Hubble flow variations as a test for inhomogeneous cosmology

Abstract

Context. Backreactions from large-scale inhomogeneities may provide an elegant explanation for the observed accelerated expansion of the universe without the need to introduce dark energy. Aims. We propose a cosmological test for a specific model of inhomogeneous cosmology, called timescape cosmology. Using largescale galaxy surveys such as SDSS and 2MRS, we test the variation of expansion expected in the Λ-CDM model versus a more generic differential expansion using our own calibrations of bounds suggested by timescape cosmology. Methods. Our test measures the systematic variations of the Hubble flow towards distant galaxies groups as a function of the matter distribution in the lines of sight to those galaxy groups. We compare the observed systematic variation of the Hubble flow to mock catalogues from the Millennium Simulation in the case of the Λ-CDM model, and a deformed version of the same simulation that exhibits more pronounced differential expansion. Results. We perform a series of statistical tests, ranging from linear regressions to Kolmogorov-Smirnov tests, on the obtained data. They consistently yield results preferring Λ-CDM cosmology over our approximated model of timescape cosmology. Conclusions. Our analysis of observational data shows no evidence that the variation of expansion differs from that of the standard Λ-CDM model.

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

“…Simple ray-tracing estimates using the Lemaître-Tolman-Bondi model show that the precision to definitively distinguish nonkinematic differential expansion from the standard expectation is not reached with present data (Dam 2016), however. That test, comparisons of the integrated Sachs-Wolfe effect (Beck et al 2018), and the Saulder et al (2019) test are all complementary ways for testing the possibility of nonkinematic differential expansion once substantial advances in observational precision are made.…”

“…whereH 0 =H(t 0 ) and H 0 = H(t 0 ) are respectively the bare and dressed Hubble constants for the timescape model as above, f v0 = f v (t 0 ), and in the last equality we have used the exact tracking limit solution (Wiltshire 2007b(Wiltshire , 2009). Using the above definitions, Saulder et al (2019) empirically define average fractions of lines of sight in finite infinity regions from the mock catalogues according to…”

“…This can compound any problems relating to observational and statistical biases. Saulder et al (2019) analyse fundamental plane distances (Saulder et al 2013(Saulder et al , 2015 which they combine with information from the SDSS (Alam et al 2015) and 2MRS (Huchra et al 2012) surveys to create a catalogue of structures in the local Universe covering some 22.7% of the northern hemisphere sky. Setting aside possible systematic uncertainties arising from incomplete sky coverage, which Saulder et al (2019) discuss, then the test that they propose based on determining the fraction of "finite infinity regions" (Ellis 1984;Wiltshire 2007a) along individual lines of sight is a perfectly reasonable one, given robust modelindependent estimates of the masses of all galaxies along the lines of sight.…”

“…While certainly justified in the case of the ΛCDM model, any rescaling of the simulation to attempt to mimic non-FLRW inhomogeneous expansion has inherent problems. In addition to various systematic issues discussed by Saulder et al (2019), I will point out a further geometrical issue that they have not considered.…”

“…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.…”