The publishers have kindly agreed to allow me to include a brief appendix in order to update some of the observational results contained in Chapters 4, 6 and 7. As noted in the preface, references in the main body of the text are complete to early 1992; here I provide summaries of some of the results reported in 1992, 1993 and early 1994. In the space of a few pages, I cannot hope to cover all the work in this field, in which hundreds of papers were published in that time interval. Instead, I will emphasize the most crucial observational results, and provide a few references to theoretical papers which establish new results or provide good reviews of the field.

Spectrum

The COBE team (Mather et al., Fixsen et al., and Wright et al., all 1994) have reported new and more complete results on the short-wavelength spectrum of the CBR. From a direct measurement of the spectrum using the FIRAS instrument (Section 4.9), Mather et al. (1994) determine T0 = 2.726±0.010 K. Their observations allow them to place limits on distortions of the spectrum; in the notation introduced in Chapter 5, these limits are |y| <2.5×10-5 and |µ| <3.3×10-4. These values, like the error in T0, are given at the 95% confidence level.