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Paper IPM / P / 18102 | ||||||||||||||||||||||||||||||
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Within the Friedmann-Lema\^itre-Robertson-Walker (FLRW) framework, the Hubble constant H0 is an integration constant. Thus, consistency {of the model demands observational constancy of H0.} We demonstrate redshift evolution of best fit ΛCDM parameters (H0,Ωm) 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 ΛCDM 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 ΛCDM 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 (Ωm>1) to be in the 1−2σ 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 (H0,Ωm) 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 χ2. Our findings can be interpreted as a statistical fluctuation or unexplored systematics in Pantheon+ or ΛCDM model breakdown. The first two possibilities are disfavoured by similar trends in independent probes.
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