Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

8.3. Generation of 3K Background

If the dust-reprocessed radiation is itself absorbed by the dust, then the radiation could be completely thermalized, leaving no residual distortions at all. Some people have therefore proposed that the entire CMB is grain-thermalized starlight (Layzer & Hively 1973, Rees 1978). This is possible in principle - and Equation (8.1) shows that this idea is certainly not precluded energetically - but the grains would have to form at a high redshift and be very elongated in order to thermalize at long wavelengths (Wright 1982, Hoyle & Wickramsinghe 1989, Hawkins & Wright 1988, Arp et al 1990). The FIRAS results now make this model rather hard to sustain. An alternative proposal is that black hole accretion generates the CMB at a somewhat higher redshift (z ~ 10³), when thermalization by free-free processes is possible (Carr 1981b). Of course, any scheme that envisages the CMB deriving from Population III stars or black holes also requires that the early Universe be cold or tepid (with the primordial photon-to-baryon ratio being much less than its present value of 10⁹). In this case, one must also invoke VMOs or their remnants to generate the observed light element abundance, as discussed in Section 5.2.