Spectral appearance of non-uniform gas at high z

The spectrum of non-uniform gas emitting line radiation in an expanding universe displays frequency structure on scale $$\delta v/v \sim {10}^{-3}$$ if the scale of lumpiness is ~ 10^–3cH^–1, but the detailed spectral structure is correspondingly uncorrelated on angular scales larger than ~ 10^–3 rad ( ~ 4 arcmin). Thus, lumpy high-redshift gas may appear as a distinctive signature in the variation of the spectrum of background radiation across the sky, even if the bulk of the background comes from other sources. 21-cm emission or absorption might be visible with present techniques out to 1 + z ~. 10 (150 MHz), Ly-α emission out to 1 + z ~ 5 (6000 Å). These observations would be a valuable probe of cosmic structure at early times.