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L. Fernández-Jambrina

Publications and source records attributed to L. Fernández-Jambrina.

At least 19 recordsLinked to original sources
arXiv2021

Singularities in inflationary cosmological models

Due to the accelerated expansion of the universe, the possibility of formation of singularities has changed from the classical Big Bang and Big Crunch singularities to include a number of new scenarios. In recent papers it has been shown that such singularities may appear in inflationary cosmological models with a fractional power scalar field potential. In this paper we enlarge the analysis of singularities in scalar field cosmological models by the use of generalised power expansions of their Hubble scalar and their scalar field in order to describe all possible models leading to a singularity, finding other possible cases. Unless a negative scalar field potential is considered, all singularities are weak and of type IV.

gr-qc↗
arXiv2021

New futures for cosmological models

The discovery of accelerated expansion of the universe opened the possibility of new scenarios for the doom of our spacetime, besides aeternal expansion and a final contraction. In this paper we review the chances which may await our universe. In particular, there are new possible singular fates (sudden singularities, big rip...), but there also other evolutions which cannot be considered as singular. In addition to this, some of the singular fates are not strong enough in the sense that the spacetime can be extended beyond the singularity. For deriving our results we make use of generalised power and asymptotic expansions of the scale factor of the universe.

gr-qc↗
arXiv2020

Curvature of planar aesthetic curves

Farin proposed a method for designing Bezier curves with monotonic curvature and torsion. Such curves are relevant in design due to their aesthetic shape. The method relies on applying a matrix M to the first edge of the control polygon of the curve in order to obtain by iteration the remaining edges. With this method, sufficient conditions on the matrix $M$ are provided, which lead to the definition of Class A curves, generalising a previous result by Mineur et al for plane curves with M being the composition of a dilatation and a rotation. However, Cao and Wang have shown counterexamples for such conditions. In this paper, we revisit Farin's idea of using the subdivision algorithm to relate the curvature at every point of the curve to the curvature at the initial point in order to produce a closed formula for the curvature of planar curves in terms of the eigenvalues of the matrix M and the seed vector for the curve, the first edge of the control polygon. Moreover, we give new conditions in order to produce planar curves with monotonic curvature. The main difference is that we do not require our conditions on the eigenvalues to be preserved under subdivision of the curve. This facilitates giving a unified derivation of the existing results and obtain more general results in the planar case.

math.NA↗
arXiv2019

Grand Rip and Grand Bang/Crunch cosmological singularities

The present accelerated expansion of the universe has enriched the list of possible scenarios for its fate, singular or not. In this paper a unifying framework for analyzing such behaviors is proposed, based on generalized power and asymptotic expansions of the barotropic index $w$, or equivalently of the deceleration parameter $q$, in terms of the time coordinate. Besides well known singular and non-singular future behaviors, other types of strong singularities appear around the phantom divide in flat models, with features similar to those of big rip or big bang/crunch, which we have dubbed grand rip and grand bang/crunch respectively, since energy density and pressure diverge faster than $t^{-2}$ in coordinate time. In addition to this, the scale factor does not admit convergent generalized power series around these singularities with a finite number of terms with negative powers.

gr-qc↗
arXiv2017

Bezier developable surfaces

In this paper we address the issue of designing developable surfaces with Bezier patches. We show that developable surfaces with a polynomial edge of regression are the set of developable surfaces which can be constructed with Aumann's algorithm. We also obtain the set of polynomial developable surfaces which can be constructed using general polynomial curves. The conclusions can be extended to spline surfaces as well.

cs.GR↗
arXiv2016

Behavior of Friedmann-Lemaitre-Robertson-Walker Singularities

A regularization procedure has been recently suggested for regularizing Big Bang singularities in Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetimes. We argue that this procedure is only appliable to one case of Big Bang singularities and does not affect other types of singularities.

gr-qc↗
arXiv2016

Geometric elements and classification of quadrics in rational Bézier form

In this paper we classify and derive closed formulas for geometric elements of quadrics in rational Bézier triangular form (such as the center, the conic at infinity, the vertex and the axis of paraboloids and the principal planes), using just the control vertices and the weights for the quadric patch. The results are extended also to quadric tensor product patches. Our results rely on using techniques from projective algebraic geometry to find suitable bilinear forms for the quadric in a coordinate-free fashion, considering a pencil of quadrics that are tangent to the given quadric along a conic. Most of the information about the quadric is encoded in one coefficient, involving the weights of the patch, which allows us to tell apart oval from ruled quadrics. This coefficient is also relevant to determine the affine type of the quadric. Spheres and quadrics of revolution are characterised within this framework.

