Large-scale peculiar motions and cosmic acceleration

Abstract

Recent surveys seem to support bulk peculiar velocities well in excess of those anticipated by the standard cosmological model. In view of these results, we consider here some of the theoretical implications of large-scale drift motions. We find that observers with small, but finite, peculiar velocities have generally different expansion rates than the smooth Hubble flow. In particular, it is possible for observers with larger than the average volume expansion at their location, to experience apparently accelerated expansion when the universe is actually decelerating. Analogous results have been reported in studies of inhomogeneous (nonlinear) cosmologies and within the context of the Lemaitre-Tolman-Bondi models. Here, they are obtained within the linear regime of a perturbed, dust-dominated Friedmann-Robertson-Walker cosmology.Comment: Revised approach, discussion and references. To appear in MNRA

Light propagation in statistically homogeneous and isotropic universes with general matter content

“…However, it is possible to say what would would be necessary for the non-dust terms to have a significant effect. For theṅ α terms to be important in (3.14)-(3.16),ṅ αṅ α would have to be of the order of the square of the expansion rate in a large fraction of space (contrary to what was argued in [75]; see also [76]), or the contraction of the energy flux andṅ α would have to be of the order of the product of the average energy density and the average expansion rate. In order for the pressure or the anisotropic stress to be important, they would have to be on average of the same order of magnitude as the average energy density.…”

Non-comoving cold dark matter in a $$\varLambda $$CDM background

“…However, we can achieve an approximation to a Copernican principle by considering arrays of localized Szekeres-II regions embedded in a spatially flat ΛCDM background by smooth matchings and thus evolving jointly with it, as in the "pancake models" derived and discussed in [22]. In fact, in that paper we presented a brief illustrative example of Szekeres-II regions characterized by an energy-momentum tensor like (6), with q a associated with CDM peculiar velocities as in ( 19), but with Λ = 0 (hence the matched FLRW background was an Einstein de Sitter model). We will consider in the following sections the case Λ > 0 of the above mentioned example.…”

“…To complement the interpretation of energy flux as noncomoving CDM, it is useful to compute the Hubble scalar for the non-comoving 4-velocityΘ =ĥ a bû b ;a withû a defined by (16). In the linear regime of peculiar velocities v a v a /c 2 1 we obtain the following relation [6]…”

“…Different congruences of observers define different Hubble flows characterized by the kinematic quantities associated with their 4-velocities. In particular, Tsagas has examined the effect of considering peculiar velocities under this approach on the interpretation of cosmological observations (see [6,7] and references therein). Another example of non-comoving matter can be found in [8], which examined the evolution of cosmic voids formed by baryons and CDM.…”

Integrability conditions of quasi-Newtonian cosmologies in modified gravity

“…19,[23][24][25] On the other hand, when linearized around a FriedmannLemaître-Robertson-Walker (FLRW) model, these quasi-Newtonian models are consistent and provide a basis for the study of peculiar velocities in almost FLRW models. 18,19 It has been pointed out 26 that peculiar motion can locally mimic the effects of dark matter in regions of a dust-dominated FRW universe endowed with bulk peculiar velocities.…”