Limits on dust and metallicity evolution of Ly alpha forest clouds from COBE

We consider the possible observational consequences of dust and metals in L y alpha forest clouds. We relate the dust content, Omega(d)(Ly alpha), to t he metal evolution of the absorbers and assume that dust is heated by the u ltraviolet background radiation and by the cosmic microwave background (CMB ). We find that the dust temperature deviates from T-CMB by at most 10 per cent at redshift z = 0. The Lya cloud dust opacity to redshift similar to 5 sources around the observed wavelength lambda(0)similar to 1 mu m is tau s imilar to 0.13, and could affect observations of the distant universe in th at band. The expected CMB spectral distortions arising from high-z dust in Ly alpha clouds is similar to 1.25-10 smaller than the current COBE upper l imit, depending on the metallicity evolution of the clouds. If Ly alpha clo uds are clustered, the corresponding CMB anisotropy due to dust is similar to 10(-1) on angular scales theta less than or similar to 10 arcsec at freq uencies probed by various future/ongoing far-infrared (FIR) missions, which makes these fluctuations potentially detectable in the near future. Emissi on from C-II fine-structure transitions could considerably contribute to th e submillimetre (submm) range of the FIR background radiation. Depending on the ionization of carbon and on the density of metal-enriched regions, thi s contribution can be comparable with the observed residual flux at lambda approximate to 0.15 mm, after CMB subtraction. We argue that constraints on metal evolution versus redshift can be obtained from the observed flux in that range.