibility of testing the equivalence principle (EP) in the gravitational field of the Earth from the orbits of LAGEOS and LAGEOS II satellites, which are very accurately tracked from ground by laser ranging. The orbital elements that are affected by an EP violation and can be used to measure the corresponding dimensionless parameter ? are semimajor axis and argument of pericenter. We show that the best result is obtained from the semimajor axis, and it is limited-with all available ranging data to LAGEOS and LAGEOS II-to ? = 2 × 10-9, more than 3 orders of magnitude worse than experimental results provided by torsion balances. The experiment is limited because of the non uniformity of the gravitational field of the Earth and the error in the measurement of semimajor axis, precisely in the same way as they limit the measurement of the product GM of the Earth. A better use of the pericenter of LAGEOS II can be made if the data are analyzed searching for a new Yukawa-like interaction with a distance scale of one Earth radius. It is found that the pericenter of LAGEOS II is 3 orders of magnitude more sensitive to a composition dependent new interaction with this particular scale than it is to a composition dependent effect expressed by the ? parameter only. Nevertheless, the result is still a factor 500 worse than EP tests with torsion balances in the gravitational field of the Earth (i.e. at comparable distance), though a detailed data analysis has yet to be performed. While EP tests with satellite laser ranging are not competitive, laser ranging to the Moon has been able to provide a test of the EP almost 1 order of magnitude better than torsion balances. We show that this is due to the much greater distance of the test masses (the Earth and the Moon) from the primary body (the Sun) and the correspondingly smaller gradients of its gravity field. We therefore consider a similar new experiment involving the orbit of LAGEOS: testing LAGEOS and the Earth for an EP violation in the gravitational field of the Sun. We show that this test may be of interest, though it is a factor 300 less sensitive than in the case of the Moon due to the fact that LAGEOS is closer to the Earth than theMoon and consequently its orbit is less affected by the Sun. The limitations we have pointed out for laser ranging can be overcome by flying in low Earth orbit a spacecraft carrying concentric test masses of different composition with the capability, already demonstrated in ground laboratories, to accurately sense in situ any differential effects between them
Force-free twisted magnetospheres of neutron stars
The X-ray spectra observed in the persistent emission of magnetars are
evidence for the existence of a magnetosphere. The high-energy part of the
spectra is explained by resonant cyclotron upscattering of soft thermal photons
in a twisted magnetosphere, which has motivated an increasing number of efforts
to improve and generalize existing magnetosphere models. We want to build more
general configurations of twisted, force-free magnetospheres as a first step to
understanding the role played by the magnetic field geometry in the observed
spectra. First we reviewed and extended previous analytical works to assess the
viability and limitations of semi-analytical approaches. Second, we built a
numerical code able to relax an initial configuration of a nonrotating
magnetosphere to a force-free geometry, provided any arbitrary form of the
magnetic field at the star surface. The numerical code is based on a
finite-difference time-domain, divergence-free, and conservative scheme, based
of the magneto-frictional method used in other scenarios. We obtain new
numerical configurations of twisted magnetospheres, with distributions of twist
and currents that differ from previous analytical solutions. The range of
global twist of the new family of solutions is similar to the existing
semi-analytical models (up to some radians), but the achieved geometry may be
quite different. The geometry of twisted, force-free magnetospheres shows a
wider variety of possibilities than previously considered. This has
implications for the observed spectra and opens the possibility of implementing
alternative models in simulations of radiative transfer aiming at providing
spectra to be compared with observations.Comment: 16 pages, 17 figures, A&A accepte
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Lipid-induced hepatocyte-derived extracellular vesicles regulate hepatic stellate cell via microRNAs targeting PPAR-γ.
