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| Paper IPM / P / 18102 |
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Within the Friedmann-Lema\^itre-Robertson-Walker (FLRW) framework, the Hubble constant $H_0$ is an integration constant. Thus, consistency {of the model demands observational constancy of $H_0$.} We demonstrate redshift evolution of best fit $\Lambda$CDM parameters $(H_0, \Omega_{m})$ in Pantheon+ supernove ({SNe}). Redshift evolution of best fit cosmological parameters is a prerequisite to finding {a statistically significant evolution as well as identifying alternative models that are competitive with $\Lambda$CDM in a Bayesian model comparison}. To assess statistical significance, we employ three different methods: i) Bayesian model comparison, ii) mock simulations and iii) profile distributions. {The first shows a marginal preference for the vanilla $\Lambda$CDM model over an ad hoc model with 3 additional parameters and an unphysical jump in cosmological parameters at $z=1$. From mock simulations}, we estimate the statistical significance of redshift evolution of best fit parameters and negative dark energy density ($\Omega_m > 1$) to be in the $1-2 \sigma$ range, depending on the criteria employed. Importantly, in direct comparison to the same analysis with the earlier Pantheon sample we find that statistical significance {of redshift evolution of best fit parameters} has increased, as expected for a physical effect. Our profile distribution analysis {demonstrates a shift in $(H_0, \Omega_m)$ in excess of $95\%$ confidence level for SNe with redshifts $z > 1$ and also shows} that a degeneracy in MCMC posteriors is not equivalent to a curve of constant $\chi^2$. Our findings can be interpreted as a statistical fluctuation or unexplored systematics in Pantheon+ or $\Lambda$CDM model breakdown. The first two possibilities are disfavoured by similar trends in independent probes.
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