Cosmological applications of H-alpha surveys

We briefly review three main applications of H-alpha surveys in cosmology, namely:(1) The diffuse H-alpha emission as a tracer of the free-free foreground that contaminates the fluctuations in the cosmic microwave background radiation; (2) the H-alpha emission from galaxies as a measure of the formation rate of massive stars, both at low and high redshift; and (3) the diffuse H-alpha emission from ionized clouds as a constraint on the local ionizing background radiation.