Background&aimsHepatic stellate cells (HSCs) play a key role in liver fibrosis in various chronic liver disorders including nonalcoholic fatty liver disease (NAFLD). The development of liver fibrosis requires a phenotypic switch from quiescent to activated HSCs. The triggers for HSCs activation in NAFLD remain poorly understood. We investigated the role and molecular mechanism of extracellular vesicles (EVs) released by hepatocytes during lipotoxicity in modulation of HSC phenotype.MethodsEVs were isolated from fat-laden hepatocytes by differential centrifugation and incubated with HSCs. EV internalization and HSCs activation, migration and proliferation were assessed. Loss- and gain-of-functions studies were performed to explore the potential role of PPAR-γ-targeting miRNAs carried by EVs into HSC.ResultsHepatocyte-derived EVs released during lipotoxicity are efficiently internalized by HSCs resulting in their activation, as shown by marked up-regulation of pro-fibrogenic genes (Collagen-I, α-SMA and TIMP-2), proliferation, chemotaxis and wound healing responses. These changes were associated with miRNAs shuttled by EVs and suppression of PPAR-γ expression in HSC. Hepatocyte-derived EVs miRNA content included various miRNAs that are known inhibitors of PPAR-γ expression with miR-128-3p being the most effectively transferred. Furthermore loss- and gain-of-function studies identified miR-128-3p as a central modulator of the effects of EVs on PPAR-γ inhibition and HSC activation.ConclusionOur findings demonstrate a link between fat-laden hepatocyte-derived EVs and liver fibrosis and have potential implications for the development of novel anti-fibrotic targets for NAFLD and other fibrotic diseases
Explicit formulation for the Rayleigh wave field induced by surface stresses in an orthorhombic half-plane
We develop an explicit asymptotic model for the Rayleigh wave field arising in case of stresses prescribed on the surface of an orthorhombic elastic half-plane. The model consists of an elliptic equation governing the behaviour within the half-plane, with boundary values given on the half-plane surface by a wave equation. Consequently, propagation along the surface is entirely accounted for by the hyperbolic equation, which, besides, may be immediately recast in terms of the associated surface displacement. The model readily solves otherwise involved dynamic problems for prescribed surface stresses, and its effectiveness is demonstrated for the classical Lamb's problem, as well as for the steady-state moving load problem. The latter example shows that the proposed model is really obtained by perturbation around the steady-state solution for a load moving at the Rayleigh speed
Acute and cumulative effects of rTMS on behavioural and EMG parameters in Focal Hand Dystonia
Previous studies suggest that low-frequency repetitive Transcranial Magnetic Stimulation (rTMS) over contralateral premotor cortex (PMC) might ameliorate Focal Hand Dystonia (FHD) symptoms. In the present study behavioral and muscle activity outcomes were explored in a patient with FHD following a single and multiple sessions of rTMS. The patient's behavior was assessed on handwriting tasks, while surface EMG signals were recorded. In Experiment 1 evaluations were performed before and after one session of active and sham 1Hz rTMS over contralateral PMC. In Experiment 2, evaluations were performed before and after six sessions of the same treatment. In Experiment 1 active rTMS improved the patient's performance, although the EMG amplitude did not change. In Experiment 2, the patient showed an improvement of performance along with a decrease of 20% in the EMG amplitude. These results demonstrated that a single session of rTMS ameliorated the patient's performance, while multiple sessions were necessary to reduce muscles activity
Dynamical response of the Galileo Galilei on the ground rotor to test the equivalence principle: Theory, simulation, and experiment. II. The rejection of common mode forces
"Galileo Galilei on the ground" (GGG) is a fast rotating differential accelerometer designed to test the equivalence principle (EP). Its sensitivity to differential effects, such as the effect of an EP violation, depends crucially on the capability of the accelerometer to reject all effects acting in common mode. By applying the theoretical and simulation methods reported in Part I of this work, and tested therein against experimental data, we predict the occurrence of an enhanced common mode rejection of the GGG accelerometer. We demonstrate that the best rejection of common mode disturbances can be tuned in a controlled way by varying the spin frequency of the GGG rotor. (c) 2006 American Institute of Physics
Dynamical response of the "GGG" rotor to test the Equivalence Principle: theory, simulation and experiment. Part I: the normal modes
Recent theoretical work suggests that violation of the Equivalence Principle
might be revealed in a measurement of the fractional differential acceleration
between two test bodies -of different composition, falling in the
gravitational field of a source mass- if the measurement is made to the level
of or better. This being within the reach of ground based
experiments, gives them a new impetus. However, while slowly rotating torsion
balances in ground laboratories are close to reaching this level, only an
experiment performed in low orbit around the Earth is likely to provide a much
better accuracy.
We report on the progress made with the "Galileo Galilei on the Ground" (GGG)
experiment, which aims to compete with torsion balances using an instrument
design also capable of being converted into a much higher sensitivity space
test.
In the present and following paper (Part I and Part II), we demonstrate that
the dynamical response of the GGG differential accelerometer set into
supercritical rotation -in particular its normal modes (Part I) and rejection
of common mode effects (Part II)- can be predicted by means of a simple but
effective model that embodies all the relevant physics. Analytical solutions
are obtained under special limits, which provide the theoretical understanding.
A simulation environment is set up, obtaining quantitative agreement with the
available experimental data on the frequencies of the normal modes, and on the
whirling behavior. This is a needed and reliable tool for controlling and
separating perturbative effects from the expected signal, as well as for
planning the optimization of the apparatus.Comment: Accepted for publication by "Review of Scientific Instruments" on Jan
16, 2006. 16 2-column pages, 9 figure
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