arXiv subjects

Roger Bieli

Publications and source records attributed to Roger Bieli.

5 recordsLinked to original sources

arXiv2010

Large data pointwise decay for defocusing semilinear wave equations

We generalize the pointwise decay estimates for large data solutions of the defocusing semilinear wave equations which we obtained earlier under restriction to spherical symmetry. Without the symmetry the conformal transformation we use provides only a weak decay. This can, however, in the next step be improved to the optimal decay estimate suggested by the radial case and small data results. This is the first result of that kind.

math.AP↗

arXiv2009

Accelerated expansion by non-minimally coupled scalar fields

In a class of spatially homogeneous cosmologies including those of Bianchi type I--VIII mathematical results are presented which show that a scalar field non-minimally coupled to the scalar curvature of spacetime can dynamically yield a positive cosmological constant without the potential being required to include one. More precisely, it is shown that in an exponential potential any positive coupling constant leads eventually to late-time de Sitter expansion and isotropization corresponding to a positive cosmological constant and that this behaviour is independent of the steepness of the potential. This is in marked contrast to the minimally coupled case where power-law inflation occurs at most, provided the potential is sufficiently shallow.

astro-ph↗

arXiv2009

Global pointwise decay estimates for defocusing radial nonlinear wave equations

We prove global pointwise decay estimates for a class of defocusing semilinear wave equations in $n=3$ dimensions restricted to spherical symmetry. The technique is based on a conformal transformation and a suitable choice of the mapping adjusted to the nonlinearity. As a result we obtain a pointwise bound on the solutions for arbitrarily large Cauchy data, provided the solutions exist globally. The decay rates are identical with those for small data and hence seem to be optimal. A generalization beyond the spherical symmetry is suggested.

math.AP↗

arXiv2006

Coupled quintessence and curvature-assisted acceleration

Spatially homogeneous models with a scalar field non-minimally coupled to the space-time curvature or to the ordinary matter content are analysed with respect to late-time asymptotic behaviour, in particular to accelerated expansion and isotropization. It is found that a direct coupling to the curvature leads to asymptotic de Sitter expansion in arbitrary exponential potentials, thus yielding a positive cosmological constant although none is apparent in the potential. This holds true regardless of the steepness of the potential or the smallness of the coupling constant. For matter-coupled scalar fields, the asymptotics are obtained for a large class of positive potentials, generalizing the well-known cosmic no-hair theorems for minimal coupling. In this case it is observed that the direct coupling to matter does not impact the late-time dynamics essentially.

gr-qc↗

arXiv2005

Algebraic expansions for curvature coupled scalar field models

A late time asymptotic perturbative analysis of curvature coupled complex scalar field models with accelerated cosmological expansion is carried out on the level of formal power series expansions. For this, algebraic analogues of the Einstein scalar field equations in Gaussian coordinates for space-time dimensions greater than two are postulated and formal solutions are constructed inductively and shown to be unique. The results obtained this way are found to be consistent with already known facts on the asymptotics of such models. In addition, the algebraic expansions are used to provide a prospect of the large time behaviour that might be expected of the considered models.

gr-qc↗