Following a hiatus of some years, experimental efforts to detect CMB polarization are now underway. At the large angular scale, an experiment based in Wisconsin ([Keating et al.] 1997) plans to make a 30GHz measurement of polarization on angular scales with a sensitivity of a few K per pixel. The main goal of the experiment would be to look for the signature of reionization (see Fig. 14). Several groups are currently considering measuring the signature at smaller scales from recombination.
The MAP and Planck missions also plan to measure polarization. Because of their all sky nature these missions will be the first to be able to measure the polarization power spectrum and temperature cross correlation with reasonable spectral sensitivity. We show an example of the sensitivity to the which MAP should nominally obtain in the absence of foregrounds and systematic effects in Fig. 14. As can be seen, MAP should certainly detect the E-mode of polarization at medium angular scales, and obtain a significant detection of the temperature polarization cross-correlation. These signatures can be useful for separating adiabatic and isocurvature models of structure formation as we have seen in §4.
However the detailed study of the polarization power spectrum will require the improved sensitivity and expanded frequency coverage of Planck for detailed features and foreground removal respectively. Planck will have the sensitivity to make a measurement of the large-angle polarization predicted in CDM models, regardless of the epoch of reionization if foreground contamination can be removed. Likewise, it can potentially measure small levels of B-mode polarization. The separation of the E and B modes is of course crucial for the isolation of scalar, vector and tensor modes and so the reconstruction problem in general.
Clearly, the polarization spectrum represents another gold mine of information in the CMB. Though significant challenges will have to be overcome, the prospects for its detection are bright.
Acknowledgments: We thank D. Eisenstein, B. Keating, S. Staggs, M. Tegmark, P. Timbie and M. Zaldarriaga for useful discussions. W.H. acknowledges support from the W.M. Keck foundation.