Le mardi 19 octobre 2021, Phillip HELBIG présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Sciences spatiales) sous la direction de Christian BARBIER et Jean SURDEJ.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Cosmological distances: calculation of distances in cosmological models with small-scale inhomogeneities and their use in observational cosmology ».

Abstract

In cosmology, one often assumes that the universe is homogeneous and isotropic. While originally a simplifying assumption, today there is observational evidence that this is a good approximation in our Universe on scales above a few hundred megaparsecs. This approximation is often used when calculating various distances as a function of redshift, even though the scales probed by a beam of light are much smaller than the scale of homogeneity. Since our Universe is obviously not homogeneous and isotropic on small scales, it is at least conceivable that this could affect distance calculation.

I discuss some approximations which have been used to deal with this problem and present my own work in this area, including applications in both traditional observational cosmology (magnitude--redshift relation for type Ia supernovae) and various cosmological applications of gravitational lensing. Since the standard cosmological parameters are now known to good precision, one can use cosmological tests which are also affected by the degree of homogeneity to determine to what extent our Universe (behaves as if it) is homogeneous, even on very small cosmological scales.

Le Jury sera composé de :

M. M.-A. DUPRET (Président), MM. C. BARBIER (Promoteur), L. DELCHAMBRE (Secrétaire), J. SURDEJ (Co-promoteur), R. STABELL (University of Oslo ), M. TYTGAT (ULB).