An improved model-independent assessment of the late-time cosmic expansion

Abstract

In the current work, we have implemented an extension of the standard Gaussian Process formalism, namely the Multi-Task Gaussian Process with the ability to perform a joint learning of several cosmological data simultaneously. We have utilised the "low-redshift" expansion rate data from Supernovae Type-Ia (SN), Baryon Acoustic Oscillations (BAO) and Cosmic Chronometers (CC) data in a joint analysis. We have tested several possible models of covariance functions and find very consistent estimates for cosmologically relevant parameters. In the current formalism, we also find provisions for heuristic arguments which allow us to select the best-suited kernel for the reconstruction of expansion rate data. We also utilised our method to account for systematics in CC data and find an estimate of H 0 = 68.52 +0.94+2.51(sys) −0.94 km/s Mpc −1 and a corresponding r d = 145.61 +2.82 −2.82−4.3(sys)Mpc as our primary result. Subsequently, we find constraints on the present deceleration parameter q 0 = −0.52 ± 0.06 and the transition redshift z T = 0.64 +0.12 −0.09 . All the estimated cosmological parameters are found to be in good agreement with the standard ΛCDM scenario. Including the local model-independent H 0 estimate to the analysis we find H 0 = 71.40 +0.30+1.65(sys) −0.30 km/s Mpc −1 and the corresponding r d = 141.29 +1.31 −1.31−2.63(sys) Mpc. Also, the constraints on r d H 0 remain consistent throughout our analysis and also with the model-dependent CMB estimate. Using the Om(z) diagnostic, we find that the concordance model is very consistent within the redshift range z 2 and mildly discrepant for z 2.

CMB Tensions with Low-Redshift H0 and S8 Measurements: Impact of a Redshift-Dependent Type-Ia Supernovae Intrinsic Luminosity

“…Another important, although less statistically significant, tension between high-and low-redshift measurements is the one on the estimate of the clustering of matter; this is usually encoded in the parameter σ 8 , i.e., the amplitude of the (linear) power spectrum on scales of 8h −1 Mpc. Recent measurements from low-redshift galaxy surveys found results on the combined parameter S 8 = σ 8 √ Ω m which differ from those inferred from CMB constraints. In Figure 6 we compare the results of the KiDS collaboration [45] (gray band) with the results obtained with the Planck+SNIa+BAO combination (solid lines) together with the case in which we do not include BAO data (dashed line), and the Planck alone case (green solid lines), again with the left and right panels referring to JLA and Pantheon SNIa datasets respectively.…”

“…In the following, we will compare our results with the local measurement of H 0 coming from the SH0eS collaboration [10] and the low-redshift measurement of S 8 = σ 8 √ Ω m from KiDS [45]. While the first low-redshift measurement is independent from the assumed cosmological model, the same does not apply to S 8 , and therefore we re-analyze KiDS data in our extended dark energy models, using the CosmoMC module publicly released by the collaboration (https://github.com/sjoudaki/kids450).…”

“…While the one on H 0 is indeed the most striking and statistically significant tension between current data sets, other inconsistencies between high-and low-redshift data have been found in recent years. In this paper, we will investigate also the tension in the matter clustering parameter S 8 , which combines the matter density Ω m and the amplitude of perturbations encoded in σ 8 . This parameter has been measured by galaxy surveys, e.g., by the KiDS collaboration [45], and it has been found to be in tension with the value extrapolated by Planck measurements by 2.3 σ.…”

Exploring the evidence for a large local void with supernovae Ia data

“…Different calibration techniques consider the usage of strong gravitational lensing (Taubenberger et al 2019), tip of the red giant branch (Beaton et al 2016;Freedman et al 2019), HII regions (Fernández Arenas et al 2018), etc. Due to the strong correlation of cosmic expansion dynamics and H 0 value, the model-independent local estimates remain crucial for the study of late-time cosmic evolution (see also Tutusaus et al (2019); Haridasu et al (2018b)).…”

“…Certainly, as the deceleration-acceleration transition redshift is well constrained at z t ≈ 0.6 (see e.g. Haridasu et al (2018b); Gómez-Valent (2019);Mukherjee et al (2019) for model-independent estimates), this alternative explanation requires a giant ( ≈ 3Gpc) isotropic void inside EdS background, which proved to be very unlikely compared to the standard cosmological model (Zibin 2011;Vargas et al 2017;Amendola et al 2013;Luković et al 2016). However, extending the LTB model with the addition of cosmological constant Λ, one can describe large (order of 100Mpc) local inhomogeneous matter distribution that converges to the standard ΛCDM model on very large scales (Valkenburg 2012;Valkenburg et al 2014;Rigopoulos & Valkenburg 2012).…”