cs.GR↗
arXiv2015

Interpolation of a spline developable surface between a curve and two rulings

In this paper we address the problem of interpolating a spline developable patch bounded by a given spline curve and the first and the last rulings of the developable surface. In order to complete the boundary of the patch a second spline curve is to be given. Up to now this interpolation problem could be solved, but without the possibility of choosing both endpoints for the rulings. We circumvent such difficulty here by resorting to degree elevation of the developable surface. This is useful not only to solve this problem, but also other problems dealing with triangular developable patches.

cs.GR↗
arXiv2015

Singularities around w=-1

In this talk we would like to analyse the appearance of singularities in FLRW cosmological models which evolve close to w=-1, where w is the barotropic index of the universe. We relate small terms in cosmological time around w=-1 with the correspondent scale factor of the universe and check for the formation of singularities.

gr-qc↗
arXiv2011

Comment on "From geodesics of the multipole solutions to the perturbed Kepler problem"

In this comment we explain the discrepancies mentioned by the authors between their results and ours about the influence of the gravitational quadrupole moment in the perturbative calculation of corrections to the precession of the periastron of quasielliptical Keplerian equatorial orbits around a point mass. The discrepancy appears to be consequence of two different calculations of the angular momentum of the orbits.

gr-qc↗
arXiv2010

$w$-singularities in cosmological models

Recently a new type of cosmological singularity has been postulated for infinite barotropic index $w$ in the equation of state $p=w ρ$ of the cosmological fluid, but vanishing pressure and density at the singular event. Apparently the barotropic index $w$ would be the only physical quantity to blow up at the singularity. In this talk we would like to discuss the strength of such singularities and compare them with other types. We show that they are weak singularities.

gr-qc↗
arXiv2010

$w$-cosmological singularities

In this paper we characterize barotropic index singularities of homogeneous isotropic cosmological models. They are shown to appear in cosmologies for which the scale factor is analytical with a Taylor series in which the linear and quadratic terms are absent. Though the barotropic index of the perfect fluid is singular, the singularities are weak, as it happens for other models for which the density and the pressure are regular.

gr-qc↗
arXiv2009

Surface densities in General Relativity

In this lecture we deal with the construction of surface densities for the angular momentum of the sources of asymptotically flat vacuum stationary axisymmetric spacetimes. These sources arise from the discontinuities of the twist potential. The result will be applied to the Kerr metric to obtain an integrable density which can be viewed as the regularized version of the density obtained using other formalisms.

gr-qc↗
arXiv2009

Magnetic Moment Density from Lack of Smoothness of the Ernst Potential

In this talk it is shown a way for constructing magnetic surface sources for stationary axisymmetric electrovac spacetimes possessing a non-smooth electromagnetic Ernst potential. The magnetic moment density is related to this lack of smoothness and its calculation involves solving a linear elliptic differential equation. As an application the results are used for constructing a magnetic source for the Kerr-Newman field.

gr-qc↗
arXiv2009

Non-regular Potentials and Sources for Static Axisymmetric Electrovacs

In this lecture a new formalism for constructing electromagnetic surface sources for static axisymmetric electrovacs is presented. The electrostatic and magnetostatic sources are derived from the discontinuities of the scalar potentials. This formalism allows the inclusion of two kinds of dipole sources: Sheets of dipoles and the dipole moment of a distribution of monopoles. It is a generalization of a previous formalism in order to cope with asymptotically monopolar electric fields.

gr-qc↗
arXiv2009

Multipole corrections to perihelion and node line precession

In this talk relativistic corrections due to Geroch-Hansen multipoles for perihelion precession and node line precession of orbits in a stationary axially symmetric vacuum spacetime endowed with a plane of symmetry will be shown. Patterns of regularity will be discussed.

gr-qc↗
arXiv2009

Non-singular inhomogeneous stiff fluid cosmology

In this talk we show a stiff fluid solution of the Einstein equations for a cylindrically symmetric spacetime. The main features of this metric are that it is non-separable in comoving coordinates for the congruence of the worldlineS of the fluid and that it yields regular curvature invariants.

gr-qc↗