We construct an updated extended compilation of distinct (but possibly correlated) f σ8(z) redshift space distortion (RSD) data published between 2006 and 2018. It consists of 63 data points and is significantly larger than previously used similar data sets. After fiducial model correction we obtain the best fit Ω0m − σ8 ΛCDM parameters and show that they are at a 5σ tension with the corresponding Planck15/ΛCDM values. Introducing a nontrivial covariance matrix correlating randomly 20% of the RSD data points has no significant effect on the above tension level. We show that the tension disappears (becomes less than 1σ) when a subsample of the 20 most recently published data is used. A partial cause for this reduced tension is the fact that more recent data tend to probe higher redshifts (with higher errorbars) where there is degeneracy among different models due to matter domination. Allowing for a nontrivial evolution of the effective Newton's constant(ga is a parameter) and fixing a Planck15/ΛCDM background we find ga = −0.91 ± 0.17 from the full f σ8 data set while the 20 earliest and 20 latest data points imply ga = −1.28 +0.28 −0.26 and ga = −0.43 +0.46 −0.41 respectively. Thus, the more recent f σ8 data appear to favor GR in contrast to earlier data. Finally, we show that the parametrization f σ8(z) = λσ8Ω(z) γ /(1 + z) β provides an excellent fit to the solution of the growth equation for both GR (ga = 0) and modified gravity (ga = 0).
General Relativity and the ΛCDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications.All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature.We list the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research.
A rapid transition of the dark energy equation of state parameter w at a transition redshift zt < 0.1 from w −1 at z > zt to w < −1 at z < zt can lead to a higher value of the Hubble constant while closely mimicking a Planck18/ΛCDM form of the comoving distance r(z)for z > zt. Such a Late w Phantom Transition (LwP T ) avoids the discontinuity of H(z) suggested in previous studies and thus does not require a step in the Pantheon Hubble diagram which is strongly constrained. We demonstrate that such an ultra low z abrupt feature of w(z) provides a better fit to cosmological data compared to smooth late time deformations of H(z) that also address the Hubble tension. The strongly present day phantom dark energy behavior implied by this class of models hints towards a rapid approach of a Big Rip singularity which for zt = 0.02 will rip the universe in less than 3.5 billion years. Early hints of such effect may be observable in the dynamics of the nearest and largest bound systems (e.g. Virgo structures). The LwP T can be generically induced by a phantom scalar field frozen by Hubble friction mimicking the cosmological constant and currently entering its ghost instability phase as Hubble friction decreases below the field dynamical scale.
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