arXiv subjects

A. F. F. Teixeira

Publications and source records attributed to A. F. F. Teixeira.

At least 19 recordsLinked to original sources

arXiv2017

Schwarz-Christoffel: piliero en rivero (a pillar on a river)

La transformoj de Schwarz-Christoffel mapas, konforme, la superan kompleksan duon-ebenon al regiono limigita per rektaj segmentoj. Cxi tie ni priskribas kiel konvene kunigi mapon de la suba duon-ebeno al mapo de la supera duon-ebeno. Ni emfazas la bezonon de klara difino de angulo de kompleksa nombro, por tiu kunigo. Ni diskutas kelkajn ekzemplojn kaj donas interesan aplikon pri movado de fluido. ------- Schwarz-Christoffel transformations map, conformally, the complex upper half plane into a region bounded by right segments. Here we describe how to couple conveniently a map of the lower half plane to the map of the upper half plane. We emphasize the need of a clear definition of angle of a complex, to that coupling. We discuss some examples and give an interesting application for motion of fluid.

math.CV↗

arXiv2014

Future dynamics in f(R) theories

The $f(R)$ gravity theories provide an alternative way to explain the current cosmic acceleration without invoking dark energy matter component. However, the freedom in the choice of the functional forms of $f(R)$ gives rise to the problem of how to constrain and break the degeneracy among these gravity theories on theoretical and/or observational grounds. In this paper to proceed further with the investigation on the potentialities, difficulties and limitations of $f(R)$ gravity, we examine the question as to whether the future dynamics can be used to break the degeneracy between $f(R)$ gravity theories by investigating the future dynamics of spatially homogeneous and isotropic dust flat models in two $f(R)$ gravity theories, namely the well known $f(R) = R + αR^{n}$ gravity and another by A. Aviles et al., whose motivation comes from the cosmographic approach to $f(R)$ gravity. To this end we perform a detailed numerical study of the future dynamic of these flat model in these theories taking into account the recent constraints on the cosmological parameters made by the Planck team. We show that besides being powerful for discriminating between $f(R)$ gravity theories, the future dynamics technique can also be used to determine the fate of the Universe in the framework of these $f(R)$ gravity theories. Moreover, there emerges from our numerical analysis that if we do not invoke a dark energy component with equation-of-state parameter $ω< -1$ one still has dust flat FLRW solution with a big rip, if gravity deviates from general relativity via $f(R) = R + αR^n $. We also show that FLRW dust solutions with $f''<0$ do not necessarily lead to singularity.

astro-ph.CO↗

arXiv2012

Large-angle non-Gaussianity in simulated high-resolution CMB maps

A detection or nondetection of primordial non-Gaussianity by using the cosmic microwave background radiation (CMB) offers a way of discriminating inflationary scenarios and testing alternative models of the early universe. This has motivated the considerable effort that has recently gone into the study of theoretical features of primordial non-Gaussianity and its detection in CMB data. Among such attempts to detect non-Gaussianity, there is a procedure that is based upon two indicators constructed from the skewness and kurtosis of large-angle patches of CMB maps, which have been proposed and used to study deviation from Gaussianity in the WMAP data. Simulated CMB maps equipped with realistic primordial non-Gaussianity are essential tools to test the viability of non-Gaussian indicators in practice, and also to understand the effect of systematics, foregrounds and other contaminants. In this work we extend and complement the results of our previous works by performing an analysis of non-Gaussianity of the high-angular resolution simulated CMB temperature maps endowed with non-Gaussianity of the local type, for which the level of non-Gaussianity is characterized by the dimensionless parameter $f_{\rm NL}^{\rm local}$

astro-ph.CO↗

arXiv2012

La relativeca tempo - I / The relativistic time - I

The relativistic time is different from the Newtonian one. We revisit some of these differences in Doppler effect, twin paradox, rotation, rigid rod, and constant proper acceleration. ------- La relativeca tempo estas malsama ol la Newtona. Ni revidos iujn el tiuj malsamoj en Dopplera efiko, gxemel-paradokso, rotacio, rigida stango, kaj konstanta propra akcelo.

physics.gen-ph↗

arXiv2011

Oscillation of a rigid rod in the special relativity

In the special relativity, a rigid rod slides upon itself, with one extremity oscillating harmonically. We discovered restrictions in the amplitude of the motion and in the length of the rod, essential to eliminate unphysical solutions. ------- Cxe la speciala relativeco, rigida stango movigxas sur si mem, kun unu fino oscilante harmonie. Ni malkovris limigajn kondicxojn pri la amplitudo de movado kaj pri la longo de stango, necesegaj por elimini ne-fizikajn solvojn.

