intensities of the red and blue wings of the Lyman-alpha line, reflecting the very small difference in energy of the hyperfine states relative to the Lyman-alpha transition. At a cosmological redshift of , Wouthuysen–Field coupling is expected to raise the spin temperature of neutral hydrogen above that of the CMB, and produce emission in the hydrogen line. Observational prospects A hydrogen line signal produced by Wouthuysen–Field coupling has not yet been observed. There are multiple experiments and radio observatories that aim to detect the neutral hydrogen line the Dark Ages and epoch of reionization, the time at which Wouthuysen–Field coupling is expected to be important. These include the Giant Metrewave Radio Telescope, the Precision Array for Probing the Epoch of Reionization, the Murchison Widefield Array, the Large Aperture Experiment to Detect the Dark Ages and . Proposed observatories that aim to detect evidence of Wouthuysen–Field coupling include the Square Kilometer Array and the Dark Ages Radio Explorer (DARE). See also Decoupling (cosmology) Notes References Space plasmas Astrophysics Physical cosmology