$Λ$CDM Tensions: Localising Missing Physics through Consistency Checks

Abstract: $\Lambda$CDM tensions are by definition model dependent; one sees anomalies through the prism of $\Lambda$CDM. Thus, progress towards tension resolution necessitates checking the consistency of the $\Lambda$CDM 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 $\Lambda$CDM 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 $\Lambda$CDM model fitting parameters. We highlight recent progress made on ${S_8:= \sigma_8 \sqrt{\Omega_{\rm m}/0.3}}$ 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 $\Lambda$CDM cosmology.