physics.gen-ph↗

arXiv2011

Algorithm for structure constants

In a $n$-dimensional Lie algebra, random numerical values are assigned by computer to $n(n-1)$ especially selected structure constants. An algorithm is then created, which calculates without ambiguity the remaining constants, obeying the Jacobi conditions. Differently from others, this algorithm is suitable even for poor personal computer. ------------- En $n$-dimensia algebro de Lie, hazardaj numeraj valoroj estas asignitaj per komputilo al $n(n-1)$ speciale elektitaj konstantoj de strukturo. Tiam algoritmo estas kreita, kalkulante senambigue la ceterajn konstantojn, obeante kondicxojn de Jacobi. Malsimile al aliaj algoritmoj, tiu cxi tauxgas ecx por malpotenca komputilo.

physics.comp-ph↗

arXiv2011

La relativeca tempo -- II / The relativistic time -- II

The theory of relativity showed that several Newtonian ideas about spacetime are imperfect. We present here some relativistic concepts related to these ideas: simultaneity of events and synchronization of clocks (both along a line in the space frame), gravitational Doppler effect, and time travel. --------- La relativeca teorio montris ke pluraj Newtonaj ideoj pri la spacotempo estas malperfektaj. Tie cxi ni prezentas kelkajn relativecajn konceptojn iel rilatajn al tiuj ideoj: samtempecon de eventoj kaj sinkronon de horlogxoj (ambaux laux linio en la spaca reto), gravitan Doppleran efikon, kaj vojagxon kun reveno al estinto.

physics.class-ph↗

arXiv2011

Tempa voja^go kaj geodezioj en ^generala relativeco / Time travel and geodesics in general relativity

In the homogeneous metric of Som-Raychaudhuri, in general relativity, we study the three types of geodesics: timelike, null, and spacelike; in particular, the little known geodesics of simultaneities. We also study the non-geodetic circular motion with constant velocity, particularly closed timelike curves, and time travel of a voyager. ------------------- ^Ce la ^Generala Relativeco, en homogena metriko de Som-Raychaudhuri, ni studas geodeziojn de la tri tipoj: tempa, nula, kaj spaca, speciale la malmulte konatajn samtempajn geodeziojn. Ni anka^u studas ne-geodezian cirklan movadon kun konstanta rapido, speciale fermitajn kurbojn de tempa tipo, kaj movadon de voja^ganto al estinto.

gr-qc↗

arXiv2010

Geodesics of simultaneity in Schwarzschild

Geodesic of simultaneity is a spacelike geodesic in which every pair of neighbour events are simultaneous ($g_{0μ}\dd x^μ=0$). These geodesics are studied in the exterior region of \Sch's metric.

physics.class-ph↗

arXiv2010

Testing large-angle deviation from Gaussianity in CMB maps

A detection of the level of non-Gaussianity in the CMB data is essential to discriminate among inflationary models and also to test alternative primordial scenarios. However, the extraction of primordial non-Gaussianity is a difficult endeavor since several effects of non-primordial nature can produce non-Gaussianity. On the other hand, different statistical tools can in principle provide information about distinct forms of non-Gaussianity. Thus, any single statistical estimator cannot be sensitive to all possible forms of non-Gaussianity. In this context, to shed some light in the potential sources of deviation from Gaussianity in CMB data it is important to use different statistical indicators. In a recent paper we proposed two new large-angle non-Gaussianity indicators which provide measures of the departure from Gaussianity on large angular scales. We used these indicators to carry out analyses of non-Gaussianity of the bands and of the foreground-reduced WMAP maps with and without the KQ75 mask. Here we briefly review the formulation of the non-Gaussianity indicators, and discuss the analyses made by using our indicators.

astro-ph.CO↗

arXiv2010

Constraints on dark energy equation of state parameters from cosmic topology

Despite our present-day inability to predict the topology of the universe it is expected that we should be able to detect it in the near future. A nontrivial detectable topology of the space section of the universe can be probed for all homogeneous and isotropic universes through the circles-in-the-sky. We discuss briefly how one can use this observable attribute to set constraints on the dark energy equation of state parameters.

astro-ph.CO↗

arXiv2010

A Study of Gaussianity in CMB band maps

The detection of non-Gaussianity in the CMB data would rule out a number of inflationary models. A null detection of non-Gaussianity, instead, would exclude alternative models for the early universe. Thus, a detection or non-detection of primordial non-Gaussianity in the CMB data is crucial to discriminate among inflationary models, and to test alternative scenarios. However, there are various non-cosmological sources of non-Gaussianity. This makes important to employ different indicators in order to detect distinct forms of non-Gaussianity in CMB data. Recently, we proposed two new indicators to measure deviation from Gaussianity on large angular scales, and used them to study the Gaussianity of the raw band WMAP maps with and without the KQ75 mask. Here we extend this work by using these indicators to perform similar analyses of deviation from Gaussianity of the foreground-reduced Q, V, and W band maps. We show that there is a significant deviation from Gaussianity in the considered full-sky maps, which is reduced to a level consistent with Gaussianity when the KQ75 mask is employed.

