Massive neutrinos, extra neutrinos, and other surprises in ΛCDM Cosmology

Recent theoretical and observational insights suggest that the six-parameter ΛCDM model may no longer adequately describe cosmology. While datasets such as CMB anisotropies, BAO, type Ia supernovae, and large-scale structure generally align with ΛCDM, they often favor conflicting parameter spaces. Laboratory experiments confirm that neutrinos have mass, affecting cosmological observations through their gravitational effects. Furthermore, the Hubble tension remains a significant challenge to standard ΛCDM. This tension arises from discrepancies in the Hubble parameter values, derived either indirectly and model-dependently from the angular scale of standard rulers or more directly from the brightness of standard candles. In this talk, we will present current limits on neutrino mass from cosmological observations, indicating a preference for massless neutrinos over a normal hierarchy and a normal hierarchy over an inverted hierarchy at the ~2σ level. We will also address the negative neutrino mass anomaly, which, although minor at <2σ level, merits investigation. Finally, assuming the Hubble tension is real, we confirm that additional radiation density, possibly from neutrinos, could alleviate this tension. Surprisingly, we find that a fully-thermalized eV-scale short baseline sterile neutrino aligns with data as well as ΛCDM.