2.6.1. Warm Plasma in the Voids

There could be a very large baryon mass in plasma in the voids. Its emission may even have been observed: plasma at temperature 2 x 10⁶K fits a component of the diffuse soft X-ray background (Wang & McCray 1993). This might be produced locally by a relatively small amount of gas in the Galaxy (for a summary of evidence for and limits on hot gas around the Milky Way see Moore & Davis 1994), or it could be produced by diffuse extragalactic gas with a mean density

(33)

where the emissivity parameter ranges from = 1 for solar to = 0.1 for primordial abundances, and the clumping parameter C < n_e² > / < n_e >² may be significantly greater than unity. Gas at this temperature at low redshift has few other detectable effects. For example, the COBE limit on the Compton distortion of the microwave background spectrum, y < 1.5 x 10^-5 (Fixsen et al. 1996), is produced by a Hubble length of gas at a mean density = (T/10⁶)^-1 h^-1, so the bound on the y-distortion is not useful here. On theoretical grounds however it is difficult to see how there could be much mass in void plasma. If primeval, it would be surprising that the voids contain little else in the form of baryons, especially as detected in diffuse helium absorption at z 3 (see below). If blown in, it would be surprising if the plasma far exceeds the density of stars, and the required temperature in this case (to achieve sufficient gas velocity to refill the voids in a Hubble time) exceeds 10⁷ K, which does produce excessive anisotropy and spectral distortion in the background radiation.