astro-ph.CO↗

arXiv2009

Doppleraj efikoj ĉe Schwarzschild

Motion of bodies and light rays are studied in the gravitational field of Schwarzschild. Several Doppler effects are described. ----- Movado de korpoj kaj lumo estas studitaj en gravita kampo de Schwarzschild. Pluraj Doppleraj efikoj estas priskribitaj.

physics.gen-ph↗

arXiv2008

Dopplera efiko de luma ebeno vidata per akcelata observanto

A plane lights on monochromatically and immediately after lights off, while an observer starts moving out from the plane, perpendicularly to it, with constant proper acceleration. Special relativity predicts that the observer will see a light circle always in the direction opposite to the plane, without Doppler effect, and that the circle will seem to progressively shrink to a point. ----- Ebeno eklumi^gas unukolore kaj tuj poste mallumi^gas, dum observanto ekmovi^gas orte el ebeno kun konstanta propra akcelo. Special-relativeco anta^udiras, ke observanto vidos luman cirklon ^ciam en direkto mala al la ebeno, sen Dopplera efiko, kaj ke tiu cirklo ^sajnos progrese eti^gi ^gis punkto.

physics.gen-ph↗

arXiv2008

Relativeca Dopplera efiko inter du akcelataj korpoj - I

We describe the Doppler effect between equally accelerated light source and observer under the special relativity. The proper accelerations are constant and parallel. An English version is available by request. - - - - - - - - - - - Ni priskribas luman Doppleran efikon inter same akcelataj fonto kaj observanto ^ce special-relativeco. La propraj akceloj estas konstantaj kaj paralelaj.

physics.gen-ph↗

arXiv2007

Relativeca Dopplera efekto ĉe unuforme akcelata movo -- II

Extending physics/0701092, a light source of monochromatic radiation, in rectilinear motion under constant proper acceleration, passes near an observer at rest. In the context of special relativity, we describe the observed Doppler effect. We describe also the interesting discontinuous effect when riding through occurs. An English version of this article is available. ----- Da^urigante physics/0701092, luma fonto de unukolora radiado ^ce rekta movo ^ce konstanta propra akcelo pasas preter restanta observanto. ^Ce la special-relativeco, ni priskribas la observatan Doppleran efikon. Ni anka^u priskribas la interesan nekontinuan efikon se trapaso okazas.

physics.gen-ph↗

arXiv2007

Relativeca Dopplera efekto ĉe unuforme akcelata movo - I

An observer, in rectilinear motion under constant proper acceleration, passes near a source of monochromatic radiation at rest. In the context of special relativity, we describe the observed Doppler effect. We describe also the interesting discontinuous effect when riding through occurs. An English version of this article is available. - - - - - - - - - - - Observanto ^ce rekta movo ^ce konstanta propra akcelo pasas preter restanta fonto de unukolora radiado. ^Ce la special-relativeco, ni priskribas la observatan Doppleran efekton. Ni anka^u priskribas la interesan nekontinuan efekton se trapaso okazas.

physics.gen-ph↗

arXiv2003

Classification of Energy Momentum Tensors in $n \geq 5$ Dimensional Space-times: a Review

Recent developments in string theory suggest that there might exist extra spatial dimensions, which are not small nor compact. The framework of a great number of brane cosmological models is that in which the matter fields are confined on a brane-world embedded in five dimensions (the bulk). Motivated by this we review the main results on the algebraic classification of second order symmetric tensors in 5-dimensional space-times. All possible Segre types for a symmetric two-tensor are found, and a set of canonical forms for each Segre type is obtained. A limiting diagram for the Segre types of these symmetric tensors in 5-D is built. Two theorems which collect together some basic results on the algebraic structure of second order symmetric tensors in 5-D are presented. We also show how one can obtain, by induction, the classification and the canonical forms of a symmetric two-tensor on n-dimensional (n > 5) spaces from its classification in 5-D spaces, present the Segre types in n-D and the corresponding canonical forms. This classification of symmetric two-tensors in any n-D spaces and their canonical forms are important in the context of n-dimensional brane-worlds context and also in the framework of 11-D supergravity and 10-D superstrings.

gr-qc↗