Cosmic Acceleration Driven by Mirage Inhomogeneities

A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X S_5 bulk is introduced. Although there is no special points in the bulk, the brane Universe has a center and is isotropic around it. The model has an accelerating expansion and its effective cosmological constant is inversely proportional to the distance from the center, giving a possible geometrical origin for the smallness of a present-day cosmological constant. Besides, if our model is considered as an alternative of early time acceleration, it is shown that the early stage accelerating phase ends in a dust dominated FRW homogeneous Universe. Mirage-driven acceleration thus provides a dark matter component for the brane Universe final state. We finally show that the model fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections, references added, version accepted in Class. Quant. Gra

Refraction of swell by surface currents

Using recordings of swell from pitch-and-roll buoys, we have reproduced the classic observations of long-range surface wave propagation originally made by Munk et al. (1963) using a triangular array of bottom pressure measurements. In the modern data, the direction of the incoming swell fluctuates by about $\pm 10^\circ$ on a time scale of one hour. But if the incoming direction is averaged over the duration of an event then, in contrast with the observations by Munk et al. (1963), the sources inferred by great-circle backtracking are most often in good agreement with the location of large storms on weather maps of the Southern Ocean. However there are a few puzzling failures of great-circle backtracking e.g., in one case, the direct great-circle route is blocked by the Tuamoto Islands and the inferred source falls on New Zealand. Mirages like this occur more frequently in the bottom-pressure observations of Munk et al. (1963), where several inferred sources fell on the Antarctic continent. Using spherical ray tracing we investigate the hypothesis that the refraction of waves by surface currents produces the mirages. With reconstructions of surface currents inferred from satellite altimetry, we show that mesoscale vorticity significantly deflects swell away from great-circle propagation so that the source and receiver are connected by a bundle of many rays, none of which precisely follow a great circle. The $\pm 10^\circ$ directional fluctuations at the receiver result from the arrival of wave packets that have travelled along the different rays within this multipath. The occasional failure of great-circle backtracking, and the associated mirages, probably results from partial topographic obstruction of the multipath, which biases the directional average at the receiver.Comment: Journal of Marine Research, in pres

Observational Features of Black Holes

Recently considered a very attracting possibility to detect retro-MACHOs, i.e. retro-images of the Sun by a Schwarzschild black hole. In this paper we discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. In some sense that is a manifestation of gravitational lens effect in the strong gravitational field near black hole horizon and a generalization of the retro-gravitational lens phenomenon. We analyze the case of a Kerr black hole rotating at arbitrary speed for some selected positions of a distant observer with respect to the equatorial plane of a Kerr black hole. We discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons and propose a procedure to measure masses and rotation parameters analyzing these forms of mirages. Some time ago suggested to search shadows at the Galactic Center. In this paper we present the boundaries for shadows calculated numerically. We also propose to use future radio interferometer RADIOASTRON facilities to measure shapes of mirages (glories) and to evaluate the black hole spin as a function of the position angle of a distant observer.Comment: Plenary talk presented at Workshop on High Energy Physics&Field Theory (Protvino, Russia, 2004

Key signal contributions in photothermal deflection spectroscopy

We report on key signal contributions in photothermal deflection spectroscopy (PDS) of semiconductors at photon energies below the bandgap energy and show how to extract the actual absorption properties from the measurement data. To this end, we establish a rigorous computation scheme for the deflection signal including semi-analytic raytracing to analyze the underlying physical effects. The computation takes into account linear and nonlinear absorption processes affecting the refractive index and thus leading to a deflection of the probe beam. We find that beside the linear mirage effect, nonlinear absorption mechanisms make a substantial contribution to the signal for strongly focussed pump beams and sample materials with high two-photon absorption coefficients. For example, the measured quadratic absorption contribution exceeds 5% at a pump beam intensity of about ${1.3}\times{10^{5}}\;{W}/{cm^{2}}$ in Si and at ${5}\times{10^{4}}\;{W}/{cm^{2}}$ in GaAs. In addition, our method also includes thermal expansion effects as well as spatial gradients of the attenuation properties. We demonstrate that these effects result in an additional deflection contribution which substantially depends on the distance of the photodetector from the readout point. This distance dependent contribution enhances the surface related PDS signal up to two orders of magnitude and may be misinterpreted as surface absorption if not corrected in the analysis of the measurement data. We verify these findings by PDS measurements on crystalline silicon at a wavelength of 1550 nm and provide guidelines how to extract the actual attenuation coefficient from the PDS signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv

