Neutrino oscillation experiments establish a hard floor on the summed neutrino masses: at least 0.059 eV in the normal mass ordering and at least 0.10 eV in the inverted ordering. Cosmology is now pushing below that floor. Combining DESI baryon acoustic oscillations with CMB data yields upper limits as tight as Sum m_nu < 0.056 to 0.072 eV at 95 percent confidence (arXiv:2407.18047), already excluding the inverted ordering with a Bayes factor near 47 and pressing against the normal-ordering minimum itself. Depending on the dataset combination and tension metric, the disagreement between cosmological and terrestrial determinations runs from 2.5 to 5 sigma. A measured physical quantity cannot be smaller than its oscillation-mandated minimum, so something in the inference chain is wrong.
The sharper symptom is that the fits do not merely want small neutrino masses; they want negative ones. When the effective neutrino mass is allowed to take unphysical negative values as a diagnostic, the data prefer them, which means the CMB and large-scale structure show more lensing and more late-time clustering than a ΛCDM universe with even the minimum allowed neutrino mass should produce. Massive neutrinos suppress small-scale power; the data show no such suppression and, if anything, the opposite. Within ΛCDM every late-time deviation of lensing and growth from the fixed prediction is absorbed by the single parameter Sum m_nu, so an unmodeled lensing-like excess drives the inferred mass below zero and the bound below the physical floor.
The standing is acute because the bound keeps tightening as BAO data improve. Known relaxation valves each carry a cost: allowing evolving dark energy (w0wa) loosens the limit to roughly 0.16 eV but imports the dynamical dark energy preference; the lensing-amplitude anomaly correlates strongly with the neutrino bound, suggesting a common unmodeled cause; systematic explanations in the optical depth tau or BAO calibration remain under investigation. If DESI Year 5 and CMB-S4 confirm a bound below 0.059 eV within strict ΛCDM, the model is internally inconsistent with laboratory neutrino physics, a falsification-grade conflict at the heart of the standard cosmology.