Can 21 cm signals reveal when the universe's dark age ended?

The early universe's reionization—when ultraviolet light from stars relit neutral hydrogen—left a fingerprint in 21 cm radio emissions. Using neural networks trained on simulations, researchers show they can reconstruct the neutral-fraction history from how the 21 cm power spectrum evolves with redshift, recovering the midpoint timing to within z ≈ 0.01. The technique works even with realistic noise levels for next-generation radio telescopes like SKA, though it struggles to pin down the sharpness of the transition itself.