Can tweaking how electrons behaved early on fix cosmology's greatest headache?

The Hubble tension—a 5-sigma disagreement between expansion rate measurements—might vanish if the electron mass varied with time in the early universe. New data from ground-based CMB experiments and galaxy surveys confirm this fix works for CMB alone, but fails when combined with late-universe measurements, revealing a fundamental incompatibility: raising today's expansion rate this way requires lowering the matter density, contradicting what we see in galaxies.