ΛCDM Tensions: Localising Missing Physics through Consistency Checks

ΛCDM tensions are by definition model dependent; one sees anomalies through the prism of ΛCDM. Thus, progress towards tension resolution necessitates checking the consistency of the ΛCDM model to localise missing physics either in redshift or scale. Since the Universe is dynamical and redshift is a proxy for time, it is imperative to first perform consistency checks involving redshift, then consistency checks involving scale, as the next steps to settle the “systematics versus new physics" debate and foster informed model building. We present a review of the hierarchy of assumptions underlying the ΛCDM cosmological model and comment on whether relaxing them can address the tensions. We focus on the lowest lying fruit of identifying missing physics through the identification of redshift dependent ΛCDM model fitting parameters. We highlight recent progress made on S_8:= σ_8 √(Ω_ m/0.3) tension and elucidate how similar progress can be made on H_0 tension. Our discussions indicate that H_0 tension, equivalently a redshift dependent H_0, and a redshift dependent S_8 imply a problem with background ΛCDM cosmology.