Solar system tests of brane world models

The classical tests of general relativity (perihelion precession, deflection of light, and the radar echo delay) are considered for the Dadhich, Maartens, Papadopoulos and Rezania (DMPR) solution of the spherically symmetric static vacuum field equations in brane world models. For this solution the metric in the vacuum exterior to a brane world star is similar to the Reissner-Nordstrom form of classical general relativity, with the role of the charge played by the tidal effects arising from projections of the fifth dimension. The existing observational solar system data on the perihelion shift of Mercury, on the light bending around the Sun (obtained using long-baseline radio interferometry), and ranging to Mars using the Viking lander, constrain the numerical values of the bulk tidal parameter and of the brane tension.Comment: 14 pages, to appear in Classical and Quantum Gravity. V2, minor corrections and references adde

Minimum mass-radius ratio for charged gravitational objects

We rigorously prove that for compact charged general relativistic objects there is a lower bound for the mass-radius ratio. This result follows from the same Buchdahl type inequality for charged objects, which has been extensively used for the proof of the existence of an upper bound for the mass-radius ratio. The effect of the vacuum energy (a cosmological constant) on the minimum mass is also taken into account. Several bounds on the total charge, mass and the vacuum energy for compact charged objects are obtained from the study of the Ricci scalar invariants. The total energy (including the gravitational one) and the stability of the objects with minimum mass-radius ratio is also considered, leading to a representation of the mass and radius of the charged objects with minimum mass-radius ratio in terms of the charge and vacuum energy only.Comment: 19 pages, accepted by GRG, references corrected and adde

Reply to ``Comment on `Hole-burning experiments within glassy models with infinite range interactions' ''

This is a reply to the comments by Richter and Chamberlin, and Diezemann and Bohmer to our paper (Phys. Rev. Lett. 85, 3448 (2000)). As further evidence for the claims in this Letter, we here reproduce the nonlinear spectral hole-burning experimental protocol in an equilibrated fully connected spin-glass model and we exhibit frequency selectivity, together with a shift in the base of the spectral hole.Comment: 1 page, two figures, to appear in Phys. Rev. Let

Can dark matter be a Bose-Einstein condensate?

We consider the possibility that the dark matter, which is required to explain the dynamics of the neutral hydrogen clouds at large distances from the galactic center, could be in the form of a Bose-Einstein condensate. To study the condensate we use the non-relativistic Gross-Pitaevskii equation. By introducing the Madelung representation of the wave function, we formulate the dynamics of the system in terms of the continuity equation and of the hydrodynamic Euler equations. Hence dark matter can be described as a non-relativistic, Newtonian Bose-Einstein gravitational condensate gas, whose density and pressure are related by a barotropic equation of state. In the case of a condensate with quartic non-linearity, the equation of state is polytropic with index $n=1$. To test the validity of the model we fit the Newtonian tangential velocity equation of the model with a sample of rotation curves of low surface brightness and dwarf galaxies, respectively. We find a very good agreement between the theoretical rotation curves and the observational data for the low surface brightness galaxies. The deflection of photons passing through the dark matter halos is also analyzed, and the bending angle of light is computed. The bending angle obtained for the Bose-Einstein condensate is larger than that predicted by standard general relativistic and dark matter models. Therefore the study of the light deflection by galaxies and the gravitational lensing could discriminate between the Bose-Einstein condensate dark matter model and other dark matter models.Comment: 20 pages, 7 figures, accepted for publication in JCAP, references adde

Dynamic heterogeneities in the out-of-equilibrium dynamics of simple spherical spin models

The response of spherical two-spin interaction models, the spherical ferromagnet (s-FM) and the spherical Sherrington-Kirkpatrick (s-SK) model, is calculated for the protocol of the so-called nonresonant hole burning experiment (NHB) for temperatures below the respective critical temperatures. It is shown that it is possible to select dynamic features in the out-of-equilibrium dynamics of both models, one of the hallmarks of dynamic heterogeneities. The behavior of the s-SK model and the s-FM in three dimensions is very similar, showing dynamic heterogeneities in the long time behavior, i.e. in the aging regime. The appearence of dynamic heterogeneities in the s-SK model explicitly demonstrates that these are not necessarily related to {\it spatial} heterogeneities. For the s-FM it is shown that the nature of the dynamic heterogeneities changes as a function of dimensionality. With incresing dimension the frequency selectivity of the NHB diminishes and the dynamics in the mean-field limit of the s-FM model becomes homogeneous.Comment: 16 pages, 8 figure

