Identifying Cosmological Parameter Anomalies in the Low-Redshift Subset of the Pantheon Dataset: A Comparison between Chi-Square and Bayesian MCMC Methods

This work investigates the possibility of cosmological parameter anomalies in the lowredshift subset of the Pantheon+ compilation (z ≤ 0.03639). Using the Pantheon+SH0ES dataset and its covariance matrix, cosmological parameters (H ₀ , Ω _m , and ΩΛ) are estimated under the assumption of near-flat curvature (k ≈ 0). The χ ² and Bayesian MCMC methods are employed to test the robustness of the identified anomaly. The χ ² method yields narrower uncertainties with broader parameter distributions, while the Bayesian MCMC approach produces smoother distributions with significantly wider uncertainties. These results indicate robust inconsistencies in the low-redshift subset across both methods, suggesting the need to re-evaluate the contribution of low-redshift data to the Hubble tension problem.