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.

A recent analysis of supernova Ia (SN Ia) data claims a "marginal" (∼3σ) evidence for a cosmic acceleration. This result has been complemented with a non-accelerating R h = ct cosmology, which was presented as a valid alternative to the ΛCDM model. In this paper we use the same analysis to show that non-marginal evidence for acceleration is truly found. We compare the standard Friedmann models to the R h = ct cosmology by complementing SN Ia data with baryon acoustic oscillations, gamma ray bursts, and observational Hubble datasets. We also study the power-law model, which is a functional generalisation of R h = ct. We find that the evidence for late-time acceleration cannot be refuted at a 4.56σ confidence level from SN Ia data alone, and at an even stronger confidence level (5.38σ) from our joint analysis. In addition, the non-accelerating R h = ct model fails to statistically compare with the ΛCDM, having a ∆(AIC) ∼ 30.

We constrain and update the bounds on the life-time of a decaying dark matter model with a warm massive daughter particle using the most recent low-redshift probes. We use Supernovae Type-Ia, Baryon Acoustic Oscillations and the time delay measurements of gravitationally lensed quasars. These data sets are complemented by the early universe priors taken from the Cosmic Microwave background. For the maximum allowed fraction of the relativistic daughter particle, the updated bounds on the life-time are found to be $\tau > 9\, \rm {Gyr}$ and $\tau >11\, \rm {Gyr}$ at $95\%$ C.L., for the two-body and many-body decay scenarios, respectively. We also comment on the recent proposal that the current two-body decaying dark matter model can provide resolution for the H0-tension, by contrasting against the standard ΛCDM model. We infer that the current dark matter decaying scenario is unlikely to alleviate the H0-tension. We find that the decaying dark matter is able to reduce the trend of the decreasing H0 values with increasing lens redshifts observed in the strong lensing dataset.