Imitating accelerated expansion of the Universe by matter inhomogeneities: corrections of some misunderstandings

A number of misunderstandings about modeling the apparent accelerated expansion of the Universe and about the ‘weak singularity’ are clarified: (1) Of the five definitions of the deceleration parameter given by Hirata and Seljak (HS), only q1 is a correct invariant measure of acceleration/deceleration of expansion. The q3 and q4 are unrelated to acceleration in an inhomogeneous model. (2) The averaging over directions involved in the definition of q4 does not correspond to what is done in observational astronomy. (3) HS’s equation (38) connecting q4 to the flow invariants gives self-contradictory results when applied at the centre of symmetry of the Lemaître–Tolman (L–T) model. The intermediate equation (31) that determines q3′ is correct, but approximate, so it cannot be used for determining the sign of the deceleration parameter. Even so, at the centre of symmetry of the L–T model, it puts no limitation on the sign of q3′(0). (4) The ‘weak singularity’ of Vanderveld et al. is a conical profile of mass density at the centre—a perfectly acceptable configuration. A. Krasiński (B) · K. Bolejko N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland e-mail: [email protected] K. Bolejko e-mail: [email protected] C. Hellaby · K. Bolejko Astrophysics, Cosmology and Gravity Centre, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch 7701, South Africa e-mail: [email protected] M.-N. Célérier Laboratoire Univers et Théories (LUTH), Observatoire de Paris, CNRS, Université Paris-Diderot, 5 place Jules Janssen, 92190 Meudon, France e-mail: [email protected]