Mirage: A New Package for the Simulation of Gravitationally Microlensed Quasars

We present Mirage, a new package for simulating gravitationally lensed quasars that allows simulation of arbitrarily sized emitting regions of the quasar’s accretion disk. We develop a robust, large-scale simulator, wirtten in Python, to model gravitationally lensed quasars. Numerical simulation of gravitationally microlensed quasars provides a tool to determine the physical size and temperature profile of quasars accretion disks which is impossible through direct observation. The method consists of ray-tracing approximately 1010 paths through a simulated starfield, taking advantage of the latest technologies in cluster computing,to calculate flux received by the observer from each lensed image from different regions of the accretion disk as the quasar moves relative to the lensing galaxy. We compare our simulations to observations of QSO2237+0305 in optical and X-ray wavebands to place constraints on the relative size of the x-ray and optical emitting regions of the quasar’s accretion disk

The geometry of sound rays in a wind

We survey the close relationship between sound and light rays and geometry. In the case where the medium is at rest, the geometry is the classical geometry of Riemann. In the case where the medium is moving, the more general geometry known as Finsler geometry is needed. We develop these geometries ab initio, with examples, and in particular show how sound rays in a stratified atmosphere with a wind can be mapped to a problem of circles and straight lines.Comment: Popular review article to appear in Contemporary Physic

Visualization, Exploration and Data Analysis of Complex Astrophysical Data

In this paper we show how advanced visualization tools can help the researcher in investigating and extracting information from data. The focus is on VisIVO, a novel open source graphics application, which blends high performance multidimensional visualization techniques and up-to-date technologies to cooperate with other applications and to access remote, distributed data archives. VisIVO supports the standards defined by the International Virtual Observatory Alliance in order to make it interoperable with VO data repositories. The paper describes the basic technical details and features of the software and it dedicates a large section to show how VisIVO can be used in several scientific cases.Comment: 32 pages, 15 figures, accepted by PAS

Visual Simulation of Flow

We have adopted a numerical method from computational fluid dynamics, the Lattice Boltzmann Method (LBM), for real-time simulation and visualization of flow and amorphous phenomena, such as clouds, smoke, fire, haze, dust, radioactive plumes, and air-borne biological or chemical agents. Unlike other approaches, LBM discretizes the micro-physics of local interactions and can handle very complex boundary conditions, such as deep urban canyons, curved walls, indoors, and dynamic boundaries of moving objects. Due to its discrete nature, LBM lends itself to multi-resolution approaches, and its computational pattern, which is similar to cellular automata, is easily parallelizable. We have accelerated LBM on commodity graphics processing units (GPUs), achieving real-time or even accelerated real-time on a single GPU or on a GPU cluster. We have implemented a 3D urban navigation system and applied it in New York City with real-time live sensor data. In addition to a pivotal application in simulation of airborne contaminants in urban environments, this approach will enable the development of other superior prediction simulation capabilities, computer graphics and games, and a novel technology for computational science and engineering

On the atmospheres of Saturn and cold gas giant extrasolar planets

Over the past few decades, short-period giant planets have been discovered in extrasolar planetary systems, allowing for new tests of planetary evolution theories. Many of these giant exoplanets have high temperatures (>1000 K) and do not directly resemble Jupiter or Saturn. Only in the past few years have exoplanets akin to the cold (~100 K) gas giants in the solar system been identified. In this dissertation, I investigated giant gaseous planets through comparative studies of Saturn and exoplanets. Saturn has been the target of numerous high-precision observations, making it the ideal candidate for comparative studies. I simulated transit observations of a Saturn-analog exoplanet and determined that cold exoplanet atmospheres are amenable to characterization via transmission spectroscopy. By casting Saturn as an exoplanet, I demonstrated the potential for exoplanets to place the solar system in a Galactic context. The transit spectrum of Saturn also highlighted the importance of atmospheric refraction in transit observations. Refraction alters the path of light propagating in an atmosphere. I showed that out-of-transit refracted light provides an opportunity to identify and characterize the atmospheres of cold transiting and non-transiting exoplanets. I searched exoplanet parameter space to locate the maximal effect and derived a criterion that predicts which atmospheres produce detectable refracted light signatures. My consideration of exoplanetary refraction also included a parallel study of Saturn's atmosphere. I developed a novel method to measure atmospheric refractivity from distorted images of the Sun. I used this method to infer Saturn's atmospheric structure for more than a dozen Saturn solar occultations and to identify seasonal variations in Saturn's stratospheric temperature. Lastly, I obtained ground-based observations of the long-period transiting exoplanet Kepler-421b to refine its transit ephemeris. Without accurate transit ephemerides, long-period exoplanet characterization with large space-based observatories cannot occur. My unique observations represent the first step toward ensuring that long-period exoplanets are characterized in the near future. In summary, this dissertation lays the foundation for investigations of cold giant exoplanets, which exist in an almost entirely unexplored regime of exoplanetary science. Using Saturn to provide context and motivation, I began confronting the challenges facing this new discipline of exoplanetary science