CMB Anomalies and the Hubble Tension

Abstract: The standard $\Lambda$CDM model of cosmology is largely successful in describing many observations, including precise measurements of the Cosmic Microwave Background (CMB) radiation. However, some intriguing anomalies remain currently unexplained within this theoretical framework. Such discrepancies can be broadly categorized into two groups: those involving CMB-independent probes and those within different CMB experiments. Examples of the former category include the $H_0$-tension between the value of the present-day expansion rate of the Universe inferred by CMB observations and local distance ladder measurements. The latter category involves anomalies between the values of cosmological parameters obtained by different CMB experiments, as well as their consistency with the predictions of $\Lambda$CDM. In this chapter, we primarily focus on this second category and study the agreement among the most recent CMB measurements to acquire a better understanding of the limitations and uncertainties underlying both the current data and the cosmological model. Finally, we discuss the implications for the $H_0$-tension.