Comment on “Hubble flow variations as a test for inhomogeneous cosmology”

Abstract

Saulder et al. (2019) have performed a novel observational test of the local expansion of the Universe for the standard cosmology as compared to an alternative model with differential cosmic expansion. Their analysis employs mock galaxy samples from the Millennium Simulation, a Newtonian N-body simulation on a ΛCDM background. For the differential expansion case the simulation has been deformed in an attempt to incorporate features of a particular inhomogeneous cosmology: the timescape model. It is shown that key geometrical features of the timescape cosmology have been omitted in this rescaling. Consequently, the differential expansion model tested by Saulder et al. (2019) cannot be considered to approximate the timescape cosmology.

A Scale-Dependent Distance Functional between Past Light Cones in Cosmology

“…In particular, it exhibits a complex pattern of fluctuations with respect to the linear FLRW Hubble flow that sets in, relatively to the standard of rest provided by the cosmic microwave background (CMB), when we probe the homogeneity scales, L 100h −1 Mpc. Again, we stress that the transitional region between the phenomenological Hubble flow and the statistical onset of the large-scale FLRW linear Hubble flow is quite uncertain and still actively debated [54]. If we adopt the weak form of the cosmological principle described in the introduction, (M, g, γ s ) can be identified with the phenomenological background spacetime or Phenomenological Background Solution (PBS) [39] associated with the actual cosmological data gathered from our past lightcone observations.…”

“…However, in a cosmographic setting and presence of a complex pattern of local inhomogeneities coupled with a non-FLRW spacetime geometry over scales 100h −1 Mpc, the peculiar motion of the phenomenological observer has a dynamical origin, driven by the gravitational interaction and not just by a kinematical velocity effect. Even if we factor out the effect of coherent bulk flows due to the non-linear local gravitational dynamics, and average the rate of expansion over spherical shells at increasing distances from (p, γ(0)), the variance in the peculiar velocity of (p, γ(0)) with respect to the average rate of expansion is significant [54]. These remarks imply that the Lorentz boosts connecting C S(p) and C S(p) acquire a dynamical meaning that plays a basic role in what follows.…”

“…The actual averaging scale marking the statistical onset of isotropy and homogeneity is still much debated. For the sake of the argument presented in this paper, we adopt the rather conservative estimate of the scales over which an average isotropic expansion is seen to emerge, namely 70 − 120 h −1 M pc, and ideally extending to a few times this scale [54]. 3 At the Hubble scale, the problem of cosmic variance may alter the statistical significance of the data samples we gather.…”

The cosmological star formation history from the Local Cosmological Volume of galaxies and constraints on the matter homogeneity

“…The accelerated expansion of the Universe could also be an apparent effect due to large-scale inhomogeneities that grow with cosmic time (Wiltshire 2007a(Wiltshire ,b, 2009(Wiltshire , 2019.…”

The many tensions with dark-matter based models and implications on the nature of the Universe

“…Wherever the loss of confidence was not computed formally, we assign a drop in confidence by 50%. The inset graph zooms into the falsifications up to 2012. while in reality significant inhomogeneities develop over time in a model without dark energy, then the inferred observational quantities appear as if the Universe has dark energy and an accelerating expansion (Wiltshire 2007a(Wiltshire ,b, 2009(Wiltshire , 2019). Wiltshire describes filaments (walls) and voids with separate homogeneous metrics, and their statistical average at large scales simplifies to a statistical homogeneity8.…”