A Hike Through the Cosmic Redwoods: Massive Neutrinos and Other Surprises in Cosmology

The standard six-parameter ΛCDM model has long guided our understanding of the Universe, yet growing tensions suggest it may be incomplete. Laboratory experiments confirm that neutrinos are massive, leaving a measurable imprint on both cosmic expansion and structure formation. Meanwhile, discrepancies such as the Hubble tension and conflicting parameter spaces in CMB, BAO, supernovae, and large-scale structure hint at cracks in ΛCDM. Additional radiation density, potentially from sterile neutrinos or other hidden ultrarelativistic relics, could help alleviate these tensions. In this talk, we will discuss current limits on neutrino mass, the subtle "negative mass anomaly", and how fully-thermalized eV-scale sterile neutrinos fit observations as well as standard ΛCDM, highlighting the surprising ways neutrinos may reshape our cosmological picture.