What do the cosmological supernova data really tell us?

Abstract

Not much by themselves, aparently. We try to reconstruct the scale factor a(t) of the universe from the SNe Ia data, i.e. the luminosity distance d L (z), using only the cosmological principle and the assumption that gravitation is governed by a metric theory. In our hence model-independent, or cosmographic study, we fit functions tois what is measured. We find that the acceleration history of the universe cannot be reliably determined in this approach due to the irregularity and parametrization-dependence of the results.However, adding the GRB data to the dataset cures most of the irregularities, at the cost of compromising the model-independent nature of the study slightly. Then we can determine the redshift of transition to cosmic acceleration as z t ∼ 0.50 ± 0.09 for a flat universe (larger for positive spatial curvature).If Einstein gravity (GR) is assumed, we find a redshift at which the density of the universe predicted from the d L (z) data is independent of curvature. We use this point to derive an upper limit on matter density, hence a lower limit on the density of dark energy. While these limits do not improve the generally accepted ones, they are derived only using the d L (z) data.

New generalizations of cosmography inspired by the Padé approximant

“…Finally, the usual cosmography with respect to the redshift z and y-shift y = z/(1 + z) has been extensively applied to various cosmological issues, for instance, the EoS of dark energy, modified gravity theories like f (R) and f (T ) theories, gamma-ray burst cosmology, and so on (see e.g. [14,[24][25][26][27][28][29][30][31][32][33][34][35][36][76][77][78][79][80][81][82][83][84][85][86][87][88]). The new generalizations of cosmography proposed in the present work can also be used in these cosmological issues, and we leave it to the future works.…”

Observational backreaction in discrete black holes lattice cosmological models

“…A key point of these results is that they hold for a vacuum spacetime without violating any energy condition and it can be interpreted as a direct consequence of using models which are both inhomogeneous and anisotropic while still possessing an approximate global isotropy. This calls for a more careful interpretation of the Hubble diagram because it suggests that dark energy may in truth be only an interpretative aspect of the current model of cosmology, and its indirect evidence from the supernova data may not constitute a solid physical result as some authors have been pointing out recently [16,43,44]. In fact, even before those observations it was argued that inhomogeneities along the line of sight would affect the motion of the light rays coming from an astrophysical source [45].…”

Pantheon update on a model-independent analysis of cosmological supernova data

“…As is well known, type Ia supernova (SNIa) observations [1,2] are taken as the first and foremost manifestation of accelerated expansion of the Universe. In a recent paper [3], we proposed a model-independent method for analysis of cosmological SNIa data, and questioned what they really tell us about the evolution of cosmic expansion. The present paper is an update of that work in several respects, necessitated by the compilation of the Pantheon dataset, containing about twice as many SNIa observations as before, with smaller relative errors in the observations.…”

“…Our proposal in that work [3] was to start with candidate relations that fit luminosity distance (d L ) versus redshift (z) of observed SNeIa well (we used the Union 2.1 data set [6]) and construct the first and second time-derivatives of scale factor using Friedman-Robertson-Walker (FRW) metric, leaving the spatial curvature free within a reasonable range. It should be emphasized that the method is a hybrid numerical/analytical algorithm, since the time-derivatives ȧ and ä can be found analytically as a function of z once a numerical fit for the parameters of the d L (z) candidate function is performed.…”