ar X iv : a st ro - p h / 99 04 18 8 v 1 1 5 A pr 1 99 9 Future Cosmic Microwave Background Experiments

We summarise some aspects of experiments currently being built or planned, and indulge in wild speculation about possibilities on the more distant horizon. The satellite missions MAP and Planck dominate any view of future measurements of the anisotropy of the Cosmic Microwave Background. We will attempt to look beyond and around those two experiments and see what sorts of physical questions other future projects might address. The reader has several advantages over the authors which we will not try to counter. First, many of the experiments which are in the near term future for us will be in the present or past for the reader, so we do not focus on evaluating detailed anticipated technical capabilities for a short list of such experiments. Readers who wish to pursue that approach might start at or other similar web-pages for up to date information and links. Second, readers of the rest this volume will be in a better position than we are to make informed judgements about the ideal strategies for measuring, avoiding or understanding foreground sources. Therefore, even though we think that this aspect of anisotropy will be an increasingly important and sophisticated part of the field, we have not put much emphasis on it here. As a crude aid to understanding how well future experiments are equipped to cope with foreground sources we have included a column giving the number of independent frequency channels for each experiment listed in Table 1. A view of the present situation, indicated in Figure 1 (see Smoot & Scott 1998 for more details), sets the context for our view of the future. Even at a casual and sceptical glance these experiments seem to be converging on a power spectrum which has a peak in it. This is confirmed by careful quantitative analysis of combined data sets (Bond, Jaffe & Knox 1998). Collectively these CMB measurements already tell us a number of fundamental things about the sort of Universe that we live in (see Lawrence, Scott & White 1999). The prospects for future measurements look very bright indeed. Announcements of the value of Ω 0 , for example, are likely to (continue to) come from experiments carried out from the best terrestrial sites or suspended from stratospheric balloons, during the next few years. However, the full belief of the community in any detailed cosmological conclusions will and should await the satellite results.