In 1998, physicists caught neutrinos doing something astonishing: the particle changes its type on the fly. That is only possible if it has mass. This became the only laboratory proof of physics beyond the Standard Model. But exactly how much a neutrino weighs remains a mystery. Direct experiments, like the giant KATRIN setup in Germany, say the electron neutrino mass is less than 0.45 electronvolts. For comparison, the electron mass is about 511,000 electronvolts. The difference is like that between a grain of sand and a truck.

Cosmos paints a very different picture. The cosmic microwave background (the afterglow of the Big Bang) and the distribution of galaxies show that the sum of the masses of all three neutrino types does not exceed 0.056 electronvolts. That is several times smaller than what the lab allows. The problem is that the cosmological "weight" relies on the standard model of the Universe — with unchanging dark energy. But recent data hint that dark energy might have changed over time. If so, the neutrino mass derived from cosmology could be an illusion.

The authors of the new work took a clever approach. They analyzed how exactly cosmological data depend on the dark energy model and built two "protected" ways to measure the neutrino mass. The first way is to account for all possible dark energy evolution scenarios and still get a constraint: the sum of masses is less than 0.152 electronvolts. The second is to drop late-Universe data altogether and use only the cosmic microwave background together with the gravitational lensing effect (bending of light by massive objects). This method gives an upper limit of 0.41 electronvolts today, and with future telescopes — down to 0.28 electronvolts.

Interestingly, these "softer" estimates fall within the sensitivity range of future terrestrial experiments, such as Project 8. It aims to catch the neutrino mass at the level of about 0.1 electronvolts. So cosmos and lab may finally meet on the same scale.