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The standard cosmological model, known as Lambda Cold Dark Matter (ΛCDM), has been highly successful in explaining a wide range of observations of the Universe, including the Cosmic Microwave Background (CMB) temperature fluctuations and baryonic acoustic oscillations. Despite these successes, several discrepancies arise within this theoretical framework. Chief among them is the “Hubble tension,” a notable discrepancy in measurements of the Hubble constant (H0), the current expansion rate of the Universe. This tension stems from differences in H0 values, which are derived either indirectly and model-dependently from the angular scale of standard rulers or model-independently using observations of standard candles.
This cosmological issue has persisted for over a decade and has even intensified as data volume and precision have increased. More recently, Planck CMB analyses and measurements from the Dark Energy Spectroscopic Instrument (DESI) Collaboration’s first data release indicate a preference for a negative neutrino mass—a nonphysical result that may be due to an excess in CMB lensing power compared to ΛCDM predictions. This presents a new discrepancy with the standard cosmological model that requires further investigation.
In this talk, I will address the negative neutrino mass anomaly and demonstrate that it remains below the two-signal level, though it still warrants further exploration. Then, assuming the Hubble tension is real, I will show that additional radiation density could alleviate this cosmological issue and present some models that might account for this extra energy density in the form of neutrinos.