Symptoms of gastroesophageal reflux disease in severely mentally retarded people: a systematic review

<p>Abstract</p> <p>Background</p> <p>Gastroesophageal reflux disease (GERD) occurs when stomach acid frequently backs up (or refluxes) into the gullet (or esophagus), and it has serious consequences for the quality of life. Usually this is felt as heartburn. Because severely mentally retarded people usually do not utter complaints of heartburn, it requires a high index of suspicion to discover possible GERD. Therefore it is relevant for care professionals such as nurses to have knowledge of those with a higher risk of GERD and of the possible manifestations of GERD.</p> <p>Methods</p> <p>Using a predefined search method, electronic databases were searched for studies relating the presence of symptoms to the presence of GERD. Relevant data were extracted and the methodological quality of the studies assessed. The results of the included studies were synthesized and conclusions about the level of evidence were drawn.</p> <p>Results</p> <p>Nineteen studies were found relating symptoms to the presence of GERD. Only four were of good methodological quality. The studies were very diverse concerning the studied population, the study method, and the kind of symptoms examined. This makes it difficult to synthesize the results of the studies. There is evidence that patients with cerebral palsy, patients using anticonvulsive drugs, and those with an IQ lower than 35 more frequently have GERD. There is also evidence that vomiting, rumination and hematemesis are associated with a higher risk of the presence of GERD, whereas there is no clear scientific evidence that particular behavior symptoms are indicative for GERD.</p> <p>Conclusion</p> <p>The possible manifestations of GERD are many and varied. A guideline will be made for care professionals to aid systematic observation of possible manifestations of GERD.</p

Hole-burning experiments within solvable glassy models

We reproduce the results of non-resonant spectral hole-burning experiments with fully-connected (equivalently infinite-dimensional) glassy models that are generalizations of the mode-coupling approach to nonequilibrium situations. We show that an ac-field modifies the integrated linear response and the correlation function in a way that depends on the amplitude and frequency of the pumping field. We study the effect of the waiting and recovery-times and the number of oscillations applied. This calculation will help descriminating which results can and which cannot be attributed to dynamic heterogeneities in real systems.Comment: 4 pages, 8 figures, RevTe

Solidity of Viscous Liquids

Recent NMR experiments on supercooled toluene and glycerol by Hinze and Bohmer show that small rotation angles dominate with only few large molecular rotations. These results are here interpreted by assuming that viscous liquids are solid-like on short length scales. A characteristic length, the "solidity length", separates solid-like behavior from liquid-like behavior.Comment: Plain RevTex file, no figure

Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate

Dielectric spectroscopy covering more than 18 decades of frequency has been performed on propylene carbonate in its liquid and supercooled-liquid state. Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric response was investigated up to frequencies well into the microscopic regime. We discuss the alpha-process whose characteristic timescale is observed over 14 decades of frequency and the excess wing showing up at frequencies some three decades above the peak frequency. Special attention is given to the high-frequency response of the dielectric loss in the crossover regime between alpha-peak and boson-peak. Similar to our previous results in other glass forming materials we find evidence for additional processes in the crossover regime. However, significant differences concerning the spectral form at high frequencies are found. We compare our results to the susceptibilities obtained from light scattering and to the predictions of various models of the glass transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.

The Einstein static universe with torsion and the sign problem of the cosmological constant

In the field equations of Einstein-Cartan theory with cosmological constant a static spherically symmetric perfect fluid with spin density satisfying the Weyssenhoff restriction is considered. This serves as a rough model of space filled with (fermionic) dark matter. From this the Einstein static universe with constant torsion is constructed, generalising the Einstein Cosmos to Einstein-Cartan theory. The interplay between torsion and the cosmological constant is discussed. A possible way out of the cosmological constant's sign problem is suggested.Comment: 8 pages, LaTeX; minor layout changes, typos corrected, one new equation, new reference [5], completed reference [13], two references adde